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Intense tropical cyclone activities in the northern Indian Ocean

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This research concerning the northern Indian Ocean demonstrates the variability of intense tropical cyclones (categories 35) both on an inter-annual and intra-seasonal scale. All the cyclones intensity have been re-analysed with the Dvorak technique using both National Oceanic and Atmospheric Administration and geostationary satellites with a 4-km infrared resolution. During the period from 1980 to 2009, 21 cyclones became intense. The decade from 1990 to 1999 was by far the most active with 11 intense cyclones while 5 intense cyclones formed in each of the other two decades. There has been no trend towards an increase in the number of categories 35 cyclones over the last 30 years. An early study, not based on re-analysed data, found a significant increase in the number of intense cyclones. Thirteen cyclones became intense when the Oceanic Nino Index was negative and matched with lower vertical wind shear. And, La Nina events have had a noticeable influence with eight intense cyclones. The monthly distribution is bimodal with seven and eight cyclones, respectively in the months of May and November. No intense cyclones were observed from July to September, being the peak of the monsoon season. Over these 3 months, the vertical wind shear is too strong to allow a significant intensification of storms. Despite particularly warm ocean waters, only 6% of categories 15 cyclones approached their maximum potential intensity. However, extreme intensities (05B at 155 knots in 1999) are comparable to other basins despite lower in terms of activity level. The proximity of land limits most cyclones from reaching a greater intensity. However, 16 of the 21 cyclones reached land with sustained winds of 100 knots and more. India and Bangladesh have been hit frequently by intense cyclones. Since 1990s, there is an increasing hit in Burma and Pakistan. Copyright (c) 2011 Royal Meteorological Society
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INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 32: 1935 –1945 (2012)
Published online 15 August 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/joc.2406
Review
Intense tropical cyclone activities in the northern Indian
Ocean
Karl Hoarau,* Julien Bernard and Ludovic Chalonge
University of Cergy-Pontoise, M.R.T.E. Laboratory, 33 Boulevard du Port, 95011 Cergy Cedex, France
ABSTRACT: This research concerning the northern Indian Ocean demonstrates the variability of intense tropical cyclones
(categories 3–5) both on an inter-annual and intra-seasonal scale. All the cyclones intensity have been re-analysed with the
Dvorak technique using both National Oceanic and Atmospheric Administration and geostationary satellites with a 4-km
infrared resolution. During the period from 1980 to 2009, 21 cyclones became intense. The decade from 1990 to 1999 was
by far the most active with 11 intense cyclones while 5 intense cyclones formed in each of the other two decades. There
has been no trend towards an increase in the number of categories 3–5 cyclones over the last 30 years. An early study,
not based on re-analysed data, found a significant increase in the number of intense cyclones. Thirteen cyclones became
intense when the Oceanic Nino Index was negative and matched with lower vertical wind shear. And, La Nina events
have had a noticeable influence with eight intense cyclones. The monthly distribution is bimodal with seven and eight
cyclones, respectively in the months of May and November. No intense cyclones were observed from July to September,
being the peak of the monsoon season. Over these 3 months, the vertical wind shear is too strong to allow a significant
intensification of storms. Despite particularly warm ocean waters, only 6% of categories 1– 5 cyclones approached their
maximum potential intensity. However, extreme intensities (05B at 155 knots in 1999) are comparable to other basins
despite lower in terms of activity level. The proximity of land limits most cyclones from reaching a greater intensity.
