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Extreme events are increasing due to global warming.
The recent heavy record deluge of rainfall over Chennai
was due to above normal sea surface temperatures in the Bay
of Bengal, presence of monsoon trough close to Chennai,
prevailing easterly waves and strong easterly winds. The 2015
El Nino phenomenon weakened South west monsoon winds
which in turn led to less churning and upwelling in the Bay of
Bengal and Arabian Sea and resulted in higher sea surface
temperatures(SSTs) due to less mixing of cold waters with
warm surface waters. This had a warming effect in Bay of
Bengal. During the post-NW monsoon withdrawal period (later
part of October and November) a monsoon trough prevailed
around 13N latitude (latitude of Chennai) over peninsular India
and Bay of Bengal.
The monsoon trough is a large area of low pressure that can
move in any direction and carry convective winds with it. The
high SSTs moistened the boundary layer (the lowest layer of
the atmosphere in touch with the ocean surface) by
evaporation. This moist air was lifted high up in the
atmosphere by easterly waves emanating some distance from
the coast at Chennai. Easterly waves are an atmospheric
phenomenon wherein the wind moves in a wave like motion
with the distance from crest to crest being as much as 2000
km (with a forward and rear sections of 1000 km each) and
the period about two to three days. ―Therefore it can be
believed that the phenomenon might have persisted for 3 -4
days with spatial scales of approximately 500-1000km,‖said
Dr. J.R. Kulkarni, Team Expert, World Meteorological
Organization, Ex-Adviser, IITM, Pune, to this Correspondent
with whom he was in telephone and email contact.
The upward motion of the winds in the wave is called
divergence and the downward, convergence. The divergence
lifted moisture bearing air with it and carried it high up into
the atmosphere as rain bearing clouds, and the easterly winds
drove these clouds towards Chennai where they precipitated
as rain. The continuous formation of clouds over Bay of
Bengal, their transformation into deep convective clouds,
their movement towards land and to Chennai city provided
continuous heavy rainfall over land and the Chennai area.
These clouds provided very heavy rainfall of the order of 150
mm/hour. If the rainfall is more than 100 mm/hour, then it is
termed as a cloud burst.
For converting water into vapour, energy is required to
provide to the water molecules. This energy is called latent
heat of vaporization. As the water vapor goes up it cools due
to ambient cooler atmosphere. It becomes saturated. The
water vapor turns into water droplets. In this process, the
energy stored in the water vapor is released to the
atmosphere. This heats the cloud air. The cloud air becomes
warm and light. The cloud air gets acceleration in upward
direction because of receiving of latent heat. Temperature in
the atmosphere decreases with height. At about 5 km above
the ground, temperature becomes 0°C. This is called freezing
level. The cloud drops freeze and become ice above freezing
level. In the freezing process heat is released which is called
latent heat of freezing. The cloud droplets and ice particles
get additional upward motion by getting this energy. This way the cloud grows to a very high altitude.
Studies have shown that south west and north east
monsoons have negative correlation. North east monsoon
during the 2009 El Nino year was stronger than normal.
Rainfall in 2009 north east monsoon was 12 per cent more than
long term normal (IMD Departmental website Chennai). This
year‘s high north east monsoon rainfall performance is
consistent with earlier findings of negative correlation between
the two monsoons in the El Nino years. The higher than normal
sea surface temperatures are the real fuel of such deep
convection. Climatologically these anomalies weaken in the
December. Therefore it may be conjectured that heavy rainfall
activity may weaken in December. Radar data are required to
check movement of clouds, their transformation into deep convective clouds, and their spatial and temporal
scales. Modeling studies are required for examining various
linkages in the proposed mechanism, said Dr. Kulkarni.
Sea surface temperatures were above normal.
Extreme weather events are indeed increasing due to
global warming. This is because a warmer climate can hold
more moisture in the atmosphere, leading to heavier rainfall
when it does occur. However, it‘s difficult to say for certain
that a particular extreme event like the Chennai floods is
attributable to anthropogenic climate change, says Dr. Roxy
Mathew Koll of The Indian Institute of Tropical Maeteorology,
Pune.
Source: www.thehindu.com
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