Formative area of ‘eighteen-degree’ water in the Sargasso sea

384

Yu. V. ISTOSmN

Formative area of ' eighteen-degree' water in the Sargasso Sea Yu. V. ISTOSHIN Central Institute of Forecasting

Okeanologiya, 1961. 1,(4), 600--607. Wogg on an interesting peculiarity of the Sargasso Sea has recently appeared in connexion with the increased research in the northern Atlantic. There is in this sea a vast, almost isothermal, volume of water which in the literature has received the not very convenient but rather precise designation: ' eighteen-degree' water. The prevalence of this water was noted there at the time of the voyage of Challenger. The profile made by this vessel in 1873 (MoRsKoI ATLAS, 1953) (FIG. 1) is from WORTI-nNGXON (1959); this prevalence can be seen well in the section. Many profiles crossing this sea in both latitudinal and meridional directions since then, show the ' eighteen-degree' water as a characteristic of the Sargasso Sea. It was found that the water from the surface down to 300 m (and at times down to 500-600 m) has a tmperature of 18 ~ 1° and a salinity of 36.5 :l: 3 ~ in the northern Sargasso Sea in winter between 33 ° and 38°N (T~t.E 1). A t t h e e n d o f w i n t e r (endofFebruary-March) the 0-300 m layer is completely mixed and has a uniform temperature and salinity due to the intense convection and mixing processes in winter.

N/tIC/T/ //5 lltlq3//g

qO 39 383"136 3/

30 g9

g8

27 26

?.52/1

500

g S O//blO" 0 ~ BopK

Sept: td6t

gS. TO~aC

FIG. 1. The Challenger profile through the Sargasso Sea, Atlantic islands (18°N, 65°W) to New York. Translation of Russian words: On the ordinate; Depth m. On the abscissa; New York, Bermuda, St. Thomas. In consequence of heating in the warm season of the year the thermocline is formed, and the 'eighteen-degree' water is observed in the layer between 200 and 400 m. The lower boundary of the seasonal thermocline gradually deepens from spring to autumn, and the thickness of the layer with predominantly 'eighteen-degree' water decreases, though this layer does not completely disappear. This proves that in summer mixing by the wind and turbulence due to vertical current-velocity gradients, are only of real significance down to a depth of not more than 200-250 m. T h e ' eighteendegree' water extending to a considerable depth in winter appears to b e ' buried' between the lower boundary of the seasonal thermocline and the upper limit of the permanent thermocline, between the 500-660 m and 1000-1200 m in the Sargasso Sea. This water can be 'renewed' only in subsequent, sufficiently cold winters. In the southern Sargasso Sea between 33 ° and 20°N 'eighteen-degree' water in winter and summer only occurs at a certain depth, the thickness of this water layer thins with distance toward the south. ' Eighteen-degree' water, however, may always be found at a depth of 300 m. ] t is supposed that it flows southward from the north above the permanent thermocline.

0 50 100 150 2OO 250 300 40O 500 600 800 1000 1200 1500 2OOO

18"37 18"37 18"38 18"37 18"39 18"36 18"31 17"70 17"13 15"49 11"94 8"38 5"13 4-I0 3.68

36"54 26"38 36"56 26"39 36"55 26"38 36"54 26"38 36"55 26"38 36"55 26"39 36"52 26"38 36"47 26"49 36"38 26"56 36.08 26"71 36"67 27"14 35"26 27"44 35.00 27.68 35-00 27-80 34"99 27"83

18"88 18-89 18-89 18"88 18"87 18"87 18.83 18.00 17-52 16-57 13"30 8"52 5.11 4-47 3.76

36.60 36.59 36"59 36.59 36.59 36-59 36"59 36-50 36"47 36"25 35-70 35.20 35.09 35"04 34"98

