Westerly wind biases over the equatorial Atlantic and their link to South American convection

Tropical Atlantic climate remains poorly captured in CMIP5 coupled ocean-atmosphere general circulation models (GCMs). One of the most conspicuous model deficiencies is the reversal of the equatorial zonal sea-surface temperature (SST) gradient relative to observations, with cold biases in the west and warm biases in the east. Previous studies have shown that weaker than observed equatorial easterlies in boreal spring are a major contributor to this reversed SST gradient, and that such wind biases exist even in uncoupled atmospheric GCMs with SSTs prescribed from observations. The wind biases in turn are closely linked to an erroneous southward shift of the intertropical convergence zone (ITCZ) in the models.

In the present study we use daily satellite-derived precipitation data as well as CMIP5 model output to investigate the reasons for the erroneous south-equatorial excursions of the simulated Atlantic ITCZ. Observational precipitation data for boreal spring show eastward-traveling disturbances on the equator, whose propagation speed is consistent with that of convectively coupled Kelvin waves. These disturbances originate from the western and central Amazon basin and typically reach the central equatorial Atlantic basin before dissipating. The disturbances help to anchor the observed Atlantic ITCZ over the equator in boreal spring, when the climatological SST are warmest south of the equator.

Atmospheric GCMs typically produce too little precipitation over the Amazon basin. Furthermore they fail to reproduce the eastward propagating features seen in observations. As a result the boreal spring ITCZ tends to move south of the equator toward the warmest SST. This erroneous southward shift is associated with a weakening of the equatorial trades and constitutes one of the main reasons for the westerly wind bias on the equator. When coupled to an oceanic GCM, the surface wind biases cool SST on and north of the equator in the western tropical Atlantic. This leads to a further southward shift of the ITCZ and amplifies the initial biases.