Role of upper ocean parameters in the genesis, intensification and tracks of cyclones over the Bay of Bengal

Abstract

It is well known that the frequency of cyclones is about four times higher over the Bay of Bengal when compared with the Arabian Sea. Most of the severe cyclones during pre-monsoon (April and May) and post-monsoon (October and November) seasons hit the east coast of India, causing colossal loss of life and damage to property. In this study an attempt has been made to show the importance of the upper ocean parameters known as the upper ocean heat content (UOHC) and the UOHC with stratification (UOHCS). The UOHC has been computed considering the stratification parameter (S) for the first time. Most of the severe cyclones are forming over the UOHC range between 40–80 kj/cm2 in the Bay of Bengal. The UOHCS was high ranging from 50–400 kj/cm2 compared to the Pacific Ocean, which is due to high stratification (S ∼2–4). Climatology of cyclone tracks and the UOHCS and in situ observation from Argo suggest that most of the tracks in the pre- and post-monsoon seasons are influenced by the high UOHCS (>120 kj/cm2). UOHC and S are the dominant factors during the pre- and post-monsoon seasons, respectively. In addition to the atmospheric parameters from ocean-side, UOHC and stratification appear to be the best parameters to examine the intensification and movement of the cyclones during the pre- and post-monsoon seasons over the Bay of Bengal.

Full text

Role of upper ocean parameters in the genesis, intensification and tracks of cyclones over the Bay
of Bengal
K. Maneesha
a
*, Y. Sadhuram
a
and K.V.S.R. Prasad
b
a
CSIR –National Institute of Oceanography, Regional Centre Physical Oceanography Division, 176, Lawsons Bay Colony,
Visakhapatnam, India;
b
Department of Meteorology & Oceanography, Andhra University, Visakhapatnam, India
It is well known that the frequency of cyclones is about four times higher over the Bay of Bengal when compared with the
Arabian Sea. Most of the severe cyclones during pre-monsoon (April and May) and post-monsoon (October and November)
seasons hit the east coast of India, causing colossal loss of life and damage to property. In this study an attempt has been made
to show the importance of the upper ocean parameters known as the upper ocean heat content (UOHC) and the UOHC with
stratification (UOHC
S
). The UOHC has been computed considering the stratification parameter (S) for the first time. Most of
the severe cyclones are forming over the UOHC range between 40–80 kj/cm
2
in the Bay of Bengal. The UOHC
S
was high
ranging from 50–400 kj/cm
2
compared to the Pacific Ocean, which is due to high stratification (S ∼2–4). Climatology of
cyclone tracks and the UOHC
S
and in situ observation from Argo suggest that most of the tracks in the pre- and post-
monsoon seasons are influenced by the high UOHC
S
(>120 kj/cm
2
). UOHC and S are the dominant factors during the
pre- and post-monsoon seasons, respectively. In addition to the atmospheric parameters from ocean-side, UOHC and
stratification appear to be the best parameters to examine the intensification and movement of the cyclones during the pre-
and post-monsoon seasons over the Bay of Bengal.
1. Introduction
It is a well-known fact that the frequency of cyclones is
about four times higher over the Bay of Bengal when com-
pared with the Arabian Sea. Most of the severe cyclones
during pre-monsoon (April and May) and post-monsoon
(October and November) seasons generally move towards
the east coast of India, causing colossal loss of life and
damage to property. A few cyclones move either north-
wards or northeast and hit the Bangladesh or Myanmar
coasts (Sadhuram et al. 2004). The essential conditions
for the formation of cyclones are: (1) Sufficient ocean
thermal energy, (2) enhanced mid-troposphere relative
humidity, (3) conditional instability, (4) enhanced lower
troposphere relative vorticity, (5) minimum weak vertical
shear and (6) displacement at least 5 deg away from the
Equator (Gray 1968). Some recent studies on cyclones indi-
cate that the shear should be less during the formation
stage, while shear greater than 20 m/sec helps in the inten-
sification provided that all the other conditions are favour-
able (Zeng et al. 2008). In addition to the favourable
atmospheric conditions, without a sufficient flux supply
from the ocean it is not possible for intensification to take
place (Byers 1944; Emanuel 1986; Lin et al. 2009).
It is now known that the vertical thermal structure and
the upper ocean heat content (UOHC, referred to as tropical
cyclone heat potential or cyclone heat potential in some
earlier studies, with an integrated vertical temperature
from surface to depth of 26°C isotherm) play a vital role
in the intensification of storms rather than Sea Surface
Temperature (SST) (Shay et al. 2000; Goni & Trinanes
2003; Lin, Liu, Wu, Chiang & Sui 2003; Lin, Liu, Wu,
Wong et al. 2003b; Lin et al. 2005; Scharroo et al. 2005;
Pun et al. 2007). Hurricane Bret intensified when it
moved over the region of high heat potential (>90 kj/cm
2
)
in the western Gulf of Mexico (Goni et al. 2003,2009;
Shay et al. 2000). Further, Hurricanes Igor (tropical
Atlantic) and Celia (Eastern North Pacific), Typhoon
Megi (Western North Pacific) and Cyclone Phet
(Arabian Sea) intensified during 2010 over areas of
high UOHC. Lin et al. (2008) showed that Hurricanes
Katrina (2005), Rita (2005) and Mitch (1998) and
Super Typhoon Maemi (2003) also intensified over
regions of high UOHC. Lin et al. (2009) also observed
that Cyclone Nargis suddenly intensified when it
entered into a region of high UOHC (77–105 kj/cm
2
).
Though UOHC variations in the Atlantic are small, the
inclusion of UOHC in the Statistical Hurricane Intensity
Prediction Scheme (SHIPS) forecast model reduced the
forecast errors up to 5% for all cases of category 5
cyclones and up to 20% for other cyclones (Mainelli
et al. 2008).
In the Bay of Bengal, seasonal variability of UOHC
was reported by Sarma et al. (1990). The UOHC was
© 2015 Institute of Marine Engineering, Science & Technology
*Corresponding author. Email: kkpalli_manisha@yahoo.com
Journal of Operational Oceanography, 2015
Vol. 8, No. 2, 133–146, http://dx.doi.org/10.1080/1755876X.2015.1087185

