Scientists still clueless on monsoon

As the south west monsoon enters a break period threatening states with rain shortage, weather scientists admit they cannot forecast the duration of the break as physical processes underlying monsoon’s variability are poorly understood.

“Causes of the break period are not known. We also cannot say with certainty how long it will last. The breaks can happen due to anything from temperature variability in April to appearance of typhoons in north west Pacific, which has adverse relations with Indian monsoon,” a seasoned weather forecaster told Deccan Herald.

An analysis of half a century of India’s monsoon data by researchers from the Indian Meteorological Department (IMD) and the Indian Institute of Science suggest that in almost one-third of cases, the break period lasts between 7 and 10 days. Only in 40 per cent cases it is of 4 days duration. Also occurrence of the break period is quite common in July and August as well.

On Friday, the IMD confirmed that the south west monsoon is entering a break period and there is little chance of revival in the first week of July, even though there may be some sporadic rain.

IMD data suggests that close to 86 per cent of Indian land mass had received either “scanty or deficient” rainfall since June 1. The overall shortage in monsoon for the country stood at 29 per cent.

The situation may not improve much in the next 48 hours when monsoon will be restricted only in the coastal areas and north eastern states. For the parched north west, there is no respite with the temperature set to cross the 44 degrees mark in the next two days.

A recent study which reviews 100 scientific papers on monsoon suggests that physical processes governing intra-seasonal breaks or extreme rainfall events are poorly understood because of which formation of breaks or heavy rainfall events on a smaller time scale cannot be predicted.

Published in the June 25 issue of Nature Climate Change, the study says regional projections for devastating droughts and floods — which are most meaningful for residents living in South Asia — are still beyond the reach of current climate models.

The study, however, suggests that South Asia can expect more rainfall, due to the increase in atmospheric moisture along with a rise in atmospheric carbon dioxide concentration of about 70 parts per million by volume and an increase in temperatures of about 0.5 degrees Celsius over the last 6 decades.  

Explaining why All India Rainfall index does not yet show the expected increase in rainfall, two reviewers — Andrew Turner from the University of Reading and H Annamalai from the University of Hawaii — cited inconsistent rainfall observations, decadal variability of the monsoon, effects of aerosols resulting from industrialisation and land-use changes as the probable causes, without pin-pointing at the culprit.