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Increasing frequency of extremely severe cyclonic storms over the Arabian Sea

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Increasing frequency of extremely severe cyclonic storms over the Arabian Sea

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Abstract

In 2014 and 2015, post-monsoon extremely severe cyclonic storms (ESCS)—defined by the WMO as tropical storms with lifetime maximum winds greater than 46 m s⁻¹—were first observed over the Arabian Sea (ARB), causing widespread damage. However, it is unknown to what extent this abrupt increase in post-monsoon ESCSs can be linked to anthropogenic warming, natural variability, or stochastic behaviour. Here, using a suite of high-resolution global coupled model experiments that accurately simulate the climatological distribution of ESCSs, we show that anthropogenic forcing has likely increased the probability of late-season ECSCs occurring in the ARB since the preindustrial era. However, the specific timing of observed late-season ESCSs in 2014 and 2015 was likely due to stochastic processes. It is further shown that natural variability played a minimal role in the observed increase of ESCSs. Thus, continued anthropogenic forcing will further amplify the risk of cyclones in the ARB, with corresponding socio-economic implications.
Letters
https://doi.org/10.1038/s41558-017-0008-6
1National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA. 2Atmospheric and Oceanic Sciences
Program, Princeton University, Princeton, NJ, USA. 3Department of Geosciences, Princeton University, Princeton, NJ, USA. 4Princeton Environmental
Institute, Princeton University, Princeton, NJ, USA. *e-mail: hir.murakami@gmail.com
In 2014 and 2015, post-monsoon extremely severe cyclonic
storms (ESCS)—defined by the WMO as tropical storms with
lifetime maximum winds greater than 46 m s1—were first
observed over the Arabian Sea (ARB), causing widespread
damage. However, it is unknown to what extent this abrupt
increase in post-monsoon ESCSs can be linked to anthropo-
genic warming, natural variability, or stochastic behaviour.
Here, using a suite of high-resolution global coupled model
experiments that accurately simulate the climatological dis-
tribution of ESCSs, we show that anthropogenic forcing has
likely increased the probability of late-season ECSCs occur-
ring in the ARB since the preindustrial era. However, the
specific timing of observed late-season ESCSs in 2014 and
2015 was likely due to stochastic processes. It is further
shown that natural variability played a minimal role in the
observed increase of ESCSs. Thus, continued anthropogenic
forcing will further amplify the risk of cyclones in the ARB,
with corresponding socio-economic implications.
In 2014, Cyclone Nilofar, the first extremely severe cyclonic storm
(ESCS (tropical storms with lifetime maximum winds greater than
46 m s1 (WMO1)), was recorded in the Arabian Sea (ARB) (west
of 77.5°E in the North Indian Ocean) during the post-monsoon
season (October–December) (Fig.1a). In the following year, two
more ESCSs (Cyclones Chapala and Megh) were observed during
the post-monsoon season in the ARB (Fig.1a), causing widespread
damage2. This was the first instance that more than one ESCS was
observed within one year in the ARB (Fig.1b). These recent severe
tropical storms in the ARB have attracted considerable attention
from the scientific community, as well as broader society, in terms
of the extent to which they were made more likely by anthropo-
genic forcing, as opposed to intrinsic natural variability. A recent
study3 reported that the increase in anthropogenic black carbon
and sulfate emissions might have led to the increase in mean storm
intensity in the ARB through a weakening of vertical wind shear (Vs,
wind speed difference between the upper and lower troposphere),
especially during the pre-monsoon season of April–June. The sug-
gested physical mechanism behind this change is that the observed
increase in anthropogenic aerosols in the lower troposphere leads
to a reduction in surface insolation in the North Indian Ocean,
which in turn leads to a decrease in the meridional gradient of sea
surface temperature (SST). This decreased meridional gradient fur-
ther leads to a weakening of the South Asian Monsoon circulation
through the thermal wind relationship, which causes a weakening of
Vs. On the other hand, another study4 argued that the recent increase
in pre-monsoon tropical storm intensity in the ARB is mainly being
caused by an earlier onset of the South Asian Monsoon, affected
by a reversal in the phase of the Pacific Decadal Oscillation (PDO)
around 1997. Overall, consensus has not been reached regarding
the main cause of the recent increase in pre-monsoon storm inten-
sity in the ARB. Alongside this debate, the recent unprecedented
Increasing frequency of extremely severe cyclonic
storms over the Arabian Sea
Hiroyuki Murakami 1,2*, Gabriel A. Vecchi 3,4 and Seth Underwood1
ESCSs and SST trend (Oct–Dec)
Number of ESCSs in the ARB
Nilofar (25–31 Oct 2014)
Chapala (28 Oct–4 Nov 2015)
Megh (5–10 Nov 2015)
30° N
20° N
10° N
40° E
0.1
1
1
1
1
1
1
11
1998
b
a
1999
2000
2001
2002
2003
2004
2005
Year
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
Jan
Feb
Mar
Apr
May
Jun
Month
Jul
Aug
Sep
Oct
Nov
Dec
0.2 0.3
0.4
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0.4 0.5 0.60.7 0.
80.9
50° E6E70° E
Fig. 1 | Observed ESCSs. a, Observed ESCSs (Nilofar (blue), Chapala
(light blue) and Megh (black)) during the post-monsoon season in 2014
and 2015, along with the observed linear trend in SST (Kelvin per 50 years,
shading). b, Observed number of ESCSs over the ARB for each month for
the period 1998–2016.
© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
NATURE CLIMATE CHANGE | VOL 7 | DECEMBER 2017 | 885–889 | www.nature.com/natureclimatechange 885
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