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Increased sea surface temperature affecting Indian monsoon: Study

New Delhi, Oct. 02 (India Science Wire): The agricultural sector plays a key role in the
economy of India and twenty-five other East Asian developing countries across a
distance of 18,000 km (from east to west) and 6,000 km (from south to north). The main
influencing factor in Indian agriculture is the southwest monsoon season from June to
September caused by the cross-equatorial winds that blow from a southwesterly
direction. The source of this wind is located more than 4,000 km away from India near
the Mascarene Islands in the Southern Indian Ocean (SIO). The state of agriculture that
includes forestry, hunting, fishing as well the cultivation of crops and livestock
production is determined by this southwest monsoon. The climate across the world is
however changing in many ways triggered by different global warming phenomena
resulting in a change in the monsoon onset time and pattern which is badly affecting the
agricultural yields as most of the important crops are seasonal and their water demand is
mainly met by the water received from rainfall. Any excess or little rainfall is therefore
detrimental to these crops.
The Mascarene Islands is a group of islands in the Indian Ocean east of Madagascar from
where the cross equatorial winds blow to India. It is also called the Indian Ocean
subtropical high, which is a high-pressure area located between 20° to 35° South latitude
and 40° to 90° East longitude. The southwest monsoon caused by this high pressure area
is the strongest component of the Indian subcontinent monsoon that contributes about
more than 80% of the annual rainfall in entire East Asia. It is also the principal water
supply source for more than a billion people in this region. Many climate modes /factors
modulate the monsoon and one of the factors is Mascarene high influence.

Now, a new study on variability in the Mascarene High (MH) during global warming
hiatus (GWH) led by National Centre for Polar and Ocean Research Goa (NCPOR Goa)
revealed that the MH region experienced significantly increased Sea Surface
Temperature (SST) during this period. A global warming hiatus is referred to as a global
warming pause or a global warming slowdown, which is a period of relatively little
change in globally averaged surface temperatures but this can result in the increase in
the SST. This warming in SST resulted in decrease in the pressure gradient between the

MH and the Indian landmass, which in turn suppresses the intensity of low-level cross-
equatorial winds over the western Indian Ocean affecting the onset of the monsoon over
the Indian subcontinent and rainfall over East Asia.

These are alarming findings for a country whose food production and economy depend
heavily on monsoon rainfall. As per NCPOR scientists, the weakening of the MH in the
Southern Indian Ocean during GWH may affect the strength of the upwelling along the
coast of Somalia and Oman and thus influence the Arabian Sea ecosystem. During the
study, it was found that increased sea level and heat content in the MH region during the
GWH (1998-2016) suppressed the intensity of low-level cross-equatorial winds.

NCPOR scientists have also investigated the recent warming trends in MH during GWH
period and found a significant positive trend in SST, sea surface height (SSH) and oceanic
heat content (OHC). Out of which, SST impacted more on the monsoon with rainfall

during the last 18 years. Scientists, therefore, call for urgent steps to reduce greenhouse
gas emission to avert the associated risk of climate change effect, including killer
heatwave and stop flooding from snow and glacier melting caused by rising
temperatures.
Led by Vidya P. J. the research team comprised of Ravichandran, M. Subeesh M. P.,
Chatterjee S., and Nuncio M. of NCPOR, Goa, Ministry of Earth Sciences. The research
paper is published in the Scientific Report of the ‘Nature’ journal. (India Science Wire)
VS/MoES/MFA/02.10.2020
Keywords: Mascarene High (MH), Southern Indian Ocean, Global Warming Hiatus, sea
surface temperature, southern Monsoon, Arabian Sea ecosystem, NCPOR

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