Impact of Lateral Mixing in the Ocean on El Nino in a Suite of Fully Coupled Climate Models

Abstract

We examine the dependence of the amplitude of the El Nino-Southern Oscillation (ENSO) on the mixing coefficient parameterizing the lateral mixing of tracers (ARedi) The value of this coefficient is very uncertain, ranging in Earth System Models between a few hundred and a few thousand m2 s−1, with some observational estimates showing even higher values. A suite of simulations is made with two spatially varying distributions of ARedi derived from satellite observations as well as four simulations where a spatially constant ARedi is varied over a factor of 6. Surprisingly, larger values of ARedi result in stronger ENSO variability despite the higher mixing coefficients producing more efficient lateral diffusive damping of anomalies. This is because lateral mixing also warms the cold tongue, increasing vertical temperature gradients and decreasing horizontal temperature gradients. Larger vertical temperature gradients make sea surface temperatures more responsive to atmospheric forcing, while smaller horizontal temperature gradients shift the location of convection and make the atmosphere more responsive to sea surface temperature anomalies. The last effect holds across simulations as well as within individual simulations and thus also helps to explain interdecadal variability in ENSO amplitude. By contrast, a previously proposed anticorrelation between the amplitude interannual and annual variability does not hold across simulations, although it does hold within simulations. The propagation of thermocline depth anomalies is relatively insensitive to the value of ARedi. Properly specifying the lateral mixing along the equator (including distinguishing the impacts of subgridscale turbulence and tropical instability waves) appears essential to simulating ENSO.