Prediction of the Indian Monsoon. Sulochana Gadgil 4 July 2011. Traditional forecasters Nandiwallas. ‘ I seek the blessings of Lord Indra to bestow on us timely and bountiful monsoons'
4 July 2011
‘I seek the blessings of Lord Indra to bestow on us timely and bountiful monsoons'
Finance Minister PranabMukherjee, budget speech in the LokSabha, Feb 2011
Note that most of the rainfall occurs during June-September i.e. the summer monsoon season
(i) long term mean =85.24 cm;
(ii) standard deviation =10% of the mean
The Indian summer monsoon is one of the most reliable elements of the tropical climate!
ISMR <90% : Drought
ISMR> 110%: Excess rainfall season
90%< ISMR< 110% : normal monsoon
(as % of the mean); (std dev is about 10% of mean)
Monsoon of 2009 is the one of the top five most severe droughts during 1876-2009
How good are the predictions of the monsoon? Consider the experience of 2009. Monthly/seasonal anomalies of all-India rainfall (as %age of the mean)
The Indian summer
rainfall(ISMR) in 2009
was 77% of its long
i.e. deficit of 23%
As in 2009, the atmospheric and coupled models at the major global centres had failed to predict deficit rainfall during the last two droughts i.e. last severe drought of 2002 and also the drought of 2004.
occurs in association with
a planetary scale dynamical system-the tropical convergence zone (TCZ) which stretches from over the Indian monsoon zone across the Bay and thence across the equatorial Pacific.
Satellite derived Outgoing Longwave Radiation (OLR) is generally used as a proxy for rainfall in the tropics. Regions with deep convective clouds are characterized by low OLR.
Note that convection over the entire region (i.e. eq. Indian Ocean, Indian region, Arabian Sea, Bay of Bengal) is suppressed (enhanced) during El Nino (La Nina)
We use EQWIN an index of EQUINOO, defined as the negative of the anomaly of the surface zonal wind averaged over 600E-900E:2.50S-2.50N, normalized by its standard deviation.
EQUINOO involves convection anomalies of opposite signs over
WEIO and EEIO
Red blobs droughts, brown: severe droughts; Blue blobs :excess;dark blue:large excess after Gadgil et al GRL2004
1988, 1961: excess monsoon both ENSO
and EQUINOO favourable
1994 : excess monsoon with EQUINOO
favourable but ENSO unfavourable
2009 (as all severe droughts): both unfavourable
1985: drought with EQUINOO unfavourable
but ENSO favourable
Atmospheric GCMs run with observed SST
Note : correct prediction for 1994, but terrible for 2009
Reversal of SST and OLR gradients over the equatorial Indian Ocean
Cold SST anomaly and suppressed convection over EEIO
SST Threshold:27.50 C
(Gadgil , Joseph and Joshi: Nature 1984)
Note that as observed, strong positive phase of EQUINOO is predicted by GFS as well as CFS. However, while CFS predicts excess ISMR, GFS (like all AGCMs) simulates negative rainfall anomalies over the Indian region
Weaker positive EQUINOO than in CFS
Need a good assimilation of the vertical variation of temperature and salinity in the equatorial Indian Ocean besides a good model for its evolution. Only if the mixed layer is realistically incorporated, can a realistic simulation of the SST anomalies and hence IOD and EQUINOO is expected.