Rodwell and Hoskins have suggested that the strong diabatic heating associated with the monsoon produces a Gill-like Rossby wave response that thermodynamically interacts with the westerlies to produce subsidence and reduced rainfall over North Africa, parts of the Mediterranean, and the Middle East. We test this proposed mechanism in the context of the monsoon onset by examining the changes to the terms of the thermodynamic energy equation and associated changes in precipitation, between the two weeks prior to the onset and the two weeks after. The NCEP/NCAR reanalysis is used for atmospheric daily data and the GPCP pentad precipitation product is used to estimate rainfall. The Fasullo and Webster index is used for the monsoon onset because it captures the onset over the ocean, where the convection is strongest, as opposed to more traditional indices which focus on the onset over India – where the societal impact is largest but the convective forcing is secondary.
A Rossby-like response to the monsoon onset is quite clear in the observational data and, as suggested by Rodwell and Hoskins, is associated with horizontal temperature advection as the westerlies intersect the warm temperature anomalies of the Rossby wave. Analysis of the thermodynamic equation verifies that the horizontal temperature advection is indeed balanced by subsidence over areas of North Africa, the Mediterranean, and the Middle East, and analysis of GPCP pentad precipitation shows an associated decrease in precipitation. Preliminary analysis suggests that the reverse occurs during the demise of the monsoon, perhaps playing a role in the onset of the fall wet season in those regions. This may be a more important factor as, while the precipitation decreases associated with the onset can be considerable, they appear to come toward the end of the wet season across most of the affected area.
Interestingly, the thermodynamic forcing of the monsoon onset also results in rising motion over northern India, which may play a role in the development of the monsoon over land, and we are examining this further.
Supplementary URL: