Twelve monthly experiments of 4D-variational assimilation in the tropical Atlantic during 1987: Part 2: Oceanographic interpretation

GEOSAT sea level anomalies, XBT temperature profiles and Levitus climatolog ies were assimilated monthly during 1987 into a non-linear primitive equati on model of the tropical Atlantic ocean. Assimilating GEOSAT sea level anom alies increased the variability of the north equatorial currents when it wa s too weak without assimilation. The assimilated run simulates both the 198 7 to 1988 sea level rise in the equatorial band and lowering in the tropics . The comparison with Reynolds satellite SST data is useful to identify the upwelling areas as the zones of highest error for the model SST. The SST w arming of early 1988 is little changed by the assimilation. It confirms tha t this warming was determined more by the wind forcing than by the internal dynamics. As previously deduced from inverted echo sounders, we find that the North Equatorial Counter Current (NECC) increased its transport during 1987. The assimilation reveals in addition that the NECC formed a strong gy re with the North Equatorial Current (NEC), Consistently with previous anal ysis, this strengthening of the gyre was a result of the anomaly in ITCZ po sition and intensity. The heat gain by the Equatorial Under Current (EUC) w as reduced because of this northward drift of the NECC. The South equatoria l Current (SEC) was strong in spring and summer, moving warm surface water to the west. The eastward transport of the underlying EUC was thus reduced, leading to a deficit of warm water in the Gulf of Guinea in 1987, Comparis ons of the assimilated northward heat transport (NHT) with climatological d ata estimates principally suggests that there was a heat gain of up to 1 pe taWatt in the northern tropics (3 degrees N-18 degrees N) in 1987, The assi milated NHT was nil at 25 degrees N and negative within 5 degrees S-15 degr ees S. The 1987-88 warm events in the tropical Atlantic ocean presents a te mporal progression from the north to the south. The assimilation presents a n increased transport of warm water by the EUC after September 1987. It con firms the positive impact of the mass redistribution on the subsurface warm ing of the Gulf of Guinea. The other significant impact in 1988 is an incre ase of the gyre formed by the SEC and the South Equatorial Counter Current (SECC) and an inhibition of the coastal upwelling off Namibia. (C) 1998 Els evier Science Ltd. All rights reserved.