Tropical Atlantic sea surface temperature variability and its relation to El Niño‐Southern Oscillation

Abstract

Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43‐year (1950–1992) record of SST [Smith et al, 1996] and other data derived from the Comprehensive Ocean‐Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions of Houghton and Tourre [1992]. Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is correlated with Pacific El Niño‐Southern Oscillation (ENSO) variability in several regions. The major region affected is the North Atlantic area of NE trades west of 40°W along 10°N–20°N and extending into the Caribbean. There, about 50–80% of the anomalous SST variability is associated with the Pacific ENSO, with Atlantic warmings occurring 4–5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO‐related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in question, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North Atlantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the development of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Brazil along 15°–30°S.