Air-sea coupling in the eastern Pacific: a regional modeling study

In the eastern tropical Pacific, coastal and equatorial upwelling reduce the sea surface temperature (SST), enhancing southern-hemisphere stratus clouds, and displacing the mean position of the intertopical convergence zone (ITCZ) into the northern hemisphere. We use the International Pacific Research Center (IPRC) coupled Regional Atmosphere-Ocean Model (IROAM) to diagnose the processes responsible for this asymmetry, and to identify sensitivities of the coupled climate system. Regional modeling allows investigation of the role of coupling on daily to seasonal timescales, and resolution of mesoscale features in the SST and the atmosphere in a coupled framework.

On seasonal time scales and regional scales, stratus clouds are found over cool SST and capped by a strong inversion. On smaller scales, stratocumulus clouds form as the planetary boundary layer advects over warmer SST. These clouds devolve into shallow cumulus convection as they move downstream. The IROAM provides a consistent dynamical framework to link the large-scale circulation, clouds, and their effect on the SST.

A sharp SST front forms on the north side of the equatorial cold tongue, causing atmospheric adjustments in the cloudiness, temperature, pressure, and wind speed. The change in sea-air heat flux across the front is decomposed into a sum of terms, one of which is the direct response to the ocean temperature difference. The other terms involve contributions from the atmospheric response to the SST front. In the IROAM, small mean meridional currents in the ocean mixed layer have a large effect on SST in the vicinity of the SST front through advection and concentration of temperature gradients.