As the Indian subcontinent swelters through what is normally the hottest time of year, forecasters are watching the oceans and atmosphere for signs as to the coming rainy Summer Monsoon.
A long-range forecast released in late April by the India Meteorology Department (IMD) has called for near-normal seasonal rainfall for the all-important rainy months of June through September. India alone gets about 80 percent of its yearly rainfall during these four months. The subcontinents roughly 1.7 billion residents are heavily dependent upon the Indian monsoon and the rain that it brings.
In the nearly four weeks since the issuing of the IMD forecast, some factors have arisen that could be point to low seasonal rainfall in parts of the subcontinent.
The leading edge of the Indian Summer, or South West, Monsoon has begun to be tracked over the eastern Indian Ocean basin by the IMD. As of Wednesday, the leading edge of the mega weather system was plotted from seas southeast of Sri Lanka northeast to near Yangon (Rangoon), the IMD website showed.
As for the subcontinent itself, the benchmark for monsoon onset has historical been the time of monsoon start in the state of Kerala, southwestern India.
The IMD have forecast monsoon onset on or about June 3, 2013. The historical average date of monsoon onset in southern Kerala is about the first of June. In 2012, monsoon onset in Kerala happened on June 5.
The IMD forecast of near-normal rainfall, namely 98 percent of the normal amount, for India as a whole was based on a set of five statistical predictors related to oceanic temperature as well as temperature and pressure in the atmosphere. These are compiled for a periods ranging between December and March. Rainfall for India as a whole in the period June-September 2012 was calculated at 93 percent of normal amount, according to the IMD website.
Among the strongest predictive indicators for seasonal rainfall tied to the Summer Monsoon is overall oceanic temperature in the equatorial Indian and Pacific Ocean basins. For instance, El Nino-Southern Oscillation (ENSO) has shown a historical relationship to the Indian Monsoon. Drought years on the Subcontinent have been correlated to years of El Nino oceanic warming in the Pacific Ocean.
Another predictive indicator for the Indian Monsoon lies closer to home in the equatorial Indian Ocean basin. A phenomenon known as the Indian Ocean Dipole (IOD) can either bolster or hinder subcontinent rainfall as it influences the rain-giving monsoon winds flowing over South Asia from the Indian Ocean.
A positive Indian Ocean Dipole implies unusually warm water over the western equatorial Indian Ocean compared to the eastern side of the basin. A negative IOD implies a relatively cool western Indian Ocean versus the eastern side of the basin.
The heat stored in tropical seawater is a prime driver for atmospheric circulation worldwide, and this is no less true for the South West Monsoon. During instances of positive IOD, the wind flow that makes up the South West Monsoon tends to be accelerated over the Arabian Sea, Bay of Bengal and the Indian subcontinent. Enhanced monsoonal winds tend to imply enhanced seasonal rainfall. Conversely, a negative IOD tends to put a drag on the monsoonal southwesterlies, and with it can leave the subcontinent with rainfall dearths.
The lack of El Nino, or its complimentary state, La Nina, known as "ENSO neutral", was a basis for the IMD long-range monsoon forecast, according to its website. Forecasts of ENSO through the summer months indicate likely persistence of neutral ENSO state over the equatorial Pacific Ocean.
At the same time, some oceanic forecasts have shown the formation of a negative IOD, something that was not referenced in the IMD background information related to the Long Range Forecast.
"A negative IOD usually results in drier than normal conditions across India, while favoring wetter than normal conditions across Indonesia and Australia," AccuWeather.com Expert Senior Meteorologist and long-time international forecast Jason Nichols said. "Many global models have been trending toward a negative Indian Ocean Dipole across the Indian Ocean from the June to August or September time period."
For Nichols, the area of greatest concern for below-normal rainfall would be the northwestern subcontinent, especially northwestern India to southern Pakistan. "The projected negative IOD for the summer months and perhaps into early fall should result in some disruptions to the normal South West Monsoon across northwest India into Pakistan. If the IOD turns negative as quickly as advertised then the arrival of the Monsoon in northwest India and Pakistan could be delayed," Nichols said.
Indian Ocean Dipole (IOD) forecast indicating (weakly) negative dipole through the four Indian Monsoon months of June-September. (Australian Bureau of Meteorology - BoM)
For the area stretching between the Indian states of Madhya Pradesh and Punjab, stretching west into southern Pakistan, Nichols forecast only 75-80 percent of normal seasonal rainfall, based upon an expected late Monsoon onset and a lack of consistency in Monsoon rains. For the rest of India, Nichols predicted 85-100 percent of normal rainfall with potential for above-normal rainfall in the northeast.
A look back to 2012 and the IOD index shows that the index was weakly negative for the first half of the India Monsoon, then a positive dipole set up, peaking in September. This may have been a factor, both in the somewhat late and deficient overall rainfall result and in the late-season flooding rain in some areas (such as Pakistan).
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