However, 16 of the 21 cyclones reached land with sustained winds of 100 knots and more. India and Bangladesh have
been hit frequently by intense cyclones. Since 1990s, there is an increasing hit in Burma and Pakistan. Copyright 2011
Royal Meteorological Society
KEY WORDS intense cyclones; northern Indian Ocean; global warming; decadal variations; La Nina event
Received 21 April 2010; Revised 30 May 2011; Accepted 21 June 2011
1. Introduction
The northern Indian Ocean is not considered to be one
of the world’s most active cyclonic basins. On average,
every year, five or six systems attain at least the stage
of tropical storm with sustained winds over a 1-min
period of 35 knots or more (Singh et al., 2001). And,
the current Joint Typhoon Warning Center (JTWC, 2009)
database shows that on average one intense cyclone
forms every 2 years in the northern Indian Ocean. In
the current debate on global warming and the change
in the number of intense cyclones, initial studies carried
out have shown very different results for the northern
Indian Ocean. Using the Saffir–Simpson scale (Simpson,
1974), Webster et al. (2005) found that there had been
a considerable increase in the number of categories 4
and 5 cyclones with a maximum sustained wind reaching
at least 115 knots. Landsea et al. (2006) demonstrated
that databases were not sufficiently reliable as cyclones
* Correspondence to: Karl Hoarau, University of Cergy-Pontoise,
M.R.T.E. Laboratory, 33 Boulevard du Port, 95011 Cergy Cedex,
France. E-mail: KHoarau@aol.com
archived as being categories 2 or 3 had been re-analysed
and assigned as categories 4 or 5 in the northern Indian
Ocean. This has the effect of questioning the trend
proposed by Webster et al. (2005). Kossin et al. (2007)
did not note any trend towards an increase in the number
of categories 4 and 5 cyclones in the northern Indian
Ocean for their period of analysis, which covered from
1983 to 2005. To provide other considerations, this article
treats the original elements of intense cyclones activity in
the northern Indian Ocean from 1980 to 2009 on the basis
of a homogenous re-analysis of satellite imagery. Intense
cyclones are those that generate sustained winds over a
1-min period of 100 knots or more, being categories 3–5
cyclones. This matches with a current intensity reaching
at least 5.5 in the 1984 Dvorak’s technique.
2. The indispensable re-analysis of databases
INSAT series geostationary satellites were launched by
India above the northern Indian Ocean as from April
1982 (Foley, 1995). However, other countries did not
have access to this satellite data (DT). In addition, the
Copyright 2011 Royal Meteorological Society
1936 K. HOARAU et al.
Figure 1. Trajectory of tropical cyclone 03B. Source: 1984 Annual Tropical Cyclones Report, JTWC.
thermal infrared images of the Indian satellites only had
a spatial resolution of 11 km before 1990 and of 8 km
as from that year with INSAT 1D. Before the arrival
of the European Meteosat geostationary satellite (1998),
the northern Indian Ocean was at the western edge of
the sector covered by Japanese GMS satellites on the
140th East meridian and on the eastern edge of the
sector covered by European satellites stationed above
the Gulf of Guinea in western Africa. Despite a 4-km
infrared resolution, GMS and Meteosat (14) could not
restore the ‘real’ temperature of the warmest pixel in
the eye of tropical cyclones. This parameter is very
important for estimating the intensity of cyclones from
infrared images in cyclonic basins where there is no
aircraft reconnaissance. Consequently, it was necessary
to use the 4-km infrared imagery resolution of orbiting
satellites belonging to National Oceanic and Atmospheric
Administration (NOAA). The intensity stage of cyclones
is in fact obtained using the Dvorak (1984) analysis
that needs to know the highest temperature of the eye
and the temperature of cloud tops within a radius of
55 km around the centre. The technique is based on
cyclone operational procedures that allow the intensity to
be estimated through the intermediary of the maximum
sustained wind over a 1-min period.
Kossin et al. (2007) carried out a re-analysis of
cyclones across the world for a period from 1983 to 2005
that included the northern Indian Ocean. These authors
warned that the aim of their research was to determine
whether there was a particular trend and that their work
could not determine the real intensity of cyclones. In fact,
only images with an 8-km resolution from geostationary
satellites were used. In concrete terms, this meant that,
for the northern Indian Ocean, the analysis by Kossin
et al. (2007) was made without resolving the angle of
view problem faced by GMS until now and Meteosat
until 1998. The intensity of the cyclones was systemati-
cally underestimated as GMS and Meteosat (14) gave
an eye temperature that was colder than in reality.
Consequently, this research is the first re-analysis
concerning the intensity of intense cyclones using satellite
data provided by a 4-km resolution. All the intense
cyclones in the northern Indian Ocean were analysed
using the Dvorak (1984) technique for the period from
1980 to 2009. Prior studies had used the archives of the
Hawaii Joint Typhoon Warning Center (JTWC, 2009).
