The Hindu Business Line : `Kerala not reliable choice for monsoon variability studies'

TWO leading US-based academicians have expressed doubts whether the choice of rainfall in a single Indian State is adequate to characterise inter-annual variability in monsoon transitions, which occur on a planetary scale.

According to Dr John Fasullo and Dr P.J. Webster of the University of Colorado, rainfall variability in Kerala, which is on the order of 200 km in breadth, has come to be arbitrarily used in an effort to simplify the determination of the transitions amid the complexity of the monsoon's large-scale evolution.

Based on Kerala rainfall, the mean onset date occurs near June 1 and varies with a standard deviation of eight to nine days from year to year while withdrawal occurs in early October.

The onset and withdrawal occur in many stages and represent significant transitions in the large-scale atmospheric and ocean circulations in the Indo-Pacific region. While there are no widely accepted definitions of these monsoon transitions, at the surface the onset is recognised as a rapid, substantial and sustained increase in rainfall over a large scale while the withdrawal marks the return to dry, quiescent conditions.

During any single year, the initiation of rainfall at a single location can be very different from the climatological pattern. In general, however, the first rains of the monsoon occur over Burma and Thailand in mid-May and extend subsequently to the North-West, so that by mid-June, rains have advanced over most of India and Pakistan. Near India, the onset occurs initially across the peninsula's southern tip in early June, progressing north-westward across most of the country in the following months.

The northward progression of the monsoon onset is symptomatic of a large-scale transition of deep convection from the equatorial to continental regions, say the academicians. The seasonal monsoon transitions can unfold in a variety of ways with abrupt, gradual or multiple transitions occurring in various years that together encompass different timings and spatial patterns.

The suddenness of rainfall fluctuations during the monsoon transitions is well established. In Kerala, the rapid, intense and sustained transition signalled by the onset is clear as rainfall increases significantly in less than five days and sustains for almost three months. The rapid transition results, in part, from instabilities that develop in the ocean-atmospheric system during springtime.

However, traditional methods of defining onset also guarantee a marked increase in rainfall. While these methods assume that variability in rainfall in Kerala during June is dictated by the monsoon transition, the relative importance of local synoptic variability, which may or may not be related to monsoon, and the climate-scale transition marking the onset are not known.

The monsoon transitions occur due to large-scale interactions between surface heating and atmospheric, dynamic, thermal, and hydrological processes. However, the extent to which rainfall in Kerala during these transitions is determined by synoptic variability unrelated to monsoon transitions is not well established. Moreover, given Kerala's relative small scales, sensitivity of any onset or withdrawal declaration solely based on the State's rainfall to spatial intricacies in monsoon transitions is also likely to be large.

More Stories on : Climate & Weather | Kerala

Article E-Mail :: Comment :: Syndication :: Printer Friendly Page

Stories in this Section
`Kerala not reliable choice for monsoon variability studies'

NEG Micon unveils `largest' wind turbine

Inflation to remain `below 5 per cent'

Garbage trucks soon with GPS/GIS equipment

CII agenda to boost trade ties with GCC

Tax collections cross Budget estimates — On target nearly after a decade, says Finance Secretary

TRAI issues 2nd consultation paper on CAS

France sees strong market for Indian automotives

Kerala film industry in soup

`Velugu' project for lac extraction yields good returns in AP

In Thiruvananthapuram today

Hardware exports up 66 pc in Dec