The low-frequency variability of the tropical atlantic ocean

Upper-ocean temperature variability in the tropical Atlantic is examined from the Comprehensive Ocean-Atmosphere Data Set (COADS) as well as from an ocean model simulation forced by COADS anomalies appended to a monthly climatology. The findings are as follows: Only the sea surface temperatures (SST) in the northern Tropics are driven by heat fluxes, while the southern tropical variability arises from wind-driven ocean circulation changes. The subsurface temperatures in the northern and southern Tropics are found to have a strong linkage to buoyancy forcing changes in the northern North Atlantic. Evidence for Kelvin-like boundary wave propagation from the high latitudes is presented from the model simulation. This extratropical influence is associated with wintertime North Atlantic oscillation (NAO) forcing and manifests itself in the northern and southern tropical temperature anomalies of the same sign at depths of 100-200 m as result of a Rossby wave propagation away from the eastern boundary in the wake of the boundary wave passage. The most apparent association of the southern tropical sea surface temperature anomalies (SSTA) arises with the anomalous cross-equatorial winds that can be related to both NAO and the remote influence from the Pacific equatorial region. These teleconnections are seasonal, so that the NAO impact on the tropical SST is the largest at midwinter but in spring and early summer the Pacific remote influence competes with NAO. However, NAO appears to have a more substantial role than the Pacific influence at low frequencies during the last 50 years. The dynamic origin of SSTA is indirectly confirmed from the SST-heat flux relationship using ocean model experiments that remove either anomalous wind stress forcing or atmospheric forcing anomalies contributing to heat exchange.