Large and mesoscale variability of the water masses and the deep chlorophyll maximum in the Azores Front

Coupling between the physical and biological fields in the upper ocean around the Azores Front (AF) was studied in summer 1998 during an oceanographic cruise to the southern Azores Islands (30°N–38°N and 20°W–33°W). On the basis of conductivity-temperature-depth (CTD), acoustic Doppler current profiler (ADCP), and fluorescence data, several large-scale, mesoscale, and submesoscale physical features are described. The ADCP data and geostrophic field revealed an eastward transport associated with the Azores Current (AC) and weak countercurrents to the south and north of the AC. The water mass field shows a strong latitudinal variation in the AF with an eastward modification. From the surface to 300 dbar, the saline distribution strongly defined the AF as a very sharp vertical front. The horizontal thermohaline gradient increased to the east and reached a maximum at approximately 150 dbar. The 16.2°C isotherm and 36.2 isohaline at 150 dbar precisely located the AF along the whole area in agreement with the dynamic field. To the west of 26°W, the Azores Current was 140 km wide while the AF was only 15–25 km wide. To the east of 26°W the AC underwent a strong meandering. The water mass characteristics in the southern part of the AF show an eastward evolution between 18°C seawater and Madeira Mode Water. Data from a CTD-fluorometer mounted on an Undulator system allowed the small-scale structure of AF to be mapped, describing several eddies and meanders. The deep chlorophyll maximum (DCM) showed highest values (0.9 to 1.2 mg Chl m−3) toward the west (32°W) on the southern side of the AF. Chlorophyll concentrations at the DCM (0.4–0.6 mg Chl m−3) on the northern side of the study area were moderate and fairly constant whereas low values (<0.3 mg Chl m−3) were measured at the very deep DCM (120–140 m) in the southeast, associated with high saline waters. Examination of a cold eddy showed similar chlorophyll concentrations to those found on the northern side of the AF. The thermohaline structure of the eddy was similar to the AF, with northern water enclosed within saline and warm southern water.