Once in a Blue Moon
very rarely, informal
In the changing dynamics of ocean warming, “climate change” etc it might not be a surprise if one gets to hear about “Once in a Blue Moon” weather events more often than not. A couple of days back New York city got flooded extensively due to record rains. JFK Airport saw nearly 22 cms of rains which was the highest for a September day in more than 60 years. Closer home the Southwest Monsoon season could indeed be called Once in a Blue Moon for Chennai and suburbs with the IMD observatory at Chennai AP recording nearly 95 cms of rains between 1st June and 30th September which is nearly twice the amount of the normal rains expected to receive during the season. The city IMD observatory at Nungambakkam recorded nearly 75 cms for the same period. To give a perspective both these number would have been classified as normal for a Northeast Monsoon season.
IMD classifies the period 1st June to 30th September as the Southwest Monsoon season statistically for better record keeping and understanding long term changes to rainfall pattern. The All India seasonal tally stands at 820 mm against the long period average of 868.6 mm. While Northwest India and Central India ended the season marginally positive both South Peninsular India and East & Northeast India ended the season negative with -8% and -18% respectively. Among the states in South Peninsular India if one were to ignore Andaman & Nicobar Islands only Telangana and Tamil Nadu ended the season with a positive anomaly, the worst being Kerala which ended the season with a -34% anomaly. It is pertinent to point here the Cauvery Basin has ended the season with a 33% deficit casting doubts on the cropping prospects for not only rest of the year but also next year as well.
In terms of monsoon dynamics as such while the withdrawal is expected to start from SW Rajasthan by 17th it is expected to complete its withdrawal over Peninsular India by 15th October. This year the withdrawal started on 25th September and as of yesterday most parts of Rajasthan, all of Punjab, Haryana and Delhi NCR region, parts of Uttar Pradesh, Himachal Pradesh, Uttarakhand and Jammu UT have seen withdrawal of monsoon.
One may wonder how today’s post is about a quick retreat of Southwest Monsoon when not one but two monsoon lows, one over Konkan coast and the other over North Bay, is seen persisting. In a way this pair of circulations is keeping the monsoon dynamics alive. The depression over the Konkan coast is expected to weaken into a Well marked low as it continues to move inland today before fading away in a couple of days time. The Well marked low over the North Bay though is expected to persist for a few more days as it moves in a WNW direction over Gangetic plains before fading away towards the second half of the coming week.
On a larger scale just as in the case of July, the above average performance by Monsoon during September was influenced by favorable Intra Seasonal Oscillation in the form of MJO over East Indian Ocean and adjoining Maritime Continent region. Now MJO is showing signs of moving further east towards western hemisphere which is expected to bring overall suppression in the atmospheric conditions over Indian Sub Continent. For the past couple of weeks Southwest Monsoon was feeding on the energy provided by favorable ISO which is expected to be cut off as MJO moves further East.
Weather ensembles indicate weak probability of any monsoon low forming once the current pair of circulations complete their life cycles. Similarly the precipitation anomaly charts also indicate weaker than normal conditions to persist during the first half of October. All in all the long term ensembles are consistent in showing a poor first half for most of the Indian Sub Continent which could be interpreted as a collapse of monsoon dynamics leading to a quick retreat of Southwest Monsoon.
Closer home after a fairly active thunderstorm season for leeward areas of Tamil Nadu, particularly North Tamil Nadu, thunderstorms are expected to greatly reduce from today leading to a reduction in rainfall. While isolated places may see thunderstorms over North TN and delta areas overall thunderstorm activity is expected to substantially reduce for the next week or so before making a return over the interior areas closer to the Western Ghats as seasonal transition of winds will make interior areas better placed for thunderstorms compared to coastal places like Chennai.
While a detailed outlook for the upcoming Northeast Monsoon season may be put up later this week initial indications are the first half of the season may be sub par for most parts of Tamil Nadu except South TN which may see some uptick in rainfall during the first couple of weeks of October.