North East Monsoon circulation and associated rainfall sets in over southern peninsular India:

 The large scale atmospheric circulation over the Asian Monsoon domain underwent a seasonal shift during the past two to three weeks and has now gradually settled towards a boreal autumn-winter time type circulation pattern. This shift includes a major change in the structure of the atmospheric column from a Baroclinic atmosphere that prevailed during the boreal summer monsoon season to a Barotropic/Equivalent Barotropic atmosphere which is currently observed. To easily understand and perceive this, one can see the recent changes that have taken place in the prevailing low level wind pattern which was predominantly south westerlies for the past 4 months has now shifted to north easterlies over the north Indian Ocean (The Arabian Sea and the Bay of Bengal) and the adjoining South China Sea. Also, one can note the associated sign reversal in the pole to equatorward temperature gradient over the south Asian region which is in association with the southward migrating quasi-stationary atmospheric heat source during this time of the year. All these changes have led to the withdrawal of the wet summer monsoon rainy season from over a larger part of India and a simultaneous start of the north east monsoon rainfall season over the south east peninsular India. Along with these seasonal shifts that is taking place in the background, the region of anomalous enhanced tropical convection, especially the low frequency component continues to be dominated by a La Nina/ Negative IOD signal over the Maritime Continent and the Eastern equatorial Indian Ocean which is prevailing as a source region for several eastward (Kelvin wave) and westward (Equatorial Rossby waves) propagating moist equatorial waves. Currently, an ER wave is seen propagating westwards over the central equatorial Indian Ocean, whose northern part is causing widespread rainfall in the southern states of India including Tamil Nadu. Global Numerical Weather Prediction Models on the medium range period, indicate that more such ER waves could form and propagate along the equatorial Indian ocean in the days to come as an MJO signal is about to constructively interfere with the low frequency signal. This could eventually lead to an uptick in the genesis of synoptic scale low pressure systems as well over the oceanic regions.