Observations and Impacts of Diurnal Warm Layers During the Pre-Onset MJO in the Tropical Indian Ocean

Diurnal air-sea coupling affects climate modes such as the Madden-Julian Oscillation (MJO) via nonlinear rectification on the regional moist static energy budget. Prior to MJO initiation (the suppressed phase; Phase C), large-scale subsidence dries the atmospheric column, thus increasing atmospheric transmissivity. Reduced mixing of the upper-ocean associated with a quiescent flow enables solar radiation to significantly warm the uppermost layer of the ocean over the course of a single day. Approximately 1-2 weeks prior to the onset of the MJO, the ocean surface layer commonly warms by 1-2° C (up to 5° C) between the morning and early evening over the tropical Indian Ocean. The observed composite diurnal cycle amplitude of low clouds, surface latent heat flux, and radiative fluxes is significantly higher for the high diurnal SST (dSST) bin compared to the low dSST bin. Moisture tendency from two reanalysis models (MERRA-2 and ERA5) show disagreement on the importance of column moistening from high dSST gridboxes; MERRA-2 shows greater moistening at high dSST prior to MJO initiation. The robustness of this local moistening effect is sensitive to spatial scale; the areal fraction of high dSST gridboxes in the days leading up to MJO onset is 5-10% lower at 1-degree resolution versus quarter-degree native resolution. Moreover, we use satellite observations to stratify composites of radiative heating rates, surface energy budget terms, precipitation, and fluxes by a high and low dSST bin during the suppressed phase over 37 MJO events between 2000-2018 to comprehensively examine this effect using state-of-the-art diurnal analysis techniques.