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Chemical anomalies in a cold core Gulf Stream ring
Deep-SeaResearch,Vol. 33, No. 10. pp. 1313-1325,1986.
0198~1149/86$3.0~)+ 0.IX) PergamonJournalsLtd.
Printed in Great Britian.
Chemical anomalies in a cold core Gulf Stream ring JANE A . ELROD* a n d DANA R . KESTERt
(Received 4 September 1985; in revised form 31 March 1986; accepted 8 May 1986) Abstract-42hanges occurring in a cold core ring were examined using anomalies of salinity, oxygen, nitrate, phosphate and silicate as a function of ~o for April and August 1977 stations. Sargasso Sea Water was used as the reference for the anomalies. All property anomalies diminished in intensity over the time interval, with stronger changes observed for the nonconservative properties" than for salinity. The Slope Water core was readily identifiable by the property anomalies, but the outer region, originally Gulf Stream Water, lost most of its characteristic anomalies by August. Nitrate and phosphate showed similar anomaly distributions, but each of the other three properties showed different distributions within the ring.
INTRODUCTION
TEMPERATUREand salinity have long been used to characterize water masses and to investigate circulation and mixing in the ocean. Both parameters are considered conservative properties of seawater away from the ocean boundaries. Although nonconservative, oxygen occasionally has been used in a manner similar to temperature and salinity (McDoWELL, 1982). Nitrate and phosphate are nonconservative properties whose distributions generally mirror that of oxygen due to their linked behavior in biological processes. Silicate is also nonconservative, but its variations are not correlated closely with those of oxygen. When these nonconservative properties are studied in conjunction with temperature, salinity and density, they often provide additional information useful in identifying water masses and in examining ocean circulation. These chemical parameters have been used as a means of examining a cyclonic (cold core) Gulf Stream ring. The general life history of cyclonic rings has been described by PARKER (1971) and RICHARDSONet al. (1979). Others have discussed the birth of cyclonic rings from southward-extending meanders that detach from the Gulf Stream (DOBLAR and CHENEY, 1977) the translation of rings within the Sargasso Sea (LAI and RICHARDSON, 1977; RICHARDSONet al., 1973, 1977) and the spindown of rings (MoLINARI, 1970; CHENEY and RICHARDSON, 1976; SCHMITZand VASTANO, 1977) or their reabsorption by the Gulf Stream (WA'rrs and OLSON,1978). Dynamic processes occurring within a ring have been studied by OLSON(1980) and VASTANOet al. (1980). In 1977, two cruises (Knorr-65 in April and Endeavor-11 in August) visited the same Gulf Stream ring, "Bob", obtaining CTD-O2 and discrete bottle data. Ring "Bob"
* Code 671, Laboratory for Oceans, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. t Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881, U.S.A. 1313
1314
J . A . ELROD and D. R. KESTER
formed in February 1977, reattached to the Gulf Stream in April, separated again in May and finally coalesced with the Gulf Stream in September 1977 (VASTANOet al., 1980). The data generated by these two cruises and several others have been the basis for many studies. In general, these studies (as summarized by BACKUSet al., 1981) examined physical and chemical properties as a function of depth or pressure. Cold core ring sections of density, temperature, salinity, oxygen, or nutrients vs depth or pressure all show the same general structure--a doming of the isopleths through the center of the ring (see e.g. Fig. 5, BACKUSet al., 1981). Using ~0 on the vertical axis effectively eliminates the firstorder dome shape of cyclonic rings and, instead, emphasizes the variations on isopycnal surfaces. Section plots of the anomalies of the properties vs Go emphasize how the ring differs from the surrounding Sargasso Sea. This study uses the method of ELROD and KESTER (1985) tO subtract Sargasso Sea reference curves from the April and August station data of cold core ring "Bob". The resulting anomaly sections illustrate how the different water masses that comprise the ring (Slope Water, Gulf Stream Water and Sargasso Sea Water) contribute to its overall properties. In addition, a comparison of the April and August anomaly sections readily reveals the changes occurring in each of the properties over the 4-month period. METHODS
The data used in this analysis are of two types: continuous salinity, potential temperature, G0 and oxygen derived from a CTD with an oxygen sensor, and discrete nitrate, phosphate and silicate from 12 Niskin bottles attached to a rosette sampler. The CTD stations formed transects along four radii of ring "Bob" in August and along two radii in April. The August sections were oriented north to south and west to east (SCHMrrz et al., 1978b), whereas the April section extended from westsouthwest to ring center to southeast, or approximately west to east (ScHMITZet al., 1978a). Contour plots against G0 were made for each of the two August sections and the April section across ring Bob. To produce the contour plots, the CTD-O2 data were obtained at 0.02 G0 unit intervals. The salinity, potential temperature, and oxygen sections cover the G0 range 26.0-27.7. For most of the ring, G0 26.0 occurs at 100 m or deeper. But in the ring center in April, the surface G0 is >26.0. Nutrient sections start at ~0 26.4 because too few samples to contour were obtained at lower Go. Nitrate and phosphate for E n d e a v o r - l l , Sta. 31 on the August north-south section, were inverted with respect to the other discrete parameters at this station. We assume they were run in reverse order and recorded incorrectly. The anomaly sections were calculated for each property using the method and Sargasso Sea reference curves of ELROD and KESTER(1985). The reference curves were obtained by fitting Sargasso Sea data with a cubic-spline equation of the form: S ( x ) = Yi + OCi,l + O2Ci,2 + D3Ci,3 .
