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How the monsoons have been propped up by the oceans and atmosphere

Vinson Kurian THIRUVANANTHAPURAM | Updated on October 22, 2019 Published on October 22, 2019

The western basin of the Indian Ocean warms up, creating lower pressure, and the rising warm air forms clouds, to go on to set up storms and heavy rain. File photo   -  K_R_DEEPAK

The ongoing frenetic run of the North-East monsoon over the South Peninsula has been supported by the same factors that propelled the preceding South-West monsoon during June-September.

The seas and the ocean have combined well to put the monsoon on song, according to meteorologists watching the proceedings.

MONSOON ENSEMBLE

The larger monsoon ensemble saw a 'neutral' Pacific (neither El Nino or La Nina) along with a positive Indian Ocean Dipole (IOD) and a positive Equatorial Indian Ocean Oscillation (EQUINOO) combine to ensure a robust monsoon season.

 

Given its location, the Indian Ocean has had a more obvious and immediate impact than the far-away Pacific, especially this year with a positive IOD and EQUINOO.

Not only the ocean (a positive IOD) but also the immediate atmosphere (EQUINOO) have acted in tandem to power the monsoon into a surplus during June-September and even into October-November.

In a positive IOD phase, the western basin of the Indian Ocean warms up, creating lower pressure, and the rising warm air forms clouds, to go on to set up storms and heavy rain.

 

The positive EQUINOO phase translates more or less into a positive IOD being replicated in the atmosphere - enhanced cloud formation in the western basin of the Indian Ocean, as the sea-surface temperatures rises to a threshold limit of 27.5 deg Celsius.

A study carried out by scientists at the Indian Institute of Science in Bengaluru with the Indian National Centre for Ocean Information Services, Hyderabad, and published recently in the peer-reviewed journal Current Science, bears this out.

According to the authors, the rainfall deficit in June (the first month of the South-West monsoon) can be attributed to the weak El Nino conditions over the Pacific Ocean.

SEA-SURFACE TEMPERATURES

An El Nino, in which the Equatorial East Pacific warms up relative to the West (note that it is the variation in sea-surface temperatures that drives both the Indian Ocean and Pacific phenomena), has been found to have an adverse impact on the Indian monsoon.

There is no bigger stage for it to unfold with the vast ocean, which covers one-third of the planet, and the atmosphere combining to send out associated bad weather half a world away and even beyond.

El Nino is a phenomenon in the equatorial Pacific, in which sea-surface temperatures rise over a threshold of +0.5 degree Celsius (and cools by the same margin during alter ego La Nina). These are averaged over five, three-month sessionsacross a stretch of water designated as the Nino 3.4 region (see graphic), to arrive at the Oceanic Nino Index (ONI).

But what perplexes scientists and researchers is that no direct correlation between the ENSO events and the monsoon has been established yet. From 1950 to 2012, there were 16 La Nina years, with the monsoon rains ending up above or around average nearly every time.

El Nino caused five droughts during this period, but on 14 other occasions, the ensuing monsoon ranged from well below-average, average, or even above-average.

To top it all, the 1997-98 El Nino, among the century’s strongest, far from heralding a drought, triggered above-average rain.

Likewise, 2002 proved to be one of the driest monsoons, despite it being a weak to moderate El Nino year. It only helped bust another myth: the strength of an individual El Nino event may not necessarily have its imprint on monsoon performance.

EQUINOO TAKES OVER

Reverting back to this year, researchers feel that as the El Nino impact on the monsoon dissipated in July, the EQUINOO took over, driving the South-West monsoon to an above-normal rainfall.

Since the warmer-than-normal sea surface temperature in the western basin of the Indian Ocean also means a positive IOD, the EQUINOO is considered as an atmospheric component of IOD.

According to Jason Nicholls, Lead International Forecaster and Senior Meterolologist with leading US-based private forecaster AccuWeather, the positive EQUINOO brought on by the strong positive could be good news for the North-East monsoon.

By the same token, the positive IOD will keep much of South and East Australia drier than normal into November, at least. Another stand-out feature of the South-West monsoon was that there were hardly any 'breaks', which inevitably occur after the monsoon runs a productive phase into July and August.

It is during these 'break' phenomena, when the monsoon pauses over large parts of the South and West, that the East Coast and foothills of the Himalayas get their due share of rainfall.

But the absence of these breaks also meant that these regions did not get their share, which is the reason why they ended up in deficit, even as the rest of the country, especially to the West, received a rainfall bounty.

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Published on October 22, 2019
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