Tropical cyclone 03B provides an example of the need
to re-analyse the intensity of cyclones and the method
used (Figure 1).
This cyclone, formed in November 1984 in the Bay
of Bengal, had been estimated at 85 knots (category 2)
by JTWC. An examination of the satellite images reveals
that 03B attained its maximum intensity on 13 November
1984 in the middle of the day as it skirted the south-east
coast of India (Figure 2). NOAA 7 revealed a system
with a small central structure and a perfectly circular
eye. The latter corresponds to a warm point of +18.5°C
surrounded by cloud tops at 70 °C (white belt).
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
INTENSE TROPICAL CYCLONES ACTIVITY IN THE NORTHERN INDIAN OCEAN 1937
Figure 2. NOAA 7 satellite image (Basic Dvorak) of cyclone 03B taken
on 13 November 1984 at 10 : 14 UTC. Source: Images treated using
raw data from NCDC.
However, using Dvorak’s basic enhancement, it is
the ‘black’ belt with cloud tops at 64/69 °Cthat
measured a minimum of 55 km across. The warmer the
eye and the colder the cloud tops suggest the greater the
cyclone intensity. The application of the Dvorak (1984)
technique to 03B reveals a substantial difference between
the maximum intensity estimated at 85 knots (category 2)
by JTWC and that re-analysed at 120 knots, which ranks
this as a category 4 cyclone (Figure 3). On the chart, the
satellite DT shows the convection intensity of the system,
characterized by the temperature of the eye and the cloud
tops. Despite the DT number is displayed every 3 h, the
intensity has been estimated every 6 h by JTWC and for
the re-analysis.
A plausible explanation for the underestimation of
03B’s intensity lies in the fact that only Japanese GMS
satellite images with a resolution of 4 km were used. This
satellite gave the eye temperature as being 38 °Con13
November at 08 : 30 UTC and 39 °C at 11 : 30 UTC. As
it was not positioned over the northern Indian Ocean, this
geostationary satellite could not reconstruct the highest
eye temperature. The NOAA7 orbiting satellite which
was at the 03B nadir at 10 : 14 UTC indicated a pixel
at +18.5°C in this eye. This difference of 57 °C between
the two satellites is sufficient to explain why 03B had an
underestimated intensity in the JTWC (2009) archives.
3. An atypical but far from innocuous intense
cyclones activity
Apart from the possible approximations in terms of
estimating the intensity, it is also necessary to take into
consideration those cyclones that moved from one basin
to another. For this research, it was decided to assign
all tropical systems to the basin in which the maximum
intensity had been observed.
The comparison between JTWC DT and those of
the re-analysis reveals a considerable difference for the
decade from 1980 to 1989 (Figure 4). The Gay (1989)
and Forrest (1992) cyclones, formed in the western North
Pacific should have been included in the northern Indian
Ocean archives as they subsequently became intense
cyclones in the Bay of Bengal (JTWC, 2009). Unlike
the JTWC best track, the re-analysis does not show a
trend towards an increase in the number of categories
3–5 cyclones over the last three decades and in fact the
1990s were twice as active as the other two periods.
Furthermore, it is clear that 30 years of data are
insufficient to update a natural activity variation cycle.
Despite the peak noted in the decade from 1990 to
1999, the information given in Figure 4 is insufficient to
judge whether there is a systematic alternation between
an active decade and another that is less so in terms of
intense cyclones. A further 30–50 years of reliable data
would be needed to better appreciate the changes. In any
case, the results, valid here for categories 3–5 cyclones,
do not confirm the conclusions of the study carried out by
Webster et al. (2005), which state that there was a 600%
Figure 3. Estimation of the intensity of cyclone 03B using the Dvorak method. Source: Chart created from raw data provided by GMS, NOAA
and JTWC. This figure is available in colour online at wileyonlinelibrary.com/journal/joc
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
1938 K. HOARAU et al.
Figure 4. Number of intense cyclones per decade (1980–2009). Source: Chart prepared using data from JTWC and from the re-analysis of
cyclone intensities in the northern Indian Ocean.
increase in the number of categories 4 and 5 cyclones in
the northern Indian Ocean over the period from 1975 to
2004.