S(x) is the value of the cubic-spline function of a property at Go = x and D = x - Ki, where Ki is the lower knot bounding interval i. The Ci's and Yi are the coefficients determined by the fit and are different for each property. The anomalies for each property, P, were calculated at every 0.02 or0 value over the range covered by the cubicspline relations (26.0-27.75) for potential temperature, salinity, and oxygen and at the
Chemical anomalies in a cold core Gulf Stream ring
1315
observed cr0 values between 26.40 and 27.75 for nitrate, phosphate and silicate, by subtracting the cubic-spline reference values from the observed data: P,,, = Pob~,,Prc|'~,,"
The anomalies of each station were then plotted with cyoon the vertical axis and radial distance on the horizontal axis to obtain anomaly sections across the ring. Because the contouring program required uniformly spaced data points, the nutrient anomaly values were interpolated linearly to standard or0 values before contouring. This led to unsatisfactory contouring in areas where the nutrient concentration gradient with cro was not uniform, as near the nutrient maximum (cy0 ~ 27.3) and near the sea surface. Therefore, the nutrient anomaly contour plots were subsequently corrected to correspond to the original data anomalies rather than the interpolated anomalies. Because the ring was translating during each cruise, ring center locations at 12-h intervals (RICHARDSON, personal communication) were determined from the trajectories of free-drifting surface buoys launched in the ring and tracked by satellite. The radial distance of each station was calculated from the interpolated location of ring center at the time the station was taken. No stations were obtained within 15 km of ring center during either of the August ring transects. To supplement the ring sections, the data from Endeavor-11 Stas 2 and 21 were used on the west to east and north to south sections, even though these stations had been sampled prior to the transect stations. In addition, Stas 13 and 14 of the Knorr-65 cruise in April did not have any nutrient data. To supplement the April nutrient sections, Knorr Sta. 5 was included; it was a near-center station sampled prior to the rest of the section. Nutrient data from Endeavor Sta. 3 were added to those of Sta. 2 and data from Sta. 22 were added to those of Sta. 21 because Stas 2 and 21 did not sample the near-surface water. The average of their radial distances was used on the August nutrient sections. The station data are not synoptic. The entire west-east April transect took 5 days to complete, while the August west-east transect took 3 days and the north-south took 4 days. The inserted center stations were sampled 1 or 2 days before the transects so that each of the August sections represents a 5-day period and together they cover an 11-day period. The rotation rate of the ring was approximately 1.9 days in April and 2.9 days in August (RICrtARDSON,1980). Therefore, the ring made over two complete rotations during the April sampling and over four rotations in August. The radial sections presented here, therefore, represent composite distributions over a period of days and several rotations of the ring. RESULTS
The salinity sections (Fig. 1) consist of negative anomaly water in the core region (within about 40 km of ring center) for % < 27.0 and slightly positive anomaly water for c~0 > 27.0, extending to 60 km on the August sections. The salinity anomalies are most intense for smaller G0 near ring center and diminish in magnitude away from the center and with increasing cy0. In April (Fig. la), the most intense anomaly, occurring at Sta. 14, is about -0.6%0. In August (Fig. lb and c), the salinity anomaly reaches -0.5%0 only at Sta. 21. The August north-south section has more intense salinity and potential temperature anomalies than the August west-east section in the core region because Sta. 21 used to supplement the north-south section has stronger anomalies than Sta. 2 used in the west-east section. The -0.1%o contour in April extends to G0 26.9 at Sta. 14, but only
1316
ELROD and D. R. KESTER
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Fig. 1. Salinity anomaly sections of cold core ring "Bob". Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 0.1%o. (a) April west-east section. (b) August west-east section. (c) August north-south section.
to a0 26.75 in August in the core region. Radially, the --0.1%o contour extends 40-45 km from the ring center. The April salinity anomaly section shows a finger of negative anomaly water (-0.1%o) protruding inward to a radius of 50 km on the western side at o0 27.3. The eastern side also shows a negative salinity anomaly of magnitude --0.2%0 centered around o0 27.0. According to ELROD and KESTER (1985), the negative and deeper positive anomalies in
1317
Chemical anomalies in a cold core Gulf Stream ring
the core region described above are both characteristic of Slope Water, whereas negative salinity anomaly between (~0 26.9 and 27.4 is indicative of Gulf Stream Water. Therefore, the finger of negative anomaly is likely to be an intrusion of fresh Gulf Stream Water (the ring in April was partly attached to the Gulf Stream). Although the August west-east section does have negative anomalies outside the core region at three stations (9, 10, 12), the north-south salinity anomaly section does not have any indication of Gulf Stream RADIAL W 26.0
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Fig. 2. Oxygen anomaly sections of cold core ring "Bob". Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 0.4 ml 1-'. (a) April west-east section. (b) August west-east section. (c) August north-south section.
1318
J.A. ELRODand D. R. KESTER
Water--that is, no negative anomaly between cy0 26.9 and 27.4. The weak Gulf Stream signal in the salinity and potential temperature anomalies has nearly disappeared by August. The oxygen anomaly section plots (Fig. 2) show positive oxygen anomalies in the core region, from cy0 26.5 to 27.25. The positive anomaly extends from ring center to a radial distance of <40 km in April for G0 < 27.0, but to nearly 60 km in August. The greatest anomaly in April is over +2 mll -~ at Sta. 14. By August, it is only +1.2 ml I ~at Sta. 2 on the west-east section and even less in the core of the north-south section. ELRODand KESTER (1985) found Slope Water characterized by positive oxygen anomalies between (Y027.0 and 27.3. In the ring, the positive oxygen anomalies in the core region extend to smaller G0 values and presumably indicate the presence of Slope Water at these c~0 values. The April oxygen anomaly section (Fig. 2a) also has a finger of weak negative anomaly in the same location as the finger of negative salinity anomaly, that is <4).2 ml I-n on the west side at G0 27.25 and a small region of 4).4 ml 1-~ anomaly water on the east at ~0 27.05. These anomalies are indicative of Gulf Stream Water (ELRoD and KESTER, 1985). Unlike the salinity and potential temperature anomaly sections, which have no distinctive anomalies for ~0 < 26.75 in the high velocity region of the ring, oxygen shows strong negative anomalies for radii >40 km between ¢s0 26.0 and 26.5 on all three sections. Tile anomaly varies in maximum intensity on the different radial sections, from -1.3 ml l-l on the April west and -1.2 ml 1 i on the August north to about -0.7 ml 1 i on the August east section. Nitrate and phosphate both have negative anomalies in the core region. The most intense anomalies occur in April, at Sta. 5 for nitrate and phosphate (Figs 3a and 4a); they are -7 p.mol kg-] nitrate and -0.4 gmol kg-~ phosphate. By August, the most intense anomalies have diminished t o - 4 lamol kg-1 nitrate and 4). 1 p.mol kg-j phosphate (Figs 3 and 4b and c). The radial extent of the negative phosphate and nitrate anomaly is not uniform for the different sections, but generally includes the area within 40 km radius. In April, the negative anomaly in the core region extends to the top of the contour plot (cs0 26.4), but in August, the water between cyo 26.4 and 26.55 has positive nitrate and phosphate anomalies. Outside the core region, the April phosphate and nitrate anomaly sections have positive anomalies extending from the top of the section (~0 26.4) to about G0 27.5. They are strongest at the smallest ~0 plotted at a radius of about 60 km, on the west side reaching +10 p.mol kg-~ nitrate and +0.5 lamol kg-l phosphate. In August, the positive anomaly is still present but does not extend to as great a cy0 as in April and is weaker in intensity. The August north radial section has a bimodal region of maximum anomaly of +5-6 lamol k g I nitrate and +0.3 Ilmol kg-~ phosphate. The silicate anomaly plots also show negative anomalies in the core region. The April section appears quite complex (Fig. 5a). There are strong negative anomalies occurring below G0 27.25, both in the core region and for greater radii. In August, however, the core region below cs0 27.25 has a weak positive anomaly. For smaller Go, there is a consistent negative anomaly in the core region. It is -2 to -3 [amol kg ~ in April and diminishes to only -1 p.mol kg-I by August (Fig, 5b and c). In both April and August, there is an area of positive silicate anomaly above the negative, between cyo 26.4 and about 26.6. The April section also shows positive silicate anomalies in the same area outside the core as did nitrate and phosphate, with a maximum of +4 gmol kg-~ at 60 km radius. By August, the silicate anomaly has diminished to +2 p.mot kg-I.
