Capturing the Atlantic cold tongue and coastal upwelling in a coupled atmosphere/ocean regional model

Coupled atmosphere-ocean global circulation models are known to have difficulties simulating the cold tongue in the equatorial Atlantic Ocean. Here a coupled regional climate model/mixed layer ocean model with Ekman dynamics is developed and used to better understand the seasonal evolution of the equatorial Atlantic cold tongue and upwelling off northwestern Africa. Parameterization improvements are made to an earlier version of the ocean model to account for the variations in temperature and shearing stress at the base of the mixed layer. 90-km resolution sensitivity tests demonstrate that the development of the equatorial Atlantic cold tongue in the boreal spring/summer is captured only if seasonal variations in the temperature at the base of the ocean mixed layer are included. The development of cold temperatures off the northwestern African coast in the late boreal winter/spring is found to be primarily associated with the net radiation balance as shortwave warming of the mixed layer is relatively low while latent cooling is relatively high yielding a net cooling of mixed layer temperatures, consistent with other studies. The development of the Atlantic cold tongue is found to be associated with primarily horizontal temperature advection of cold water in the ocean mixed layer from along the South Atlantic African coast, with cooling associated with vertical entrainment and vertical diffusion having secondary, more localized roles over the equatorial Atlantic.