- Email: [email protected]
The treatment of inconsistencies in Atlantic deep water salinity data
Pergamon
Deep-SeaResearch1, Vol. 41, No. 9, pp. 1387-1405,1994 0967-0637(94)E00016--6
Copyright~) 1994ElsevierScienceLtd Printed in Great Britain. All rightsreserved 0967~637/94 $7.00+ 0.00
The treatment of inconsistencies in Atlantic deep water salinity data A R N O L D W . MANTYLA*
(Received 3 September 1993; in revised form 20 December 1993; accepted 20 December 1993) Abstract--Salinity biases between hydrographic cruises are known to have occurred, and it has been r e c o m m e n d e d that the uniform characteristics of the basins east of the Mid-Atlantic ridge be used to discern m e a s u r e m e n t differences between cruises to the region. That approach was used with two high quality n o r t h - s o u t h eastern Atlantic expeditions (Oceanus Cruise 202 ["McTT"[ and South Atlantic Ventilation Experiment ["SAVE"] Leg III), to construct a reference to assess the salinity differences between I G Y cruises from the 1950s and some more recent expeditions compared to that reference. Tables of salinity differences were prepared so that one m a y make adjustments to data sets when mapping any characteristic that is sensitive to relative salinity errors, such as computed geostrophic currents, or when mapping any characteristic on density surfaces.
INTRODUCTION
IN order to map an ocean hemisphere property distribution, such as salinity, it is necessary to merge data from many different expeditions; no single expedition has sufficient areal coverage to reveal anything more than a general coarse scale picture. The North Atlantic now has reasonably good coverage of full water column hydrographic stations, but they are not of uniform quality. That is not unexpected, as analytical and instrumental techniques have changed over time. Prior to the late 1950s, salinity was typically determined by silver nitrate titration of seawater chloride concentration and the Knudsen formula S -- 0.03 + 1.805 Cl. That method was used by the Meteor Expedition in both hemispheres of the Atlantic in the late 1920s, and by the Discovery cruises around Antarctica in the 1930s. Ship-board titration salinities can be resolved only to two decimal places (WOOSTERand T A F T , 1958), and three place precision is desired for good density calculations and to map subtle deep salinity features. An indication of the measurement precision is shown by a O S scatter plot (Fig. la). There are, however, areas of the ocean that have no modern data in the vicinity of some Meteor and Discovery stations; so data sets that attempt to cover large areas are apt to include a few stations from both expeditions. The instruments of choice between the late 1950s and the early 1970s were conductivity salinometers (SCHLEICHER and B R A D S H A W , 1 9 5 6 ; PAQUETTE, 1958); data from those instruments were considered to be good to about 0.005 (Fig. lb and c). Inductive salinometers (BROWN and HAMON, 1961), with inherently better sensitivity than the older conductivity instruments, were widely used in the 1970s, and are still commonly used. *Marine Life Research Group, Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, C A 92093-0230, U . S . A . 1387
1388
A.W. MANTYLA
I
2.5
I
I
a) 1927 METEOR
2.1
S
°°~° o
I
I
I
b) 1957 CRAWFORD 10
0
O-c~o
18°-19°N 2.3
I
I
I c ~ ~/
16°-24°N
-+.010
+ .005~ '~°
o
°°/~°Q ° O
1.9
2.5
c) 1973 GEOSECS J e ( -
k) o ®
/
2.3 f
_+13013
_+.0016
2.1
1.9
2.5
e) 1988 McTf 17°-23°N
.~
f) 1988 McTT
O/~/
CTD
/'f Z
2.3 _+.0006
+.0015 2.1
1.9 I 34.88
t 34.90
I 34.92
I 34.94
I 34.88
I 34.90
t 34.92
I 34.94
SALINITY Fig. 1. Potential temperature vs salinity plots for selected cruises in thc northeast Atlantic basins, to illustrate salinity precision over time (assuming negligible tempcrature errors). (a) 1927 Meteor Stas 272,277,279,281,282 and 283. Salinities recalculated to three places from chlorinities listed in the original Meteor atlases. Error bar is + or - standard deviation. (b) 1957 Crawford Cr. 10, Stas 77-83. Salinities run on Schleicher and Bradshaw conductivity bridge. (c) 1973 GEOSECS Stas 114 and 115. Salinities run on U.W. conductivity bridge. (d) 1983 TTO/TAS Stas 73-93. Salinities run on a double conductivity ratio Autosal salinometer. (e) 1988 McTT Stas 83-91. Salinities run on same salinometer type as TTO/TAS. (f) 1988 McT1TMStas 86-87. Data from NBIS CTD profilers at 2 db intervals.
Treatment of inconsistenciesin Atlantic deep water salinity data
1389
They are operated at ambient temperatures and are subject to non-linear drift, so the results are often no better than from the older salinometers. The current state-of-the-art salinometer, in use since the early 1980s, is a double-sensitivity salinometer operated in a temperature controlled water bath; it has exceptional stability and a sensitivity of better than 0.001 (Fig. l d and e) (DAuPHINEEand KLEIN,1975; KNAPP and STALCUP,1991). Salinity data are also derived from continuous profiling CTD measurements. They have a high data rate, on the order of 25 data-points per second, allowing the averaging of enough observations for each decibar to result in smooth profiles in low temperaturegradient deep water areas (Fig. lf). The precision of those deep profiles can be quite high, although the accuracy is dependent upon field checks run on salinometers standardized with IAPSO Standard Seawater (SSW). Many expeditions over the past decade have exceptional salinity precision due to care in sampling and modern instrumentation, yet biases between expeditions remain. Preliminary versions of the maps of salinity on North Atlantic deep density surfaces revealed modern cruise to cruise differences, some greater than 0.005 (REID, personal communication). A salinity offset error of 0.005 results in a geopotential height error of 2 dynamic cm when integrated from 5000 db to the surface, certainly a non-trivial amount. SAUNDERS (1986) has pointed out significant salinity offsets in the deep data from cruises to the Northeast Atlantic Basins (NEAB). H e states: "The systematic differences exist only in the measurements and evidently not in the ocean", and offers a potential temperature (®) vs salinity (S) reference line S = 34.698 + 0.098 19 for N E A B stations deeper than 3000 db and below a potential temperature of 2.6°C, that can be used to "calibrate" other expedition salinities. I report here the results of using that approach with a more recent expedition data set as a reference to determine the relative salinity offsets of other expeditions to the North Atlantic. First, however, a general discussion will be given on some potential sources of bias and factors affecting precision, and on the suitability of the eastern basins of the Atlantic as a "standard reference calibration tank". SOME PAST OBSERVATIONS OF SALINITY BIAS Possible salinity biases between cruises have been mentioned often in the literature. To point out a few: FUGLISTER(1960) noted that salinities from the 1950s Atlantic IGY cruises were slightly higher than those from the Meteor Expedition over 30 years earlier. As will be shown below, they were also higher than salinities from cruises 30 years later. Fuglister also noted that salinities from the I G Y cruise at 28°N in the Eastern Basin appeared to be too low compared with those from the cruises at 24°N and 32°N. WORTHINGTONand WRIGHT (1970) agreed that the 28°N IGY salinity data was unreasonably low. They also noted that a 1958 V.F.S. Gauss cruise reported unreasonably high salinities, thought to be a result of long storage time before analysis ashore. CLARKEet al. (1980), in a three-ship operation near the Grand Banks of Newfoundland, found the Hudson salinities differed from the Chain and Cirolana salinities by about 0.006, so that amount was added to the Hudson salinities. Actually, comparisons with the Geosecs Expedition salinities taken in the same region later in the same year, and with more recent expeditions to the region, indicate that the Hudson salinities were closer to other cruise data than those from the other two ships, which were probably too high. The region is oceanographically active, the meeting place of abyssal waters from both the north
1390
A.W. MANTYLA
and the south (BROECKERand BAINBRIDGE,1978; MANTYLAand REID, 1983), so comparisons between cruises tend to be a little noisy, presumably due in part to natural variability. SAUNDERS (1986), in his careful evaluation of data taken by IOS in the deep basins of the Northern Atlantic between 1977 and 1983, found a very uniform relationship between salinity and potential temperature (O-S) for five IOS cruises, yet systematically different by 0.003 from two other 1981 cruises to the region: Transient Tracers in the Ocean (TTO), and Atlantis Cruise 109. The relative errors in various batches of IAPSO Standard Seawater used to standardize the salinity analyses were not known at. the time, but a later study (MANTYLA,1987) found relative errors in SSW that accounted for much of the differences between the IOS and the other two cruises. There is still a mystery with Atlantis 109: the low resolution CTD salinity profile data stored at N O D C differs from the bottle salinometer data from the s a m e casts by 0.004! Yet, the CTD satinities are usually derived from calibrations against the water sample data. SPEER and McCARTNEY (1991), in mapping characteristics on constant density surfaces for the Atlantic Ocean, attempted to remove salinity biases between cruises by accounting for relative SSW errors. They pointed out " . . . the salinity offset is somewhat greater along a curve of constant density than it is along the salinity axis" of the O-S diagram. That is true where the O-S slope between the deep and bottom water is positive, as it is in the deep Atlantic, Indian and South Pacific oceans; however this salinity offset error is somewhat less along constant density surfaces where the ®-S slope is negative, as it is in the North Pacific or elsewhere such as in the transition depths between the intermediate water salinity minimum and higher salinity colder deep water. Correction for SSW biases did improve inter-cruise comparisons, but the salinity on constant density maps still showed some unlikely contour distortions where cruise tracks cross, clearly an indication that some sort of salinity bias remained between cruises. Assuming that the perceived cruise salinity differences are due to biases and are not a result of real changes in the deep ocean, what could account for the differences? POTENTIAL SOURCES OF SALINITY BIAS Oceanic samplers have changed over time. Most early cruises used brass Nansen bottles with various materials such as tin plating, wax coating, or Teflon or epoxy-lining, to minimize reaction of the seawater sample with the metal. Coated or lined bottles still had exposed bare metal end valves. A comparison of samples taken from epoxy-lined Nansen bottles and PVC plastic Niskin bottles tripped close together in depth on the same Geosecs test cruise deep cast showed detectable salinity and oxygen differences; the Nansen bottle averaging 0.002 higher in salinity and 0.02 ml 1-~ lower in 0 2 than the Niskin Bottle (H. CRAIG, personal communication). Uncoated Nansen bottles would presumably have a greater effect on the deterioration of the water sample. Salinity samples collected at sea and run ashore at a later date are usually higher than they would have been if they had been analysed within a few days of collection at sea. That lesson was learned early in the C A L C O F I program in a curious way. In order to get a quick look at environmental surface conditions, the 10 m salinity samples were titrated immediately after the cruise. The remainder of the station salinity samples were titrated some months later. The resulting station curves showed a consistent and hydrostatically unstable 10 m salinity minimum, it became obvious that evaporation was occurring in the stored salinity samples (SIO, 1963). A later experiment of successive weighing of stored
Treatment of inconsistencies in Atlantic deep water salinity data
1391
salinity samples over several months showed an average evaporation rate of 0.052/100 days for salinity bottle and seals used at the time (J. L. REID, unpublished memo, 1954). Another storage test of salinity samples stored in glass citrate bottles and in screwcap polyethylene bottles had evaporation rates of 0.028/100 days and 0.076/100 days respectively (J. WELLS, 1980, personal communication). Recently, in a salinity comparison experiment between the University of Hawaii and the Monterey Naval Postgraduate School, RAGO and KENNAN (1991) found an evaporation rate of 0.007/100 days. From these observations, two things are apparent: (1) salinity bottle seals have improved over time; and (2) in order to get every digit of the third decimal place, the salinity samples must be analysed within a week of collection. Stored samples not only are apt to be too high, they are also noisier because evaporation rates very from bottle to bottle. IAPSO Standard Seawater was originally certified in terms of chlorinity, and the salinity inferred from the Knudsen relation. In practice, conductivity measurements are a more sensitive measurement of the salinity of seawater than chlorinity titrations, so IAPSO SSW was used as a conductivity standard, although it was not certified as such until 1980. The change occurred once it became clear that the conductivity-chlorinity relationship was not consistent. Salinity was re-defined in terms of the conductivity ratio relative to a standard KCI solution ( U N E S C O , 1981). When older batches of SSW were compared by conductivity, discrepancies were found in some batches between salinity calculated from the labeled chlorinity and the conductivity derived salinity. SOVLEet al. (1961) found relative errors between two batches of SSW prepared in the late 1940s of 0.018. PARK (1964) found relative differences of 0.018 for batches prepared in the late 1950s. Several investigators, as summarized in MANTYLA (1980), found relative errors of 0.009 between SSW batches P49 and P54, prepared in 1967 and 1970, respectively. Since 1980, with the start of conductivity ratio labeling of IAPSO SWW, all tested batches (P91 to Pl12) have been within 0.0015 of their labeled values, as shown in Table 1 (TAKATSUKI et al., 1991; MANTVLA, 1987, unpublished SIO comparisons). The bias in salinity measurements due to SSW offsets can be accounted for, if a record is kept of which batch of SSW was used for any particular expedition. Two examples are shown in MANXYLA(1980). Unfortunately, many older cruises did not keep track of the batch number because the fact that the standard seawater could be slightly in error was not generally appreciated. Even if the SSW batch is known, there is another type of error that can occur, one that is instrument-specific, depending upon whether the SSW batch was slightly greater, or slightly less than 35.000. Salinometers often have a response shift when the largest dial setting is moved, which occurs at 35.000. To illustrate the problem, calibration curves are shown for the U W salinometer, a conductivity salinometer that used 700.00 ohms as equivalent to 35.000 S. The conductivity of a series of samples prepared by weight dilutions of an evaporated high salinity stock solution are shown in Fig. 2a, as deviations from the salinity/conductivity slope near 35 S, and 700 ohms. Figure 2b shows a least squares polynomial curve fit to all of the calibration points. The fit turned out to be poorest near 35 and the juncture of the 600 and 700 ohm decades; curves fit to the 600 ohm and 700 ohm decades separately revealed a mismatch equivalent to 0.008 S there (Fig. 2c). The shift in the sum of the first 600 ohms is not unexpected as an instrument ages (R. PAQUETTE, personal communication, 1967), but its effect occurs at the critical point where the instrument is standardized with SSW when station salinity samples are being analysed. SSW batches with S > 35 caused a systematic error in sample salinities less than 35 when
1392
A.W. MANTVLa Table 1.