Chemical anomalies in a cold core Gulf Stream ring
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1320
J . A . ELROD and D. R. KESTER
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Chemical anomalies in a cold core Gulf Stream ring
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Fig. 5. Silicate anomaly sections of cold core ring " B o b " . Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 1 pmol k g ~ for the August sections, variable for the April section. (a) April west-east section. (b) August west-east section. (c) August north-south section.
1322
J . A . ELROD and D. R. KESTER
DISCUSSION
The three objectives of this analysis were to determine how well waters in the ring could be characterized using the guidelines and methods of ELRODand KESTER(1985) to examine the changes occurring in property anomalies between April and August, and to determine if each of the six properties provided a unique contribution to the total picture of ring structure and decay. Anomaly characterization In the previous section it was shown that the Slope Water salinity and oxygen anomalies of ELROD and KESTER (1985) are readily apparent in the core of the ring. Gulf Stream salinity and oxygen anomalies do appear outside the core region, but are not very strong. Although Elrod and Kester concluded that the available data did not adequately define nutrient anomalies in Slope and Gulf Stream Water, they did report generally positive nitrate and phosphate anomalies in Gulf Stream Water which is consistent with the positive nutrient anomalies found here in the high velocity region of the ring, Also, this study shows consistent negative nutrient anomalies in the core of the ring of Slope Water origin, which were not found systematically in Slope Water stations in the previous work. Oxygen and nutrient concentrations (and thus their anomalies) are more spatially and seasonally variable on a density surface than salinity and temperature. Ring core concentrations of these parameters would depend upon where and when the ring formed. Ring "Bob" split off from the Gulf Stream in February 1977, whereas the Slope Water and Gulf Stream stations used by Elrod and Kester were sampled in April, August and November. Thus, the seasonal sampling differences may account for the difference in anomalies observed in the ring core and in Slope Water. Changes over time The major change occurring in the ring between April and August was a decrease in the magnitude of the anomalies of all the properties. This is consistent with decay of the ring through mixing and exchange processes towards the state where its properties no longer differ from those of the Sargasso Sea. In the core region, for G0 less than about 26.9, the water is shallower than 100 m and is subject to atmospheric influences. One effect of the atmosphere between April and August is to heat the surface layer. The heat input will change not only the temperature of a parcel of water, but also the density, while having little effect on the salinity. A decrease in density for a constant salinity would result in an increase in negative salinity anomaly according to the reference curve of ELROD and KESTER (1985, Fig. 1). Since the magnitude of the negative core anomalies in salinity diminished slightly for Go < 26.6 over the 4-month period, we can conclude that this effect is more than compensated for by mixing processes. Salinity and potential temperature anomalies changed by about 20% (from -0.6 to -0.5%0 and -2 to -1.5°C) between April and August. The strongest oxygen anomaly in the core diminished by about 40% (from +2.4 ml 1-Z in April to +1.4 ml 1-1 in August) as did nitrate anomaly (from -7 to -4 p.mol kg-l). Phosphate and silicate anomalies changed by more than 60% over this period (from -0.4 to -0.15 l~mol kg-~ for phosphate and-3 to -1 ~tmol kg-1 for silicate). The decrease in density through heating near the surface would cause the nutrient anomalies to become more positive, according to the slopes of the reference curves (EL~oD and KESTER, 1985). This is the change that is observed in the
1323
Chemical anomalies in a cold core Gulf Stream ring
core region. The reference curve for oxygen is not monotonic above c~0 26.9, making it difficult to predict the effect a change in density would have on oxygen anomaly. At greater densities within the core (below approximately mt 26.9) and outside the core region, density should be conservative, and changes in anomaly would be due entirely to mixing and in situ chemical and biological processes. Outside the core region, between radii of 40 and 70 km, the oxygen anomaly changed from -1.2 ml 1-1 in April to -0.8 ml 1-~ in August within the ~0 range 26.3-26.5. The nitrate anomaly diminished from 10 to 6 lamol k g 1, phosphate anomaly from (I.5 to 0.3 ~tmol kg-1, and silicate anomaly from 4 to 2 ~tmol kg-~. These changes reflect the influence of mixing at the boundaries of the ring upon the dilution of the anomaly signature of the Gulf Stream remnant.
Uniqueness of properties Potential temperature and salinity are linked to density by the equation of state of seawater and therefore provide the same type of information when plotted on a ~0 section. Nitrate and phosphate are both nonconservative properties due to their biological utilization, and their anomaly distributions are similar. Oxygen is utilized in biological processes, but it is affected also by air-sea gas exchange. The degree to which oxygen anomalies resemble those of nitrate and phosphate should depend partly upon proximity to the sea surface and the extent of vertical mixing. Tables 1 and 2 list the slopes, intercepts, their standard deviations, and the correlation coefficients for the linear regressions of nitrate and oxygen anomalies against phosphate anomaly for all stations for cy0 ranging from 26.4 to 27.0. The regressions with nitrate anomaly show strong correlation coefficients of 0.9, while the oxygen anomaly regressions have weaker correlation, especially for the smallest cy0 value. Because G0 26.4 is located near the surface, oxygen concentrations and anomalies are affected by gas exchange, whereas phosphate is not. Therefore, the two anomalies would not be expected to correlate as closely as they would deeper in the water column. Although this study compares property anomalies rather than concentrations, the slopes of both types of regression are within one or two standard deviations of the Redfield AP:AN:AO2 ratio of 1:16:138 Table 1. Regression statistics for nitrate anomaly vs phosphate anomaly ¢~ 26.4 26.6 26.8 27.0
Table 2.
