The sensitivity of climatological SST to slab ocean model thickness

Abstract

A pronounced tropical cooling (> 4 °C) and high-latitude warming in the annual mean sea surface temperature (SST) climatology is found in a numerical experiment conducted with a coupled model consisting of an atmospheric general circulation model (AGCM) coupled to a slab ocean model (SOM) in which the time-independent SOM thickness is reduced by a factor of two. The results suggest that biases in the ocean mixed layer depth could be contributing to SST biases in coupled atmosphere–ocean general circulation models. These changes in annual mean SST are noteworthy since in simple climate models the SOM thickness controls the amplitude and phase of the SST annual cycle, but it plays no role in determining the annual mean SST. Results from the numerical experiment indicate that halving of the SOM thickness not only changes annual cycle amplitude but results in asymmetrical changes in the annual cycle that rectify onto annual mean SST. The changed SOM thickness is found to primarily affect the surface net solar flux and latent heat flux components. However, due to the SOM equilibrium energy budget constraint, the annual mean net surface heat flux does not change, and the solar and latent heat fluxes changes compensate for each other. There is no such constraint on the annual cycle of heat fluxes, where the solar and latent components act to reinforce each other over much of the ocean, with the notable exception of the tropic Pacific warm pool region. We investigate the influence of the amplitude of the SST annual cycle on the annual mean SST through the net surface heat flux and its components using a two-step approach. First, two prescribed SST AGCM simulations are carried out, both with the annual mean SST of the full-thickness SOM coupled model simulation: one forced with the SST annual cycle of the full-thickness SOM coupled simulation, and the other forced with the SST annual cycle of the half-thickness SOM coupled simulation. Second, the impacts of these changes in the annual cycle on the annual mean SST are assessed in a full-thickness AGCM-SOM coupled simulation forced by the net, annual-mean, surface heat flux changes produced by the SST annual cycle changes in the first step. It is found that these net surface heat flux changes can largely reproduce the annual mean SST sensitivity to SOM thickness found in the coupled SOM thickness halving experiment. The effects on the annual mean heat flux components and annual mean SST from changes in the SOM thickness in an idealized limiting case are also considered.