During the year 2016 IMD brought about an end to the system of calling “Droughts” and instead decided to use terms like “Deficient Year” / “Largely Deficient Year” etc. Similar changes were made at all the spatial levels as well while providing status of Monsoon Rains. IMD classifies an area deficient if the overall rains in a sub division / state / district is less than long period average by -20% to -59% while if the rains for a sub division / state / district is less than long period average by -60% to -99% then it is called largely deficient. If the country receives between 90% and 110% of the long period average then it is considered normal while for smaller spatial units like States / Sub divisions / Districts it is considered normal if the received rains are between 80% and 120% of the long period average.
It is now 4 weeks since the Southwest Monsoon made its onset over Kerala on 8th June ’23. While the onset covered India nearly a week in advance to its normal date of 8th July the overall performance of Monsoon has left a lot to be desired. As of 5th July more than 40% (304 out of 717) of the districts in the country have seen deficient or worse rains cumulatively for the season. This includes the core monsoon areas like Peninsular West Coast, Gangetic Plains, Central India etc. Large states like Maharashtra, Telangana, Odisha and Jharkhand have nearly 3/4th of the districts receive deficient or worse rains so far. The large excess districts are all skewed towards the NW India which got bonus rains from the remnant circulation of Cyclone Biparjoy. If one were to look even more closely at the district wise rainfall status more than 50% of the districts in India has so far received 90% or lesser rains compared to Long period average.
“Devil is always in the detail” goes a famous saying. This holds true for weather as well and the current state of conditions is a classic example of that. While the meteorological world and the ocean has moved towards El Nino the atmosphere is still holding onto its La Nina state resulting in an incoherent sync between Ocean and atmosphere dynamics. Atmosphere has to couple with ocean dynamics be it La Nina or El Nino, if the coupling does not happen then the incoherent signal reflects in Monsoon performance not only over the Indian Sub continent but other places as well.
While the latest ocean temperatures over the Nino 3.4 region of Pacific Ocean is nearly 1°C warmer than normal the atmospheric component which is indicated through Southern Oscillation Index (SOI) is in neutral condition moving back towards La Nina zone after a brief movement towards El Nino zone during the month of May. During the year 2014 we saw similar instance when most weather models were extremely optimistic of a quick transition towards El Nino and a subsequent super Nino. Instead we had a case of atmosphere lagging in its coupling to the ocean dynamics until well into the 4th quarter of 2014. Eventually though the atmosphere will couple with the ocean dynamics as Oceans remain the biggest drivers of global weather phenomenon.
Just as in the case of El Nino forecasts almost all the weather models were estimating the Indian Ocean to move into a Positive IOD phase by June / July with the Australian Met Agency expecting positive IOD to peak by July. In a way this development of strong positive IOD was expected to counter balance the effect of developing El Nino over the Indian summer monsoon providing some moderate and allowing the Indian summer monsoon to be near normal rather than look at a poor monsoon year during 2023. The current state of IOD has caught most model outlooks off guard with the IOD continuing to be in neutral zone with no tangible movement towards positive IOD phase. In a way the incoherent atmosphere and ocean dynamics is possibly reflecting in weak positive IOD development.
The latest long term outlooks for the period July to December has started to come from most models. Keeping in mind the current incoherent state of Atmosphere and Ocean coupling it may be prudent to take all the both excess and deficit forecasts being put out by weather models for the remaining months of 2023 with a pinch of salt as things will change for the good or worse once the coupling becomes stronger. Until then the performance of Southwest Monsoon will be primarily driven by the favorable movement of intra seasonal tropical waves like MJO, Kelvin and ER etc.