There is another element that worth mentioning, being
that the northern Indian Ocean is the basin in the world
with the least intense cyclones (Mc Bride, 1995) with,
on average, a single system every 2 years over the
decades from 1980 to 1989 and from 2000 to 2009 and
a phenomenon every year over the period from 1990 to
1999. Despite a lower activity level, extreme intensities
are quite comparable with those of other basins. During
the ‘record’ 1999 season, of the three intense systems
formed (Figure 5), 05B generated sustained winds over
a 1-min period estimated at 155 knots, being the peak of
the Saffir–Simpson (Simpson, 1974) rating system.
This intensity is similar to that of Katrina or Rita in
the North Atlantic in 2005 or that of Monica (2006) and
Geralda (1994), respectively in the South Pacific and the
southern Indian Ocean.
With only 30 years of reliable data for the northern
Indian Ocean, it is very difficult to observe any significant
trend towards an increase in extreme intensity: Gay
(1989) 140 knots, 05B (1999) 155 knots, and Gonu
(2007) 145 knots were respectively the most intense
cyclones of the last three decades. However, there is
no reason not to believe that comparable or even more
powerful cyclones may have taken place in the past.
The inter-annual distribution reveals a high level of
irregularity (Figure 5). Over a period of three decades,
13 years did not have categories 35 cyclones. In the
1990s, only 1993 did not have any intense cyclone. The
maximum of three intense cyclones in 1999 appears low
when compared with other basins across the world. The
fact is that the North Atlantic with 7 major hurricanes in
2005, the western North Pacific with 12 in 1997, as well
as the southern Indian Ocean with 6 intense cyclones in
1980, the South Pacific with 6 intense cyclones in 2003
and the northern East Pacific with 10 major hurricanes
in 1992, all show a level of activity at least twice as
great as that of the northern Indian Ocean. The statistics
mentioned here are official for the different basins.
In the northern Indian Ocean, the connection between
the number of intense cyclones and a hydro-climatic
phenomenon such as El Nino is not self-evident. The
oceanic Nino index (ONI) is the standard that NOAA
uses to identify El Nino and La Nina events in the tropical
Pacific (CPCM, 2010). It is the running 3-month mean
sea surface temperature anomaly for the Nino 3.4 region
(5 °N–5°S and 120°–170 °W). Events are defined as five
consecutive months at or above the +0.5°C anomaly
for warm events (El Nino) and at or below the 0.5°C
anomaly for cold events (La Nina). Weak, Moderate and
Strong events are those with an anomaly of 0.5–0.9,
1.0–1.4 and 1.5 or above, respectively. In the northern
Indian Ocean, among 21 intense cyclones, only 4 formed
during El Nino events, 8 during La Nina events, and 9
were associated with neutral conditions (ONI between
0.4and+0.4°C). Also, it is interesting to notice that
13 intense cyclones formed when the ONI was negative,
6 when the ONI was positive and 2 when the ONI was at
zero. The intense cyclones formed more frequently during
La Nina events or under neutral conditions associated
with a negative ONI. The record of three intense cyclones
in 1999 took place during a strong and long La Nina
event. Chang Seng and Jury (2010a, 2010b), who studied
the intense cyclones (90 knots over 10 min or 100 knots
over 1 min) in the south-west Indian Ocean, found also
that La Nina event was a governing factor.
Another indicator is not in accord with the conclusions
found by Webster et al. (2005). In fact, the proportion
of categories 3–5 cyclones did not increase in a steady
manner in all cyclones having reached an intensity
of 65 knots and more over the last three decades
(Figure 6).
The proportion of intense cyclones reached a peak
of 44% during the 1990s and, a considerable reduction
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
INTENSE TROPICAL CYCLONES ACTIVITY IN THE NORTHERN INDIAN OCEAN 1939
Figure 5. Annual number of intense Cat 3–5 cyclones (1980 2009). Source: Authors.
Figure 6. The proportion of intense cyclones per decade from among all cyclones at Cat 1– 5 (1980 –2009). Source: Authors.
can be seen since 2000 with 31.25%. Consequently, no
potential global warming influence can be noted here
as Webster et al. (2005) have demonstrated that the
northern Indian Ocean had been at its warmest during
the 2000s.
