Changing monsoon pattern, influenced by global and local factors, poses a challenge for India

As monsoon withdrew from the north this year, the seasonal total rainfall was reported to be 7% above the Long-Period Average or LPA. The distribution in time and space has been erratic again with deadly floods, reduced sowing areas and crop damages, many full reservoirs, but of course severe droughts in some states. The country will again muddle through in terms of food and water security.

The persistent patterns of surpluses and deficits with a blend of extremes are now the norm for more than half a century. They need careful attention in terms of improving forecasts as well as water resource management and region-specific crop selections.

Rainfall deficits stretched from Delhi to Uttar Pradesh, Bihar, Jharkhand, Manipur and Mizoram. The shortfall in rain ranged from 20% to 45%. One of the main reasons has been that the low-pressure systems kept trekking in a nearly westward pathway rather than their normal northwestward trajectories.

These low-pressure systems, referred to as LPSs, bring up to 60% of the seasonal rainfall. While the monsoon tends to get locked into wet and dry regimes for decades with corresponding shifts in winds and low-pressure systems, the global warming monster with its many arms is pushing and pulling the monsoon in many directions. It may also be locking the monsoon into this particular pattern for much longer than two to three decades.

Two main things to note in this context are that the rainfall deficits have settled into this pattern for nearly six decades now and the drivers of monsoon circulation and rainfall distribution are global but with a local amplification. One local amplifier is the Arabian Sea warming and the wind changes it drives.

The monsoon onset occurred earlier than normal this year but June remained largely deficit over most of the country, indicating again that the onset itself is not a good indicator of the monsoon evolution or the total rainfall. The southwesterly winds or the low-level jet jumps into action as soon as the onset happens.

However, the conditions for this jump are set up from the previous winter into the pre-monsoon season. The northern Arabian Sea has warmed rapidly and monotonically over several decades thanks to the long and direct reach of the North Atlantic during winter, and even a longer and indirect reach during the pre-monsoon via the western Pacific.

This northern Arabian Sea warming has been causing the low-level jet to swing northward after the onset. Once the shift happens, more rainfall begins to occur over the northern Western Ghats and northwestern India. The heating associated with this rainfall begins to demand more moisture and the jet continues to persist in this northwardly position. This is the local amplification of a pattern that is driven from the North Atlantic and the western Pacific. El Niño and La Niña and the Arctic warming impact the monsoon on top of this shift that is occurring year after year.

The other local amplifier is that the northward shift of the low-level jet tends to create a more westward corridor for the low-pressure systems. This begins to amplify the rainfall deficits that extend from Delhi down into the Northeastern states.

Delhi, Uttar Pradesh, Bihar and Jharkhand received normal to above-normal rainfall during September. Late season extreme events are often driven by waves generated in the Arctic. The Arctic has been warming faster than most regions of the planet due to the so-called Polar Amplification.

Loss of ice and snow reduces the reflection of solar radiation back to space and the additional energy that stays melts more ice and snow. This ice-albedo feedback has been a key player during all glacial and deglacial periods during Earth’s history and continues to be a key player during global warming as well.

Parts of the Arctic experienced much warmer than normal or record warmest temperatures during the summer of 2022. In this context of global drivers, local amplifiers and the resulting uneven monsoon distribution, India has its task cut out for itself. India has to pay much more attention to longer term planning of managing the erratic monsoon distribution with shifting of crops as needed.

The cropping patterns have been locked in since the 1960s with the government’s procuring system and the public distribution system, reinforcing the anachronistic choices. Food, water and energy securities now require that a more holistic strategy be developed to deal with the monsoon shift and persistent dry and wet patterns with continued extremes and crop damages during kharif, from June to October. While the rabi cropping – sown around November and harvested around March – will benefit overall from excess monsoon rains, total crop yields have to more than compensate for the kharif losses.

The improving weather and climate forecasts have to be combined with agricultural, hydrological and energy tools for near- and long-term adaptation plans for safely navigating the future of food-water-energy nexus.

Raghu Murtugudde is a Professor of Atmospheric and Oceanic Science and Earth System Science at the University of Maryland and visiting faculty at the Indian Institute of Technology, Bombay.