o0 26.4 26.6 26.8 27.0
Slope 17.4 18.7 16.5 14.3
_+ 1.6 + 1.3 _+ 1.7 + 1.4
Intercept -0. l -0.2 0.1 -0.2
_+ 0.4 + 0.2 _+_0.2 + 0.2
Correlation coefficient /I.90 11.93 0.88 0.88
Regression statistics for oxygen anomaly (converted to ~mol kg ~) vs phosphate anomaly Slope -101 -166 -163 -131
_+ 30 +_ 28 +_ 18 _+ 17
Intercept
Correlation coefficient
-1 _+ 8 16 + 4 14 +2 10 _+ 2
-0.56 -0.75 -0.87 0.82
1324
J . A . ELROD and D. R. KESTER
(REDFmLD et al., 1963). TAKAHASHIet al. (1985) revised this ratio to 1:16:172, based on analyses conducted in water at ~0 27.0 and 27.2. The slopes determined at or0 26.6 and 26.8 for oxygen in this study are in closer agreement with the revised ratio than the traditional one. Table 3 lists the correlation coefficients of linear regressions of silicate vs oxygen and phosphate anomalies. Silicate anomaly shows poor correlations with both anomalies, but with generally higher correlation with phosphate than oxygen. Regressions of oxygen, phosphate, nitrate, and silicate anomalies against salinity anomaly for G0 26.4, 26.6 and 26.8 have poor correlations, indicating that the anomalies of salinity and the nonconservative properties provide independent information. Salinity and potential temperature anomalies indicate a region of water colder and saltier than Sargasso Sea Water in the ring within about 40 km of ring center, down to about ~0 26.75. Oxygen, however, indicates that ring water is significantly different from Sargasso Sea Water out to a radius of at least 60 km, and down to a ~0 of about 27.25. Nitrate and phosphate anomaly plots indicate that the ring contains water anomalous to the Sargasso Sea out to 80 km and down to about % 27.25. Silicate provides a similar picture but shows some anomalous water below or0 27.25, especially in April. Silicate is the only property examined which has strong anomalies within the ring for or0 values >27.2. The negative anomaly appearing in potential temperature and salinity below ~0 26.75 on the August west section at Stas 9, 10 and 12 is reflected in the oxygen anomaly plot by two regions of positive anomaly near ~0 27.7 at Stas 9 and 12. However, the silicate section displays anomalies of opposite sign at Stas 9 and 12 for or0 > 27.25, indicating waters of different histories at these two stations. The extension of-0.4°C potential temperature and-0.1%o salinity anomaly contours to 60 km on the April west radial section might be interpreted as an extension of the Slope Water beyond the core region. This area does not have the same appearance for the other property anomalies. The silicate anomaly resembles potential temperature and salinity most in that the positive silicate anomaly water near the surface of the ring core continues beyond 40 km radius, to about 80 km. Nitrate and phosphate, however, change sign around 35 km radius from negative anomalies in the core to positive anomalies outside, indicating the presence of two different waters. Oxygen anomaly also changes sign around 35 km. Although the signs of the anomalies usually are the same for the three nutrients in a particular region of the ring, there are several exceptions. The August south section has positive phosphate anomalies over most of the % range at Stas 32-34. Nitrate and silicate, however, both have negative anomalies at Sta. 34. The negative silicate anomalies occurring in April for % > 27.25 over the whole ring section are not echoed in the nitrate and phosphate anomaly sections. Table 3.
Correlation coefficients (r) of silicate anomaly vs oxygen and phosphate anomalies
%
r, oxygen
r, phosphate
26.4 26.6 26.8 27.0
0.25 0.44 0.65 0.64
0.81 0.64 0.74 0.66
Chemical anomalies in a cold core Gulf Stream ring
1325
It is concluded that of the six properties examined, a subset of four, including oxygen, silicate, either salinity or potential temperature, and either phosphate or nitrate, will provide usefully distinctive information on ¢Y0sections. Acknowledgement-Funding for this research was provided to the University of Rhode Island by the Office of Naval Research, grant no. N00014-81-C-0062. REFERENCES BACKUSR. H., G, R. FLIERL, D. R. KESTER, D. B. OLSON, P. L. RICHARDSON,A. C. VASTANO,P. H. W1EBE and J. H. WORMUTH(1981) Gulf Stream cold-core rings: Their physics, chemistry, and biology. Science, 212, 1091-1100. CHENEY R. E. and P. L. RICHARDSON(19776) Observed decay of a cyclonic Gulf Stream ring. Deep-Sea Research, 23, 143-155. DOBLAR R. A. and R. E. CHENEY (1977) Observed formation of a Gulf Stream cold core ring. Journal of Physical Oceanography, 7,944-946. ELROD J. A. and D. R. "KESTER (1985) Sargasso Sea reference curves for salinity, potential temperature, oxygen, nitrate, phosphate, and silicate as functions of sigma-theta. Deep-SeaResearch, 32,391-405. LAID. Y. and P. L. RICHARDSON(1977) Distribution and movement of Gulf Stream rings. Journal of Physical Oceanography, 7,670-683. McDOWELL S. E. (1982) Analyses of North Atlantic Intermediate Waters upon isopycnal surfaces and within mesoscale eddies. Ph.D. Dissertation, University of Rhode Island, 227 pp. MOLINARI R. L. (1970) Cyclonic ring spin-down in the North Atlantic. Ph.D. Dissertation, Texas A&M University, 105 pp. OLSON D. B. (1980) The physical oceanography of two rings observed by the cyclonic ring experiment. Part 11. Dynamics. Journal of Physical Oceanography, 10, 514-528. PARKER C. E. (1971) Gulf Stream rings in the Sargasso Sea. Deep-SeaResearch, 18, 981-993. REDFIELD A. C., 1~. H. KETCHUMand F. A. RICHARDS(1963) The influence of organisms on the composition of seawater. In: The sea: ideas and observations on progress in the study of the seas, Vol. 2, M. N. HILL, editor, Interscience, New York, pp. 26-77. RICHARDSONP. L, (1980) Gulf Stream ring trajectories. Journal of Physical Oceanography, 10, 90--104. RICHARDSON P. L., A. E. STRONG and J. A. KNAUSS(1973) Gulf Stream eddies: recent observations in the western Sargasso Sea. Journal of Physical Oceanography, 3, 297-301. RICHARDSONP. L., R. E. CHENEYand L, A. MANTINI(1977) Tracking a Gulf Stream ring with a free drifting surface buoy. Journal of Physical Oceanography, 7,580-590. RICHARDSON P. L., C. MAILLARD and T. B. STANFORD(1979) The physical structure and life history of cyclonic Gulf Stream ring Alien. Journal of Geophysical Research, 84, 7727-7741. SCHMITZ.1. E. and A. C. VASTANO(1977) Decay of a shoaling Gulf Stream cyclonic ring. Journal of Physical Oceanography, 7, 479-481. SCHMITZJ. E., O. E. HAGAN and A. C. VASTANO(1978a) Physical Oceanography Data Report I. RV Knorr cruise 65, April 1977, Texas A&M University, 145 pp. SCHMITZ J. E., J. R. HAUSTEIN, D. E. HAGAN and A. C. VASTANO(1978b) Physical Oceanography Data Report II. RV Endeavor 011, July-August 1977, Texas A&M University, 198 pp, TAKAHASHIT., W. S. BROECKERand S. LANGER(1985) Redfield ratio based on chemical data from isopycnal surfaces. Journal of Geophysical Research, 90, 6907-6924. VASTANOA. C., J. E. SCHMITZand D. E. HAGAN (1980) The physical oceanography of two rings observed by the cyclonic ring experiment. Part I: Physical structures. Journal of Physical Oceanography, 10, 493-513. WATTS D. R. and D. B. OLSON (1978) Gulf Stream ring coalescence with the Gulf Stream off Cape Hatteras. Science, 202, 971-972.
