Positive Indian Ocean Dipole May Help Summer Monsoon

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By Drew Lerner

Kansas City, April 10 (World Weather Inc.) – Another monsoon season is approaching in southern Asia and worry over El Nino has already grabbed the headlines a number of times this year. Recent data still does not support an El Nino event prior to June and the odds are still high that the event – if it is going to occur – will be weak, non-traditional and most likely sometime in the second half of the year. Even though El Nino is not expected at the beginning of the 2017 monsoon season there is still some concern that lighter than usual rainfall could occur this year because of a recent trend in neutral ENSO years to generate less than usual rain and because of potentially decreasing rainfall in latter part of the monsoon season due to El Nino. However, developments in the Indian Ocean are suggesting a positive Indian Ocean Dipole (IOD) event is evolving and if it is present at the beginning of the monsoon season it will contribute more rain for the nation reducing the potential for moisture shortages late in the year if El Nino does actually evolve.

The latest data on El Nino released today suggests little change from that of recent weeks. A large buildup of warmer than usual water along the northwestern South America Coast remains in place and continues to contribute to heavy rainfall in Peru and parts of Ecuador at times and the reduction of cool water fish activity off the coast. Ocean surface temperatures in the central equatorial Pacific Ocean remain near normal and the trend for cooling near Indonesia remains in place as well.

Even though warming in the far eastern Pacific Ocean has subsided in recent weeks the large buildup of warm water in the region at the same time cooling is occurring in the western equatorial Pacific Ocean still leaves the door open for possible El Nino evolution later this year. However, the ocean temperature trends have not shown much change recently. There is a pool of cooler than usual subsurface water in between the International Dateline and 140 degrees west longitude at a depth between 75 and 175 meters below the ocean surface that has grown in size over the past few weeks. The cooling water is expected to inhibit any further evolution toward El Nino reinforcing the World Weather, Inc. outlook that no El Nino will be possible prior to June, although it may come later in the year.

The fear this year has been over the potential development of El Nino again raising a threat of reduced rainfall in southern Asia. India has been a little more concerned about this trend than Southeast Asia because recent weeks of rain in Indonesia have been timely and sufficient in supporting normal crop development. The start of this year’s rainy season in mainland areas of Southeast Asia was normal for the first time in three years and as long as there is no further evolution toward El Nino conditions the southwest monsoon season is expected to begin normally this year, as well.

Recent statistics have shown a lighter than usual bias to rainfall in India during recent neutral ENSO conditions. The situation has not been critical, but there has been a tendency for drier than usual conditions to impact a part of India during neutral ENSO and that bias is still prevailing today, according to World Weather, Inc.

However, the biggest difference potentially for 2017 is the possible development of positive IOD (Indian Ocean Dipole). This phenomenon has been evolving in recent weeks with a building trend toward warmer than usual ocean temperatures in the western Indian Ocean, including parts of the Arabian Sea. At the same time, there has been a cooling trend in Indian Ocean water off the Sumatra, Indonesia and northwestern Australia coasts. If these anomalous ocean temperature trends continue for a few more weeks and are present when the monsoon begins there is likely to be some enhanced rainfall – at least at the beginning of the monsoon season.

Warm ocean water out in the western Indian Ocean and western Arabian Sea will prove to be a great source of water vapor when the southwest monsoon kicks in during June. The extra water vapor rising from the ocean’s surface resulting from warmer than usual water will be picked by southwest to northeast surface winds that are associated with the monsoon season bringing the greater amounts of low-level moisture into India. The influx of moisture will then be lifted into the atmosphere by naturally rising air over the Indian subcontinent raising the potential for heavier rainfall in the west and central parts of the nation during the first weeks of the monsoon.

The potential contribution of positive phased IOD must still be in place during June and July to have the impact that is suggested here. World Weather, Inc. will continue to monitor its evolution and then a continued watch will be warranted for signs of its demise.

An official monsoon forecast will be released by World Weather, Inc. in May. However, based on the above discussion this year’s monsoon would have potential to be close to 100% of normal, but that statement is based on continued positive IOD development in the next few weeks, no El Nino before June and preferably after July and a normal startup of the southwest monsoon over southeast Asia in May.

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