S

26"29 26-28 26-28 26-29 26"29 26"29 26"30 26-44 26"54 26.60 26.89 27.37 27.76 27"81 27.82

ot

t

t

ot

M. Lomonosov

M. Lomonosov

S

Stn. 550 25"11"1960 35°05'.6; 67°07'.5

Stn. 559 29.11.1960 36°31".8; 57°01'.2

S

19"18 36"59 19"22 36"59 19"20 36"60 19"21 36"59 19"22 36"59 19"14 36"58 18"44 36"53 17"74 36"43 17"43 36"41 15"96 36.28 12-48 35.62 7"78 35.09 5"42 35.07 4.32 35-01 3"65 34"97

t 26"21 26-60 26"21 26"20 26"20 26"21 26"35 26"45 26"51 26"76 26"99 27'40 27'70 27"78 27"82

ot

M. Lomonosov

Stn. 553 26.11" 1960 33°21'.6; 64°34'.8

I

S at

18"48 36"35 26"20 18"50 36"36 26"21 18"48 36"37 26"22 18"19 36"42 26"33 17"67 36"42 26"46 17'11 36"34 26"53 16"51 36"26 26"62 14"84 35"94 26'75 14"34 35"94 26"86 13"38 35"76 26"92 10"16 35"38 27"24 8'04 35"27 27"50 5"27 35-25 27-86 4"65 35"04 27"77 3"75 35'00 27"83

t

M. Lomonosov

Stn. 524 29.1.1960 35°20'.6; 43°36'.7

18"46 18"48 18"46 18"49 18"55 18.60 18"15 16.37 14.37 12.18 8-13 5-54 4.64 3'96

t

ot

36"50 26"32 36'50 36"32 36"50 36"32 36"52 36.34 36"53 36-32 36'54 26.32 36.49 26.39 36.20 26.60 35"86 26-79 35"50 26-96 35"12 27.37 35-02 27.65 35.01 27"75 35.00 27-75

S

M. Lomonosov

Stn. 557 28"11-1960 34°30'.0; 59°37'.6

1 25 50 99 149 198 297 396 495 594 693 792 891 990 1188

17"72 17"70 17"68 17"42 17-46 17"31 17-13 15-79 14-15 12"56 10"61 8'40 7-23 6"17 5"02

Chain

t

S

02 0 17"94 36"55 5"08 18.84 36"50 30 17"92 36"51 5"02 18.82 36.50 61 17"92 36"50 4"97 18"82 36"50 18-71 36"50 [ 122 17"96 36"50 5"05 183 17"91 36"50 5"00 18-58 36"50 242 17"91 36"51 5"00 18"35 36"48 363 17"00 36"51 5"01 17.64 36-43 484 17-04 36'35 4"53 17"46 36"42 628 15"65 36"08 4"19 17"41 36"42 17"30 36"39 16"60 36'25 14"62 35"89 12'52 35"61 9"76 35"25 6-62 35'12

S

Atlant~ Chain t

Stn. 1153 28.11.1932 34°02";54°05 '

Stn. 48 29-IV'1959 36°15';46°49 '

36.39 1 36.39 24 36"38 47 36"36 95 36"39 142 36-37 190 36-35 285 36"11 381 35'85i 477 35"64! 574 35"38 671 35-18 768 35"13 866 35-10 964 35"05i 1159

S

Stn. 42 25.IV.1959 30°15'; 56050'

Table 1. Vertical distribution o f t °, S%, and ot in the region where ' eighteen-degree' water is formed in winter

O0

o

386

Yu. V. ISTOSmN

O°

5 * I

70 ° I

20*

75" I

I i I

TX

"=

25*

:.--T--q- I ")j

.-'".j,..

tO0

gO0

300

i

500

60~

?Ot~

FIG. 2. Curves for the vertical distribution of the water temperature on the Atlantis profile. Translation of Russian words: Countercurrent on the right side of the G u If Stream. Gulf Stream.

The prevalence of ' eighteen-de~,e' water is conditionally assumed here to be considerable when the ratio of the thickness of the layer between the 17° and 19° isotherms to the thickness of the layers between the 19-21 ° and 15-17 ° isotherms equals 1.5 or more (TA]3LE2). If these ratios lie in the range 1.0--1.4, its prevalence is considered to be negligible; if one of these ratios should be less than 1.0, then it is not prevalent. This method of appraisal as proposed here outlines the boundary of the region where ' eighteendegree' water predominates and delimits the zone where it is clearly expressed.