0198~1149/86$3.0~)+ 0.IX) PergamonJournalsLtd.
Printed in Great Britian.
Chemical anomalies in a cold core Gulf Stream ring JANE A . ELROD* a n d DANA R . KESTERt
(Received 4 September 1985; in revised form 31 March 1986; accepted 8 May 1986) Abstract-42hanges occurring in a cold core ring were examined using anomalies of salinity, oxygen, nitrate, phosphate and silicate as a function of ~o for April and August 1977 stations. Sargasso Sea Water was used as the reference for the anomalies. All property anomalies diminished in intensity over the time interval, with stronger changes observed for the nonconservative properties" than for salinity. The Slope Water core was readily identifiable by the property anomalies, but the outer region, originally Gulf Stream Water, lost most of its characteristic anomalies by August. Nitrate and phosphate showed similar anomaly distributions, but each of the other three properties showed different distributions within the ring.
INTRODUCTION
TEMPERATUREand salinity have long been used to characterize water masses and to investigate circulation and mixing in the ocean. Both parameters are considered conservative properties of seawater away from the ocean boundaries. Although nonconservative, oxygen occasionally has been used in a manner similar to temperature and salinity (McDoWELL, 1982). Nitrate and phosphate are nonconservative properties whose distributions generally mirror that of oxygen due to their linked behavior in biological processes. Silicate is also nonconservative, but its variations are not correlated closely with those of oxygen. When these nonconservative properties are studied in conjunction with temperature, salinity and density, they often provide additional information useful in identifying water masses and in examining ocean circulation. These chemical parameters have been used as a means of examining a cyclonic (cold core) Gulf Stream ring. The general life history of cyclonic rings has been described by PARKER (1971) and RICHARDSONet al. (1979). Others have discussed the birth of cyclonic rings from southward-extending meanders that detach from the Gulf Stream (DOBLAR and CHENEY, 1977) the translation of rings within the Sargasso Sea (LAI and RICHARDSON, 1977; RICHARDSONet al., 1973, 1977) and the spindown of rings (MoLINARI, 1970; CHENEY and RICHARDSON, 1976; SCHMITZand VASTANO, 1977) or their reabsorption by the Gulf Stream (WA'rrs and OLSON,1978). Dynamic processes occurring within a ring have been studied by OLSON(1980) and VASTANOet al. (1980). In 1977, two cruises (Knorr-65 in April and Endeavor-11 in August) visited the same Gulf Stream ring, "Bob", obtaining CTD-O2 and discrete bottle data. Ring "Bob"
* Code 671, Laboratory for Oceans, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. t Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881, U.S.A. 1313
1314
J . A . ELROD and D. R. KESTER
formed in February 1977, reattached to the Gulf Stream in April, separated again in May and finally coalesced with the Gulf Stream in September 1977 (VASTANOet al., 1980). The data generated by these two cruises and several others have been the basis for many studies. In general, these studies (as summarized by BACKUSet al., 1981) examined physical and chemical properties as a function of depth or pressure. Cold core ring sections of density, temperature, salinity, oxygen, or nutrients vs depth or pressure all show the same general structure--a doming of the isopleths through the center of the ring (see e.g. Fig. 5, BACKUSet al., 1981). Using ~0 on the vertical axis effectively eliminates the firstorder dome shape of cyclonic rings and, instead, emphasizes the variations on isopycnal surfaces. Section plots of the anomalies of the properties vs Go emphasize how the ring differs from the surrounding Sargasso Sea. This study uses the method of ELROD and KESTER (1985) tO subtract Sargasso Sea reference curves from the April and August station data of cold core ring "Bob". The resulting anomaly sections illustrate how the different water masses that comprise the ring (Slope Water, Gulf Stream Water and Sargasso Sea Water) contribute to its overall properties. In addition, a comparison of the April and August anomaly sections readily reveals the changes occurring in each of the properties over the 4-month period. METHODS
The data used in this analysis are of two types: continuous salinity, potential temperature, G0 and oxygen derived from a CTD with an oxygen sensor, and discrete nitrate, phosphate and silicate from 12 Niskin bottles attached to a rosette sampler. The CTD stations formed transects along four radii of ring "Bob" in August and along two radii in April. The August sections were oriented north to south and west to east (SCHMrrz et al., 1978b), whereas the April section extended from westsouthwest to ring center to southeast, or approximately west to east (ScHMITZet al., 1978a). Contour plots against G0 were made for each of the two August sections and the April section across ring Bob. To produce the contour plots, the CTD-O2 data were obtained at 0.02 G0 unit intervals. The salinity, potential temperature, and oxygen sections cover the G0 range 26.0-27.7. For most of the ring, G0 26.0 occurs at 100 m or deeper. But in the ring center in April, the surface G0 is >26.0. Nutrient sections start at ~0 26.4 because too few samples to contour were obtained at lower Go. Nitrate and phosphate for E n d e a v o r - l l , Sta. 31 on the August north-south section, were inverted with respect to the other discrete parameters at this station. We assume they were run in reverse order and recorded incorrectly. The anomaly sections were calculated for each property using the method and Sargasso Sea reference curves of ELROD and KESTER(1985). The reference curves were obtained by fitting Sargasso Sea data with a cubic-spline equation of the form: S ( x ) = Yi + OCi,l + O2Ci,2 + D3Ci,3 .