found to be > 60 kj/cm
2
in the Andaman Sea, southern and
central Bay of Bengal, where generally the cyclones gen-
erate and move during the post-monsoon (October and
November) season (Sadhuram et al. 2004). The relation-
ship between the UOHC and the efficiency of intensifica-
tion (EI, ratio between the severe storms and the total
number of storms in a grid) under different seasons was
examined by Sadhuram et al. (2006). The UOHC was
found to be above 70 kj/cm
2
in the potential zones (5
deg N to 12 deg N post-monsoon; north of 15 deg N
summer monsoon) of cyclogenesis in the Bay of Bengal
(Kumar & Chakraborthy 2011). The importance of the
UOHC in the translation speed of the storms in the Bay
of Bengal was studied by Sadhuram et al. (2010). Goni
et al. (2010) reported that there was a decrease in
UOHC during 2009/10 in the Gulf of Mexico and the
southwestern Pacific Ocean, while there was an increase
in the western Pacific Ocean, Arabian Sea and Bay of
Bengal.
All the above studies emphasize the importance of the
UOHC in the genesis and intensification of storms.
Though the Bay of Bengal is a highly stratified ocean,
the importance of stratification while estimating the
UOHC has not been considered so far.
1.1. Importance of stratification in the Bay of Bengal
A recent study (Shay & Brewster 2010) highlighted the
importance of the stratification parameter (S) in addition
to the UOHC in the East Pacific Ocean. The authors
observed that UOHC with stratification (UOHC
S
) was
high along the hurricane passage areas in the East Pacific.
However, the Bay of Bengal is highly stratified due to a
high influx of fresh water, and thus study of UOHCs is
very important but such studies are not done so far over
this region.
The Bay of Bengal (north-eastern Indian Ocean) is
bounded by land mass except in the south and covers an
area of 2.2 × 10
6
km
2
(Lafond 1966). The winds blow
from the northeast from November to February, southerly
from March to May, and southwest from June to October
(Varkey et al. 1996). Several major river systems –the
Ganges-Brahmaputra, the Irrawadi-Salwan and the
Krishna-Godavari –drain into the Bay of Bengal. The
total run-off from the peninsular rivers, which peaks
during the summer monsoon (June to September)
amounts to 2.95 × 10
12
m
3
/yr (Sengupta et al. 2006). This
huge river run-off and excess precipitation (Prasad 1997)
results in low-salinity surface waters. The weak winds
coupled with the low salinity makes the Bay of Bengal a
Figure 1. Monthly distribution of climatological wind shear (m/s, 200–850 mb) and the genesis locations of cyclones in the Bay of Bengal
from 1981 to 2008.
134 K. Maneesha et al.

