The interaction between moist convection and regional tropical and subtropical gravity wave breaking

Previous studies have explored the linear and nonlinear responses to idealized regional tropical and subtropical gravity wave drags. Using a hierarchy of analytical and numerical models it was shown that equatorial retrograde drag induces upwelling to the east, and subtropical retrograde drag induces upwelling to the south. It was also shown that, in the presence of sufficient moisture, the induced vertical motion is amplified due to a reduction in the effective static stability.

Here we apply the above theoretical and model insights to a more realistic scenario. Using a comprehensive global circulation model we explore the interaction of orographic gravity wave breaking over the East African Plateau and the Tibetan Plateau with tropical moist convection over the Indian ocean. By isolating the effect of the gravity wave drag we show that precipitating ascent over the Indian Ocean is enhanced by drag to the west and north of that ocean basin. This enhanced upwelling increases cloud cover, decreases outgoing longwave radiation, and projects strongly on tropical variability. To explore possible implications for the real tropical atmosphere, we use reanalysis data to examine how enhanced gravity wave breaking over Africa and South Asia is related to precipitating convection in the onset phases of the Madden-Julian Oscillation.