S(x) is the value of the cubic-spline function of a property at Go = x and D = x - Ki, where Ki is the lower knot bounding interval i. The Ci's and Yi are the coefficients determined by the fit and are different for each property. The anomalies for each property, P, were calculated at every 0.02 or0 value over the range covered by the cubicspline relations (26.0-27.75) for potential temperature, salinity, and oxygen and at the
Chemical anomalies in a cold core Gulf Stream ring
1315
observed cr0 values between 26.40 and 27.75 for nitrate, phosphate and silicate, by subtracting the cubic-spline reference values from the observed data: P,,, = Pob~,,Prc|'~,,"
The anomalies of each station were then plotted with cyoon the vertical axis and radial distance on the horizontal axis to obtain anomaly sections across the ring. Because the contouring program required uniformly spaced data points, the nutrient anomaly values were interpolated linearly to standard or0 values before contouring. This led to unsatisfactory contouring in areas where the nutrient concentration gradient with cro was not uniform, as near the nutrient maximum (cy0 ~ 27.3) and near the sea surface. Therefore, the nutrient anomaly contour plots were subsequently corrected to correspond to the original data anomalies rather than the interpolated anomalies. Because the ring was translating during each cruise, ring center locations at 12-h intervals (RICHARDSON, personal communication) were determined from the trajectories of free-drifting surface buoys launched in the ring and tracked by satellite. The radial distance of each station was calculated from the interpolated location of ring center at the time the station was taken. No stations were obtained within 15 km of ring center during either of the August ring transects. To supplement the ring sections, the data from Endeavor-11 Stas 2 and 21 were used on the west to east and north to south sections, even though these stations had been sampled prior to the transect stations. In addition, Stas 13 and 14 of the Knorr-65 cruise in April did not have any nutrient data. To supplement the April nutrient sections, Knorr Sta. 5 was included; it was a near-center station sampled prior to the rest of the section. Nutrient data from Endeavor Sta. 3 were added to those of Sta. 2 and data from Sta. 22 were added to those of Sta. 21 because Stas 2 and 21 did not sample the near-surface water. The average of their radial distances was used on the August nutrient sections. The station data are not synoptic. The entire west-east April transect took 5 days to complete, while the August west-east transect took 3 days and the north-south took 4 days. The inserted center stations were sampled 1 or 2 days before the transects so that each of the August sections represents a 5-day period and together they cover an 11-day period. The rotation rate of the ring was approximately 1.9 days in April and 2.9 days in August (RICrtARDSON,1980). Therefore, the ring made over two complete rotations during the April sampling and over four rotations in August. The radial sections presented here, therefore, represent composite distributions over a period of days and several rotations of the ring. RESULTS
The salinity sections (Fig. 1) consist of negative anomaly water in the core region (within about 40 km of ring center) for % < 27.0 and slightly positive anomaly water for c~0 > 27.0, extending to 60 km on the August sections. The salinity anomalies are most intense for smaller G0 near ring center and diminish in magnitude away from the center and with increasing cy0. In April (Fig. la), the most intense anomaly, occurring at Sta. 14, is about -0.6%0. In August (Fig. lb and c), the salinity anomaly reaches -0.5%0 only at Sta. 21. The August north-south section has more intense salinity and potential temperature anomalies than the August west-east section in the core region because Sta. 21 used to supplement the north-south section has stronger anomalies than Sta. 2 used in the west-east section. The -0.1%o contour in April extends to G0 26.9 at Sta. 14, but only
1316
ELROD and D. R. KESTER
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Fig. 1. Salinity anomaly sections of cold core ring "Bob". Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 0.1%o. (a) April west-east section. (b) August west-east section. (c) August north-south section.
to a0 26.75 in August in the core region. Radially, the --0.1%o contour extends 40-45 km from the ring center. The April salinity anomaly section shows a finger of negative anomaly water (-0.1%o) protruding inward to a radius of 50 km on the western side at o0 27.3. The eastern side also shows a negative salinity anomaly of magnitude --0.2%0 centered around o0 27.0. According to ELROD and KESTER (1985), the negative and deeper positive anomalies in
1317
Chemical anomalies in a cold core Gulf Stream ring
the core region described above are both characteristic of Slope Water, whereas negative salinity anomaly between (~0 26.9 and 27.4 is indicative of Gulf Stream Water. Therefore, the finger of negative anomaly is likely to be an intrusion of fresh Gulf Stream Water (the ring in April was partly attached to the Gulf Stream). Although the August west-east section does have negative anomalies outside the core region at three stations (9, 10, 12), the north-south salinity anomaly section does not have any indication of Gulf Stream RADIAL W 26.0
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1318
J.A. ELRODand D. R. KESTER
Water--that is, no negative anomaly between cy0 26.9 and 27.4. The weak Gulf Stream signal in the salinity and potential temperature anomalies has nearly disappeared by August. The oxygen anomaly section plots (Fig. 2) show positive oxygen anomalies in the core region, from cy0 26.5 to 27.25. The positive anomaly extends from ring center to a radial distance of <40 km in April for G0 < 27.0, but to nearly 60 km in August. The greatest anomaly in April is over +2 mll -~ at Sta. 14. By August, it is only +1.2 ml I ~at Sta. 2 on the west-east section and even less in the core of the north-south section. ELRODand KESTER (1985) found Slope Water characterized by positive oxygen anomalies between (Y027.0 and 27.3. In the ring, the positive oxygen anomalies in the core region extend to smaller G0 values and presumably indicate the presence of Slope Water at these c~0 values. The April oxygen anomaly section (Fig. 2a) also has a finger of weak negative anomaly in the same location as the finger of negative salinity anomaly, that is <4).2 ml I-n on the west side at G0 27.25 and a small region of 4).4 ml 1-~ anomaly water on the east at ~0 27.05. These anomalies are indicative of Gulf Stream Water (ELRoD and KESTER, 1985). Unlike the salinity and potential temperature anomaly sections, which have no distinctive anomalies for ~0 < 26.75 in the high velocity region of the ring, oxygen shows strong negative anomalies for radii >40 km between ¢s0 26.0 and 26.5 on all three sections. Tile anomaly varies in maximum intensity on the different radial sections, from -1.3 ml l-l on the April west and -1.2 ml 1 i on the August north to about -0.7 ml 1 i on the August east section. Nitrate and phosphate both have negative anomalies in the core region. The most intense anomalies occur in April, at Sta. 5 for nitrate and phosphate (Figs 3a and 4a); they are -7 p.mol kg-] nitrate and -0.4 gmol kg-~ phosphate. By August, the most intense anomalies have diminished t o - 4 lamol kg-1 nitrate and 4). 1 p.mol kg-j phosphate (Figs 3 and 4b and c). The radial extent of the negative phosphate and nitrate anomaly is not uniform for the different sections, but generally includes the area within 40 km radius. In April, the negative anomaly in the core region extends to the top of the contour plot (cs0 26.4), but in August, the water between cyo 26.4 and 26.55 has positive nitrate and phosphate anomalies. Outside the core region, the April phosphate and nitrate anomaly sections have positive anomalies extending from the top of the section (~0 26.4) to about G0 27.5. They are strongest at the smallest ~0 plotted at a radius of about 60 km, on the west side reaching +10 p.mol kg-~ nitrate and +0.5 lamol kg-l phosphate. In August, the positive anomaly is still present but does not extend to as great a cy0 as in April and is weaker in intensity. The August north radial section has a bimodal region of maximum anomaly of +5-6 lamol k g I nitrate and +0.3 Ilmol kg-~ phosphate. The silicate anomaly plots also show negative anomalies in the core region. The April section appears quite complex (Fig. 5a). There are strong negative anomalies occurring below G0 27.25, both in the core region and for greater radii. In August, however, the core region below cs0 27.25 has a weak positive anomaly. For smaller Go, there is a consistent negative anomaly in the core region. It is -2 to -3 [amol kg ~ in April and diminishes to only -1 p.mol kg-I by August (Fig, 5b and c). In both April and August, there is an area of positive silicate anomaly above the negative, between cyo 26.4 and about 26.6. The April section also shows positive silicate anomalies in the same area outside the core as did nitrate and phosphate, with a maximum of +4 gmol kg-~ at 60 km radius. By August, the silicate anomaly has diminished to +2 p.mot kg-I.
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Fig. 3. Phosphate anomaly sections of cold core ring "'Bob". Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 0.1 Iamol kg -~, except in high gradient regions. (a) April west-east section. (b) August west-east section. (c) August northsouth section.