highly-stratified basin. The static stability of the water
column is three to four times higher than the Arabian Sea
(Prasannakumar et al. 2002). The near-surface stratified
layer plays a major role in the genesis of cyclones in the
Bay of Bengal (Murty et al. 2000). Hence, in this present
study, the UOHC is computed including the stratification
parameter (S; Shay & Brewster 2010) which has not been
reported on before for the Bay of Bengal. The role of
UOHC and UOHC
S
in the genesis and intensification of
storms in the Bay of Bengal is thus addressed in this study.
2. Data and methods
UOHC and UOHC
S
and the stratification parameter (S) in
the Bay of Bengal are computed from the following
equations.
UOHC =
r
CPD26
0
(T−26)dZ (1)
where ρis the density of water column above 26°C iso-
therm, C
P
is the specific heat of seawater at constant
pressure, Tis the average temperature of two consecutive
layers of a depth increment dZ and D26 is the depth of
the 26°C isotherm (Sarma et al. 1990; Sadhuram et al.
2004).
UOHC
S
is computed following the method of Shay and
Brewster (2010):
UOHCS=UOHC∗S(2)
S is calculated from the equation
S=
Nmax
N0
,(3)
where N
0
is the reference buoyancy frequency and N
max
is the maximum value of the Brunt Vaisala frequency,
Where N=
−g
r
d
r
dZ
(4)
World Ocean Atlas-2009 (WOA09) Data (Antonov et al.
2010; Locarnini et al. 2010) is available from the National
Oceanic and Atmospheric Administration (NOAA)
website. A 1 × 1 grid was used in the computations. The
data on cyclone tracks were taken from the Cyclone Atlas
2007 of the Indian Meteorological Department.
Figure 2. Monthly distribution of climatological absolute vorticity (s
−1
, 850 mb) and the genesis locations of cyclones in the Bay of
Bengal from 1981 to 2008.
Journal of Operational Oceanography 135

Daily Argo data was taken from the Coriolis Global
Data Center. Daily atmospheric data was taken from
National Centers for Environmental Prediction/National
Center for Atmospheric Research (NCEP-NCOAR) reana-
lysis data.
3. Results and discussion
3.1. Atmospheric parameters
In the present study three parameters –namely, wind shear,
vorticity and relative humidity –are studied with respect to
their role in the genesis of cyclones. Figure 1 shows that
most of the genesis locations occurred over the regions
where the wind shear is less than 10 m/sec. Wind shear is
found to be greater than 23 m/sec in the month of Decem-
ber over the northern part of Bay of Bengal. Figure 2 shows
that most of the genesis locations occurred in the regions
where the relative vorticity is above 2 × 10
−05
S
−1
.Figure
3shows that the relative humidity varies from 20 to 70%
over the Bay of Bengal, and most of the cyclones form
under conditions where the relative humidity is greater
than 40%. After the genesis, studies on the intensification
and movement of cyclones are essential for the proper fore-
cast mechanism. Hence there is a need to study the ocean
parameters which play a major role in the intensification
of cyclones.
3.2. Daily atmospheric data during different
cyclones (case studies)
Daily atmospheric data (Figures 4–6) during different
cyclones shows that near the formation locations there is
a low shear, high humidity and sufficient relative vorticity.
Wind shear was almost less than 10 m/s along the track in
all the cases (Figure 4). Minimum relative vorticity was
also maintained along the track (Figure 5). After formation,
in some cases cyclones are sustained even under conditions
of low humidity (Figure 6). So, these atmospheric par-
ameters are necessary for the formation and sustainability
of the cyclones, but they are not the sole causal factors.
The other major factors that affect the cyclones originate
in the ocean itself; thus, we need to study the ocean par-
ameters that influence cyclones.
3.3. Upper ocean heat content (UOHC)
In general, the UOHC in the Bay of Bengal is found to vary
from 20–130 kj/cm
2
and most of the cyclones have their
genesis where the UOHC is greater than 40 kj/cm
2
. From
Figure 3. Monthly distribution of climatological relative humidity (%, 600 mb) and the genesis locations of cyclones in the Bay of Bengal
from 1981 to 2008.
136 K. Maneesha et al.