Recent comparisons o f l A P S O Standard Seawater Batches P91-P112
(S...... - Slabcl)X 103
Batch
Prep. d/m/y
P91 P92 P93 P94 P95 P96
10/5/80 29/10/81 31/10/81 18/I 1/81 8/3/83 3/3/83
P97
P98 P99 PI00 P101 P102 P103 PI04 P105 P106 P107 P 108 P109 P110 PI 11 PII2
Kj5
S
1.000117 0.99988 0.99990 0.99992 0.99997 1.000116
35.0027 34.9953 34.9961 34.9969 34.9988 35.01123
3/3/ 83
1.00002
35.0008
3/3/83 27/7/84 29/11/84 4/6/85 29/11/84 11/10/85 21/2/86 21/2/86 8/6/87 11/11/87 7/4/88 7/4/88 20/7/88 7/2•89 4/7/89
0.99993 0.99997 1.00003 1.00002 1.00001 0.99987 0.99994 1t.99988 0.99989 0.99991 0.99980 0.99976 0.99999 0.99982 1t.99984
34.9973 34.9988 35.0012 35.0008 35.0004 34.9949 34.9976 34.9953 34.9957 34.9965 34.992 I 34.9905 34.9996 34.9929 34.9937
MANTYLA(1989) Sept. 1985
S10 July 1987
SIO June 1989
TAKATSUK! et al..
1990
1.0
-l.5 -0.4 -0 . 2 0.9 1.2
-
1.4
1.0
-
1.2
0.7
0.8 0.8 -11.4 (1.3 (}.5 0.2
0.0
0.2
-0.3 0.6
0.3
- 1.0
-0.6
0.8 0.2 0.3 0.9 (1.6
(1.4
11.8
0.4
Note: Last digit of Kl~ conductivity ratio is equivalent to 0.4 × 10 3salinity. a n a l y s e d o n i n s t r u m e n t s that had the decade shift p r o b l e m , a p r o b l e m that was n o t usually k n o w n by the operator. This is critical b e c a u s e the m a j o r i t y of deep a n d b o t t o m waters have S < 35 (WORTHINGTON, 1981). A 1965 S I O calibration of all three of its 7-year old U W conductivity s a l i n o m e t e r s had a shift at 700 o h m s (MANTYLA, 1966, u n p u b l i s h e d m a n u script). T h e e x p e d i e n t solution was to reset the 35 S s t a n d a r d setting from 700 o h m s to 675 ohms, which placed salinities of a b o u t 30.5-36.5 on the same 600 o h m decade a n d e l i m i n a t e d the shift at the critical s t a n d a r d i z a t i o n point n e a r 35 S. All of the Geosecs salinities were r u n at sea on these i n s t r u m e n t s . A l t h o u g h there were n o i n s t r u m e n t d e p e n d e n t s t a n d a r d i z a t i o n errors for the Geosecs salinities, there were biases of + 0 . 0 0 2 to - 0 . 0 0 3 because of SSW batch errors a m o n g the seven different batches used. T h e i n s t r u m e n t response shift occurs in m o d e r n s a l i n o m e t e r s as well, as illustrated in Fig. 1 of MANTYLA'S (1987) SSW c o m p a r i s o n study. Most of the recent batches of SSW have S < 35, so recent deep a n d b o t t o m salinities, for which inter-cruise c o m p a r i s o n s are usually m a d e , should n o t suffer from the s a l i n o m e t e r shift p r o b l e m . Errors could occur for salinities greater than 35 of course, but those are usually in a shallower part of the water c o l u m n where there is g r e a t e r n a t u r a l variability a n d w h e r e the three place salinity accuracy may not be quite as critical as it is in abyssal waters. Differences b e t w e e n down-cast C T D salinity profiles a n d bottle sample salinities collected o n the up cast were alluded to above. A n up C T D profile can be adjusted to agree
1393
Treatment of inconsistencies in Atlantic deep water salinity data
0
(o) 3
.040
(9
I I o3
.020
- 80
I
R --700
I
- 60
-40
I
I
( ,t"Z )
I
O
I
01.0 ~. --20 4)~i~ 4)*' g® ' -.010
-~(9.1.* 20 I I I~I .,~(~.~**~pSt**
®
,,~*I** I
60
I
I
80
I
(b)
-80
-60
t R-
I
-40
I
700(
~
I
~-20 I
I ~ 1
.I.,
}-~.
~ ~"'~'----~
-.010-
/600
OHM
.040
l-
li
.020
-80
I
R-700
II
-60
I
I
~1 I {A/, ) -----.)-
-40
I
I
o
7 0 0 OHM
/
DECADE~/
.{a
" ' " . ~ 20
II ~
co)
80
I 0 ~ " - ~""
-.o,0
/
',
I
---~-"
Fig. 2. Calibration of U.W. conductivity salinometer No. 9, October 1965. (a) Calibration samples derived from weight dilutions of the high salinity stock solution, plotted as deviations from the salinity conductivity slope at 35%O. (b) Least squares polynomial curve fitted to all of the calibration points. (c) Curve fitted to the 600 and 700 ohm decades separately. The displacement between the two decades is equivalent to 0.008%o in salinity.
1394
A . W . MANTYLA
perfectly with the salinometer water sample data collected on the up cast, and yet differ from the down C T D profile if there is any hysteresis in the pressure, temperature or conductivity sensors. A temperature difference of one millidegree or a pressure difference of 2-3 db will result in a calculated salinity difference of 0.001. Each pressure sensor has a different hysteresis error. C T D pressure sensor calibrations performed by SIO's Oceanographic Data Facility ( O D F ) have had differences between increasing pressure calibration series (loading curve) and decreasing pressure calibration series (unloading curve) of typically 5-10 db, and as much as 20 db for different pressure sensors (M. C. JOHNSON, personal communication). A pressure hysteresis error of 10-12 db could account for a salinity difference of 0.005 between part of the down and up C T D traces if the pressure error is not accounted for. WILLIAMSand DELAHOYDE(1993) show procedures for handling the C T D hysteresis problem. Of the sources of bias listed above, only the SSW bias can easily be corrected for, if the SSW was less than 35 S. Use of SSW batches greater than 35 S may have produced instrument-dependent errors apart from the SSW batch bias itself; the salinity error in that case must be assessed some other way. Salinity samples stored for analyses much later are likely to be noisy and to have a high bias, and may not be correctable. CONSTANCY OF THE EASTERN ATLANTIC BASINS SAUNDERS (1986) has documented the uniformity of the deeper waters of the northeastern Atlantic basins and has r e c o m m e n d e d the locality as an environmental standard that could be used to calibrate the results from various other cruises. Time-variations in deep and bottom waters that have been noted close to their source regions (FosTER and MIDDLETON,1979; Swwx, 1984; LAZIER,1988) are not likely to be detectable in regions more remote from the source waters. The bottom waters of the N E A B are derived from abyssal waters from the south in the Western Atlantic basins by way of the V e m a Fracture Zone (McCARTNEYe t al., 1991; MANTYLA and REID, 1983). Newer northern deep water sources sdch as the Iceland-Scotland Overflow Water, Labrador Sea Water and Mediterranean Overflow Water are confined to depths shallower than about 3000 m (TsucHIYA et al., 1992). The eastern North Atlantic chlorofluorocarbon data set discussed by DONEY and BULLISTER(1992) confirms that the ventilation of the deep N E A B is on a much longer than decadal time scale. A 14C section along the deep N E A B (SCHLITZER,1985) shows a 14C minimum ("oldest" water) at a depth of about 3000-4500 m and at a density of about o4 = 45.86, a density that is found at the bottom of the N E A B between 40°N and 50°N. That suggests a scenario similar to those described for: (1) the deep North Pacific (MANTYLA, 1975) where the bottom waters have become less dense and return southward above the bottom water as deep water; (2) the modification of Weddell Sea Bottom Water (CARMACK, 1977) whereby it becomes less dense (and enriched in silica [LOEF and BENNEKOM, 1989]) as it spreads in the Atlantic-Antarctic basin and returns above the newly formed W S B W in a spiral fashion; and (3) the circulation schematic prepared by MCCARTNEY et al. (1991) for bottom waters colder than 2°(0) in the N E A B upwelling across the 2 ° isotherm as it spreads and becomes less dense. With no outlet for the densest water from the N E A B , it appears that the uniformity of the deep waters there is the product of dense waters passing through the V e m a Passage over sill depths of about 4700 m (McCARTNEY et al., 1991), cascading down to depths as great as 6000 m in the G a m b i a Abyssal Plain, and becoming less dense as they spread eastward and northward, thereby
Treatment of inconsistenciesin Atlantic deep water salinity data
1395
producing a thick body of uniform water between 20°N and the Azores Rise (TsucHIYA et al., 1992) that can ride back over the denser incoming water and has the oldest 14C age in the region (ScHLITZERet al., 1985). Saunders' O-S regression line for N E A B waters below 2 . 6 0 encompasses this water mass and also falls on WORTmNCa'Orq'S (1981) first seven greatest-volume O-S classes for the Atlantic Ocean. Detection of any deep or bottom water time variations in the N E A B is not expected because of the large volume of the present water mass and the remote source of the bottom water. Thus, the N E A B is an ideal locality to compare cruise results, as recommended by Saunders. Eastern Atlantic basins to the south of the Gambia Abyssal Plain are filled with dense western basin waters through the Romanche Fracture Zone (MANTYLAand REID, 1983). The chlorofluoromethane data shown by WARNER and WEISS (1992) indicate no recent ventilation there either, so perhaps the eastern and southeastern Atlantic basins might be a suitable location for expedition intercomparisons also. REVISION OF S A U N D E R S ' O-S REFERENCE LINE
Saunders' visual O--S fit of S = 34.698 + 0.0980 to N E A B cruises in 1972 and 1981 between 30°N and 41.5°N had a standard deviation of 0.002 S and the reference line also fit well with three other cruises in 1982 and 1983 between 26°N and 46°N. The agreement may have been fortuitous, because five different batches of IAPSO SSW were used that had offset errors from +0.001 to - 0 . 0 0 2 S, and three of the five batches were greater than 35 in salinity, which could have led to some bias for the deep salinities which are all less than 35. Also, according to WORTmNCTONand WRICHT'SNorth Atlantic Atlas (1970), there is some salinity variation at O = 2.6 °, from about 34.94 to 34.96, so there should be some latitudinal variation, at least in the O-S slope. Two expeditions of 1988 (Fig. 3) that have high quality salinity data in north-south transects of the eastern Atlantic basins were selected to evaluate the O-S relations. Oceanus Cr. 202 ("McTT" [McCARTNEY et al., 1991]) went from Iceland to the equator between 20°W and 29°W taking stations at half degree latitude intervals or less; and the South Atlantic Ventilation Experiment (SAVE) Leg III (JENKINS and OLSON, 1992) between the Walvis Ridge and Abidjan, Ivory Coast was between 5°W and I°E with station spacing of 1° or less. McTT used IAPSO SSW batch P108 and SAVE III used P106; the offset errors for both are 0.001 or less (Table 1). Closer examination of individual McTF O-S curves showed some curvature above 2.5°O in the north, and a bend at 2°0 and less to the south (the familar "two-degree discontinuity" [BRoECKERet al., 1976]); therefore the regressions were confined to the potential temperature range between 2.0 ° and 2.5 ° . In order to have enough data points for good statistics, the regressions were run for all of the data grouped from stations over 5 ° latitude, at 2.5 ° latitude intervals; about 25-50 data points were included in each latitude group. The linear regressions generally had correlation coefficients of 0.985 or higher and salinity standard deviations of 0.0015 or less. That is not to say that the O-S relationship over this temperature range is perfectly linear, but only that the non-linearity is too small to affect comparisons between cruises significantly. The results of the regressions for McTT and SAVE III cruises are shown in Fig. 4, plotted versus latitude. The salinities shown are calculated from the regression equations at the bracketing potential temperatures of 2.5 ° and 2.0 ° and the mid-range potential temperature of 2.25 °. Straight line fits over segments of latitude were made for the ®-S slope and for the 2.25 ° salinity, and the dashed lines for the 2.0 ° and 2.5 ° salinities
1396
A . W . MANTYLA
60°
60 °
40 °
40 o
20°
20°
0°
0o
Fig. 3. Location of north-south Oceanus Cr. 202 (McTI') stations (August 1988) and Knorr South Atlantic Ventilation Experiment (SAVE) Leg Ill stations (February-March 1988) used to establish reference salinity-potential temperature relationships of the uniform deep waters of the eastern Atlantic basins (O). East-west stations are from the late 1950s IGY cruises (+). 40(X)m depth contour shown.
