The Implication of Summer Monsoon Onset from Seasonally Migrating Linear Rossby Wave and Wave-breaking Regimes

A global Hovmöller diagrams of column water vapor (CWV) at 30°N latitude based on ERA-interim reanalysis data reveal two "ribbons" of enhanced CWV, with one each in northern subtropical Pacific and Atlantic Oceans. The ribbons of enhanced CWV propagate from respective east sides of the basins during boreal winter into the western basins during summer and they appear to foreshadow the onset of monsoons as the ribbons make landfall. As such, we investigate the potential connections between these CWV features and monsoon onset. Potential vorticity maps at 250mb reveal eastward propagating linear Rossby waves west of CWV ribbons, which may be associated with mid-latitude atmospheric rives (AR). Rossby wave breaking (RWB), as revealed by large finite-amplitude local wave activity, propagates westward with CWV ribbons as spring gives way to summer. Our preliminary analysis suggests that in this linear Rossby wave regime, west of the ribbons, momentum convergence sustain linear waves and generate surface westerlies. While in the RWB region, east of the ribbons, momentum divergence and surface easterlies are observed. These features co-propagate westward across the ocean basins as spring progresses, and we speculate about a possible mechanism involving critical layer dynamics. We will explore ongoing work to test this mechanism in a moist idealized GCM described in of Frierson et al., (2006) with simple subtropical continental configurations.