No consistent improvement in tropical Pacific sea surface temperature pattern in high-resolution climate models

Abstract

Most of the coarse resolution Coupled Model Intercomparison Project (CMIP) climate models simulate a weakening of the equatorial Pacific east-west sea surface temperature (SST) gradient, contrary to the observation since the mid-1970s. Proposed reasons for this model-observation discrepancy are the equatorial Pacific cold tongue bias and the underestimation of equatorial trade wind strength in enhancing the upwelling in the central to eastern Pacific Ocean. Higher oceanic resolution has been known to improve eddy-induced heat transport, equatorial Pacific mean state SSTs, and precipitation. Here, we assess SST mean state biases and trend responses in a multi-model and multi-resolution ensemble from the High-Resolution Model Intercomparison Project (HighResMIP). Some models show an alleviation of the cold tongue bias in simulations of higher resolution compared to their respective low-resolution simulations, however, there is no consistent improvement across models in the trend response of the equatorial eastern Pacific SSTs at a higher resolution. Models are deficient in simulating the synchrony of trends and the causal relationships between surface zonal wind, SSTs, and thermocline structure in the eastern equatorial Pacific Ocean on multidecadal timescales. An underestimated ocean thermostat mechanism might explain climate models’ inability to simulate equatorial Pacific SST patterns since the mid-1970s. Simulating air-sea coupling correctly on multidecadal timescales in models might reduce the uncertainty of the projected tropical Pacific SST gradient.