were calculated from those fits. T h e S a u n d e r s regression (SR) is also shown b e t w e e n 20°N a n d 50°N. T h e Mcq~F data is 0.002-0.003 lower t h a n SR, but that is close to the c o m b i n e d error estimates of both data sets. T h e ® - S slope does vary with latitude, though by only 10% b e t w e e n 20°N a n d 50°N (0.093-0.104). T h e 6)-S r e l a t i o n s h i p changes m o r e rapidly south of 20°N, b u t the c o r r e l a t i o n coefficients r e m a i n consistently good until 15°S, where they begin to d e t e r i o r a t e rapidly as waters from o t h e r sources e n t e r the region a n d result in curved O - S plots, especially south of 20°S. T h e r e is only a 0.009 salinity range at O = 2.0 ° from 20°S to 35°N where it intersects the b o t t o m (salinities at O - S = 2 ° b e t w e e n 35 ° a n d 50°N e x t r a p o l a t e d ) ; most of the latitudinal change in the O - S slope is due to salinity
(a)
z~/A''z~ ~
I
~
~"~/
I
EQ
~
S~0=2.50
I o
I
.00 ~
I
20 °
/.O.I"O"e-°
LATITUDE
~ s ~ ° = 2o ' 2c s~. . . ~ , - ' e
"
I
10o
I
30°
I
I
40°
I
I
S0°N
0
,3 n
0 Mc'rr Z~ SAVEIll
0 n
SR
0
O
(b)
O
O O
@
__E)__ea3__C)__O_~(3---O----O---~ -~9-
[
Fig. 4. Salinity-potential temperature linear regression results versus latitude from the McT1 ~ ((D) and S A V E I l l ( A ) expeditions. Regressions over ® = 2.0 ° to 2.5 ° and over 5 ° latitude bands, at 2.5 ° latitude intervals. (a) Regression calculated salinities at 2.5 °, 2.25 ° and 2.0°®. S at ® = 2.0 ° north of 35°N is extrapolated. (b) O - S regression slopes versus latitude. Saunders' reference indicated by lines labeled "SR" between 20°N and 50°N.
092
0.04
0,06
0.08
34.890
34.900
34.910
34.920
34.930
34.940
I
I
I
10°
20°S
tao
e~
,7
>
9.
8
g
,.q
1398
A . W . MANTYLA
Table 2. Linearfits to ®-S slopes (between 19 = 2.0 ° and 2.5 °) versua" latitude and S at ® = 2.25 ° versus latitude over segments o f latitude in the eastern Atlantic S=A
+B(O-2.25
°)
where A = 34.9144 + 0.0001N5¢ between ¢ = 22.5°N and 50°N 34.9000 + 0.000747¢ between 5°S and 22.5°N 34.8992 + 0.000388¢ between 19°S and 5°S where B = 0.08569 0.05044 0.05808 0.06490
+ + + +
0.000345¢ 0.001950¢ 0.000987¢ 0.002087¢
between between between between
¢ = 22.5°N and 50°N 7.5°N and 22.5°N 5°S and 7.5°N 19°S and 5°S
Note: Relationships are valid only in the vicinity of the McTT and SAVE III cruises. They may not be applicable in the region between the two cruises at 0°-5°N.
variation at the higher temperatures. The standard deviation of the latitudinal fits to the McTT and SAVE 2.25°0 regression salinities is 0.0005, so the two cruises form a remarkably consistent data set that can be used to evaluate salinity data from other expeditions that have taken station data in the eastern Atlantic. The coefficients for the latitudinal variations of the 2.25 ° salinity and the O-S slope are given in Table 2. COMPARISON
OF OTHER
EXPEDITIONS
WITH
THE McTT--SAVE REFERENCE
The Atlantic IGY data used in the FUGLISTER(1960) and WORTHINGTONand WRIGHT (1970) atlases form a large area data set (Fig. 3) taken over a period of a few years that can be usefully compared with the M c T T - S A V E reference regressions. Each of the zonal IGY lines between 16°S and 52°N was evaluated in the same way as with M c T T - S A V E data: least-squares regression lines were fit to the eastern basin stations over the potentialtemperature range of 2.00-2.5 °, and the regression-calculated salinity at 2.25 ° was then compared to the reference line (Fig. 5). The I G Y O-S slopes were generally the same as for McT-I'-SAVE, but the IGY salinities were uniformly higher than the recent salinities. Interestingly, where duplicate salinity samples were collected from the same Nansen bottles, but run separately by the UK and the US, the U K results were closer to the modern data by 0.003 to 0.004 S (Fig. 5, at 24°N and 43°N, Table 3). Thus it is not likely that the deep N E A B salinity was higher in 1958-1959 than it is now. Further support for the lack of salinity change in the N E A B may be found in the 1927 Meteor Expedition data, they agree very well with Geosecs (1973), T T O (1983) and McTT (1988) (Fig. 6). The IGY salinity offsets are well within possible IAPSO SSW batch-to-batch offsets, typically about 0.007 high for the US salinities. In order to bring the IGY salinities into agreement, modern cruise data salinity corrections for each latitude crossing are tabulated in Table 3. The 2.25 ° regression salinities are shown for "modern" cruises in Fig. 7, along with the M c T T - S A V E reference lines. As noted by Saunders, the agreement between modern cruises is not as good as modern analytical salinity precision would lead one to expect. IAPSO SSW offsets have been accounted for in all of the cruises shown in Fig. 7 except for the Meteor 56 cruise, yet biases between cruises clearly remain. Atlantis 109 bottle data,
Fig. 5.
[]
I
I
EQ
S @ 0 = 2.25 °
I
I
10°
I
[]
LATITUDE
I
~
/
[]
20 °
us
=
Qus
t
30 °
[]
13
I
[]
- - - ~ ~ £ ) ~
13 w
13
I
~
13
~
uJ
•
[]
I
40*
us
[]
m
13
~
~
'
"uux 13 13
~
13 u~
13
I
I
,
IS
.50°N
Results of I G Y cruise salinity-potential temperature regressions plotted versus latitude, as in Fig. 4. Reference lines are from McTT and S A V E data. Note cruises at 24°N and 43°N had salinities determined by both the U.S. and U . K .
0.02
0.04
0.06
0.08
0.10
34.890
34.900
34.910
34.920
34.930
I
I
I
lO °
20°S
~"
taJ
e~
.=. ,.7
g
m
w
>
8
1400
A.W. MANTYLA
2.4 1
~
2.2
19571GY
1927MT
2.0
I
I
I
34.88
I
I
34.90
I
34.92
I
r
34.94
S Fig. 6. Comparison of linear regression fit to 1927 Meteor, 1957 Crawford IGY, 1973 Geosecs, and 1988 McTT O-S data between 2.0 ° and 2.5°C. The 1927 Meteor data agrees well with modern data, while the IGY data is systematically higher in salinity than the others. Regression lines are from Fig. 1.
Table 3.
O-S relationships for 1GY Atlantic cruises in order of north to south transects'
S/® Expedition
Date
Discovery I1-3 Discovery II-1 Discovery II-3 Discovery I1-3
Aug. Apr. Nov. Nov.
'58 '57 '58 '58
Crawford 16 Crawford 16 Chain 7 Discovery II-2 Discovery II-2
Oct. Oct. May Dec. Oct.
'57 "57 '59 '57 '57
.
.
.
. Crawford Crawford Crawford Crawford Crawford Crawford Crawford
. 10 16 10 22 10 10 22
.
.
Feb. "57 Nov. '57 May '57 Nov. '58 Mar. '57 Apr. '57 Oct. '58
Stations
Latitude
N
slope
3853-3860 3531-3542 3869-3880 3889-3906
50°N-53°N 47°N-49°N 46°N 43°N
20 34 32 38 37 42 25 42 49 52 57 24 58 65 61 47 63 --
0.08773 0.09743 0.09322 0.10185 0.09905 0.09817 (/.09669 0.09436 0.10268 0.09477 0.09847 0.08924 0.08785 0.06761 0.05531 0.0528 0.03607 non-linear
.
.
.
.
245-253 259-264 61-71 3635-3647 3592-3606
.
. 77-83 278-297 155-170 464-481 86-104 137-151 437-455
40°N 37°N 36°N 32°N 24°N
. 16°N-24°N 16°N 8°N 0°S 8°S 16°S 24°S
S at (9 = 2.25 °
S corr. × 103
34.9249 34.9235 34.9207 34.9217 34.9184 34.9272 34.9233 34.9252 34.9169 34.9218 34.9173 34.9261 34.9180 34.9125 34.9068 34.9036 34.9011
8 -7 -4 -5 (U.S. salts) - 2 (U.K. salts) - 11 -7 9 - I - 6 (U.S. salts) - 2 (U.K. salts) - 10 -6 -6 7 -7 -8 (-~ 7)
Fig. 7.
0
©
I
<>
(3
I
0
I
I
EQ
©
I
I
10o
(3
I
0
LATITUDE
I
~"
20 °
c~Br~ A
(3 (3 cro (D
I
I
30 °
O
(3
I
©cro (3 Br~
I
40 °
0
0
I
50°N
Results of some modern cruise salinity-potential temperature regressions, as in Figs 4 and 5. Note the cruises at 24°N and 36°N show both C T D and bottle salinity results, the bottle data agrees well with the M c T T reference line.
0,02
0.04
0.06
0.08
0.I0
34.890
34.900
34.910
34.920
34.930
20°S
10 o
4~
e~
e~
o 3" >
o.
8
3'
¢D
3
1402
A.W.
Tabh"
Expedition GEOSECS GEOSECS GEOSECS GEOSECS M E T E O R 56-5 TTO-NAS TI'O-NAS A T L A N T I S 11t9 BTL A T L A N T I S 109 CTD A T L A N T I S 1119 BTL A T L A N T I S 109 CTD H U D S O N 82002 TTO-TAS O C E A N U S 133~5 K N O R R 104 K N O R R 104 K N O R R 104 K N O R R 104 K N O R R 1(14 AJAX AJAX AJAX AJAX AJAX SAVE SAVE SAVE S A V E II TAS 8
Date Mar. '73 Mar. '73 Feb. '73 Feb. "73 Apr. "81 Jun.'81 May "81
4.
