Friday, September 3, 2010

How will La~ Nina affect India's weather in 2010

Akshay Deoras,
Severe Weather Forecaster

The cooling phase of equatorial pacific also termed as La Nina which had begun last month is now in full swing as per the datas of the Sea Surface Temperature (SST) and the precipitation anomaly in the la nina affected areas. A strong rainfall anomaly already happening in South,East Australia and Western USA.. Noted heavy rains or above normal rains are happening in parts of Australia and drought conditions happening in Western USA.
A La Nina is a global climatological phenomena happening in Equatorial pacific when the sea waters in central equatorial pacific starts cooling down! The reason for this is very simple. During La Nina, the trade winds blowing from East to West strengthen as a result of which the cooler water near South America rushes in the place of warm waters which happen during an opposite El~ Nino phase. This actually brings a very cool pool of water in Eastern Asia stretching to Australia and thus it causes more than average rains in these areas and also in parts of asia.
This phenomena always is preceded by El Nino and can last as long as 9 months to 12months! La Nina happens in 3-5yrs.Since the warm/cool water replacement is followed by high/low pressure formations. At places where warm SST is, warm air rises and a low pressure is formed.

From the present Sea surface temperature plots in the equatorial pacific,SST at the Central EQ.pacific have cooled upto to 2C.
This La Nina condition is expected to stay uptil early 2011 so its definite that the winters in India and in Asia are going to be affected!

How can La Nina affect India's weather
In La Nina, the global easterlies strengthen as a result of weakening of the westward moving winds in the pacific. In USA, a high pressure area actually forms in Northern Pacific which blocks the jet stream path as a result the polar jet stream flows through central USA thus making very cold there esp NW USA
In India, the cool water shifts towards the seas near the mainland and causes more than average rains since there is a strong and entire shift in the Indian Ocean Dipole which is a sort of ENSO or Elnino southern oscillation in Indian ocean where there is a regular change in the Sea surface temperatures in west and east Indian ocean.
In India, winters are expected to remain cooler than normal since the local jet streams dig more inland. However at this stage, the exact details of winter can't be described but I w'd expect a cooler than normal winter in Northern India and slightly slightly cool in central India.

When will the SW Monsoon 2010 end over Central India?
The NE Monsoon or Monsoon retreat begins in October and brings the cool air from Northern India to central parts. Right now in early september, this cycle hasn't initiated!
An active MJO cell currently stationed in the 140E Pacific is not expected to move towards India i.e oceans near India which means that the SW 2010 season can get over as per the predicted time
I have an estimation that in Central India, Northern India the season will conclude by mid september

Possible End of SW Monsoon in central India in an active mode
The anticipated low pressure currently situated over Eastern India is likely to shift westwards crossing central India and causing an additional moisture deposition over Central India.
Another low pressure just NW of the present one in bay of bengal also expected to move inwards
However, the intensity of these low pressures is not expected to be very strong as a result of which I think Rains Won't create a havoc in central India.
But if some vortices or Multiple Vortex systems builds over Central India as being depicted by GFS, then Nagpur and other nearby areas can get sure and good rainfall starting from Sunday night uptil Tuesday.
I will update separately as the situation unfurls!

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