Apart from the low annual number of cyclones when
compared with other basins across the world (Mc Bride,
1995), the characteristics of the northern Indian Ocean
lies in the unique bimodal distribution of its activity
(Figure 7).
The first part of the season concerns the quarter from
April to June, with a peak of seven intense cyclones
in May. The second seasonal cyclone activity occurs
in October to November with a maximum of eight
intense cyclones in November. No intense cyclones were
observed during the July to September quarter between
1980 and 2009. In fact, maps produced by the University
of Wisconsin show that this period, corresponding to
the middle of the monsoon season, is characterized by
a strong vertical wind shear preventing storms from
becoming intense cyclones (Krishna, 2009). The south-
west flows of the lower and middle troposphere are
crowned by winds with an easterly component in the
upper troposphere. In addition, from July to September,
the monsoon trough responsible for the tropical storms
genesis is positioned near or on the landmass. Five of
the 21 intense cyclones in the northern Indian Ocean are
formed in the Arabian Sea which has its greatest activity
level in May and June. However, although it has a smaller
ocean surface area, the Bay of Bengal generates three
times more categories 3–5 cyclones. To attain the high
intensity stage, 17 of the 21 cyclones in the northern
Indian Ocean (81%) developed rapidly over a 24-h period
(Figure 8). This means that an increase occurred in the
speed of the sustained wind over a 1-min period of at least
35 knots, being from 65 to 100 knots. This minimum
threshold is considered as representative in the Dvorak
(1984) method.
To assess the intensification speed of tropical cyclones,
DeMaria and Kaplan (1999) for the North Atlantic and
Holliday and Thompson (1979) for the western North
Pacific used the drop in the central atmospheric pressure
measured or estimated in the eye. However, the dropson-
des used since 1997 for aerial reconnaissance in the North
Atlantic have permitted better wind measurements. These
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1940 K. HOARAU et al.
Figure 7. Monthly distribution of intense cyclones (1980– 2009). Source: Authors.
Figure 8. The intensification speed of intense cyclones over a 24-h period (1980–2009). Source: Authors.
dropsondes revealed that for an identical central pres-
sure, two cyclones could have very different sustained
winds (Hock et al., 1999). This very clearly signifies that
the wind is the most significant parameter for evaluating
cyclone intensity variation.
By increasing from 70 to 145 knots in 24 h, being a 75
knots wind increase, Gonu (2A in June 2007) intensified
in a remarkable manner in the Arabian Sea. This value,
which represents a record for the northern Indian Ocean
over the period from 1980 to 2009, is comparable with
the values estimated in cyclones having taken place in the
southern Indian Ocean, South Pacific and northern East
Pacific. Only the North Atlantic and the western Pacific
had greater values with a 95 knots increase in 24 h for
Hurricane Wilma (October 2005) and Typhoon Forrest
(September 1983). Fourteen systems developed rapidly
in the Bay of Bengal and three in the eastern Arabian
Sea, including Gonu (Figure 9).
Nine of the 17 cyclones completed the rapid intensi-
fication process at less than 200 km from the coast. An
additional intensification but at a more moderate rate can
continue up to landfall.
4. Intense cyclones considerably influenced by
nearby land masses
Intense cyclones considerably influenced by nearby land
masses 13 intense cyclones attained their intensity peak at
less than 200 km from the coast (Figure 10). This repre-
sents a majority of 62% for which it is possible to believe
that the proximity of land could have impeded intensifi-
cation. Chang Seng and Jury (2010a, 2010b) found that
Madagascar could also block the flow converging towards
the cyclone in the south-west Indian Ocean. The fact is
that when the cyclonic circulation interacts with a land-
mass, there is less humidity and therefore less energy
in the central part of a tropical system. However, rela-
tively small cyclones can continue to intensify as they
approach coastlines. This was the case for cyclone 03B
in November 1984 which intensified by 40 knots over
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
INTENSE TROPICAL CYCLONES ACTIVITY IN THE NORTHERN INDIAN OCEAN 1941
Figure 9. The location of the rapid intensification phase of intense cyclones (1980– 2009). Source: Authors. This figure is available in colour
online at wileyonlinelibrary.com/journal/joc
24 h before attaining category 4 along the coastline near
Chennai (Figures 1 and 2). The very cold cloud tops and
the excellent organization of the structure in the upper
troposphere lead one to suppose that 03B would have
attained category 5 had the distance from the coast been
any greater. This is why it is always difficult to make
significant comparisons, in this case between the extreme
cyclones to be found in the northern Indian Ocean. What
can be stated is that two category 5 systems were formed
in each of the three decades.
