Sensitivity of tropical Atlantic climate to mixing in a coupled ocean-atmosphere model

The sensitivity of tropical Atlantic climate to upper ocean mixing is investigated using an ocean-only model and a coupled ocean-atmosphere model. The upper ocean thermal structure and associated atmospheric circulation prove to be strongly related to the strength of upper ocean mixing. Using the heat balance in the mixed layer it is shown that an excessively cold equatorial cold tongue can be attributed to entrainment flux at the base of the oceanic mixed layer, that is too large. Enhanced entrainment efficiency acts to deepen the mixed layer and causes strong reduction in the upper ocean divergence in the central equatorial Atlantic. As a result, the simulated sea surface temperature, thermocline structure, and upwelling velocities are close to the observed estimates. In the coupled model, the seasonal migration of the Intertropical Convergence Zone (ITCZ) reduces when the entrainment efficiency in the oceanic mixed layer is enhanced. The precipitation rates decrease in the equatorial region and increase along 10 degrees N, resulting in a more realistic Atlantic Marine ITCZ. The reduced meridional surface temperature gradient in the eastern tropical Atlantic prohibits the development of convective precipitation in the southeastern part of the tropical Atlantic. Also, the simulation of tropical Atlantic variability as expressed in the meridional gradient mode and the eastern cold tongue mode improves when the entrainment efficiency is enhanced.