F o r m a t i v e area o f 'eighteen-degree' water in the Sargasso Sea

387

Table 2. Thickness of the layers between the isotherms and the ratios of these thicknesses characterizing the predominant 'eighteen-degree' water in the Sargasso Sea on the Atlantis profile along meridian 50°30'W in November 1956 Station nos.

Dates

Coordinates N W

Thicknesses of Inter-isotherm layers 19-21

5428 5432 5433 5436 5437 5438 5446 5450 5454 5456

14"XI 15"XI 15"XI

16-XI 17"XI 17"XI

20.XI 23"XI 25"XI 26-XI

40 ° 48' 39 ° 37' 39 ° 18' 38 ° 34' 38 ° I 1' 37046 ' 30 ° 13' 25 ° 26' 20 ° 46' 18027 '

50 ° 29' 50 ° 37' 50 ° 26" 50 ° 24' 50 ° 30' 50031 ' 50 ° 34" 50 ° 34' 5O° 33' 50o32 '

17-19

Ratios of the layer thicknesses

15-17

1~19 19--2f

17-19 15-17

1"0

0'5 3-4 2-7 1'9 1-9 1.0 0"8

m

m

m

50

50 207 335 279 233 117 56

159 47 43 54 34

m

95 60 123 146 120 117 73

--

2-1 6.0 5.4 2.2 1"6

It is evident (FIG. 2, TABLE 2) that at Atlantis Stn. 5428, in the G u l f Stream, a n d also at Stns. 5432 a n d 5433, in the countercurrent to the right side o f the Stream, ' e i g h t e e n - d e g r e e ' water either does n o t p r e d o m i n a t e or else is n o t observed at all. It b e c o m e s prevalent, starting f r o m Stn. 5437, a n d then is very o b v i o u s between Stns. 5438 a n d 5450. T o the s o u t h o f 24°N (as far as 20°N) the prevalence is n o t as obvious (Stn. 5454), a n d it is n o t observed at all s o u t h o f 20°N. F r o m the profiles completed o n t h e 6th, 7th, a n d 8th voyages o f t h e Mikhail Lomonosov a n d also t h o s e p u b l i s h e d in FUGLISTER'S atlas (1960), a n d m a k i n g use o f the specified criterion, it was possible to delimit t h e area o f ' e i g h t e e n - d e g r e e ' water. I n m o s t cases t h e thicknesses o f the layer between t h e 17 ° a n d 19 ° i s o t h e r m s exceed 200 m , whereas the thickness o f t h e 19-21 ° isotherm layer is usually less t h a n I00 m , being less t h a n 150 m for t h e layer between the 15 ° a n d 17 ° i s o t h e r m (TABLE 3).

Table 3. Thicknesses of the inter-isotherm layers and the ratios of these thicknesses characterizing the prevalence of Sargasso Sea's 'eighteen-degree' Station nos.

Vessel

Date

Coordinates N W

Thicknesses of the inter-isotherm layers

Ratios of the layer thicknesses

"?'7

"7 O0

321 533 537 3623 529 547 5192 559 3630 5754

Crawford M. Lomonosov M. Lomonosov Discovery M. Lomonosov M. Lomonosov Atlantis M. Lomonosov Discovery Atlantis

15'11'58 11 .II'60 13"II'60

27"X'57 4-9.II-60 24.II.60 23"IX'54 29.II.60 26.XI'57 12"11"59

23°30 ' 25°37'5 28°15'7 24032 , 28°39'7 37009'5 37o52 ' 36°31'8 32°14 ' 28°15 '

