The APEC Climate Centre at Busan, South Korea, has extended its La Niña watch for the next 3-5 months (until August), but good tidings from this monsoon-friendly mode in the tropical Pacific could be partly undone by its near-door cousin, the negative phase of the Indian Ocean Dipole (IOD).  

The outcome is below-normal rainfall for Kerala, Tamil Nadu and Karnataka before footprint of the deficiency spreads to Rayalaseema, Telangana, Coastal Andhra Pradesh, Madhya Maharashtra, Marathwada, Goa and Konkan (practically most of the West Coast) variously in June, July and August. 

Pre-monsoon rain

The rest of the country is forecast to receive normal to above-normal rainfall before September to bring some respite for parts of the South Peninsula (but not Kerala and South Tamil Nadu), according to this outlook. The South Korean agency is, however, predicting normal rain for the country as a whole till August. It is also a fact that, except Andhra Pradesh and Telangana, the rest of the Peninsula has received excess or large excess rainfall so far in the ongoing pre-monsoon season. The quantum of excess ranges from 192 per cent in Andaman & Nicobar Islands (large excess) to 184 per cent in Lakshadweep (large excess); 110 per cent in Kerala (large excess); 102 per cent in Karnataka (large excess); and 48 per cent in Tamil Nadu and 27 per cent in Puducherry (both excess). 

Telangana (-87 per cent, large deficit) and Andhra Pradesh (-68 per cent, large deficit) are significant exceptions. But the South Korean agency expects that May and, to some extent, June would make amends and turn in normal to above-normal rain, though Kerala and parts of adjoining south Tamil Nadu may likely miss out on the gains others may make. 

Early summer heat

During the La Nina phase, surface winds (blowing from East to West) across the entire tropical Pacific are stronger than usual, and most of the tropical Pacific Ocean is cooler than average (and warmer to West). As warm water enters the nearby Indian Ocean, La Nina-induced surface wind anomalies result in large-scale anomalous heat transport in the equatorial Indian Ocean. 

As a consequence, the IOD (a coupled ocean and atmosphere phenomenon) in the equatorial Indian Ocean is also perturbed and the change in temperature gradients across the Indian Ocean results in changes in the preferred regions of rising and descending moisture and air (wet and dry conditions).

This year, heatwaves affected North and North-West India in March itself and the effect of early summer heat is being felt in Delhi, Maharashtra, Madhya Pradesh, Rajasthan and Uttar Pradesh, Lal said.