Stations
O-S
MANTYI,A
relationships/br modern Atlantic" cruises
Latitude
114-117 21°N-31°N 112-113 6 ° N - I I ° N 109-1 I 1 2°N-2~S 106108 5°S- 16°S 513-517 25°N-31°N 1111-119 37°N 51°N 48--109 29°N 35°N
N
O/,,; slope
.S'at (") , 2.25 °
SSW batch
Corr. × l0 :~
('orr. S at O = 2.25 °
S corr. x 103
fl 0 0 0
34.9147 34.9079 34.8998 34.8091
nil - 1.6 nil nil nil nil nil
37 12 t6 18 33 46 1211
(I.fl9310 11.068(19 0.05428 11.113852 0.09799 I).119851 0.09574
34.9147 34.9117t) 34.8998 34.8(t91 34.9162 34.9167 34.9164
P59 P59 P59 P59 '? P81) P80
wn ~a~
Jun. '81
72 86
36.25~'N
46
11.1193~H
34.9160
PSI
+I
34.9176
nil
Jun. '81
77-81
36.25°N
54
11.1197211 34.tH97
P81
t I
34.92117
- 4
Aug. '81
158-171
25. F'N
38
{1.(18886
34.9149
PSI
+1
34.9159
nil
Aug. '81 Apr. '82 Dec. '82 Mar. "83 Aug. '83 Aug. '83 Aug. '83 Aug. '83 Aug. '83 Oct. '83 Oct. '83 Oct. "83 Oct. '83 Oct. "83 Dec. '87 Dec. "87 Dec. "87 Jan. '88 Mar. '89
165-169 8-24 81~93 15-27 45-49 50-55 56-62 63~9 70-81 4-8 8-14 14-18 1823 23-28 34-40 47-52 52-56 61-77 4~3
24.5°N 37 48°N-49°N 92 20°N-27°N 37 I 1.25°S 28 30°N-35°N 22 25°N-30°N 37 20°N-25°N 28 15°N-20°N 311 10°N-15°N 46 0°N-3°N 18 I1°S-5°S 29 5°S-10°S 23 10°S-15°S 36 15°S-20°S 41 I°N-5°N 37 f)°S-5°S 27 5°S-111°S 27 11)°S-14°S 100 I 1.3°N
11.(19562 1).1024 0.091)98 11.1t4241 0.08952 0.09471) I).09064 I).08078 0.07258 0.06421 0.05072 0.04998 11.11392 0.02539 0.06016 0.05651 0.05591 [).1)4060 0.117380
34.9192 34.92114 34.9147 34.8989 34.921)4 34.92311 34.9229 34.9187 34.9153 34.91165 34.9011 34.8994 34.8972 34.8931 34.9t144 34.91X)2 34.91X18 34.8954 34.9084
PSI P85/86 P9()/811 P93 P93 P93 P93 P93 P93 P90 P90 P90 Pg0 Pg0 PII16 P106 PI06 P106 P108
-~ I I ~ 1.4 0.4 -11.4 0.4 -11.4 0.4 -11.4 2 2 2 2 2 1 I 1 I +11.3
34.92112 34.9194 34.9161 34.8985 34.921t11 34.9226 34.9224 34.9183 34.9149 34.9045 34.8991 34.8974 34.8952 34.891 I 34.9034 34.8992 34.8998 34.8944 34.9087
4 - 3 nil 4 -5 --7 -7 5 -6 --3 nil nil nil nil nil nil 3 nil nil
TAS 8, TTO/NAS, S A V E I, A J A X , Meteor 56, and parts of TTO/TAS, S A V E II, and GEOSECS agree well with the M c T T - S A V E III reference data; but Atlantis 109 CTD, Knorr 104, Hudson 82-002 and Oceanus 133-5 do not. Offset adjustments needed to bring modern cruise salinities into agreement are tabulated in Table 4, differences of 0.001 or less are not significant and are listed as "nil". Evaluations of a few key western Atlantic basin cruises were done by comparison of deep salinities with the westward extension of the Atlantis 109 lines at 36°N and 24°N. The Endeavor 129 cruise at about 64°W was found to be in agreement with the reference cruise, while Oceanus 133, Leg 7 at about 53°W appeared to be about 0.003 higher than the reference cruises. The individual expedition salinity corrections listed here have been used by REID (1994) in his study on the total geostrophic circulation of the Atlantic Ocean. Also, the abyssal Atlantic characteristics maps shown in MANTYLA and RE~D (1983) have been revised on the basis of the corrected data set. Agreement between cruise crossings showed good improvement in both salinity and density, further supporting the internal consistency of the revised Atlantic deep data array.
Treatment of inconsistencies in Atlantic deep water salinity data
1403
DISCUSSION
Although the M c T T - S A V E data set yields a gratifyingly precise salinity reference for the eastern basins of the Atlantic, only future cruises will tell if the data are correct or not. The selection of these two cruises as a standard is entirely arbitrary, of course. The McTI'S A V E salinities tended to be lower than many other data sets; it is quite possible that some unknown source of bias could exist for the data set (such as possible non-conductive material added to the sample by the plastic rosette bottles or the coated inner springs within the rosette bottle); but several other expeditions agreed closely with M c T T - S A V E , so the reference line is believed to be good. One value of comparing stations taken in the eastern Atlantic lies in cruises that have stations on both sides of the Mid-Atlantic Ridge. If a cruise shows good agreement with the reference stations in the deep eastern Atlantic, that would lend credence to differences seen at mid-water depths or to bottom water differences seen between cruises in the western Atlantic basins. However, with temperature and salinity both currently being measured to three decimal places in the deep ocean, the approximately ten to one ® - S slope in the deep north Atlantic makes temperature the more sensitive indicator of any secular changes that might be going on in the abyssal ocean. The technique of evaluating expedition data taken in deep, uniform basins should work in other oceans as well. The northeast Pacific basin is the home of the largest volume O-S class in the world oceans (MoNT6OMERY, 1958); it should be a good region to compare results from different expeditions, taking care to avoid the real near-bottom variability observed by JOYCE et al. (1986). In the Indian Ocean, the central Indian basin is remote from bottom water sources (MANa'YLA and REID, 1983) and appears to be quite uniform in characteristics (WARREN, 1981); it tOO should serve as an environmental calibration area to compare Indian Ocean cruises. Acknowledgements--The work reported here was supported by the National Science Foundation and by the Marine Life Research Program of the Scripps Institution of Oceanography.
REFERENCES BROECKER W. S. and A. BAINBRIDGE(1978) An abyssal shear zone. Journal of Geophysical Research, 83(C4), 1963-1966. BROECKER W. S., T. TAKAHASHIand Y. -H. LI (1976) Hydrography of the central Atlantic--I. The two-degree discontinuity. Deep-Sea Research, 23, 1083-1104. BROWN N. L~ and B. V. HAMON (1961) A n inductive salinometer. Deep-Sea Research, 8, 65-75. CARMACK E. C. (1977) Water characteristics of the Southern Ocean south of the Polar Front. In: Voyage of discovery, M. ANGEL,editor, Pergamon, Oxford, pp. 15-41. CLARKER. A., H. W. HILL,R. F. REINIGERand B. A. WARREN(1980) Current system south and east of the Grand Banks of Newfoundland. Journal of Physical Oceanography, 10, 25-65. DAUPHINEET. M. and H. P. KLEIN (1975) A new automated laboratory salinometer. Sea Technology, 16, 23-25. DONEY S. C. and J. L. BULLISTER(1992) A chlorofluorocarbon section in the eastern North Atlantic. Deep-Sea Research, 39, 1857-1883. FOSTER T. D. and J. H. MIDDLETON (1979) Variability in the bottom water of the Weddell Sea. Deep-Sea Research, 26, 743-762. FUGLISTERF. C. (1960) Atlantic Ocean atlas of temperature and salinity profiles and data from the International Geophysical Year of 1957-1958. Woods Hole Oceanographic Institution Atlas Series I, 209 pp. JENKINS W. J. and D. B. OLSON (1992) Chemical, Physical, and CTD Data Report. South Atlantic Ventilation Experiment (SAVE) Leg 3, $10 Ref. 92-9, Scripps Institution of Oceanography, University of California San Diego, 447-729.
1404
A . W . MANTYLA
JOYCE T. M,, B. A. WARREN and L. D. TALLEY (1986) The geothermal heating of the abyssal subarctic Pacific Ocean, Deep-Sea Research, 33, 1003-1015. KNAPP G. P. and M. C. STALCUP(1987) Progress in the measurement of salinity and oxygen at the Woods Hole Oceanographic Institution. Technical Report, WHOI-87-4, Woods Hole Oceanographic Institution, 27 pp. LAZIER J. R. N. (1988) Temperature and salinity change in the deep Labrador Sea, 1962-1986. Deep-Sea Research, 35, 1247-1253. LOEE M. M. R. v. and A. J. v. BENNEKOM(1989) Weddell Sea contributes little to silicate enrichment in Antarctic Bottom Water. Deep-Sea Research 36, 1341-1357. MANTYLA A. A. W. (1975) On the potential temperature in the abyssal Pacific Ocean. Journal of Marine Research, 33,341-354. MANTYLA A. W. (1980) Electrical conductivity comparisons of Standard Seawater batches P29-P84. Deep-Sea Research, 27, 837-846. MANTYEA A. W. (1987) Standard Seawater comparisons updated. Journal of Physical Oceanography, 17, 543-548. MANTYLAA, W. and J. L. REID (1983) Abyssal characteristics of the World Ocean waters. Deep-Sea Research, 30, 805-833. MCCARTNEY M. S., S. L. BENNETT and M. E. WOODGATE-JONES(1991) Eastward flow through the Mid-Atlantic Ridge at 11°N and its influence on the abyss of the Eastern Basin. Journal of Physical Oceanography, 21, 1089-1121. MCCARTNEY M. S., L. D. TALLEYand M. TSUCHIYA(1991) Physical, Chemical and CTD Data Report. Oceanus 202 (McTT). SI0 Ref. 91-16, Scripps Institution of Oceanography, University of California San Diego, 305 Pp. MONTGOMERY R. B. (1958) Water characteristics of the Atlantic Ocean and of the world oceans. Deep-Sea Research, 5,134-148. PAQUETrE R. G. (1958) A modification of the Wcner-Smith-Soule salinity bridge for the determination of salinity in sea water, with details of construction, operation, and maintenance. University of Washington, Department of Oceanography, Technical Report No. 54-14, 1-57. PARK K. (1964) Reliability of Standard Sea Water as a conductivity standard. Deep-Sea Research, 11, 85-87. RAGOT. A. and S. C. KENNAN(1991) A 2-year intercomparison of measurements of salt water samples between the University of Hawaii and the Naval Postgraduate School. Transactions of the American Geophysical Union, 72,260. REID J. L. (1994) On the total geostrophic circulation of the North Atlantic Ocean: ltow patterns, tracers and transports. Progress in Oceanography, 33, 1-92. SAUNDERS P. M. (1986) The accuracy of measurement of salinity, oxygen, and temperature m the deep ocean. Journal of Physical Oceanography, 16, 189-195. SCHLEICHERK. E. and A. BRADSHAW(1956) A conductivity bridge for measurement ol the salinity of sea water. Journal Conseil Permanent lnternationalpour l'Exploration de la Mer, 22, 9-2(I. SCHIJTZERR., W. ROETttER, U. WEIDMANN, P. KALTand H. H. LoosIJ (1985) A meridiona114C and 3~Ar section in Northeast Deep Water. Journal of Geophysical Research, 90(C4), 6945-6952. Scripps Institution of Oceanography, University of California (1963) Oceanic observations of the Pacific: 1951. Berkeley and Los Angeles, University of California Press, 477 pp. SOULE F. M., P. A. MORRILLand A. P. FRANCESCHETTI(1961) Physical oceanography of the Grand Banks region and the Labrador Sea in 1960. Bulletin U.S. Coast Guard, 46, 31-114. SPEER K. G. and M. S. McCARTNEY (1991) Tracing lower North Atlantic deep water across the equator. Journal of Geophysical Research, 96, 20,443-20,448. S w l n J. H. (1984) A recent 0)-S shift in the deep water of the northern North Atlanlic. In: Climateprocesses and climate sensitivity, Geophysical Monograph 29, Maurice Ewing, 5, 3947. TAKATSUKIV., M. AOYAMA,T. NAKANO, H. MIYAGI, T. ISHIHARAand T. TSUTSUMIDA(1991) Standard seawater comparison of some recent batches. Journal of Atmospheric and Oceanic Technology, 8,895-897. TSUCHIYA M., L. D. TALLEr and M. S. McCARrNEY (1992) An eastern Atlantic section from Iceland southward across the cquator. Deep-Sea Research, 39, 1885-1918. UNESCO (1981) Background papers and supporting data on the Practical Salinity Scale, 1978. UNESCO Technical Paper, Marine Science, 37, 144 pp. WARNER M. J. and R. F. WEISS (1992) Chlorofluoromethane in South Atlantic Antarctic Intermediate Water. Deep-Sea Researeh, 39, 2053-2075.
Treatment of inconsistencies in Atlantic deep water salinity data
1405
WARRENB. A. (1981) Transindian hydrographic section at lat. 18°S: property distributions and circulation in the south Indian Ocean. Deep-SeaResearch, 28,759-788. WILLIAMSR. T. and F. W. DELAHOYDE(submitted) Improving the measurements of pressure in the NBIS Mark III CTD. WOOSTERW. S. and B. A. TAFT(1958) On the reliability of field measurements of temperature and salinity in the ocean. Journal of Marine Research, 17,552-566. WORTHINGTON L. V. (1981) The water masses of the World Ocean: some results of a fine-scale census. In: Evolution of physical oceanography, B. A. WARRENand C. WUNSCH,editors, The MIT Press, Cambridge, MA, pp. 42-69. WORTHINGTONL. V. and W. R. WRIGHT(1970) North Atlantic Ocean Atlas of potential temperature and salinity in the deep water including temperature, salinity and oxygen profiles from the Erika Dan cruise of 1962. Woods Hole Oceanographic Institution Atlas Series 2,24 pp. and 58 plates.
