The Diurnal Cycle and Large-scale Tropical Climate

This study is provoked by new observations from DYNAMO (Dynamics of the MJO) that demonstrate the high degree to which column humidity is modulated by the diurnal cycle of clouds under suppressed conditions. In this regime, large-scale subsidence precludes the development of widespread deep convection. Owing to a mostly clear sky, the large diurnal cycle in shortwave heating in this regime drives a diurnal cycle in sea surface temperature (SST) that can reach magnitudes change of ~2 degC. Convective clouds deepen in response to this daytime boundary layer warming in a manner that resembles the diurnal cycle over land. A modeling study is performed to 1) better understand the complex diurnal cycle of oceanic shallow convection, which exhibits both nocturnal and daytime peaks, and 2) elucidate the manner in which this diurnal cycle rectifies onto longer timescales. The model framework (CM1) fully resolves clouds, and includes a diurnal cycle in both SST and fully interactive shortwave and longwave radiation.