Upper ocean circulation in the western tropical Atlantic in boreal fall 2000

Abstract

The upper ocean large-scale circulation of the western tropical Atlantic from 11.5°S to the Caribbean in November and December 2000 is investigated from a new type of shipboard ADCP able to measure accurate velocities to 600 m depth, combined with lowered ADCP measurements. Satellite data and numerical model output complement the shipboard measurements to better describe the large-scale circulation. In November 2000 the North Brazil Undercurrent (NBUC) was strongly intensified between 11 and 5°S by inflow from the east, hence the NBUC was formed further to the north than in the mean. The NBUC was transporting 23.1 Sv northward at 5°S, slightly less than the mean of six cruises (Geophysical Research Letters (2002) 29 (7) 1840). At 35°W the North Brazil Current (NBC) transported 29.4 Sv westward, less than the mean of 13 cruises (Geophysical Research Letters (2003) 30 (7) 1349). A strong retroflection ring had just pinched off the NBC retroflection according to the satellite information. The inflow into the Caribbean south of 16.5°N originated in part of a leakage from the NBC retroflection zone and in part from the North Equatorial Current. A thermocline intensified ring with a transport of about 30 Sv was located off Guadeloupe carrying South Atlantic Central Water towards the north. Observed deviations of the November/December 2000 flow field from the November long-term mean flow field were related to an enhanced Intertropical Convergence Zone (ITCZ) associated with an increased North Equatorial Countercurrent (NECC), as well as to boundary current rings and Rossby waves with zonal wavelength of the order of 1000 km. At 44°W the presence of a Rossby wave associated with an anticyclonic circulation led to a strongly enhanced NBC of 65.0 Sv as well as to a combined NECC and Equatorial Undercurrent transport of 52.4 Sv, much stronger than during earlier cruises. While the 1/3°-FLAME model is unable to reproduce details of the vertical distribution of the observed horizontal flow at 44 °W for November 2000 as well as the horizontal distribution of some of the observed permanent current bands, a climatological simulation with the 1/12°-FLAME agrees much better with the observations and provides information on the spreading path between the sections. E.g., the interpretation that the widening in the Antarctic Intermediate Water layer of the westward flowing NBC at 44°W in November was caused by water from the Equatorial Intermediate Current was further supported by the model results.