Mesoscale pigment fields in the Gulf Stream: Observations in a meander crest and trough

Abstract

In September‐October 1988 and April 1989 a series of hydrographic transects were completed across the Gulf Stream front in a meander crest and meander trough, respectively. One of the main experimental objectives was to relate the spatial distribution of chlorophyll to the physical fields of Gulf Stream meanders. Chlorophyll distributions are derived from conductivity‐temperature‐depth (CTD)/fluorescence profiles calibrated with discrete pigment samples collected from bottles at several depths at various stations. Objective analysis (OA) maps and vertical sections, in stream coordinates, of chlorophyll on density surfaces show the chlorophyll distribution was strongly related to the structure 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 determined from the horizontal and temporal correlation function are the same order (50–100 km) as length scales of the physical variables. The maximum chlorophyll concentrations in the vertical were from the surface to 50‐m depth on the western flank of the meander crest and deepened to 75‐ to 100‐m depth on the eastern flank. This coincides with the deepening of the 24.4–25.7 σθ 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 the surface and 100 m, there were no clear distinctions in pigment distributions between the western and eastern transects of a relatively weak trough. Maximum chlorophyll concentrations, 1.0–1.5 mg m−3, from the 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 this 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.