Deep-SeaResearch1, Vol. 41, No. 9, pp. 1387-1405,1994 0967-0637(94)E00016--6
Copyright~) 1994ElsevierScienceLtd Printed in Great Britain. All rightsreserved 0967~637/94 $7.00+ 0.00
The treatment of inconsistencies in Atlantic deep water salinity data A R N O L D W . MANTYLA*
(Received 3 September 1993; in revised form 20 December 1993; accepted 20 December 1993) Abstract--Salinity biases between hydrographic cruises are known to have occurred, and it has been r e c o m m e n d e d that the uniform characteristics of the basins east of the Mid-Atlantic ridge be used to discern m e a s u r e m e n t differences between cruises to the region. That approach was used with two high quality n o r t h - s o u t h eastern Atlantic expeditions (Oceanus Cruise 202 ["McTT"[ and South Atlantic Ventilation Experiment ["SAVE"] Leg III), to construct a reference to assess the salinity differences between I G Y cruises from the 1950s and some more recent expeditions compared to that reference. Tables of salinity differences were prepared so that one m a y make adjustments to data sets when mapping any characteristic that is sensitive to relative salinity errors, such as computed geostrophic currents, or when mapping any characteristic on density surfaces.
INTRODUCTION
IN order to map an ocean hemisphere property distribution, such as salinity, it is necessary to merge data from many different expeditions; no single expedition has sufficient areal coverage to reveal anything more than a general coarse scale picture. The North Atlantic now has reasonably good coverage of full water column hydrographic stations, but they are not of uniform quality. That is not unexpected, as analytical and instrumental techniques have changed over time. Prior to the late 1950s, salinity was typically determined by silver nitrate titration of seawater chloride concentration and the Knudsen formula S -- 0.03 + 1.805 Cl. That method was used by the Meteor Expedition in both hemispheres of the Atlantic in the late 1920s, and by the Discovery cruises around Antarctica in the 1930s. Ship-board titration salinities can be resolved only to two decimal places (WOOSTERand T A F T , 1958), and three place precision is desired for good density calculations and to map subtle deep salinity features. An indication of the measurement precision is shown by a O S scatter plot (Fig. la). There are, however, areas of the ocean that have no modern data in the vicinity of some Meteor and Discovery stations; so data sets that attempt to cover large areas are apt to include a few stations from both expeditions. The instruments of choice between the late 1950s and the early 1970s were conductivity salinometers (SCHLEICHER and B R A D S H A W , 1 9 5 6 ; PAQUETTE, 1958); data from those instruments were considered to be good to about 0.005 (Fig. lb and c). Inductive salinometers (BROWN and HAMON, 1961), with inherently better sensitivity than the older conductivity instruments, were widely used in the 1970s, and are still commonly used. *Marine Life Research Group, Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, C A 92093-0230, U . S . A . 1387
1388
A.W. MANTYLA
I
2.5
I
I
a) 1927 METEOR
2.1
S
°°~° o
I
I
I
b) 1957 CRAWFORD 10
0
O-c~o
18°-19°N 2.3
I
I
I c ~ ~/
16°-24°N
-+.010
+ .005~ '~°
o
°°/~°Q ° O
1.9
2.5
c) 1973 GEOSECS J e ( -
k) o ®
/
2.3 f
_+13013
_+.0016
2.1
1.9
2.5
e) 1988 McTf 17°-23°N
.~
f) 1988 McTT
O/~/
CTD
/'f Z
2.3 _+.0006
+.0015 2.1
1.9 I 34.88
t 34.90
I 34.92
I 34.94
I 34.88
I 34.90
t 34.92
I 34.94
SALINITY Fig. 1. Potential temperature vs salinity plots for selected cruises in thc northeast Atlantic basins, to illustrate salinity precision over time (assuming negligible tempcrature errors). (a) 1927 Meteor Stas 272,277,279,281,282 and 283. Salinities recalculated to three places from chlorinities listed in the original Meteor atlases. Error bar is + or - standard deviation. (b) 1957 Crawford Cr. 10, Stas 77-83. Salinities run on Schleicher and Bradshaw conductivity bridge. (c) 1973 GEOSECS Stas 114 and 115. Salinities run on U.W. conductivity bridge. (d) 1983 TTO/TAS Stas 73-93. Salinities run on a double conductivity ratio Autosal salinometer. (e) 1988 McTT Stas 83-91. Salinities run on same salinometer type as TTO/TAS. (f) 1988 McT1TMStas 86-87. Data from NBIS CTD profilers at 2 db intervals.
Treatment of inconsistenciesin Atlantic deep water salinity data
1389
They are operated at ambient temperatures and are subject to non-linear drift, so the results are often no better than from the older salinometers. The current state-of-the-art salinometer, in use since the early 1980s, is a double-sensitivity salinometer operated in a temperature controlled water bath; it has exceptional stability and a sensitivity of better than 0.001 (Fig. l d and e) (DAuPHINEEand KLEIN,1975; KNAPP and STALCUP,1991). Salinity data are also derived from continuous profiling CTD measurements. They have a high data rate, on the order of 25 data-points per second, allowing the averaging of enough observations for each decibar to result in smooth profiles in low temperaturegradient deep water areas (Fig. lf). The precision of those deep profiles can be quite high, although the accuracy is dependent upon field checks run on salinometers standardized with IAPSO Standard Seawater (SSW). Many expeditions over the past decade have exceptional salinity precision due to care in sampling and modern instrumentation, yet biases between expeditions remain. Preliminary versions of the maps of salinity on North Atlantic deep density surfaces revealed modern cruise to cruise differences, some greater than 0.005 (REID, personal communication). A salinity offset error of 0.005 results in a geopotential height error of 2 dynamic cm when integrated from 5000 db to the surface, certainly a non-trivial amount. SAUNDERS (1986) has pointed out significant salinity offsets in the deep data from cruises to the Northeast Atlantic Basins (NEAB). H e states: "The systematic differences exist only in the measurements and evidently not in the ocean", and offers a potential temperature (®) vs salinity (S) reference line S = 34.698 + 0.098 19 for N E A B stations deeper than 3000 db and below a potential temperature of 2.6°C, that can be used to "calibrate" other expedition salinities. I report here the results of using that approach with a more recent expedition data set as a reference to determine the relative salinity offsets of other expeditions to the North Atlantic. First, however, a general discussion will be given on some potential sources of bias and factors affecting precision, and on the suitability of the eastern basins of the Atlantic as a "standard reference calibration tank". SOME PAST OBSERVATIONS OF SALINITY BIAS Possible salinity biases between cruises have been mentioned often in the literature. To point out a few: FUGLISTER(1960) noted that salinities from the 1950s Atlantic IGY cruises were slightly higher than those from the Meteor Expedition over 30 years earlier. As will be shown below, they were also higher than salinities from cruises 30 years later. Fuglister also noted that salinities from the I G Y cruise at 28°N in the Eastern Basin appeared to be too low compared with those from the cruises at 24°N and 32°N. WORTHINGTONand WRIGHT (1970) agreed that the 28°N IGY salinity data was unreasonably low. They also noted that a 1958 V.F.S. Gauss cruise reported unreasonably high salinities, thought to be a result of long storage time before analysis ashore. CLARKEet al. (1980), in a three-ship operation near the Grand Banks of Newfoundland, found the Hudson salinities differed from the Chain and Cirolana salinities by about 0.006, so that amount was added to the Hudson salinities. Actually, comparisons with the Geosecs Expedition salinities taken in the same region later in the same year, and with more recent expeditions to the region, indicate that the Hudson salinities were closer to other cruise data than those from the other two ships, which were probably too high. The region is oceanographically active, the meeting place of abyssal waters from both the north
1390
A.W. MANTYLA
and the south (BROECKERand BAINBRIDGE,1978; MANTYLAand REID, 1983), so comparisons between cruises tend to be a little noisy, presumably due in part to natural variability. SAUNDERS (1986), in his careful evaluation of data taken by IOS in the deep basins of the Northern Atlantic between 1977 and 1983, found a very uniform relationship between salinity and potential temperature (O-S) for five IOS cruises, yet systematically different by 0.003 from two other 1981 cruises to the region: Transient Tracers in the Ocean (TTO), and Atlantis Cruise 109. The relative errors in various batches of IAPSO Standard Seawater used to standardize the salinity analyses were not known at. the time, but a later study (MANTYLA,1987) found relative errors in SSW that accounted for much of the differences between the IOS and the other two cruises. There is still a mystery with Atlantis 109: the low resolution CTD salinity profile data stored at N O D C differs from the bottle salinometer data from the s a m e casts by 0.004! Yet, the CTD satinities are usually derived from calibrations against the water sample data. SPEER and McCARTNEY (1991), in mapping characteristics on constant density surfaces for the Atlantic Ocean, attempted to remove salinity biases between cruises by accounting for relative SSW errors. They pointed out " . . . the salinity offset is somewhat greater along a curve of constant density than it is along the salinity axis" of the O-S diagram. That is true where the O-S slope between the deep and bottom water is positive, as it is in the deep Atlantic, Indian and South Pacific oceans; however this salinity offset error is somewhat less along constant density surfaces where the ®-S slope is negative, as it is in the North Pacific or elsewhere such as in the transition depths between the intermediate water salinity minimum and higher salinity colder deep water. Correction for SSW biases did improve inter-cruise comparisons, but the salinity on constant density maps still showed some unlikely contour distortions where cruise tracks cross, clearly an indication that some sort of salinity bias remained between cruises. Assuming that the perceived cruise salinity differences are due to biases and are not a result of real changes in the deep ocean, what could account for the differences? POTENTIAL SOURCES OF SALINITY BIAS Oceanic samplers have changed over time. Most early cruises used brass Nansen bottles with various materials such as tin plating, wax coating, or Teflon or epoxy-lining, to minimize reaction of the seawater sample with the metal. Coated or lined bottles still had exposed bare metal end valves. A comparison of samples taken from epoxy-lined Nansen bottles and PVC plastic Niskin bottles tripped close together in depth on the same Geosecs test cruise deep cast showed detectable salinity and oxygen differences; the Nansen bottle averaging 0.002 higher in salinity and 0.02 ml 1-~ lower in 0 2 than the Niskin Bottle (H. CRAIG, personal communication). Uncoated Nansen bottles would presumably have a greater effect on the deterioration of the water sample. Salinity samples collected at sea and run ashore at a later date are usually higher than they would have been if they had been analysed within a few days of collection at sea. That lesson was learned early in the C A L C O F I program in a curious way. In order to get a quick look at environmental surface conditions, the 10 m salinity samples were titrated immediately after the cruise. The remainder of the station salinity samples were titrated some months later. The resulting station curves showed a consistent and hydrostatically unstable 10 m salinity minimum, it became obvious that evaporation was occurring in the stored salinity samples (SIO, 1963). A later experiment of successive weighing of stored
Treatment of inconsistencies in Atlantic deep water salinity data
1391
salinity samples over several months showed an average evaporation rate of 0.052/100 days for salinity bottle and seals used at the time (J. L. REID, unpublished memo, 1954). Another storage test of salinity samples stored in glass citrate bottles and in screwcap polyethylene bottles had evaporation rates of 0.028/100 days and 0.076/100 days respectively (J. WELLS, 1980, personal communication). Recently, in a salinity comparison experiment between the University of Hawaii and the Monterey Naval Postgraduate School, RAGO and KENNAN (1991) found an evaporation rate of 0.007/100 days. From these observations, two things are apparent: (1) salinity bottle seals have improved over time; and (2) in order to get every digit of the third decimal place, the salinity samples must be analysed within a week of collection. Stored samples not only are apt to be too high, they are also noisier because evaporation rates very from bottle to bottle. IAPSO Standard Seawater was originally certified in terms of chlorinity, and the salinity inferred from the Knudsen relation. In practice, conductivity measurements are a more sensitive measurement of the salinity of seawater than chlorinity titrations, so IAPSO SSW was used as a conductivity standard, although it was not certified as such until 1980. The change occurred once it became clear that the conductivity-chlorinity relationship was not consistent. Salinity was re-defined in terms of the conductivity ratio relative to a standard KCI solution ( U N E S C O , 1981). When older batches of SSW were compared by conductivity, discrepancies were found in some batches between salinity calculated from the labeled chlorinity and the conductivity derived salinity. SOVLEet al. (1961) found relative errors between two batches of SSW prepared in the late 1940s of 0.018. PARK (1964) found relative differences of 0.018 for batches prepared in the late 1950s. Several investigators, as summarized in MANTYLA (1980), found relative errors of 0.009 between SSW batches P49 and P54, prepared in 1967 and 1970, respectively. Since 1980, with the start of conductivity ratio labeling of IAPSO SWW, all tested batches (P91 to Pl12) have been within 0.0015 of their labeled values, as shown in Table 1 (TAKATSUKI et al., 1991; MANTVLA, 1987, unpublished SIO comparisons). The bias in salinity measurements due to SSW offsets can be accounted for, if a record is kept of which batch of SSW was used for any particular expedition. Two examples are shown in MANXYLA(1980). Unfortunately, many older cruises did not keep track of the batch number because the fact that the standard seawater could be slightly in error was not generally appreciated. Even if the SSW batch is known, there is another type of error that can occur, one that is instrument-specific, depending upon whether the SSW batch was slightly greater, or slightly less than 35.000. Salinometers often have a response shift when the largest dial setting is moved, which occurs at 35.000. To illustrate the problem, calibration curves are shown for the U W salinometer, a conductivity salinometer that used 700.00 ohms as equivalent to 35.000 S. The conductivity of a series of samples prepared by weight dilutions of an evaporated high salinity stock solution are shown in Fig. 2a, as deviations from the salinity/conductivity slope near 35 S, and 700 ohms. Figure 2b shows a least squares polynomial curve fit to all of the calibration points. The fit turned out to be poorest near 35 and the juncture of the 600 and 700 ohm decades; curves fit to the 600 ohm and 700 ohm decades separately revealed a mismatch equivalent to 0.008 S there (Fig. 2c). The shift in the sum of the first 600 ohms is not unexpected as an instrument ages (R. PAQUETTE, personal communication, 1967), but its effect occurs at the critical point where the instrument is standardized with SSW when station salinity samples are being analysed. SSW batches with S > 35 caused a systematic error in sample salinities less than 35 when
1392
A.W. MANTVLa Table 1.
