Meteorology and Atmospheric Physics
Investigation on the Bay of Bengal branch of summer monsoon
during normal and delayed onset over Gangetic West Bengal
· Paramita Mondal
· Poulomi Saha
· Sutapa Chaudhuri
Received: 19 September 2017 / Accepted: 29 May 2018
© Springer-Verlag GmbH Austria, part of Springer Nature 2018
The regional features of Bay of Bengal (BOB) branch of summer monsoon (SM) are examined to identify the causes of
delayed onset over Gangetic West Bengal (GWB) in the years having normal onset over Kerala coast. The normal onset
over both GWB and Kerala is designated as Normal–Normal (NN) years, while delayed onset over GWB and normal onset
over Kerala is termed Normal–Delayed (ND) years. The temperature gradient (TTg), winds at 850 and 150 hPa pressure
levels, sea-surface temperature (SST), outgoing long wave radiation (OLR), low-level moisture convergence, instability,
and rainfall rate (RR) are analyzed in this study using National Centers for Environmental Prediction and National Oceanic
and Atmospheric Administration reanalysis dataset during the period from 1981 to 2015. The result shows that TTg over
BOB plays a signiﬁcant role in controlling the movement of BOB branch of SM. Warm SST is observed to prevail over
north BOB during NN years. The divergence at 150 hPa and convergence at 850 hPa pressure levels are found to inﬂuence
the propagation of BOB branch of SM during both NN and ND years. The winds at 850 hPa level converge over BOB and
GWB during NN years, whereas winds converge more over eastern BOB and Indo-Chinese peninsula during ND years.
Result depicts abundance of low-level (850–1000 hPa) moisture over eastern BOB and Indo-Chinese peninsula during ND
years, whereas moisture is observed to converge over north and north-eastern BOB during NN years. The RR is observed
to be slightly higher during NN than ND years. However, it may not be concluded from the analysis that delayed onset over
GWB will be responsible for less RR over the study region.
The onset of summer monsoon (SM) is of immense signiﬁ-
cance over Indian states. The normal, advanced, and delayed
onset of SM rain has colossal impact on Indian agriculture
and economy (Chaudhuri et al. 2016).
According to Soman and Kumar (1993), the onset of
monsoon is recognized as a rapid increase and characteristic
persistency of rainfall when the rainfall amount increases
from 5 to 15 mm/day.
The southern tip of Indian coast (Kerala) receives the
ﬁrst onset of SM around 1st June. The progress of SM may
be divided into two branches: Arabian Sea (AS) branch and
Bay of Bengal (BOB) branch. BOB branch propagates north-
wards into central BOB and reaches Assam by ﬁrst week
of June and subsequently deﬂected by southern barrier of
Himalaya and progresses westwards (Das 1968). This branch
then propagates towards Gangetic plain rather than Myan-
mar (Pai and Rajeevan 2009).
The investigation on BOB branch of SM is pertinent in
this study to understand the reason of variability in the onset
over Gangetic West Bengal (GWB). The study might help
in deciding the time of harvesting. The normal onset of SM
over GWB is around 10th June. However, there are occa-
sions when BOB branch takes longer time to reach GWB
even with a standard deviation of 8 days with normal onset.
Various scientists have investigated on dynamic and ther-
modynamic characteristics of Asian SM (ASM) to predict
the onset and its variability. The climatology of SM onset
Responsible Editor: A.-P. Dimri.
* Sutapa Chaudhuri
Department of Atmospheric Science, University of Calcutta,
51/2, Hazra Road, Kolkata 700 019, India