January to March, the north and western Bay of Bengal has low
(<30 kj/cm
2
) UOHC, while in May high (>90 kj/cm
2
)values
are observed in the western and central bay, with a peak
(>110 kj/m
2
). In the post-monsoon season (October and
November), UOHC greater than 70 kj/cm
2
is observed in the
north-eastern area of the bay. Most of the genesis locations
are found to coincide with high UOHC values (Figure 7)and
similar observations have been reported by Sadhuram et al.
(2004), who found that the UOHC was greater than 60 kj/
cm
2
in the eastern part of the bay during post-monsoon
season, which is close to the present results. It is observed
that in the month of May cyclones intensify in the western
bay, where the UOHC is >110 kj/cm
2
, whereas in the post-
monsoon season most of the cyclones intensify near the
coast. From these results it is inferred that a threshold value
of about 40 kj/cm
2
is necessary for the genesis and intensifica-
tion of storms in the Bay of Bengal.
The intensification and dissipation of the cyclones
mostly takes place in the north and west of the Bay of
Bengal, where high UOHC values persist in the months
of April and October, then low valuese of UOHC during
November and December (Figure 8).
3.4. Stratification parameter (S)
In the present study, the stratification parameter (S) is
calculated from Equation (3) using the maximum Brunt
Vaisala frequency (N
max
) and the reference frequency
(N
o
; Shay & Brewster 2010). Typical vertical structures
of the Brunt Vaisala frequency at four different locations
in the Bay of Bengal are shown in Figure 9.N
max
is high
(29 cph) at a depth of 25 m in the eastern area of the bay,
and low (20 cph) in the western area. It is above 22 cph
in the northern and southern areas. Stratification (S)
values in the Bay of Bengal are found to vary between
2 and 5; they are high in the northern area (S>4),
which extends to the south along the coast in the
eastern area. From June to December there is high
stratification (S>3) in the northern and central areas of
the Bay of Bengal. High stratification (S>3) is also
observed in the eastern and western areas during the
post-monsoon months due to high river discharge
during the summer monsoon season. It was found that
most of the cyclones are generating where S is greater
than 2.8, except during December. The maximum
number of cyclones form during October, when high S
Figure 4. Distribution of climatological wind shear (m/s, 200–850 mb) and the tracks of cyclones in the Bay of Bengal from 2007
to 2013.
Journal of Operational Oceanography 137

Figure 5. Distribution of climatological absolute vorticity (s
−1
, 850 mb) and the tracks of cyclones in the Bay of Bengal from 2007 to 2013.
Figure 6. Distribution of climatological relative humidity (%, 600 mb) and the tracks of cyclones in the Bay of Bengal from 2007 to 2013.
138 K. Maneesha et al.

values (S>3) prevail over the bay (Figure 10). Most of
the intensification locations in the pre-monsoon (April
and May) and post-monsoon (October and November)
months correspond with locations of high (S>3) stratifi-
cation (Figure 11).
3.5. Upper ocean heat content with stratification
(UOHC
S
)
UOHC
S
is found to vary from 50–400 kj/cm
2
, with
maximum values occurring in the central and northern
areas of the Bay of Bengal during the pre-monsoon
months, as well as in June. From July to November,
UOHC
S
is high in the eastern and western areas. Most of
the cyclones form over regions where UOHC
S
>120 kj/
cm
2
, which is almost double the value of 60 kj/cm
2
without stratification (Figure 12).
Shay and Brewster (2011) reported UOHC
S
values of
114–129 kj/cm
2
in the east and west Pacific and 120–150
kj/cm
2
in the North West Caribbean Sea. It is interesting
to note that UOHC
S
values are almost double in the Bay
of Bengal compared to those of the Pacific, which is
Figure 7. Monthly distribution of climatological UOHC and the genesis locations of cyclones from 1981 to 2008.
Figure 8. Climatology of the intensification (red dots) and dissipation (blue dots) locations of cyclones overlaid on UOHC.
Journal of Operational Oceanography 139

mainly due to the high stratification in the bay. In general
cyclones are found to intensify over regions where
UOHC >120 kj/cm
2
; most of the cyclones in the pre-
monsoon (April and May) season intensify over regions
where UOHC
S
>224 kj/cm
2
in the northern area of the
bay. But in the post-monsoon season, most of the intensifi-
cation is takes place in the eastern area of the bay, where
UOHC
S
>150 kj/cm
2
(Figure 13).
Figure 9. Typical vertical profiles of Brunt Vaisala frequencies at four different locations in the Bay of Bengal.
Figure 10. Distribution of stratification (S) and the genesis locations of the cyclones.
140 K. Maneesha et al.

