369 
Note on plankton and 
cold-core rings in the Gulf of Mexico 
Douglas C. Biggs* 
Robert A. Zimmerman 
Department of Oceanography 
Texas A&M University, College Station, Texas 77843-3146 
*E-mail address: dbiggs@ocean.tamu.edu 
Rebeca Gasca 
Eduardo Suarez-Morales 
Ivan Castellanos 
ECOSUR-Unidad Chetumal 
A.P 424, Chetumal, QR 77000, Mexico 
Robert R. Leben 
Colorado Center for Astrodynamics Research 
University of Colorado, Boulder, Colorado 80309 
Data from ship and aircraft hydro- 
graphic surveys, supplemented 
with data from current meter moor- 
ings and drifters, have demon- 
strated that one or more cyclonic 
circulation features, 100-200 km in 
diameter, are often present in the 
Gulf of Mexico. In the eastern Gulf, 
these cold-core rings (CCR’s) occur 
in close association with the Loop 
Current (LC) (Lee et ah, 1994), and 
in the central and western Gulf, 
they are companions of the anticy- 
clonic eddies that are shed during 
northern excursions of the LC 
(Hamilton, 1992). Cyclone-anticy- 
clone dipoles and cyclone-anticy- 
clone-cyclone triads have been de- 
scribed (Lewis and Kirwan, 1985; 
Rouse et al., 1994; Vidal et aL, 1994). 
Temperature-salinity relationships 
document that cyclones and anti- 
cyclones in the Gulf of Mexico form 
from the same water types, but con- 
vergence flow within the anticy- 
clones causes the surface waters of 
these gyres to be regions of low pro- 
duction. The upper 100 m are de- 
pleted in nitrate and chlorophyll 
concentrations, primary productiv- 
ity, and zooplankton biomass are 
generally extremely low (Biggs, 
1992). In contrast, the companion 
cyclones are mesoscale regions of 
divergence flow. From nutrient- 
chlorophyll data collected during 
several cruises when Gulf of Mexico 
CCR’s were tracked, Biggs et al. 
(1988) hypothesized that cyclones 
were regions of locally high primary 
productivity which could support 
elevated stocks of zooplankton. 
In March 1993, a CCR was de- 
tected by remote sensing of the 
western central Gulf of Mexico, and 
the opportunity arose to study its 
hydrographic and biological signa- 
ture as RV Gyre transited this fea- 
ture while proceeding along a 
TOPEX ground track. This meso- 
scale cyclonic circulation was vis- 
ible in remote sensing data as a 
region of surface temperatures 1- 
2°C cooler than the adjacent oce- 
anic surface waters (Fig. 1; Table 
1) and as an elliptical local depres- 
sion in sea surface height (SSH) 
(-15 to -20 dyn cm of SSH anomaly; 
see Table 1). Expendable bathy- 
thermographs (XBT’s), dropped to 
profile isotherm depths in the up- 
per 760 m, resolved strong doming 
of subsurface isotherms within this 
CCR (Fig. 2), and Gyre’s hull- 
mounted 153 kHz acoustic Doppler 
current profiler (ADCP) confirmed 
that cyclonic near-surface currents 
were associated with this feature. 
Both the amplitude and direction 
of these ADCP-measured currents 
were found to be in close agreement 
with those computed from the 
along-track horizontal geopotential 
gradient in relation to a reference 
level of 800 db: a low of 88 dyn cm 
in the interior of the CCR, versus 
>102 dyn cm to the north and south 
(Table 1). This -14 cm SSH gradi- 
ent between interior and periphery 
of the CCR over a distance of 100 
km should have driven a cyclonic 
transport (relative to 800 db) of 6- 
7 Sverdrup, with mixed-layer ve- 
locities of 40-60 cm/s (see p. 165- 
166 in Texas A&M 1 ). Altimeter-de- 
rived dynamic height anomalies 
from TOPEX Cycle 18, which flew 
over the ship’s track just as Gyre 
completed the XBT survey, showed 
agreement with the hydrographic 
estimates to better than 2 cm re- 
sidual mean squares difference 
with respect to a corrected along- 
track mean surface, which is within 
the generally accepted error range 
for altimeter measurements (Leben 
et al, 1993). A comparison with 
TOPEX Cycle 17 data from 10 days 
earlier also demonstrates the pres- 
ence of this cylone (see Fig. 2 in 
Leben et al, 1993). 
Time constraints did not allow us 
to divert the ship to make a more 
detailed hydrographic survey of the 
CCR or to stop to make time-series 
measurements. However, we were 
1 Texas A&M University. 1993. Ship-of- 
opportunity hydrographic data from PUV 
Gyre cruise 93G-03. Tech. Rep. 93-04-T, 
Dep. Oceanography, TAMU, College Sta- 
tion, TX, 216 p. Available from NTIS, 
Springfield, VA: PB94-123957. 
Manuscript accepted 26 November 1996. 
Fishery Bulletin 95:369-375 (1997). 
