MESOSCALE PIGMENT FIELDS IN THE GULF-STREAM - OBSERVATIONS IN A MEANDER CREST AND TROUGH

In September-October 1988 and April 1989 a series of hydrographic tran sects were completed across the Gulf Stream front in a meander crest a nd meander trough, respectively. One of the main experimental objectiv es was to relate the spatial distribution of chlorophyll to the physic al fields of Gulf Stream meanders. Chlorophyll distributions are deriv ed from conductivity-temperature-depth (CTD)/fluorescence profiles cal ibrated with discrete pigment samples collected from bottles at severa l depths at various stations. Objective analysis (OA) maps and vertica l sections, in stream coordinates, of chlorophyll on density surfaces show the chlorophyll distribution was strongly related to the structur e of the Gulf Stream front. In particular, chlorophyll concentrations greater than 0.4 mg m-3 were at, or inshore of, the Gulf Stream north wall. Characteristic length scales of chlorophyll distribution determi ned from the horizontal and temporal correlation function are the same order (50-100 km) as length scales of the physical variables. The max imum chlorophyll concentrations in the vertical were from the surface to 50-m depth on the western flank of the meander crest and deepened t o 75- to 100-m depth on the eastern flank. This coincides with the dee pening of the 24.4-25.7 sigma(theta) surfaces from the western to the eastern flank of the meander crest. Although in the spring cruise the maximum chlorophyll concentrations were also found at depths between t he surface and 100 in, there were no clear distinctions in pigment dis tributions between the western and eastern transects of a relatively w eak trough. Maximum chlorophyll concentrations, 1.0-1.5 mg m-3, from t he 1989 spring bloom in slope waters were about double those observed in the 1988 fall data. The primary physical mechanisms influencing the mesoscale pigment distribution in Gulf Stream meanders observed in th is study are (1) outcropping of nutrient-bearing strata in the spring, (2) meander-induced upwelling of nutrients along sloping isopycnals, and (3) Gulf Stream-ring interactions.