This study, comprising of various upper ocean par-
ameters during the intensification of the cyclones,
showed that some minimum threshold values are required
in all parameters such as D26, UOHC, S and UOHC
S
for
the intensification of cyclones over the Bay of Bengal. In
the southern part of the bay (5 to 15 deg N), the cyclones
are likely to intensify where UOHC >90 kj/cm
2
in the pre-
monsoon season and >30 kj/cm
2
in the post-monsoon
season. The stratification parameter (S) is generally >2.4
in the pre-monsoon season and >2.8 in the post-
monsoon season, whereas in the northern area (>15 deg
N), most of the cyclones intensify where UOHC >60 kj/
cm
2
in the pre-monsoon season and >40 kj/cm
2
in the
post-monsoon season. Also, S is generally >2.6 in the
pre-monsoon season and >2.8 in the post-monsoon
season. The threshold values in the parameter UOHC
S
,
which is the combined effect of both UOHC and S were
also observed.
Figure 11. Climatology of the intensification (red dots) and dissipation (blue dots) locations of cyclones overlaid on stratification (S).
Figure 12. Distribution of UOHC
S
(kj/cm
2
) and the genesis locations of the cyclones.
Journal of Operational Oceanography 141

3.6. Cyclone tracks in relation to UOHC, S and
UOHC
S
during the pre- and post-monsoon seasons
Climatology of the monthly frequencies of cyclones from
1891 to 2008 shows that most of the severe cyclones
occurred in the pre- and post-monsoon seasons. From the
climatology of the cyclone tracks from 1891 to 2008, it
can be seen that cyclones in the pre-monsoon season gen-
erally travel north and northeast, while in the post-
monsoon season they tend to move in the north and
northwest directions (Figure 14). The spatial distributions
of UOHC, S and UOHC
S
in the pre- and post-monsoon
months are plotted along with cyclone tracks, and it was
found that the tracks generally follow the high UOHC
S
regions. But UOHC
S
is a combination of both UOHC
and S. In the pre-monsoon season (April and May), S is
comparatively less (<3) than in the post-monsoon season
–but the increase in UOHC during the same period is
caused by the high insolation. So, in the pre-monsoon
Figure 13. Climatology of the intensification (red dots) and dissipation (blue dots) locations of cyclones overlaid on UOHC
S
(kj/cm
2
).
Figure 14. Distribution of stratification (S) and the climatology of cyclone tracks.
142 K. Maneesha et al.

Figure 15. Distribution of UOHC
S
(kj/cm
2
) and the climatology of cyclone tracks.
Figure 16. Distribution of stratification (S), UOHC (kj/cm
2
) and UOHC
S
(kj/cm
2
) a few days before the occurrence of Cyclones Sidr,
Nargis and Aila.
Journal of Operational Oceanography 143

season, the increase in UOHC
S
is mainly due to high
UOHC (Figure 15). In the post-monsoon season, S is on
the higher side (>3) in the central, eastern and western
areas of the Bay of Bengal due to the freshwater discharge
during the monsoon season. The UOHC in the post-monsoon
season is comparatively less (<85 kj/cm
2
) than in the pre-
monsoon season. Here the high UOHC
S
is mainly due to strati-
fication. So, UOHC appears to be the dominant factor in the pre-
monsoon season, while stratification is the dominant factor in the
post-monsoon season. The combined effect can be seen in
UOHC
S
, which in turn influences the genesis, intensification
and tracks of the cyclones.
Figure 17. Distribution of stratification (S), UOHC (kj/cm
2
) and UOHC
S
(kj/cm
2
) a few days before the occurrence of Cyclones Laila,
Thane, Neelam and Mahasen.
144 K. Maneesha et al.

