Sunday, April 10, 2011

Monsoon Watch - 1

These reports are my personal views, study and observations. Estimates and forecasts are also my own calculations and views, and may/may not tally with any other estimates. The Monsoon progress as analysed will be discussed and explained in Vagaries.

This annual series is a follow up and chasing of the South-West Monsoon. For now, this series will be chasing and closely following up the actual developments of the monsoon, and analyse its progress. We are not yet contemplating the quantum of rains or the monsoon strength as yet.

Very very briefly, let us first understand, that the "power house" of our SWM is actually based far away in the southern Hemisphere. The main "generating factory" is the Mascrene Highs, SE of the Madagascar coast. The proper formation of these high regions,(1028 mb at least) will boost and create proper SE winds to lash the East African Coast near the Equator, and then turn SW in the Northern Hemisphere.

And the formation of the "Tibetan High" is another factor. An anti-cyclone in the upper atmosphere over the Tibetan Plateau. During the formation stages, there will be thunderstorm activity in the SE parts of Tibet during the months of April and May.

To generate a powerfull SW wind, the cross equatorial flow must be strong and gusty. SW winds (ultimately upto 600 mb levels) then are capable of riding on the warmer Arabian Sea waters and bring cloud masses towards the West Coast of India. In the Bay, similar SW winds are generated, but from SE winds below the equator, just below Sri Lanka.

And to attract the SW winds, again, we need a good proper "seasonal low" to develop in the Sub-Continent. This seasonal low, normally should start forming around Barmer, Rajasthan, by mid April, and at its peak stretch upto Arabia. Seasonal low at its peak, in June, should normally be around 994 mb, and the ridge around Kerala should be 1008 mb. This enables a good gradiant to pull the SWM inland.

In this initial stage, we will observe the basic root and foundation,the initial parameters and the initial seed of the monsoon.

1. Cross Equtorial Winds from Southern Hemisphere.

2. Seasonal Low over India/Pakistan.

3. El-Nino Status.

4. Pre Monsoon Low in the Bay.

1. Cross Equatorial Winds should start forming, and taking shape from mid -April, especially in the South Indian ocean region.

The Mascrene Highs are the main "Power House" of the South -West Monsoons.

This generates the South -East winds, which after gaining strength, cross the equator and become south west. For a proper High to form in the South Indian Ocean, the cyclone season (Low pressures) should completely die down from that region.

Weak signs of initial SE winds forming, but not yet getting organised. Initial forming has commenced off the East African Coast. But the winds are NE above equator. Also, very Strong wind currents are required to be observed near the Eastern side of the Indian Ocean, as the Monsoon stikes the Andaman Sea by mid May.

2. Now, we have just about "normal to below normal" heating in the plains and Southern regions of the Sub-Continent. This is not much of a plus point in the formation, or a quicker formation of one of the important pre- monsoon weather requirement, the seasonal low. The seasonal low, stretch over Arabia thru India, normally starts forming in the Barmer and adjoining regions in the second week of April. Today, the MSL is around 1010 mb in the Sindh/Rajasthan region.

In the "core seed"area, it 1008 mb. (The ultimate low in the core, in June when ready, should be 994mb.). Today, there is a "high" of 1008 mb covering almost the entire Indian region.

The Seasonal Low normally is required to start taking shape for the monsoon by the second week of April.

3. La - Nina factor is favouring SWM as of now. It seems to be weakening, and approaching near normal levels. Forecasted to go neutral by June. Last SOI reading observed was +23. This actually indicates an surviving La-Nina.

• The Oceanic Nino Index (ONI) is based on SST departures from average in the Niño 3.4 region, and is a principal measure for monitoring, assessing, and predicting ENSO. The readings for the last 2 quarters were:-1.4, -1.3 and -1.2. La Niña: characterized by a negative ONI less than or equal to -0.5 C, so today La-Nina prevails.

4. Another pre monsoon seasonal normal requirment is the creation of a low in the Bay by the second week of April. This is necessary to trigger off the flow and formation of lows from the bay.

Progress this year is bad, as the Bay shows a ridge today. High pressure is prevelent in the Bay. No forecast of any low as of now. (in 2009,a low formed by the 12th. of April, and later it intensified into cyclone "Bijli", 2010 had no low till 3rd week of April).

Summary: Parameter; 1. normal, Parameter 2. -ve, Parameter 3. -ve. Parameter 4. -ve.

New Readers, please note, Parameters are normally (every year) considered in MW Series to guage and estimate the arrival cate of SWM, not strength. Time of arrival normally indicated after mid-April.

Next update on 17th. April.


Neeraj said...

It is still early days. I think we should look at all the parameters with a bit of caution for now. By the way, when is IMD coming up with its first seasonal forecast?

emkay said...

Seems thundershowers in Mumbai are also delayed. Not expecting before May 1st week.

Anonymous said...

would be interesting to keep an eye on "below normal" heating up of northern plains

rajesh said...

@Neeraj:MWseries starts every year around 10 April.The developments from stage 1 are followed, and as it should be at that period of time.In short, parameters of development are tracked.
@emkay:Not necessary. In fact, seeing some thundery developments over ghats by Thursday. Shall confirm this in Vagaries.
@anonymous. In fact, the heating has lessened on Sunday. As per Vagaries'blog put up on saturday.Delhi was 32c and Simla 16c on Sunday. All North dropped by about 3c.

NT said...

News of rains from Pune just now (@6:15 pm, 11th April). The lightning map also shows lightning activity just south and east of Mumbai.

rajesh said...

Dear Rajesh,
IMD recorded 6mm rain yesterday. Overcast again to-day and rain expected later in the day. Roof work is still on and hope it gets covered up by the evening. It is quite normal to receive showers at this time of the year.
Regards, Shiraz.By Email.

Cmdr. Potey Reports by SMS-"Rain in Talegoan, Kamshet and Mahableshwar".

Ron said...

Thanks for the update