Intraseasonal SST Forcing of Atmospheric Convection in the Western Indian Ocean Using an Idealized Model

The mechanisms by which downwelling equatorial Rossby waves in the ocean are able to drive convective initiation and organization in the western Indian Ocean that sparks the onset of certain intraseasonal oscillation events are investigated using an idealized atmospheric model, Cloud Model 1 (CM1). In particular the atmospheric response to sea surface temperature (SST) variations forced by oceanic downwelling equatorial Rossby waves are quantitatively analyzed in a vertically integrated boundary layer divergence budget. Downwelling equatorial Rossby (ER) waves have been observed to sharpen SST gradients in the western Indian Ocean. Changes in SST cause the atmosphere to hydrostatically adjust, subsequently modulating the low-level wind field. In the idealized cloud model, wind speeds, surface moisture fluxes, and precipitable water maximize in regions of strongest SST gradients. SST-induced convergence also encourages shallow convection that moistens the free troposphere such that deep, precipitating convection can then occur. The roles of the low-level moistening by increased wind speeds and convergence forcing by the shape of the SST are compared. The greatest free tropospheric moistening occurs near the region of maximum wind forcing by the SST gradient. This is also the region of greatest surface latent heat flux. Through these processes, oceanic downwelling equatorial Rossby waves are hypothesized to increase upscale convective organization and rainfall sufficient for onset of select intraseasonal oscillation events. These results suggest that modulation of the SST distribution, in addition to SST magnitude, by oceanic equatorial Rossby waves primes the western Indian Ocean atmospheric boundary layer for intraseasonal convective development. The slow transit speed of downwelling ER waves across the Indian Ocean (75 – 90 days) suggests increased lead times for predicting intraseasonal convective initiation and organization in the western Indian Ocean.