WESTERN BOUNDARY CURRENTS AND TRANSPORTS OFF FRENCH-GUIANA AS INFERRED FROM PEGASUS OBSERVATIONS

Pegasus current measurements carried out along a section located off F rench Guiana are presented; the section was repeated five times from S eptember 1989 to June 1991 during the NOE (region Nord-Ouest equatoria le)/STACS (Sub Tropical Atlantic Climate Studies) cruises on board the NOAA (National Oceanographic and Atmospheric Administration) research vessels. The data (vertical and horizontal distributions) show, as su ggested by earliest observations, the presence of different currents. At the surface, and in winter, the North Brazil Current (NBC) flows no rthwestward (positive), is strong (120 cm/s), confined in the first 25 0 m, coastally trapped (within 250 km) and fed by offshore waters; in summer the NBC strengthens (the velocity increases to 150 cm/s), verti cally extends (down to 800 m) and veers offshore; the mean NBC mass fl ux computed from all the sections available is 34 +/- 9 Sv with absolu te maximum and minimum respectively in September 1989 (81 +/- 4 Sv) an d February 1990 (13 +/- 1 Sv). Subsurface, a southeastward (negative) undercurrent (herein after named Western Boundary UnderCurrent) is pre sent in winter-spring and located in the layer 250-800 m with similar velocities in February 1990 (- 33 cm/s) and June 1991 (- 29 cm/s) but with largest vertical and horizontal extensions in the latter case; in summer (September 1989 and September 1990) this undercurrent is absen t; the mean mTBUC mass flux is - 9 +/- 3 Sv; the maximum is observed i n June 1991 (- 19 +/- 2 Sv) and the minimum in February 1990 (- 3 +/- 0.4 Sv) and January 1991 (- 5 +/- I Sv). Deeper, the equatorward Deep Western Boundary Current is trapped against the continental shelf (wit hin 100 km of the shelf break), extends downward from 1 200 to 3 000 m depth with the velocity core centred in the 1 700-2 000 m layer, is m aximum (- 50 cm/s) in spring-summer and minimum (- 23 cm/s) in winter; the absolute velocity (- 92 cm/s) has been recorded at 2 000 m depth in September 1989 suggesting a strong variability at this level; the m ean equatorward DWBC mass flux is - 30 +/- 14 Sv with absolute maximum and minimum respectively in September 1989 (- 59 +/- 6 Sv) and Septem ber 1990 (- 7 +/- 1 Sv). The Integrated Mass, Temperature and Salt Flu xes (IMF, ITF and ISF) with the cumulated errors, computed across the whole section and down to 3 000 m depth for the September 1990, Januar y 1991 and June 1991 cruises, are all positive in September 1990 (resp ectively 1.9 +/- 19 Sv, 3.1 +/- 30.1 PW and 74 +/- 1 845 Tt/s) but all negative both in January 1991 (- 13.3 +/- 17.0 Sv, - 15.6 +/- 26.5 PW and - 478 +/- 1594 Tt/s) and June 1991 (- 3.8 +/- 10.1 Sv, - 4.0 +/- 16.3 PW, - 130 +/- 989 Tt/s) showing a strong variability between the summer and winter periods. The mean IMF, ITF and ISF values (respectiv ely - 5.1 +/- 46.4 Sv, - 5.2 +/- 73 PW and - 178 +/- 4428 Tt/s) are hi gh and negative, indicating the large influence of the DWBC and the as sociated North Atlantic Deep Water (upper part) off French Guiana at 5 degrees N.