The Sea Surface Temperature Pattern Effect on Outgoing Longwave Radiation: The Role of Large-Scale Convective Aggregation

Observations and climate models show a strong increase/decrease of tropical low clouds, and hence reflected solar radiation, in response to an increase/decrease of the west-east sea surface temperature (SST) gradient in the tropical Pacific due to its impact on boundary layer inversion strength. Here, we discuss an accompanied increase/decrease of outgoing longwave radiation due to the contraction/expansion of the tropical deep convection area (decreasing/increasing the high cloud amount and relative humidity) when the SST gradient between regions with high and low SST increases/decreases. In targeted amip-piForcing style GFDL-AM4 model simulations, the negative longwave radiation response due to large-scale convective aggregation resulting from the La-Nina-like warming pattern over the period 1980–2010 is comparable to the negative shortwave cloud feedback. CMIP6 models show that the multi-model-mean is similar to that in our simulations. However, the relative magnitude of shortwave and longwave effects differs substantially between models, revealing an underappreciated climate model uncertainty.