Despite a fundamental role in certain cases, the sea
surface temperature is not always a determining factor
governing intensity. In fact, the correlation coefficient at
0.018 does not indicate any link between the maximum
intensity of cyclones and the sea surface temperature in
the northern Indian Ocean for the period between 1980
and 2009 (Figure 11). Only 3 of the 21 intense cyclones,
being 14.3%, approached their maximum potential inten-
sity defined by the energy available in a given oceanic
space (Emanuel, 1988): the three cyclones had an inten-
sity of 125, 140 and 155 knots, over a sea surface tem-
perature, respectively of 27.7, 28.2 and 28.7 °C. This
concept assumes that there are no thermodynamic con-
straints in the atmosphere and that conditions are almost
ideal. Chang Seng and Jury (2010a, 2010b) found that
the intense cyclones in the south-west Indian Ocean
depended mainly on the warm sea surface temperature
and the favourable vertical wind shear.
Working on the North Atlantic, DeMaria and Kaplan
(1994) found that only 20% of hurricanes (65 knots
and more) attained at least 80% of their maximum
potential intensity. For the western North Pacific, Baik
and Paek (1998) advanced a proportion of 37%. This
clearly signifies that factors such as a strong vertical wind
shear between the lower and the upper troposphere, the
intrusion of dry air or the weakness of the divergence
in the upper troposphere have a considerable influence
in limiting the intensification of cyclones (Merril, 1988).
In the northern Indian Ocean, if one simply considers
cyclones having reached a minimum of 65 knots, only
5.66% (3 of 53) approached or attained their maximum
potential intensity over the last three decades. In addition
to the limiting factors mentioned above, it is clear that
the influence of landmasses represents a considerable
element for explaining this very small proportion. More
than elsewhere, the considerable presence of landmasses
largely explains variations in the sea surface temperature
over the months (Figure 12). The northern Indian Ocean
is closed off from subtropical latitudes by the southern
tip of the Asian continent. As a result, the sea surface
reaches a very high temperature much earlier than in
most of the other basins. Along with the western North
Pacific, it is the only basin where a category 5 cyclone,
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
1942 K. HOARAU et al.
Figure 10. The location of the maximum intensity of intense cyclones (1980–2009). Source: Authors. This figure is available in colour online
at wileyonlinelibrary.com/journal/joc
02B (1991), formed as from the month of April (there is
no equivalent in October for the southern hemisphere).
On average, the northern Indian Ocean is at its warmest
in May when the water temperature exceeds 29 °C over
large areas (Figure 12). It is the month of the first peak
for intense cyclones. As from June, the monsoon flows
begins to considerably cool the ocean and, in August,
there exists an upwelling accompanied by masses of cool
water in the western part of the Arabian Sea (Benestad,
2009). As soon as the monsoon ‘ends’ around the end of
September, the ocean begins to slowly reheat, although it
does not reach the sea surface temperatures to be found in
the month of May and the vertical wind shear diminishes.
Chronologically, November corresponds to the second
peak of intense cyclones, although quantitatively to the
first peak, with eight intense systems as opposed to seven
in May. Globally, the sea surface temperature remains far
more favourable in the Bay of Bengal than in the Arabian
Sea. In addition, in the latter, the intrusion of dry air
limiting convection is more frequent than in the Bay of
Bengal which is better ‘protected’ from the continental air
mass by the Himalayan barrier. This somewhat explains
why there were three times more intense cyclones in
the Bay of Bengal over the last three decades. However,
this does not exclude the intensification of a cyclone to
category 5, as was the case with Gonu on 4 June 2007.