Recent comparisons o f l A P S O Standard Seawater Batches P91-P112
(S...... - Slabcl)X 103
Batch
Prep. d/m/y
P91 P92 P93 P94 P95 P96
10/5/80 29/10/81 31/10/81 18/I 1/81 8/3/83 3/3/83
P97
P98 P99 PI00 P101 P102 P103 PI04 P105 P106 P107 P 108 P109 P110 PI 11 PII2
Kj5
S
1.000117 0.99988 0.99990 0.99992 0.99997 1.000116
35.0027 34.9953 34.9961 34.9969 34.9988 35.01123
3/3/ 83
1.00002
35.0008
3/3/83 27/7/84 29/11/84 4/6/85 29/11/84 11/10/85 21/2/86 21/2/86 8/6/87 11/11/87 7/4/88 7/4/88 20/7/88 7/2•89 4/7/89
0.99993 0.99997 1.00003 1.00002 1.00001 0.99987 0.99994 1t.99988 0.99989 0.99991 0.99980 0.99976 0.99999 0.99982 1t.99984
34.9973 34.9988 35.0012 35.0008 35.0004 34.9949 34.9976 34.9953 34.9957 34.9965 34.992 I 34.9905 34.9996 34.9929 34.9937
MANTYLA(1989) Sept. 1985
S10 July 1987
SIO June 1989
TAKATSUK! et al..
1990
1.0
-l.5 -0.4 -0 . 2 0.9 1.2
-
1.4
1.0
-
1.2
0.7
0.8 0.8 -11.4 (1.3 (}.5 0.2
0.0
0.2
-0.3 0.6
0.3
- 1.0
-0.6
0.8 0.2 0.3 0.9 (1.6
(1.4
11.8
0.4
Note: Last digit of Kl~ conductivity ratio is equivalent to 0.4 × 10 3salinity. a n a l y s e d o n i n s t r u m e n t s that had the decade shift p r o b l e m , a p r o b l e m that was n o t usually k n o w n by the operator. This is critical b e c a u s e the m a j o r i t y of deep a n d b o t t o m waters have S < 35 (WORTHINGTON, 1981). A 1965 S I O calibration of all three of its 7-year old U W conductivity s a l i n o m e t e r s had a shift at 700 o h m s (MANTYLA, 1966, u n p u b l i s h e d m a n u script). T h e e x p e d i e n t solution was to reset the 35 S s t a n d a r d setting from 700 o h m s to 675 ohms, which placed salinities of a b o u t 30.5-36.5 on the same 600 o h m decade a n d e l i m i n a t e d the shift at the critical s t a n d a r d i z a t i o n point n e a r 35 S. All of the Geosecs salinities were r u n at sea on these i n s t r u m e n t s . A l t h o u g h there were n o i n s t r u m e n t d e p e n d e n t s t a n d a r d i z a t i o n errors for the Geosecs salinities, there were biases of + 0 . 0 0 2 to - 0 . 0 0 3 because of SSW batch errors a m o n g the seven different batches used. T h e i n s t r u m e n t response shift occurs in m o d e r n s a l i n o m e t e r s as well, as illustrated in Fig. 1 of MANTYLA'S (1987) SSW c o m p a r i s o n study. Most of the recent batches of SSW have S < 35, so recent deep a n d b o t t o m salinities, for which inter-cruise c o m p a r i s o n s are usually m a d e , should n o t suffer from the s a l i n o m e t e r shift p r o b l e m . Errors could occur for salinities greater than 35 of course, but those are usually in a shallower part of the water c o l u m n where there is g r e a t e r n a t u r a l variability a n d w h e r e the three place salinity accuracy may not be quite as critical as it is in abyssal waters. Differences b e t w e e n down-cast C T D salinity profiles a n d bottle sample salinities collected o n the up cast were alluded to above. A n up C T D profile can be adjusted to agree
1393
Treatment of inconsistencies in Atlantic deep water salinity data
0
(o) 3
.040
(9
I I o3
.020
- 80
I
R --700
I
- 60
-40
I
I
( ,t"Z )
I
O
I
01.0 ~. --20 4)~i~ 4)*' g® ' -.010
-~(9.1.* 20 I I I~I .,~(~.~**~pSt**
®
,,~*I** I
60
I
I
80
I
(b)
-80
-60
t R-
I
-40
I
700(
~
I
~-20 I
I ~ 1
.I.,
}-~.
~ ~"'~'----~
-.010-
/600
OHM
.040
l-
li
.020
-80
I
R-700
II
-60
I
I
~1 I {A/, ) -----.)-
-40
I
I
o
7 0 0 OHM
/
DECADE~/
.{a
" ' " . ~ 20
II ~
co)
80
I 0 ~ " - ~""
-.o,0
/
',
I
---~-"
Fig. 2. Calibration of U.W. conductivity salinometer No. 9, October 1965. (a) Calibration samples derived from weight dilutions of the high salinity stock solution, plotted as deviations from the salinity conductivity slope at 35%O. (b) Least squares polynomial curve fitted to all of the calibration points. (c) Curve fitted to the 600 and 700 ohm decades separately. The displacement between the two decades is equivalent to 0.008%o in salinity.
1394
A . W . MANTYLA
perfectly with the salinometer water sample data collected on the up cast, and yet differ from the down C T D profile if there is any hysteresis in the pressure, temperature or conductivity sensors. A temperature difference of one millidegree or a pressure difference of 2-3 db will result in a calculated salinity difference of 0.001. Each pressure sensor has a different hysteresis error. C T D pressure sensor calibrations performed by SIO's Oceanographic Data Facility ( O D F ) have had differences between increasing pressure calibration series (loading curve) and decreasing pressure calibration series (unloading curve) of typically 5-10 db, and as much as 20 db for different pressure sensors (M. C. JOHNSON, personal communication). A pressure hysteresis error of 10-12 db could account for a salinity difference of 0.005 between part of the down and up C T D traces if the pressure error is not accounted for. WILLIAMSand DELAHOYDE(1993) show procedures for handling the C T D hysteresis problem. Of the sources of bias listed above, only the SSW bias can easily be corrected for, if the SSW was less than 35 S. Use of SSW batches greater than 35 S may have produced instrument-dependent errors apart from the SSW batch bias itself; the salinity error in that case must be assessed some other way. Salinity samples stored for analyses much later are likely to be noisy and to have a high bias, and may not be correctable. CONSTANCY OF THE EASTERN ATLANTIC BASINS SAUNDERS (1986) has documented the uniformity of the deeper waters of the northeastern Atlantic basins and has r e c o m m e n d e d the locality as an environmental standard that could be used to calibrate the results from various other cruises. Time-variations in deep and bottom waters that have been noted close to their source regions (FosTER and MIDDLETON,1979; Swwx, 1984; LAZIER,1988) are not likely to be detectable in regions more remote from the source waters. The bottom waters of the N E A B are derived from abyssal waters from the south in the Western Atlantic basins by way of the V e m a Fracture Zone (McCARTNEYe t al., 1991; MANTYLA and REID, 1983). Newer northern deep water sources sdch as the Iceland-Scotland Overflow Water, Labrador Sea Water and Mediterranean Overflow Water are confined to depths shallower than about 3000 m (TsucHIYA et al., 1992). The eastern North Atlantic chlorofluorocarbon data set discussed by DONEY and BULLISTER(1992) confirms that the ventilation of the deep N E A B is on a much longer than decadal time scale. A 14C section along the deep N E A B (SCHLITZER,1985) shows a 14C minimum ("oldest" water) at a depth of about 3000-4500 m and at a density of about o4 = 45.86, a density that is found at the bottom of the N E A B between 40°N and 50°N. That suggests a scenario similar to those described for: (1) the deep North Pacific (MANTYLA, 1975) where the bottom waters have become less dense and return southward above the bottom water as deep water; (2) the modification of Weddell Sea Bottom Water (CARMACK, 1977) whereby it becomes less dense (and enriched in silica [LOEF and BENNEKOM, 1989]) as it spreads in the Atlantic-Antarctic basin and returns above the newly formed W S B W in a spiral fashion; and (3) the circulation schematic prepared by MCCARTNEY et al. (1991) for bottom waters colder than 2°(0) in the N E A B upwelling across the 2 ° isotherm as it spreads and becomes less dense. With no outlet for the densest water from the N E A B , it appears that the uniformity of the deep waters there is the product of dense waters passing through the V e m a Passage over sill depths of about 4700 m (McCARTNEY et al., 1991), cascading down to depths as great as 6000 m in the G a m b i a Abyssal Plain, and becoming less dense as they spread eastward and northward, thereby
Treatment of inconsistenciesin Atlantic deep water salinity data
1395
producing a thick body of uniform water between 20°N and the Azores Rise (TsucHIYA et al., 1992) that can ride back over the denser incoming water and has the oldest 14C age in the region (ScHLITZERet al., 1985). Saunders' O-S regression line for N E A B waters below 2 . 6 0 encompasses this water mass and also falls on WORTmNCa'Orq'S (1981) first seven greatest-volume O-S classes for the Atlantic Ocean. Detection of any deep or bottom water time variations in the N E A B is not expected because of the large volume of the present water mass and the remote source of the bottom water. Thus, the N E A B is an ideal locality to compare cruise results, as recommended by Saunders. Eastern Atlantic basins to the south of the Gambia Abyssal Plain are filled with dense western basin waters through the Romanche Fracture Zone (MANTYLAand REID, 1983). The chlorofluoromethane data shown by WARNER and WEISS (1992) indicate no recent ventilation there either, so perhaps the eastern and southeastern Atlantic basins might be a suitable location for expedition intercomparisons also. REVISION OF S A U N D E R S ' O-S REFERENCE LINE
Saunders' visual O--S fit of S = 34.698 + 0.0980 to N E A B cruises in 1972 and 1981 between 30°N and 41.5°N had a standard deviation of 0.002 S and the reference line also fit well with three other cruises in 1982 and 1983 between 26°N and 46°N. The agreement may have been fortuitous, because five different batches of IAPSO SSW were used that had offset errors from +0.001 to - 0 . 0 0 2 S, and three of the five batches were greater than 35 in salinity, which could have led to some bias for the deep salinities which are all less than 35. Also, according to WORTmNCTONand WRICHT'SNorth Atlantic Atlas (1970), there is some salinity variation at O = 2.6 °, from about 34.94 to 34.96, so there should be some latitudinal variation, at least in the O-S slope. Two expeditions of 1988 (Fig. 3) that have high quality salinity data in north-south transects of the eastern Atlantic basins were selected to evaluate the O-S relations. Oceanus Cr. 202 ("McTT" [McCARTNEY et al., 1991]) went from Iceland to the equator between 20°W and 29°W taking stations at half degree latitude intervals or less; and the South Atlantic Ventilation Experiment (SAVE) Leg III (JENKINS and OLSON, 1992) between the Walvis Ridge and Abidjan, Ivory Coast was between 5°W and I°E with station spacing of 1° or less. McTT used IAPSO SSW batch P108 and SAVE III used P106; the offset errors for both are 0.001 or less (Table 1). Closer examination of individual McTF O-S curves showed some curvature above 2.5°O in the north, and a bend at 2°0 and less to the south (the familar "two-degree discontinuity" [BRoECKERet al., 1976]); therefore the regressions were confined to the potential temperature range between 2.0 ° and 2.5 ° . In order to have enough data points for good statistics, the regressions were run for all of the data grouped from stations over 5 ° latitude, at 2.5 ° latitude intervals; about 25-50 data points were included in each latitude group. The linear regressions generally had correlation coefficients of 0.985 or higher and salinity standard deviations of 0.0015 or less. That is not to say that the O-S relationship over this temperature range is perfectly linear, but only that the non-linearity is too small to affect comparisons between cruises significantly. The results of the regressions for McTT and SAVE III cruises are shown in Fig. 4, plotted versus latitude. The salinities shown are calculated from the regression equations at the bracketing potential temperatures of 2.5 ° and 2.0 ° and the mid-range potential temperature of 2.25 °. Straight line fits over segments of latitude were made for the ®-S slope and for the 2.25 ° salinity, and the dashed lines for the 2.0 ° and 2.5 ° salinities
1396
A . W . MANTYLA
60°
60 °
40 °
40 o
20°
20°
0°
0o
Fig. 3. Location of north-south Oceanus Cr. 202 (McTI') stations (August 1988) and Knorr South Atlantic Ventilation Experiment (SAVE) Leg Ill stations (February-March 1988) used to establish reference salinity-potential temperature relationships of the uniform deep waters of the eastern Atlantic basins (O). East-west stations are from the late 1950s IGY cruises (+). 40(X)m depth contour shown.
