Thursday, April 29, 2010

Monsoon Watch - 4

The parameter of utmost concern to me now is the Bay factor. The consistently -ve factor in the first 3 MWs.
The latest winds in the Bay show a strong current rushing Northwards along the East coast of India, into the trough in sub-Himalayan West Bengal. The North-West flow of winds into the Myanmar coast also exists. Thereby, to some extent, the hitherto prevailing "high" is somewhat breaking up.
Also, the SST in the Bay is quite conducive to host a low anytime now. Like previously mentioned, a low, according to me, should have normally formed by 15th. April. Infact, last year we had a cyclone in the Bay in April. A delay here can effect the Bay branch of the Monsoon. Now, the delay in the formatiion of the wanted low is due to the trough running along the North-Eastern states, which has caused very heavy rains there. Moisture laden winds are gushing into the region, causing heavy rains.
Now, according to ECMWF forecasts, and other models,no low is predicted in the Bay till the 6th.May at least. Indicator :-ve

The Seasonal Low in the Thar Desert is deepening gradually. As on end April, a pressure of 1000 mb in the central core region around Barmer is normal.(See Thai Met Dept Map).Now onwards, the core should expand, and stretch into the West, into Pakistan, and within a week further West. Finally, by June, the low has to stretch from India to Arabia, with its core centre (994mb) somewhere near Jacobabad. The intense heat in the sub continent is an useful aide to this formation.
Also, further heavy moisture inflow from the North-West winds into Kerala and South Karnataka coast may prevent a proper gradiant to form to give the Monsoon a final momentum and push along the west coast. During end of May,the low in the North (Rajasthan/Sindh) may be normal at 996mb,but the 1008 mb required off the Southern tip may not materise on time. And that is a must for creating a pull Northwards for the Monsoon.Still, we have the month of May for the situation to "cool off", or rather "warm up" along the south- west coast. Indicator :Normal.

Another parameter to observe now are the 200 hpa jet stream. Normally, they should be Easterlies right upto 20N by May end. This would facilitate the Northward movement of the Monsoon Current. Today, the jet stream is developed upto the Equator, and are gaining strenght over Sri Lanka.(See COLA/IGES Map) Forecasts on models show these winds getting organised around the first week of May. Should march up to the 10N by middle of May, to be in time for the Andaman Sea Monsoon arrival. Indicator: Normal

The cross equatorial flow,is surely picking up. A very crucial factor this one. The Mascrene highs are establishing and winds hitting the East African coast are getting stronger, and the Somali Current is getting defined. The lacking factor is the cross flow south of the Bay, that is, the Eastern parameter.(See Aust. Bur. of Met.TXLAPS Wind chart attached). The Bay branch should be getting stronger by now, but lacks the momento. But, a push is there in the offing. As reported by the Australian Weather buraeu "The latest guidance suggests that an active MJO event will develop in the western Indian Ocean over the coming week or two. In response to this, we would expect to see an increase in convective activity over the northern Indian Ocean, although is unlikely to trigger the Indian Monsoon onset this early in the year."
However, I feel, this will be a good booster for the Bay branch of the Monsoon. Remember, the due date for the Monsoon in the South Andaman Sea is May 12th. Indicator: Normal.

The cyclone in the Southern Indian Ocean has dissipitated, hence there is no longer any hindrance now for the ITCZ to start moving North. Situated at 5S, it should normally cross the equator around 10th.May,for a proper setting of the Monsoon in both the branches.

El-Nino: According to the latest report from the Australian Weather Bureau, "the majority of ENSO indicators have now dropped below El Niño thresholds. The cooling of the tropical Pacific Ocean ..has resulted in trade winds and tropical cloudiness returning to normal."
But, my estimates show that a weak El-Nino is still hanging around.
Regions Previous Current Temperature change
(2 weeks)
Nino 3        0.3°C cooler
Nino 3.4    0.1°C cooler
Nino 4        0.1°C cooler

While checking out all the other parameters, we cannot neglect this very important event. Still, as we have time in hand, and if all goes well, SST can start declining , and come to neutral levels by May end. SOI indicates this favourably, as it is now gained a few notches up to +15 on April 25th.. Very very encouraging. In fact,sustained positive values of the SOI above +10 may indicate La Niña episodes.
Indicator: +ve

Really, we are still at the mercy of Nature and Providence in spite of our so called "developments."
Conclusion: As of today, there seems to be a slight delay in the Bay branch of the Monsoon. And, the Arabian Sea branch needs a little trigger to get going in May. Maybe a little less rain in the first half of May for Kerala? Arrival date should be around 1st. June for Kerala.(This Monsoon chase is my personal view, and my personal analysis and deciphering is done as per my assumptions).

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