Good El Nino? How a bad monsoon left Delhi drenched

Delhi had a season’s rainfall surplus of 8% in a meteorological subdivision (Haryana)

NEW DELHI: The capital defied the odds this monsoon. As of Sept 16, Delhi had a season's rainfall surplus of 8% in a meteorological subdivision (Haryana) that's reeling under a 23% deficit and a region (northwest India) that has logged a 10% shortfall.

The season's peculiar rainfall pattern indicates that a strengthening El Nino as well as climate change may have played a role in giving the capital an unlikely monsoon bounty.A look at the season's district-level rainfall map of the region shows Delhi literally at the edge of the good monsoon zone this year (see graphic). Just north of the capital lies a swathe of deficient districts starting with neighbouring Sonipat (-32%) and Rohtak (-47%) and all the way up to Hoshiarpur (-53%) and Kapurthala (-69%) in Punjab - a cluster of over 30 rain-starved districts in Haryana and Punjab as well as adjoining Rajasthan, Himachal and west UP.So how did Delhi turn into a wet pocket next to a rain-deficient region? The answer lies in the way low-pressure systems coming from Bay of Bengal shifted direction this season, probably under the influence of El Nino, and how their interactions with westerly disturbances uniquely aided rains in Delhi-NCR.

Wet capital

A majority of monsoon rainfall in northwest and central India happens under the influence of moisture-laden low-pressure systems (LPSs), including depressions, travelling inland from Bay of Bengal.

Contrary to what one would expect, the number of such systems does not decrease in an El Nino year. In fact, a 2024 study in Climate Dynamics showed that the number of Bay of Bengal depressions increased in El Nino years in the 1979-2020 period, averaging 3.9 during July-Aug versus 2.4 in normal years.This year there were 10 LPSs, including four depressions in July-Aug, against a normal of less than nine.What generally changes in an El Nino year is the direction of the LPSs.

"In El Nino years, there is subsidence of air over south India and its adjoining northern parts. This prevents formation of LPSs in the southern latitudes, say in central Bay of Bengal, which would normally move towards the north Andhra-south Odisha coast," said IMD director general Mrutyunjay Mohapatra.Here's where Delhi benefited, particularly in Aug, when as many as six LPSs, including two depressions, impacted India.

With a northward shift, most of these systems entered inland over north Odisha-Bengal and tracked north-northwest, affecting Odisha, Bengal, Jharkhand, UP, Uttarakhand and the Delhi region, with lesser impact in Rajasthan, Haryana and Punjab.

There was another factor in play, likely linked to climate change. WDs are moist wind systems moving east from the Mediterranean region that usually bring wet weather to north India in winter through early summer.

Historically, WDs are rare during the monsoons as the jet stream, which causes the disturbance, typically migrates north of the Himalayas by April-May. However, in recent years, the jet stream has persistently been retreating late, a trend scientists attribute to global warming.

This year, there were 16 WDs in the June-Aug period - eight in Aug alone, as per IMD.There were several instances of the easterly monsoon system interacting with a WD, intensifying rainfall over parts of north India, including Delhi. For instance, monsoon's onset over the capital on July 2 happened during an active WD spell. The heaviest rain spells in Delhi (July 8-9, July 29, Aug 7-8, Aug 25) occurred when both a monsoon system and a WD were simultaneously active in the region. These interactions also brought heavy rains in west UP and left a catastrophic footprint in the hills - the Bandipora (J&K) cloudburst, fatal flash floods in Chamba (HP) and forced closure of around 290 roads across HP and Uttarakhand - but don't appear to have majorly impacted Haryana and Punjab.