Structure of the equatorial Atlantic cold tongue and meridional flow

A generalized method is developed to determine the position of the northern cold tongue front in the equatorial Atlantic from satellite sea surface temperature (SST) data. The methodology is based on a median frontal SST and frontal characteristics are generally consistent with tropical instability waves (TIWs). Application to drifter observations shows how the new methodology can be used to better understand circulation features near the northern cold tongue front: A drifter pair deployed on the eastern side of a passing TIW crest north of the front revealed that the trajectories of the drifters were clearly influenced by the shape of the front and - in agreement with model studies in both the Atlantic and Pacific - they did not cross the front, but rather stayed close together approximately 2° north of the front. At the western edge of the cold tongue, the longer-transmitting drifter entered the northern branch of the South Equatorial Current (nSEC) and rapidly moved westward. Analyses in an along- and cross-frontal frame of reference complement isopycnal coordinate mapping; for example, tropical Atlantic drifter velocities averaged in frontal coordinates indicate a broadened shear zone between the nSEC and North Equatorial Countercurrent as well as meridional convergence near the front. This new methodology will be used to examine aspects of the near-frontal circulation such as the flow associated with the shallow overturning cells known as tropical cells (TCs) from two-dimensional observations. Shipboard and lowered acoustic Doppler current profiler (ADCP) measurements collected in the central and eastern equatorial Atlantic during the past sixteen years are for the first time utilized to describe the mean cross-equatorial structure of the meridional currents associated with the TCs. These meridional currents are an order of magnitude smaller than velocity fluctuations associated with transient phenomena in the region like TIWs, and to date individual in situ current measurements have been too sparse in their temporal and spatial coverage to estimate their mean without large uncertainties. Due to surface reflections, the currents between 30 m and the ocean surface are typically not resolved by the ADCP measurements and 15-m currents from the drifter annual climatology are used to derive those currents.