67°05 , 69°56'1 76'16°4 74o56 , 62°12'8 69o45'5 64°57' 57°01'2 41o42 ' 43o59 ,

72 79 168 64 94 81 55

180 234 223 229 300 239 251 - - 508 34 208 36 263

115 93 69 104 139 119 74 118 154 103

54 92 82 76 40 79 52 -36 29

2"5 3'0 1.4 3"3 3'2 3'0 4'6 6"2 7"3

1-6 2'5 3"3 2"2 2"2 2.0 3"4 4"3 1"5 2-6

A t t e n t i o n is called to the fact that at the 300 m level the t e m p e r a t u r e o f the water everywhere equals 18 4- 1 ° (FIG. 3) a n d the salinity 36'5 4- 0-3 ~0o. T h i s was d r a w n u s i n g d a t a f r o m winter stations (January-April), b u t it w o u l d be t h e s a m e for the w a r m e r m o n t h s if the presence o f t h e m a r k e d s e a s o n a l thermocline in s u m m e r is overlooked. T h e area (Fio. 4) in w h i c h the prevalence o f ' eighteen-degree ' water is unclearly s h o w n (the ratios between t h e thicknesses o f the layers 17-19°/ 19-21 ° a n d 17-19°]15-17 ° is less t h a n 1-5 °) is recorded by a b r o k e n line. W i t h i n this zone the ratio

388

Y u . V. I s ~ s m N

5* I

I0 ° I

15" I

g5*

20 ° I

I

I |1

Illt-~,~l

]

I

I

I

900

tlO0

FIG. 3. Curves for the vertical distribution o f the t e m p e r a t u r e in the ' eighteen degree ' water area. 17-19°/15-17 ° s o m e t i m e s appears to be less than one, indicative o f the prevalence o f water with a temperature o f 16 °, 17 ~ a n d 18 ° at the s a m e time. T h i s water is f o r m e d in the north-eastern part of the area m a r k e d by the broken line (FIG. 4). T h e distribution area o f ' e i g h t e e n - d e g r e e ' water is b o u n d e d by the Antilles current in the south-west a n d by the G u l f Stream in the west a n d n o r t h (FIG. 4). It is s o m e t i m e s f o u n d at a b o u t 400 m u n d e r the Antilles current a n d G u l f Stream. T h i s region spreads to the 40th meridian in the east - - as far as the N o r t h Atlantic R a n g e (Mid-Atlantic Ridge). Instances have been recorded, however, when the m a r k e d prevalence of ' eighteen-degree ' water occurred to the east o f the ridge. It should be n o t e d that in the north-east the regional b o u n d a r y o f this water is not directly adjacent to the G u l f Stream, but is separated f r o m it by a counter-current to the right o f the G u l f Stream. In his article WORTSIN~TOr~ (1959) accounts for this ' e i g h t e e n - d e g r e e ' water by its s i n k i n g in winter f r o m the surface in the region to the n o r t h o f 33°N, which he calls the ' f o r m a t i v e ' area. A l t h o u g h he h a d extremely little material at his disposal, the data from the three Atlantis stations 1153 (TABLE 1), 4170 (35°18'N, 69°26'W - - 3'111"1941), a n d 4171 (35°18'N, 68°24'W - - 4.111.1941) the formative area for ' eighteen-degree ' water was outlined very accurately. T h e two latter showed that f r o m the surface d o w n to 284 a n d 297 m the water h a d a temperature o f 17"84 ° ± 0-04°C a n d a salinity o f 36.5 ± 0 . 0 2 ~ respectively. W e h a d m a n y more data, which permitted a m o r e accurate definition o f the b o u n d a r y o f the e i g h t e e n - d e g r e e ' water a n d the factors determining its formation. The north-eastern b o u n d a r y o f t h e ' f o r m a t i v e ' area was outlined on the basis o f actual data. It is in the vicinity o f the M. Lomonosov Stations 549-551 to the north-west, o f Stations 556-560 to the north, a n d o f Chain Stations 41--49 a n d o f the M. Lomonosov Station 524 to the north-east. There is evidence that the A m e r i c a n weather-ship E (35~N, 48°W) is situated in the ' f o r m a t i v e ' area o f the ' eighteen-degree ' water. F r o m the weather-ship data, the m e a n m o n t h l y temperature o f the surface water in M a r c h varied f r o m 17 ° in 1953 to 19 ° in 1951 with the multi-year average for the water temperature in M a r c h being 18 ° (Scrmo~o~R, et al., 1959). '