were calculated from those fits. T h e S a u n d e r s regression (SR) is also shown b e t w e e n 20°N a n d 50°N. T h e Mcq~F data is 0.002-0.003 lower t h a n SR, but that is close to the c o m b i n e d error estimates of both data sets. T h e ® - S slope does vary with latitude, though by only 10% b e t w e e n 20°N a n d 50°N (0.093-0.104). T h e 6)-S r e l a t i o n s h i p changes m o r e rapidly south of 20°N, b u t the c o r r e l a t i o n coefficients r e m a i n consistently good until 15°S, where they begin to d e t e r i o r a t e rapidly as waters from o t h e r sources e n t e r the region a n d result in curved O - S plots, especially south of 20°S. T h e r e is only a 0.009 salinity range at O = 2.0 ° from 20°S to 35°N where it intersects the b o t t o m (salinities at O - S = 2 ° b e t w e e n 35 ° a n d 50°N e x t r a p o l a t e d ) ; most of the latitudinal change in the O - S slope is due to salinity
(a)
z~/A''z~ ~
I
~
~"~/
I
EQ
~
S~0=2.50
I o
I
.00 ~
I
20 °
/.O.I"O"e-°
LATITUDE
~ s ~ ° = 2o ' 2c s~. . . ~ , - ' e
"
I
10o
I
30°
I
I
40°
I
I
S0°N
0
,3 n
0 Mc'rr Z~ SAVEIll
0 n
SR
0
O
(b)
O
O O
@
__E)__ea3__C)__O_~(3---O----O---~ -~9-
[
Fig. 4. Salinity-potential temperature linear regression results versus latitude from the McT1 ~ ((D) and S A V E I l l ( A ) expeditions. Regressions over ® = 2.0 ° to 2.5 ° and over 5 ° latitude bands, at 2.5 ° latitude intervals. (a) Regression calculated salinities at 2.5 °, 2.25 ° and 2.0°®. S at ® = 2.0 ° north of 35°N is extrapolated. (b) O - S regression slopes versus latitude. Saunders' reference indicated by lines labeled "SR" between 20°N and 50°N.
092
0.04
0,06
0.08
34.890
34.900
34.910
34.920
34.930
34.940
I
I
I
10°
20°S
tao
e~
,7
>
9.
8
g
,.q
1398
A . W . MANTYLA
Table 2. Linearfits to ®-S slopes (between 19 = 2.0 ° and 2.5 °) versua" latitude and S at ® = 2.25 ° versus latitude over segments o f latitude in the eastern Atlantic S=A
+B(O-2.25
°)
where A = 34.9144 + 0.0001N5¢ between ¢ = 22.5°N and 50°N 34.9000 + 0.000747¢ between 5°S and 22.5°N 34.8992 + 0.000388¢ between 19°S and 5°S where B = 0.08569 0.05044 0.05808 0.06490
+ + + +
0.000345¢ 0.001950¢ 0.000987¢ 0.002087¢
between between between between
¢ = 22.5°N and 50°N 7.5°N and 22.5°N 5°S and 7.5°N 19°S and 5°S
Note: Relationships are valid only in the vicinity of the McTT and SAVE III cruises. They may not be applicable in the region between the two cruises at 0°-5°N.
variation at the higher temperatures. The standard deviation of the latitudinal fits to the McTT and SAVE 2.25°0 regression salinities is 0.0005, so the two cruises form a remarkably consistent data set that can be used to evaluate salinity data from other expeditions that have taken station data in the eastern Atlantic. The coefficients for the latitudinal variations of the 2.25 ° salinity and the O-S slope are given in Table 2. COMPARISON
OF OTHER
EXPEDITIONS
WITH
THE McTT--SAVE REFERENCE
The Atlantic IGY data used in the FUGLISTER(1960) and WORTHINGTONand WRIGHT (1970) atlases form a large area data set (Fig. 3) taken over a period of a few years that can be usefully compared with the M c T T - S A V E reference regressions. Each of the zonal IGY lines between 16°S and 52°N was evaluated in the same way as with M c T T - S A V E data: least-squares regression lines were fit to the eastern basin stations over the potentialtemperature range of 2.00-2.5 °, and the regression-calculated salinity at 2.25 ° was then compared to the reference line (Fig. 5). The I G Y O-S slopes were generally the same as for McT-I'-SAVE, but the IGY salinities were uniformly higher than the recent salinities. Interestingly, where duplicate salinity samples were collected from the same Nansen bottles, but run separately by the UK and the US, the U K results were closer to the modern data by 0.003 to 0.004 S (Fig. 5, at 24°N and 43°N, Table 3). Thus it is not likely that the deep N E A B salinity was higher in 1958-1959 than it is now. Further support for the lack of salinity change in the N E A B may be found in the 1927 Meteor Expedition data, they agree very well with Geosecs (1973), T T O (1983) and McTT (1988) (Fig. 6). The IGY salinity offsets are well within possible IAPSO SSW batch-to-batch offsets, typically about 0.007 high for the US salinities. In order to bring the IGY salinities into agreement, modern cruise data salinity corrections for each latitude crossing are tabulated in Table 3. The 2.25 ° regression salinities are shown for "modern" cruises in Fig. 7, along with the M c T T - S A V E reference lines. As noted by Saunders, the agreement between modern cruises is not as good as modern analytical salinity precision would lead one to expect. IAPSO SSW offsets have been accounted for in all of the cruises shown in Fig. 7 except for the Meteor 56 cruise, yet biases between cruises clearly remain. Atlantis 109 bottle data,
Fig. 5.
[]
I
I
EQ
S @ 0 = 2.25 °
I
I
10°
I
[]
LATITUDE
I
~
/
[]
20 °
us
=
Qus
t
30 °
[]
13
I
[]
- - - ~ ~ £ ) ~
13 w
13
I
~
13
~
uJ
•
[]
I
40*
us
[]
m
13
~
~
'
"uux 13 13
~
13 u~
13
I
I
,
IS
.50°N
Results of I G Y cruise salinity-potential temperature regressions plotted versus latitude, as in Fig. 4. Reference lines are from McTT and S A V E data. Note cruises at 24°N and 43°N had salinities determined by both the U.S. and U . K .
0.02
0.04
0.06
0.08
0.10
34.890
34.900
34.910
34.920
34.930
I
I
I
lO °
20°S
~"
taJ
e~
.=. ,.7
g
m
w
>
8
1400
A.W. MANTYLA
2.4 1
~
2.2
19571GY
1927MT
2.0
I
I
I
34.88
I
I
34.90
I
34.92
I
r
34.94
S Fig. 6. Comparison of linear regression fit to 1927 Meteor, 1957 Crawford IGY, 1973 Geosecs, and 1988 McTT O-S data between 2.0 ° and 2.5°C. The 1927 Meteor data agrees well with modern data, while the IGY data is systematically higher in salinity than the others. Regression lines are from Fig. 1.
Table 3.
O-S relationships for 1GY Atlantic cruises in order of north to south transects'
S/® Expedition
Date
Discovery I1-3 Discovery II-1 Discovery II-3 Discovery I1-3
Aug. Apr. Nov. Nov.
'58 '57 '58 '58
Crawford 16 Crawford 16 Chain 7 Discovery II-2 Discovery II-2
Oct. Oct. May Dec. Oct.
'57 "57 '59 '57 '57
.
.
.
. Crawford Crawford Crawford Crawford Crawford Crawford Crawford
. 10 16 10 22 10 10 22
.
.
Feb. "57 Nov. '57 May '57 Nov. '58 Mar. '57 Apr. '57 Oct. '58
Stations
Latitude
N
slope
3853-3860 3531-3542 3869-3880 3889-3906
50°N-53°N 47°N-49°N 46°N 43°N
20 34 32 38 37 42 25 42 49 52 57 24 58 65 61 47 63 --
0.08773 0.09743 0.09322 0.10185 0.09905 0.09817 (/.09669 0.09436 0.10268 0.09477 0.09847 0.08924 0.08785 0.06761 0.05531 0.0528 0.03607 non-linear
.
.
.
.
245-253 259-264 61-71 3635-3647 3592-3606
.
. 77-83 278-297 155-170 464-481 86-104 137-151 437-455
40°N 37°N 36°N 32°N 24°N
. 16°N-24°N 16°N 8°N 0°S 8°S 16°S 24°S
S at (9 = 2.25 °
S corr. × 103
34.9249 34.9235 34.9207 34.9217 34.9184 34.9272 34.9233 34.9252 34.9169 34.9218 34.9173 34.9261 34.9180 34.9125 34.9068 34.9036 34.9011
8 -7 -4 -5 (U.S. salts) - 2 (U.K. salts) - 11 -7 9 - I - 6 (U.S. salts) - 2 (U.K. salts) - 10 -6 -6 7 -7 -8 (-~ 7)
Fig. 7.
0
©
I
<>
(3
I
0
I
I
EQ
©
I
I
10o
(3
I
0
LATITUDE
I
~"
20 °
c~Br~ A
(3 (3 cro (D
I
I
30 °
O
(3
I
©cro (3 Br~
I
40 °
0
0
I
50°N
Results of some modern cruise salinity-potential temperature regressions, as in Figs 4 and 5. Note the cruises at 24°N and 36°N show both C T D and bottle salinity results, the bottle data agrees well with the M c T T reference line.
0,02
0.04
0.06
0.08
0.I0
34.890
34.900
34.910
34.920
34.930
20°S
10 o
4~
e~
e~
o 3" >
o.
8
3'
¢D
3
1402
A.W.
Tabh"
Expedition GEOSECS GEOSECS GEOSECS GEOSECS M E T E O R 56-5 TTO-NAS TI'O-NAS A T L A N T I S 11t9 BTL A T L A N T I S 109 CTD A T L A N T I S 1119 BTL A T L A N T I S 109 CTD H U D S O N 82002 TTO-TAS O C E A N U S 133~5 K N O R R 104 K N O R R 104 K N O R R 104 K N O R R 104 K N O R R 1(14 AJAX AJAX AJAX AJAX AJAX SAVE SAVE SAVE S A V E II TAS 8
Date Mar. '73 Mar. '73 Feb. '73 Feb. "73 Apr. "81 Jun.'81 May "81
4.
