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Monsoon Onset And Seasonal Rainfall - Poor Correlation

May 25, 2022 11:00 AM |

Onset time of southwest  monsoon is drawing closer.  Predictions are for before time arrival over mainland Kerala. Monsoon arrived over North Andaman Sea and Bay Islands on 16thMay, about a week before the due date. Thereafter, it has advanced further to some parts of Southeast Arabian Sea, nearly touching Sri Lanka and small portion of Northeast and South-Central Bay of Bengal. Progress so far, has just been incremental. Earlier, cyclone Asani set the pace for monsoon by enhancing cross equatorial flow and replacing anticyclone over Arabian Sea. 

The onset date of monsoon has hardly any correlation with its further progress and overall seasonal performance. It remains a matter of academic interest.  Ontime  or early arrival of monsoon is a feel good factor and surely builds up morale of stake holders.  Earlier, there was a general reluctance in announcing delayed onset or poor monsoon forecast.  Why plunge the country in to despair even before the 1st rain hit the ground.  Therefore, it was prudent to couch the unpleasant news in some comforting terms. With the changed times and technology and better resources at the disposal of allied partners, ill effects of monsoon, if any, can be managed. Bold forecasts, without shielding the bleak aspects have now become a norm.

 Monsoon Onset Date – Seasonal Performance ( Normal-N : Above Normal-AN : Below Normal- BN : Drought-D )

Southwest monsoon has a record of earliest arrival  on 18thMay 2004 and it ended up in a ‘drought’ with a seasonal deficiency of 14%.  Similarly, the monsoon made onset on 13thJune in 1983 and finished in excess with a seasonal surplus of 13%.  More importantly, monsoon should remain decent in the core months of July and August. Timely and adequate spatial distribution carry more weightage than shear numbers of Above Normal and Excess.   

Oceanic parameters like La Nina/El Nino and Indian Ocean Dipole (IOD) are closely related with the monsoon rainfall.  While the former is placed favorably this season, IOD has inclination of developing -ve phase, specially during 2nd half of the season. Accordingly, monsoon may show volatility in the later half. Opening monsoon month of June and the rainiest month of July hold large stakes for the rainfed areas, to facilitate timely sowing and giving stable start to the crops. Both these months are expected to behave approvingly.        

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