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

Abstract

GEOSAT sea level anomalies, XBT temperature profiles and Levitus climatologies were assimilated monthly during 1987 into a non-linear primitive equation model of the tropical Atlantic ocean. Assimilating GEOSAT sea level anomalies increased the variability of the north equatorial currents when it was too weak without assimilation. The assimilated run simulates both the 1987 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 warming of early 1988 is little changed by the assimilation. It confirms that 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 gyre with the North Equatorial Current (NEC). Consistently with previous analysis, this strengthening of the gyre was a result of the anomaly in ITCZ position and intensity. The heat gain by the Equatorial Under Current (EUC) was reduced because of this northward drift of the NECC. The South equatorial 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. Comparisons of the assimilated northward heat transport (NHT) with climatological data estimates principally suggests that there was a heat gain of up to 1 petaWatt in the northern tropics (3°N–18°N) in 1987. The assimilated NHT was nil at 25°N and negative within 5°S–15°S. The 1987–88 warm events in the tropical Atlantic ocean presents a temporal progression from the north to the south. The assimilation presents an increased transport of warm water by the EUC after September 1987. It confirms the positive impact of the mass redistribution on the subsurface warming of the Gulf of Guinea. The other significant impact in 1988 is an increase of the gyre formed by the SEC and the South Equatorial Counter Current (SECC) and an inhibition of the coastal upwelling off Namibia.