Stations
O-S
MANTYI,A
relationships/br modern Atlantic" cruises
Latitude
114-117 21°N-31°N 112-113 6 ° N - I I ° N 109-1 I 1 2°N-2~S 106108 5°S- 16°S 513-517 25°N-31°N 1111-119 37°N 51°N 48--109 29°N 35°N
N
O/,,; slope
.S'at (") , 2.25 °
SSW batch
Corr. × l0 :~
('orr. S at O = 2.25 °
S corr. x 103
fl 0 0 0
34.9147 34.9079 34.8998 34.8091
nil - 1.6 nil nil nil nil nil
37 12 t6 18 33 46 1211
(I.fl9310 11.068(19 0.05428 11.113852 0.09799 I).119851 0.09574
34.9147 34.9117t) 34.8998 34.8(t91 34.9162 34.9167 34.9164
P59 P59 P59 P59 '? P81) P80
wn ~a~
Jun. '81
72 86
36.25~'N
46
11.1193~H
34.9160
PSI
+I
34.9176
nil
Jun. '81
77-81
36.25°N
54
11.1197211 34.tH97
P81
t I
34.92117
- 4
Aug. '81
158-171
25. F'N
38
{1.(18886
34.9149
PSI
+1
34.9159
nil
Aug. '81 Apr. '82 Dec. '82 Mar. "83 Aug. '83 Aug. '83 Aug. '83 Aug. '83 Aug. '83 Oct. '83 Oct. '83 Oct. "83 Oct. '83 Oct. "83 Dec. '87 Dec. "87 Dec. "87 Jan. '88 Mar. '89
165-169 8-24 81~93 15-27 45-49 50-55 56-62 63~9 70-81 4-8 8-14 14-18 1823 23-28 34-40 47-52 52-56 61-77 4~3
24.5°N 37 48°N-49°N 92 20°N-27°N 37 I 1.25°S 28 30°N-35°N 22 25°N-30°N 37 20°N-25°N 28 15°N-20°N 311 10°N-15°N 46 0°N-3°N 18 I1°S-5°S 29 5°S-10°S 23 10°S-15°S 36 15°S-20°S 41 I°N-5°N 37 f)°S-5°S 27 5°S-111°S 27 11)°S-14°S 100 I 1.3°N
11.(19562 1).1024 0.091)98 11.1t4241 0.08952 0.09471) I).09064 I).08078 0.07258 0.06421 0.05072 0.04998 11.11392 0.02539 0.06016 0.05651 0.05591 [).1)4060 0.117380
34.9192 34.92114 34.9147 34.8989 34.921)4 34.92311 34.9229 34.9187 34.9153 34.91165 34.9011 34.8994 34.8972 34.8931 34.9t144 34.91X)2 34.91X18 34.8954 34.9084
PSI P85/86 P9()/811 P93 P93 P93 P93 P93 P93 P90 P90 P90 Pg0 Pg0 PII16 P106 PI06 P106 P108
-~ I I ~ 1.4 0.4 -11.4 0.4 -11.4 0.4 -11.4 2 2 2 2 2 1 I 1 I +11.3
34.92112 34.9194 34.9161 34.8985 34.921t11 34.9226 34.9224 34.9183 34.9149 34.9045 34.8991 34.8974 34.8952 34.891 I 34.9034 34.8992 34.8998 34.8944 34.9087
4 - 3 nil 4 -5 --7 -7 5 -6 --3 nil nil nil nil nil nil 3 nil nil
TAS 8, TTO/NAS, S A V E I, A J A X , Meteor 56, and parts of TTO/TAS, S A V E II, and GEOSECS agree well with the M c T T - S A V E III reference data; but Atlantis 109 CTD, Knorr 104, Hudson 82-002 and Oceanus 133-5 do not. Offset adjustments needed to bring modern cruise salinities into agreement are tabulated in Table 4, differences of 0.001 or less are not significant and are listed as "nil". Evaluations of a few key western Atlantic basin cruises were done by comparison of deep salinities with the westward extension of the Atlantis 109 lines at 36°N and 24°N. The Endeavor 129 cruise at about 64°W was found to be in agreement with the reference cruise, while Oceanus 133, Leg 7 at about 53°W appeared to be about 0.003 higher than the reference cruises. The individual expedition salinity corrections listed here have been used by REID (1994) in his study on the total geostrophic circulation of the Atlantic Ocean. Also, the abyssal Atlantic characteristics maps shown in MANTYLA and RE~D (1983) have been revised on the basis of the corrected data set. Agreement between cruise crossings showed good improvement in both salinity and density, further supporting the internal consistency of the revised Atlantic deep data array.
Treatment of inconsistencies in Atlantic deep water salinity data
1403
DISCUSSION
Although the M c T T - S A V E data set yields a gratifyingly precise salinity reference for the eastern basins of the Atlantic, only future cruises will tell if the data are correct or not. The selection of these two cruises as a standard is entirely arbitrary, of course. The McTI'S A V E salinities tended to be lower than many other data sets; it is quite possible that some unknown source of bias could exist for the data set (such as possible non-conductive material added to the sample by the plastic rosette bottles or the coated inner springs within the rosette bottle); but several other expeditions agreed closely with M c T T - S A V E , so the reference line is believed to be good. One value of comparing stations taken in the eastern Atlantic lies in cruises that have stations on both sides of the Mid-Atlantic Ridge. If a cruise shows good agreement with the reference stations in the deep eastern Atlantic, that would lend credence to differences seen at mid-water depths or to bottom water differences seen between cruises in the western Atlantic basins. However, with temperature and salinity both currently being measured to three decimal places in the deep ocean, the approximately ten to one ® - S slope in the deep north Atlantic makes temperature the more sensitive indicator of any secular changes that might be going on in the abyssal ocean. The technique of evaluating expedition data taken in deep, uniform basins should work in other oceans as well. The northeast Pacific basin is the home of the largest volume O-S class in the world oceans (MoNT6OMERY, 1958); it should be a good region to compare results from different expeditions, taking care to avoid the real near-bottom variability observed by JOYCE et al. (1986). In the Indian Ocean, the central Indian basin is remote from bottom water sources (MANa'YLA and REID, 1983) and appears to be quite uniform in characteristics (WARREN, 1981); it tOO should serve as an environmental calibration area to compare Indian Ocean cruises. Acknowledgements--The work reported here was supported by the National Science Foundation and by the Marine Life Research Program of the Scripps Institution of Oceanography.
REFERENCES BROECKER W. S. and A. BAINBRIDGE(1978) An abyssal shear zone. Journal of Geophysical Research, 83(C4), 1963-1966. BROECKER W. S., T. TAKAHASHIand Y. -H. LI (1976) Hydrography of the central Atlantic--I. The two-degree discontinuity. Deep-Sea Research, 23, 1083-1104. BROWN N. L~ and B. V. HAMON (1961) A n inductive salinometer. Deep-Sea Research, 8, 65-75. CARMACK E. C. (1977) Water characteristics of the Southern Ocean south of the Polar Front. In: Voyage of discovery, M. ANGEL,editor, Pergamon, Oxford, pp. 15-41. CLARKER. A., H. W. HILL,R. F. REINIGERand B. A. WARREN(1980) Current system south and east of the Grand Banks of Newfoundland. Journal of Physical Oceanography, 10, 25-65. DAUPHINEET. M. and H. P. KLEIN (1975) A new automated laboratory salinometer. Sea Technology, 16, 23-25. DONEY S. C. and J. L. BULLISTER(1992) A chlorofluorocarbon section in the eastern North Atlantic. Deep-Sea Research, 39, 1857-1883. FOSTER T. D. and J. H. MIDDLETON (1979) Variability in the bottom water of the Weddell Sea. Deep-Sea Research, 26, 743-762. FUGLISTERF. C. (1960) Atlantic Ocean atlas of temperature and salinity profiles and data from the International Geophysical Year of 1957-1958. Woods Hole Oceanographic Institution Atlas Series I, 209 pp. JENKINS W. J. and D. B. OLSON (1992) Chemical, Physical, and CTD Data Report. South Atlantic Ventilation Experiment (SAVE) Leg 3, $10 Ref. 92-9, Scripps Institution of Oceanography, University of California San Diego, 447-729.
1404
A . W . MANTYLA
JOYCE T. M,, B. A. WARREN and L. D. TALLEY (1986) The geothermal heating of the abyssal subarctic Pacific Ocean, Deep-Sea Research, 33, 1003-1015. KNAPP G. P. and M. C. STALCUP(1987) Progress in the measurement of salinity and oxygen at the Woods Hole Oceanographic Institution. Technical Report, WHOI-87-4, Woods Hole Oceanographic Institution, 27 pp. LAZIER J. R. N. (1988) Temperature and salinity change in the deep Labrador Sea, 1962-1986. Deep-Sea Research, 35, 1247-1253. LOEE M. M. R. v. and A. J. v. BENNEKOM(1989) Weddell Sea contributes little to silicate enrichment in Antarctic Bottom Water. Deep-Sea Research 36, 1341-1357. MANTYLA A. A. W. (1975) On the potential temperature in the abyssal Pacific Ocean. Journal of Marine Research, 33,341-354. MANTYLA A. W. (1980) Electrical conductivity comparisons of Standard Seawater batches P29-P84. Deep-Sea Research, 27, 837-846. MANTYEA A. W. (1987) Standard Seawater comparisons updated. Journal of Physical Oceanography, 17, 543-548. MANTYLAA, W. and J. L. REID (1983) Abyssal characteristics of the World Ocean waters. Deep-Sea Research, 30, 805-833. MCCARTNEY M. S., S. L. BENNETT and M. E. WOODGATE-JONES(1991) Eastward flow through the Mid-Atlantic Ridge at 11°N and its influence on the abyss of the Eastern Basin. Journal of Physical Oceanography, 21, 1089-1121. MCCARTNEY M. S., L. D. TALLEYand M. TSUCHIYA(1991) Physical, Chemical and CTD Data Report. Oceanus 202 (McTT). SI0 Ref. 91-16, Scripps Institution of Oceanography, University of California San Diego, 305 Pp. MONTGOMERY R. B. (1958) Water characteristics of the Atlantic Ocean and of the world oceans. Deep-Sea Research, 5,134-148. PAQUETrE R. G. (1958) A modification of the Wcner-Smith-Soule salinity bridge for the determination of salinity in sea water, with details of construction, operation, and maintenance. University of Washington, Department of Oceanography, Technical Report No. 54-14, 1-57. PARK K. (1964) Reliability of Standard Sea Water as a conductivity standard. Deep-Sea Research, 11, 85-87. RAGOT. A. and S. C. KENNAN(1991) A 2-year intercomparison of measurements of salt water samples between the University of Hawaii and the Naval Postgraduate School. Transactions of the American Geophysical Union, 72,260. REID J. L. (1994) On the total geostrophic circulation of the North Atlantic Ocean: ltow patterns, tracers and transports. Progress in Oceanography, 33, 1-92. SAUNDERS P. M. (1986) The accuracy of measurement of salinity, oxygen, and temperature m the deep ocean. Journal of Physical Oceanography, 16, 189-195. SCHLEICHERK. E. and A. BRADSHAW(1956) A conductivity bridge for measurement ol the salinity of sea water. Journal Conseil Permanent lnternationalpour l'Exploration de la Mer, 22, 9-2(I. SCHIJTZERR., W. ROETttER, U. WEIDMANN, P. KALTand H. H. LoosIJ (1985) A meridiona114C and 3~Ar section in Northeast Deep Water. Journal of Geophysical Research, 90(C4), 6945-6952. Scripps Institution of Oceanography, University of California (1963) Oceanic observations of the Pacific: 1951. Berkeley and Los Angeles, University of California Press, 477 pp. SOULE F. M., P. A. MORRILLand A. P. FRANCESCHETTI(1961) Physical oceanography of the Grand Banks region and the Labrador Sea in 1960. Bulletin U.S. Coast Guard, 46, 31-114. SPEER K. G. and M. S. McCARTNEY (1991) Tracing lower North Atlantic deep water across the equator. Journal of Geophysical Research, 96, 20,443-20,448. S w l n J. H. (1984) A recent 0)-S shift in the deep water of the northern North Atlanlic. In: Climateprocesses and climate sensitivity, Geophysical Monograph 29, Maurice Ewing, 5, 3947. TAKATSUKIV., M. AOYAMA,T. NAKANO, H. MIYAGI, T. ISHIHARAand T. TSUTSUMIDA(1991) Standard seawater comparison of some recent batches. Journal of Atmospheric and Oceanic Technology, 8,895-897. TSUCHIYA M., L. D. TALLEr and M. S. McCARrNEY (1992) An eastern Atlantic section from Iceland southward across the cquator. Deep-Sea Research, 39, 1885-1918. UNESCO (1981) Background papers and supporting data on the Practical Salinity Scale, 1978. UNESCO Technical Paper, Marine Science, 37, 144 pp. WARNER M. J. and R. F. WEISS (1992) Chlorofluoromethane in South Atlantic Antarctic Intermediate Water. Deep-Sea Researeh, 39, 2053-2075.
Treatment of inconsistencies in Atlantic deep water salinity data
1405
WARRENB. A. (1981) Transindian hydrographic section at lat. 18°S: property distributions and circulation in the south Indian Ocean. Deep-SeaResearch, 28,759-788. WILLIAMSR. T. and F. W. DELAHOYDE(submitted) Improving the measurements of pressure in the NBIS Mark III CTD. WOOSTERW. S. and B. A. TAFT(1958) On the reliability of field measurements of temperature and salinity in the ocean. Journal of Marine Research, 17,552-566. WORTHINGTON L. V. (1981) The water masses of the World Ocean: some results of a fine-scale census. In: Evolution of physical oceanography, B. A. WARRENand C. WUNSCH,editors, The MIT Press, Cambridge, MA, pp. 42-69. WORTHINGTONL. V. and W. R. WRIGHT(1970) North Atlantic Ocean Atlas of potential temperature and salinity in the deep water including temperature, salinity and oxygen profiles from the Erika Dan cruise of 1962. Woods Hole Oceanographic Institution Atlas Series 2,24 pp. and 58 plates.