3.7. A few case studies using Argo daily data
The upper ocean parameters, comprising stratification,
UOHC and UOHC
S
, prior to the genesis of the cyclones
have been examined for a few case studies during the
period from 2007 to 2013 (Cyclones Sidr to Mahasen)
using daily ARGO data.
Cyclone Sidr formed in the southern area of the Bay of
Bengal and was sustained during 9–16 November 2007,
during which time it moved north and started intensifying
(based on IMD cyclone track data) in the central Bay of
Bengal, where UOHC
S
>250 kj/cm
2
persists (Figure 16).
UOHC and S varied from 60–75 kj/cm
2
and 3–3.5
respectively.
Cyclone Nargis also formed in the southern area of the
Bay of Bengal and was sustained during 27 April–3May
2008, during which time it moved northwest but was
then diverted to the northeast. As shown in Figure 16,
Cyclone Nargis intensified twice during the course of its
existence. It was found that first intensification at 85 deg
E, 14 deg N occurred under conditions of high UOHC
S
(>350 kj/cm
2
), while the second intensification occurred
under conditions of high stratification (S∼3).
Cyclone Aila formed in the northern area of the Bay of
Bengal and was sustained during 23–27 May 2009. It
formed under conditions of high UOHC
S
(Figure 16). In
this case, the intensification cannot be explained based on
our argument due to the unavailability of Argo data near
to coast.
Cyclone Laila formed in the southern area of the Bay of
Bengal and was sustained during 17–22 May 2010, during
which time it moved in northwest. It formed and intensified
as a category-1 cyclone under conditions of high UOHC
S
(>400 kj/cm
2
). Due to its high energy input, it was sustained
for two days over land after hitting the coast, causing a lot of
damage along the east coast of India (Figure 17).
Cyclone Thane formed in the southern area of the Bay
of Bengal and was sustained during 25–30 December 2011,
during which time it moved west and intensified as cat-
egory-1 cyclone under conditions of high stratification
near to the coast. It crossed the coast in the north of Sri
Lanka (Figure 17).
Cyclone Neelam was sustained during 29–31 October
2012 near to Sri Lanka, during which time it moved north-
west and crossed the coast as a tropical storm within a short
period. It was formed and sustained under conditions of
high stratification (3.5–4; Figure 17).
Cyclone Mahasen formed in the southern area of the
Bay of Bengal and was sustained during 10–16 May
2013, during which time it primarily moved northwest,
later changing course and moving northeast. Landfall
occurred over the northern part. It was formed under con-
ditions of high UOHC
S
(>400 kj/cm
2
) and was sustained
for five days under the conditions of UOHC
S
of 300–
400 kj/cm
2
(Figure 17).
The results of the above case studies align well with the
conclusions drawn from the climatological studies. Most of
the case studies suggest that pre-monsoon cyclones occur
and are sustained in regions of high UOHC
S
, whereas
post-monsoon cyclones occur and are sustained in
regions of high stratification. Hence, in addition to the
atmospheric parameters, oceanic parameters like stratifica-
tion and UOHC
S
are also important in the intensification of
cyclones.
4. Conclusions
In this study, an attempt has been made to identify the role
of the upper ocean parameters (UOHC and UOHC
S
) in the
intensification and movement of tropical cyclones. Case
studies using Argo data were also performed in support
of the results drawn from the climatology. Freshwater dis-
charge plays a vital role in the stratification of the upper
layer, which was not considered in the previous studies
while estimating the UOHC. Here, the stratification par-
ameter (S) is used to estimate UOHC
S
following Shay
and Brewster (2010).
(1) It was found that a UOHC of >40 kj/cm
2
is usual
during the genesis and intensification of cyclones
in the Bay of Bengal. Stratification (S) varies
from 2–5 and most of the cyclones in October
form in the region where S>3. UOHC
S
varies
from 50–400 kj/cm
2
, which is higher than in the
east and west Pacific and also the Caribbean
(Shay & Brewster 2011). Most of the cyclones
form, intensify and moving over regions of high
UOHC
S
(>120 kj/cm
2
). The minimum threshold
UOHC and UOHC
S
required for the intensification
of cyclones are 30 and 120 kj/cm
2
, respectively.
(2) Severe pre- and post-monsoon cyclones over the
Bay of Bengal are mainly influenced by the
UOHC
S
and stratification.
Acknowledgements
The authors are thankful to Dr. SWA Naqvi, Director, National
Institute of Oceanography, and Dr. V.S.N. Murty, Scientist-in-
charge, NIO Regional centre, Visakhapatnam for their support
and encouragement. Thanks is offered to the NOAA for pro-
viding the climatology data sets (WOA-09) and the IMD,
New Delhi for the data on cyclone tracks. IRI/LDEO Climate
Data Library for the NCEP-NCAR reanalysis data. Dr. Mrs.
Maneesha is thankful to SERB-Department of Science and
Technology for funding. This is NIO contribution number
5782.
Disclosure statement
No potential conflict of interest was reported by the authors.
Journal of Operational Oceanography 145

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