Impact of Diurnal Warm Layers on Atmospheric Convection

Abstract

The phenomenon of sea surface temperature (SST) anomalies created by oceanic diurnal warm layers has been extensively studied for the last decades, but the assessment of its importance for atmospheric convection has come within reach only very recently, thanks to the development of kilometre-scale simulations. We use the output of a global coupled simulation with a 5km horizontal grid spacing and near-surface ocean layers of order O(0.5m) to explicitly resolve both atmospheric convection and diurnal warm layers. As expected, the simulations produce daily SST fluctuations of up to several degrees. The increase of SST during the day causes an abrupt afternoon increase of atmospheric moisture due to enhanced latent heat flux. This increase is followed by an increase in cloud cover and cloud liquid water content. However, although the daily SST amplitude is exaggerated in comparison to reanalysis, the impact on cloud cover and cloud liquid water content only lasts for 5-6 hours. Moreover, the global daily average of these quantities is not influenced by their increase. All in all, we conclude that the global short-timescale impact of diurnal warm layers is negligible.