1320
J . A . ELROD and D. R. KESTER
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Fig. 4. Nitrate anomaly sections of cold core ring " B o b " . Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 2 lamol kg t or as labeled. (a) April west-east section. (b) August west-east section. (c) August north-south section.
Chemical anomalies in a cold core Gulf Stream ring
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Fig. 5. Silicate anomaly sections of cold core ring " B o b " . Solid lines are positive anomalies and dashed lines are negative anomalies. The contour interval is 1 pmol k g ~ for the August sections, variable for the April section. (a) April west-east section. (b) August west-east section. (c) August north-south section.
1322
J . A . ELROD and D. R. KESTER
DISCUSSION
The three objectives of this analysis were to determine how well waters in the ring could be characterized using the guidelines and methods of ELRODand KESTER(1985) to examine the changes occurring in property anomalies between April and August, and to determine if each of the six properties provided a unique contribution to the total picture of ring structure and decay. Anomaly characterization In the previous section it was shown that the Slope Water salinity and oxygen anomalies of ELROD and KESTER (1985) are readily apparent in the core of the ring. Gulf Stream salinity and oxygen anomalies do appear outside the core region, but are not very strong. Although Elrod and Kester concluded that the available data did not adequately define nutrient anomalies in Slope and Gulf Stream Water, they did report generally positive nitrate and phosphate anomalies in Gulf Stream Water which is consistent with the positive nutrient anomalies found here in the high velocity region of the ring, Also, this study shows consistent negative nutrient anomalies in the core of the ring of Slope Water origin, which were not found systematically in Slope Water stations in the previous work. Oxygen and nutrient concentrations (and thus their anomalies) are more spatially and seasonally variable on a density surface than salinity and temperature. Ring core concentrations of these parameters would depend upon where and when the ring formed. Ring "Bob" split off from the Gulf Stream in February 1977, whereas the Slope Water and Gulf Stream stations used by Elrod and Kester were sampled in April, August and November. Thus, the seasonal sampling differences may account for the difference in anomalies observed in the ring core and in Slope Water. Changes over time The major change occurring in the ring between April and August was a decrease in the magnitude of the anomalies of all the properties. This is consistent with decay of the ring through mixing and exchange processes towards the state where its properties no longer differ from those of the Sargasso Sea. In the core region, for G0 less than about 26.9, the water is shallower than 100 m and is subject to atmospheric influences. One effect of the atmosphere between April and August is to heat the surface layer. The heat input will change not only the temperature of a parcel of water, but also the density, while having little effect on the salinity. A decrease in density for a constant salinity would result in an increase in negative salinity anomaly according to the reference curve of ELROD and KESTER (1985, Fig. 1). Since the magnitude of the negative core anomalies in salinity diminished slightly for Go < 26.6 over the 4-month period, we can conclude that this effect is more than compensated for by mixing processes. Salinity and potential temperature anomalies changed by about 20% (from -0.6 to -0.5%0 and -2 to -1.5°C) between April and August. The strongest oxygen anomaly in the core diminished by about 40% (from +2.4 ml 1-Z in April to +1.4 ml 1-1 in August) as did nitrate anomaly (from -7 to -4 p.mol kg-l). Phosphate and silicate anomalies changed by more than 60% over this period (from -0.4 to -0.15 l~mol kg-~ for phosphate and-3 to -1 ~tmol kg-1 for silicate). The decrease in density through heating near the surface would cause the nutrient anomalies to become more positive, according to the slopes of the reference curves (EL~oD and KESTER, 1985). This is the change that is observed in the
1323
Chemical anomalies in a cold core Gulf Stream ring
core region. The reference curve for oxygen is not monotonic above c~0 26.9, making it difficult to predict the effect a change in density would have on oxygen anomaly. At greater densities within the core (below approximately mt 26.9) and outside the core region, density should be conservative, and changes in anomaly would be due entirely to mixing and in situ chemical and biological processes. Outside the core region, between radii of 40 and 70 km, the oxygen anomaly changed from -1.2 ml 1-1 in April to -0.8 ml 1-~ in August within the ~0 range 26.3-26.5. The nitrate anomaly diminished from 10 to 6 lamol k g 1, phosphate anomaly from (I.5 to 0.3 ~tmol kg-1, and silicate anomaly from 4 to 2 ~tmol kg-~. These changes reflect the influence of mixing at the boundaries of the ring upon the dilution of the anomaly signature of the Gulf Stream remnant.
Uniqueness of properties Potential temperature and salinity are linked to density by the equation of state of seawater and therefore provide the same type of information when plotted on a ~0 section. Nitrate and phosphate are both nonconservative properties due to their biological utilization, and their anomaly distributions are similar. Oxygen is utilized in biological processes, but it is affected also by air-sea gas exchange. The degree to which oxygen anomalies resemble those of nitrate and phosphate should depend partly upon proximity to the sea surface and the extent of vertical mixing. Tables 1 and 2 list the slopes, intercepts, their standard deviations, and the correlation coefficients for the linear regressions of nitrate and oxygen anomalies against phosphate anomaly for all stations for cy0 ranging from 26.4 to 27.0. The regressions with nitrate anomaly show strong correlation coefficients of 0.9, while the oxygen anomaly regressions have weaker correlation, especially for the smallest cy0 value. Because G0 26.4 is located near the surface, oxygen concentrations and anomalies are affected by gas exchange, whereas phosphate is not. Therefore, the two anomalies would not be expected to correlate as closely as they would deeper in the water column. Although this study compares property anomalies rather than concentrations, the slopes of both types of regression are within one or two standard deviations of the Redfield AP:AN:AO2 ratio of 1:16:138 Table 1. Regression statistics for nitrate anomaly vs phosphate anomaly ¢~ 26.4 26.6 26.8 27.0
Table 2.
