Large Scale Circulation

More On The Loop Curent

The bathymetry of the Gulf of Mexico includes major oceanic inflow and outflow ports in the south and the southeast, respectively, with most river inflow occurring along the northern boundary. Wide shelves are found along the Florida and Campeche coasts; narrower ones around the western basin. A very narrow shelf is found between Pensacola and Apalachicola, Florida, where the De Soto Canyon penetrates almost to the shoreline. Monthly composites of inferred chlorophyll concentration show the generally hot, clear waters from the Caribbean Sea flowing into and out of the Gulf of Mexico in a clockwise looping behavior. Chlorophyll contrast for Gulf of Mexico waters is greatest in the summer (August 1979) and least in the winter (December 1980).

Sea-surface temperature behaves in the opposite way. During the summer ( Summer 1993Summer 1994Summer 1995), the surface waters of the Gulf of Mexico are quite homogeneous. The winter ( Winter 1993/1994Winter 1994/1995Winter 1995/1996) months show the hot Caribbean water intruding into the cold Gulf water, creating sharp temperature gradients. Thus, a combination of chlorophyll pigment and sea-surface temperature provides a year-round look at the large-scale, surface processes occurring in the Gulf of Mexico.

Eddy Separation and Shedding from the Loop Current:

A comparison of satellite altimetry data from TOPEX and ERS-1 (provided by Dr. Robert Leben of the University of Colorado, corrected for orbit, geopotential, and tidal effects) of the Gulf of Mexico with ocean thermal imagery (AVHRR) shows that the SST signature of the Loop Current matches the dynamic height high shown in the altimetry imagery. Data obtained through altimetry sensors can be used to calculate the first order or geostrophic currents in the ocean. Looking at the altimetry image referenced above, we can see that the actual Loop Current is positioned around the largest dynamic height signature, where the lines of constant sea-surface height are closest together. A time-series image of scenes from January to March 1995 shows the separation of a warm core eddy from the main body of the Loop Current. The altimetric highs in the Gulf have a Caribbean source, so they are warm, clock-wise circulation features in the eastern part of the basin, and the lows are cool, counter-clockwise features consisting of older Gulf water. Farther to the west, away from the Caribbean source of warm water, the thermal distinction disappears as thermal exchange with the atmosphere dominates the thermal inertia of the warm rings.

Coastal Zone Color Scanner data has depicted some rings of blue water that were transported almost to the western shelf of the basin before their low-chlorophyll signatures were lost ( July 1982July 1980). This suggests that altimetry data combined with both ocean color and thermal imagery will better depict the circulation features than any of them separately. Effects of heat transport and exchange with the atmosphere, as well as diffusion with the adjacent waters, will be apparent in time sequences of the imagery. When appropriate models of the circulation, thermodynamics, and primary production of the Gulf need testing, initialization or updating, a combination of altimetric, thermal, and color imagery will be much more useful than just one of these. A composite of the sea-surface temperature and altimetry images shown in the previous paragraph clearly indicates the correlation of sea-surface temperature to sea-surface height in the Loop Current. The resulting observation of the warm-core eddy that separated from the Loop Current might lead to new insights on the frequency and conditions of such separation events, previously overlooked by infrequent ship data. Time series of such overlay images will provide significant understanding on the dynamics of and various forcing functions on the surface flow.

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