Direct and disequilibrium effects on precipitation in transient climates

Abstract. Climate models are in broad agreement that global precipitation increases with surface temperature as atmospheric CO₂ concentrations rise, but recent studies have shown that climates that are not yet in equilibrium exhibit additional "transient precipitation effects". In conditions of rising CO₂, for example, precipitation at a given temperature is suppressed relative to its equilibrium value. Some authors argue that the primary driver of these effects is ocean heat uptake, but most recent studies assume that they result from some direct radiative effect. We show here that global precipitation and temperature anomalies are insufficient to resolve mechanisms, since the conventional "fast/slow" representation of transient precipitation effects is degenerate with a "disequilibrium" representation that posits control only by ocean heat uptake. We use regional anomalies instead to show in multiple ways that ocean heat uptake is the dominant driver of transient precipitation effects in CO₂-forced climates. Precipitation suppression appears predominantly over the ocean, with response over land of the opposite sign. The coefficients of a disequilibrium representation are uncorrelated, suggesting that they capture physically meaningful processes, while those of a fast/slow representation are highly correlated. Further, the regional patterns of transient precipitation response are highly similar for both CO₂ and solar forcing, with a relatively small and homogeneous offset between them. Examination of the surface energy budget allows us to conclude that energy balance in solar-forced climates is achieved by the superposition of both disequilibrium and direct processes. Our results highlight the importance of using regional information rather than global aggregates for understanding the physics of transient climate change and its impacts on societies.