o0 26.4 26.6 26.8 27.0
Slope 17.4 18.7 16.5 14.3
_+ 1.6 + 1.3 _+ 1.7 + 1.4
Intercept -0. l -0.2 0.1 -0.2
_+ 0.4 + 0.2 _+_0.2 + 0.2
Correlation coefficient /I.90 11.93 0.88 0.88
Regression statistics for oxygen anomaly (converted to ~mol kg ~) vs phosphate anomaly Slope -101 -166 -163 -131
_+ 30 +_ 28 +_ 18 _+ 17
Intercept
Correlation coefficient
-1 _+ 8 16 + 4 14 +2 10 _+ 2
-0.56 -0.75 -0.87 0.82
1324
J . A . ELROD and D. R. KESTER
(REDFmLD et al., 1963). TAKAHASHIet al. (1985) revised this ratio to 1:16:172, based on analyses conducted in water at ~0 27.0 and 27.2. The slopes determined at or0 26.6 and 26.8 for oxygen in this study are in closer agreement with the revised ratio than the traditional one. Table 3 lists the correlation coefficients of linear regressions of silicate vs oxygen and phosphate anomalies. Silicate anomaly shows poor correlations with both anomalies, but with generally higher correlation with phosphate than oxygen. Regressions of oxygen, phosphate, nitrate, and silicate anomalies against salinity anomaly for G0 26.4, 26.6 and 26.8 have poor correlations, indicating that the anomalies of salinity and the nonconservative properties provide independent information. Salinity and potential temperature anomalies indicate a region of water colder and saltier than Sargasso Sea Water in the ring within about 40 km of ring center, down to about ~0 26.75. Oxygen, however, indicates that ring water is significantly different from Sargasso Sea Water out to a radius of at least 60 km, and down to a ~0 of about 27.25. Nitrate and phosphate anomaly plots indicate that the ring contains water anomalous to the Sargasso Sea out to 80 km and down to about % 27.25. Silicate provides a similar picture but shows some anomalous water below or0 27.25, especially in April. Silicate is the only property examined which has strong anomalies within the ring for or0 values >27.2. The negative anomaly appearing in potential temperature and salinity below ~0 26.75 on the August west section at Stas 9, 10 and 12 is reflected in the oxygen anomaly plot by two regions of positive anomaly near ~0 27.7 at Stas 9 and 12. However, the silicate section displays anomalies of opposite sign at Stas 9 and 12 for or0 > 27.25, indicating waters of different histories at these two stations. The extension of-0.4°C potential temperature and-0.1%o salinity anomaly contours to 60 km on the April west radial section might be interpreted as an extension of the Slope Water beyond the core region. This area does not have the same appearance for the other property anomalies. The silicate anomaly resembles potential temperature and salinity most in that the positive silicate anomaly water near the surface of the ring core continues beyond 40 km radius, to about 80 km. Nitrate and phosphate, however, change sign around 35 km radius from negative anomalies in the core to positive anomalies outside, indicating the presence of two different waters. Oxygen anomaly also changes sign around 35 km. Although the signs of the anomalies usually are the same for the three nutrients in a particular region of the ring, there are several exceptions. The August south section has positive phosphate anomalies over most of the % range at Stas 32-34. Nitrate and silicate, however, both have negative anomalies at Sta. 34. The negative silicate anomalies occurring in April for % > 27.25 over the whole ring section are not echoed in the nitrate and phosphate anomaly sections. Table 3.
Correlation coefficients (r) of silicate anomaly vs oxygen and phosphate anomalies
%
r, oxygen
r, phosphate
26.4 26.6 26.8 27.0
0.25 0.44 0.65 0.64
0.81 0.64 0.74 0.66
Chemical anomalies in a cold core Gulf Stream ring
1325
It is concluded that of the six properties examined, a subset of four, including oxygen, silicate, either salinity or potential temperature, and either phosphate or nitrate, will provide usefully distinctive information on ¢Y0sections. Acknowledgement-Funding for this research was provided to the University of Rhode Island by the Office of Naval Research, grant no. N00014-81-C-0062. REFERENCES BACKUSR. H., G, R. FLIERL, D. R. KESTER, D. B. OLSON, P. L. RICHARDSON,A. C. VASTANO,P. H. W1EBE and J. H. WORMUTH(1981) Gulf Stream cold-core rings: Their physics, chemistry, and biology. Science, 212, 1091-1100. CHENEY R. E. and P. L. RICHARDSON(19776) Observed decay of a cyclonic Gulf Stream ring. Deep-Sea Research, 23, 143-155. DOBLAR R. A. and R. E. CHENEY (1977) Observed formation of a Gulf Stream cold core ring. Journal of Physical Oceanography, 7,944-946. ELROD J. A. and D. R. "KESTER (1985) Sargasso Sea reference curves for salinity, potential temperature, oxygen, nitrate, phosphate, and silicate as functions of sigma-theta. Deep-SeaResearch, 32,391-405. LAID. Y. and P. L. RICHARDSON(1977) Distribution and movement of Gulf Stream rings. Journal of Physical Oceanography, 7,670-683. McDOWELL S. E. (1982) Analyses of North Atlantic Intermediate Waters upon isopycnal surfaces and within mesoscale eddies. Ph.D. Dissertation, University of Rhode Island, 227 pp. MOLINARI R. L. (1970) Cyclonic ring spin-down in the North Atlantic. Ph.D. Dissertation, Texas A&M University, 105 pp. OLSON D. B. (1980) The physical oceanography of two rings observed by the cyclonic ring experiment. Part 11. Dynamics. Journal of Physical Oceanography, 10, 514-528. PARKER C. E. (1971) Gulf Stream rings in the Sargasso Sea. Deep-SeaResearch, 18, 981-993. REDFIELD A. C., 1~. H. KETCHUMand F. A. RICHARDS(1963) The influence of organisms on the composition of seawater. In: The sea: ideas and observations on progress in the study of the seas, Vol. 2, M. N. HILL, editor, Interscience, New York, pp. 26-77. RICHARDSONP. L, (1980) Gulf Stream ring trajectories. Journal of Physical Oceanography, 10, 90--104. RICHARDSON P. L., A. E. STRONG and J. A. KNAUSS(1973) Gulf Stream eddies: recent observations in the western Sargasso Sea. Journal of Physical Oceanography, 3, 297-301. RICHARDSONP. L., R. E. CHENEYand L, A. MANTINI(1977) Tracking a Gulf Stream ring with a free drifting surface buoy. Journal of Physical Oceanography, 7,580-590. RICHARDSON P. L., C. MAILLARD and T. B. STANFORD(1979) The physical structure and life history of cyclonic Gulf Stream ring Alien. Journal of Geophysical Research, 84, 7727-7741. SCHMITZ.1. E. and A. C. VASTANO(1977) Decay of a shoaling Gulf Stream cyclonic ring. Journal of Physical Oceanography, 7, 479-481. SCHMITZJ. E., O. E. HAGAN and A. C. VASTANO(1978a) Physical Oceanography Data Report I. RV Knorr cruise 65, April 1977, Texas A&M University, 145 pp. SCHMITZ J. E., J. R. HAUSTEIN, D. E. HAGAN and A. C. VASTANO(1978b) Physical Oceanography Data Report II. RV Endeavor 011, July-August 1977, Texas A&M University, 198 pp, TAKAHASHIT., W. S. BROECKERand S. LANGER(1985) Redfield ratio based on chemical data from isopycnal surfaces. Journal of Geophysical Research, 90, 6907-6924. VASTANOA. C., J. E. SCHMITZand D. E. HAGAN (1980) The physical oceanography of two rings observed by the cyclonic ring experiment. Part I: Physical structures. Journal of Physical Oceanography, 10, 493-513. WATTS D. R. and D. B. OLSON (1978) Gulf Stream ring coalescence with the Gulf Stream off Cape Hatteras. Science, 202, 971-972.