Has the India Meteorological Department (IMD) sought to play it safe in its latest forecast downgrade of the 2011 south-west monsoon from ‘normal' to ‘below normal'?
The IMD's monsoon forecasts employ statistical models comprising various parameters – relating to temperature, pressure and wind conditions – identified to be significantly correlated with the observed rainfall over the four-month season from June to October.
In its first forecast issued on April 19, the IMD predicted a normal monsoon based on two out of the five parameters, considered for that time, being ‘favourable'. The remaining three were deemed to be ‘neutral', which included the parameter relating to La Nina (Equatorial Pacific Warm Water Volume).
The first-stage forecast assumed that the prevailing La Nina conditions would weaken to a neutral phase by June and not transform into the dreaded El Nino, associated with monsoon failure.
El Nino and La Nina, collectively known as ENSO, basically involve unusual spurts (as in the former) or dips (as in the latter) in sea surface temperatures across the tropical Pacific Ocean.
In its forecast update released on Tuesday, the IMD, in fact, reiterated that a majority of the latest dynamical and statistical models “indicate strong probability for the present ENSO-neutral conditions to continue during the current monsoon season and the remaining part of 2011”.
Moreover, it was emphasised that in the last 11 La Nina years, there were only two instances — 1964 and 1971 — where the subsequent year (1965 and 1972) recorded an El Nino event. Therefore, there was no reason to believe that the current year might witness an El Nino just because the period from mid-August 2010 to early February saw moderate-to-strong La Nina conditions, delivering copious rainfall across the country.
About Turn
Yet, the IMD, in its latest forecast, has taken the parameter relating to El Nino (Central Pacific Sea Surface Temperature Tendency) to be ‘unfavourable'. Overall, two out of the six parameters used in the forecast update have been considered as ‘unfavourable', with three being ‘neutral' and only one ‘favourable'. These, taken together, have translated into a ‘below normal' monsoon prediction.
Besides El Nino and La Nino, the IMD has also taken note of other factors, such as the Indian Ocean Sea Surface Temperatures, seen to have a significant influence on the monsoon. Here, too, “the latest forecasts do not suggest development of either a positive or a negative Indian Ocean Dipole event during the 2011 monsoon season”.
If despite all this, the IMD has still scaled down its monsoon forecast from ‘normal' to ‘below normal', it probably indicates a preference to err on the side of caution.