This is also why systems may not be intensifying into cyclones, which usually brings more rain during the season. Nevertheless, the two depressions on November 10 and 18 have already dumped 61% excess rainfall in Tamil Nadu and 59% surplus in Chennai.
Prof Raghu Murtugudde, climate expert from University of Maryland, and a visiting faculty at IIT-B, said the southwest monsoon, which brought excess rain in September and October, did not dissipate completely and the northeast monsoon, with a delayed onset, did not organise well. Normally, when the monsoon is well organised, conditions favour systems growing into cyclones. When cyclones form, strong winds cause an upwelling of ocean leading to cold sub-surface water moving to the sea surface, while warm sea surface water would have given energy to the system. So, the ocean surface takes a few days to warm again and give energy to another system. This year, the systems remained depressions. “Conditions are still favourable for more systems to form till December… What we have to watch out for is whether they’ll bring rain or grow into cyclones,” he added.
One of the reasons for the not-so-well organised northeast monsoon, Prof Murtugudde said, could be heavy southwest monsoon spells in September and October, which has a link to the melting of arctic ice, and the strong southwesterly winds not letting the northeasterly winds, a criterion for NE monsoon, get strong and streamlined.
While no single global phenomenon could infl uence a monsoon, former IMD deputy director general Y E A Raj said a strong jet stream around 12km above sea level and an intense Siberian anticyclone on the surface level could bring heavy rain. “There are also other parameters like Indian Ocean Dipole (difference in sea surface temperature between two areas). At times, these relations could change with time or intra-seasonally. A monsoon will not normally obey a single parameter. It is unclear why NE monsoon becomes active or inactive,” he said.
N Puviarasan, director, Area Cyclone Warning Centre, IMD said sea surface temperature may have remained warmer as the systems intensifi ed into depression near the coast. Hence winds it may be carrying may not have affected sea surface and the warmer sea may have continued to give energy to systems back-to-back.