Precipitation projections in the tropical Pacific are sensitive to different types of SST bias adjustment

Many coupled climate models suffer from large sea surface temperature (SST) biases in the tropical Pacific, and for this reason studies have used a combination of observed SSTs and model-generated SST warming patterns to obtain climate projections. However, the existence of biases in the present-day coupled model SSTs implies that SST future warming patterns may also be biased. Models with large biases in tropical Pacific SST simulate warming that does not align with the observed cold tongue, producing a dynamically unrealistic pattern when added to observed SSTs. We demonstrate that projections of precipitation are highly sensitive to the warming pattern of SSTs by using idealized annually forced atmospheric model experiments. Simple bias adjustment of the warming pattern is applied, leading to precipitation changes that are around 60% weaker in the Western Tropical Pacific. Sensitivity of precipitation to small changes in the SST may explain the large discrepancies in Coupled Model Intercomparison Project Phase 5 precipitation projections in this region.