Weather & Climate | VOL. 9, ISSUE 55, July-August 2009

Investigating Monsoon 2009

Q-IMD has a 133 year history of attempting to predict weather. How close are the 2009 predictions to reality?

As on 26th July, cumulative seasonal rainfall realised over the country as a whole is around 80 per cent of the normal. However, IMD expects good rainfall activity in August and a few good spells of rainfall in the remaining season can substantially change the scenario. In the past two days Northeast and Northwest regions have received very good amount of rainfall and the condition is likely to remain for a couple of days. This will definitely reduce the deficiency over the NE and NW regions.

Q-What are your interpretations of the short fall this year?

Monsoon is characterised by intra seasonal rainfall variations. This year the country received abnormally low rainfall in June, around 52 per cent of the long period average (LPA). In the past, in years 1905, 1923 and 1926, June rainfall was of the similar order.

Q-How have the upper air and surface circulations been affected in the present monsoon period?

This year a quasi stationary anticyclonic circulation prevailed over the north western parts of the country. This circulation acted as a barrier for the intrusion of monsoon current over the region. It may be mentioned that most of NW/NE regions receive rainfall, either when a low pressure system forming in the Bay of Bengal travels northwesterly, re-curves or when an extra tropical westerly system moves eastwards through the northern parts of the country and interacts with monsoon circulation. This year, so far, one intense system (depression) and a low pressure area formed in the Bay of Bengal. This may be due to unfavourable conditions (persistently descending motion) over the Bay of Bengal (probably because of systems in the west Pacific Ocean in June and development of the El-Nino). Similarly, the anticyclonic circulation over the Northwest region did not allow the westerly trough to penetrate in the region.

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Q-Are such long dry spells just an aberration common to the monsoons, or are there larger manifestations in the context of climate change?

A- Such long dry spells have occurred in the past too and may be an aberration. For example last year, the first three weeks of July witnessed very little rainfall throughout the country. Further, it may be mentioned that latest IPCC report suggests that monsoon may be slightly stronger due to global warming.

Q- We understand that tropical climate cycles are indeed turbulent and difficult to predict. However, what are the chances of facing similar situations in the near future or is the next monsoon period likely to be normal?

A- At this juncture, it would be difficult to foresee the next monsoon season. In fact all dynamical models with April-initial-conditions predicted excess rainfall over India. However, with May-initial-conditions, the same models suggest deficient monsoon rainfall.

Q- What role do the agromet services play in dissemination of information during the monsoons?

A- IMD has 130 agromet field units that issue district specific agro advisory bulletins on Tuesday and Friday. The five day forecast is disseminated through print and electronic media including mobile phones.

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Q- Is the Department confident of weather predictions during the Commonwealth Games 2010 to be held in October?

A- Yes, IMD is fully capable and is establishing the necessary technology. To provide venue and event specific now cast and short range forecast during the Commonwealth Games 2010, observational systems is proposed in and around Delhi. A meso-network of Automatic Weather Stations will eventually help the country establish a City Nowcast System for predicting high impact weather events at any location. An auto-nowcast decision support system, capable of generating nowcasts (upto 6 hours), based on Doppler weather radar and model prognosis will provide information on convective weather phenomenon. A number of visualisation products of weather forecasts and information will be also be displayed in the web site.

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