80

,

'75

,, .S'

.,

75

70

7o

,3

9

~

/ 65

_

o650

~'~

65

_

60

,

"569e

561 I ~

i

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55

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o,0, l ....... 50

40

45

/ 40

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30

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45

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25

25

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20

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,

'1

I

20

15

t5

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i

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25

30

,351

~01

FtG. 4. Distribution o f ' eighteen degree ' water. 1 - - region o f the m a r k e d prevalence o f ' eighteen degree ' water; 2 - - region o f less obvious prevalence o f ' eighteen degree ' water; 3 - - line o f subtropical convergence in winter; 4 - - line o f subtropical convergence in s u m m e r ; 5 - - currents ( G u l f Stream, Antilles, Canary); 6 - - position o f the American weather-ship; 7 - - isotherms for the multi-year m e a n - m o n t h l y air-temperature in M a r c h ; 8 - - MidAtlantnc Range; 9 - no. o f stations whose data were used in the analysis ( a - absent; ~ - weak prevalence; B - a b r u p t prevalence).

35t

40 I ~

4 5~

80

OO

O

g,

,=

<

B

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390

Yu. V. ISTOSHIN

The formation o f ' eighteen-degree ' water then is, above all, due to the processes of winter convection and mixing by the wind. Processes of vertical and horizontal mixing, due to vertical and horizontal current-velocity gradients, and also the sinking of water in the subtropical convergence play is, as we see it, a minor role. The average multi-year isotherms for the air temperature at the sea surface in March, and also the position of subtropical convergence in winter and summer based on data in the Marine Atlas of the U.S.S.R. (FIG. 4) shows that water with a temperature of 18° and below can only be formed at the surface to the north of latitude 33°N. It should be noted that at the end of February 1960, on the 7th voyage of the M. Lomonosov (TABLE 1), instances were recorded when the upper (to 200 m) layer of water in the Sargasso Sea between 34 ° and 32°N had the same densities and temperatures, somewhat in excess of 19°. It is probable that by March these temperatures may drop somewhat to the range 18-19 °, but it is not excluded that the formation of expressly 'eighteen-degree' water occurs in this region (32-24°N). It has been pointed out above that there are areas where water with a temperature of 16° and 17° is formed. Systematic observations made near the Bermuda Islands from 1954-1958, revealed an astonishing stability of 'eighteen-degree' water at 200-400m (SCHROEDER, et al., 1959). A comparison of this data with that of the Challenger indicates how little the temperature and salinity of the mid-water of the Sargasso Sea varied over the last 85 years. Thus, the Sargasso Sea is characterized as follows:

(1) the prevalence o f ' eighteen-degree ' water: (2) a surface t h e r m o c l i n e - seasonal in the northern part and permanent in the southern; (3) the presence of a permanent ' chief thermocline ' from 500-600 m to 1000-1200 m.

REFERENCES

APPENDIX TO THE METEOROLOGICAL BULLETIN (1960). World maps of multi-year mean-monthly values for the atmospheric pressure and air temperature at sea-level. Tsentr. Inst. Prognozov. FUGLISTER,F. C. (1960) Atlantic Ocean Atlas, Temperature and Salinity Profiles and Data. International Geophysical Year of 1957-1958, Woods Hole Oceanographic Institute, Woods Hole, Mass. MARINE ATLAS (MORSKOI ATLAS) (1953) II, Gos. Uchrezh. Voyenn. Morsk. Flot. SCHROEDER,E., STOMMEL,H., MENZEL,D. and SUTCL1FFE,W. (1959) Climatic stability o f ' eighteendegree' water at Bermuda, J. Geophys. Res. 64, (3) 363-366. THOMSON,WYVILLE (1877). The Voyage of The ' Challenger ', I. The Atlantic. Macmillan, London. U.S. NAV'¢ (1956) Marine Climatic Atlas of The World 1. WORTHINGTON, L. V. (1959) The ' eighteen-degree' water in the Sargasso Sea. Deep-Sea Res. 5, (4), 297-305.