Leading Modes of the Upper-Ocean Temperature Interannual Variability along the Equatorial Atlantic Ocean in NCEP GODAS

In this work, the authors analyze the physical mechanisms of interannual variability of the upper-ocean temperature anomaly (OTA) in the equatorial Atlantic Ocean, using ocean reanalysis from the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System. The variability of equatorial Atlantic OTA is dominated by two leading modes. The first mode is characterized by same-sign variation along the thermocline with pronounced amplitude in the central and eastern equatorial Atlantic. This mode represents the modulation of the overall thermocline depth at the equator generated by net heat convergence in the equatorial ocean, with heat content first accumulated mainly in the off-equatorial northwestern Atlantic in response to anomalous wind curl associated with Atlantic meridional mode.The second leading mode shows an opposite variation between the western and eastern Atlantic. This mode is mainly driven by the zonal wind stress fluctuation confined in the southwestern tropical and equatorial Atlantic and reflects the equatorial balanced response between the zonal slope of the equatorial thermocline depth and the atmospheric zonal wind variations with pronounced surface wind and ocean anomalies in the southwestern and equatorial ocean. The different characteristics of these two modes suggest that they may occur independently. In fact, evolution of the two leading modes is approximately in quadrature, and they may also occur in sequence on interannual time scales. The two leading mode-associated air-sea interaction processes suggest that the Atlantic meridional mode and zonal mode are statistically and physically connected in their evolution.