This very powerful system was presented by the media as
being the result of global warming, but the fact is that on
average (1971–2000), the central and eastern parts of the
Arabian Sea a sea surface temperature greater than 29 °C
in May and June (Figure 12). This high threshold should
have resulted in a larger number of intense cyclones
than the five that developed between 1980 and 2009.
Incidentally, Hoarau (2001) cites the case of the Daniela
cyclone which reinforced explosively up to 125 knots in
the south-west of the Indian Ocean in December 1996
above oceanic masses having a temperature profile of
27 °C. The case of the Arabian Sea confirms that the sea
surface temperature alone is insufficient to comprehend
the intensity of cyclones (Lal, 2001).
The considerable presence of landmass in the northern
Indian Ocean is also translated by a considerable number
of landfalls by intense cyclones. Among the 21 systems
formed, 16 made a landfall at an intensity at least equal
to 100 knots (Figure 13).
This decadal distribution, true to that of the number of
intense systems (Figure 4), did not increase on a regular
basis between 1980 and 2009. A reduction in the number
of cyclones landfalls’ was obvious in the 2000s.
This was accompanied by a reduction in the num-
ber of intense cyclones when compared with the decade
from 1990 to 1999. Four countries in the northern Indian
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
INTENSE TROPICAL CYCLONES ACTIVITY IN THE NORTHERN INDIAN OCEAN 1943
Figure 11. Sea surface temperature and maximum intensity of intense cyclones (1980– 2009). Source: Chart prepared using data from
NOAA/ESRL and from the re-analysis of cyclone intensities.
Figure 12. Average monthly sea surface temperatures (1971–2000) of the northern Indian Ocean.
Ocean were affected by intense systems over the last
three decades: India, Bangladesh, Burma and Pakistan
(Figure 14). While India is the country most frequently
affected due to the length of its coast, it is remarkable
that it has been spared in the 2000s. Despite the same
number of five intense cyclones formed in the northern
Indian Ocean during the two decades 19801989 and
2000–2009, India suffered three intense cyclones ‘land-
falls’ in the 1980s. It is also worth mentioning the case of
Burma as the Nargis cyclone, which killed over 135 000
people.
The fact is that this catastrophe took place in a highly
vulnerable and densely populated region and the archives
cannot provide any comparable examples over the last
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
1944 K. HOARAU et al.
Figure 13. Number of intense cyclones having hit the coast, by decade (1980–2009). Source: Chart prepared using data from JTWC and from
the re-analysis of cyclone intensities.
Figure 14. Number of intense cyclones having affected various countries, by decade (1980– 2009). Source: Chart prepared using data from JTWC
and from the re-analysis of cyclone intensities.
30 or 40 years. Like Gonu, Nargis formed in a period
(2000–2009) characterized by a reduction in the number
of categories 3–5 cyclones when compared with the
previous decade.
While countries such as the Sultanate of Oman, Soma-
lia or Yemen have not been concerned by intense
cyclones since 1980, this possibility cannot be ruled out
despite the relatively unfavourable thermodynamic con-
ditions generally to be found along the north and west
coasts of the Arabian Sea.
5. Conclusion
This study highlights the particularities of categories 35
intense tropical cyclones in the northern Indian Ocean
over the last three decades. The re-analysis of intensity
using satellite images indicates that 21 intense cyclones
were formed over a 30-year period. The decadal distribu-
tion does not reveal a regular trend towards an increase
in the number of these cyclones despite a doubling of
activity in the 1990s when compared with the 1980s and
2000s which were wholly comparable. Nor does new data
reveal a continuous increase in the proportion of intense
cyclones among all cyclones (categories 15) over the
three decades studied. The multi-year analysis shows that
13 intense cyclones formed when the ONI was negative.
Eight intense cyclones formed during La Nina events,
whereas only four intense cyclones formed during El
Nino events. Associated with a weak vertical wind shear,
tropospheric conditions of La Nina events could inten-
sify the cyclones further. The 1999 year, a strong and
long La Nina event, was the most active season with three
cyclones having attained at least the category 3. With 155
knots in October 1999, 05B reached the highest intensity
estimated for a cyclone of the northern Indian Ocean in
the 1980–2009 period.
Apart from the small annual number of cyclones when
compared with the number in other basins across the
world, the particular uniqueness of the northern Indian
Ocean lies in the bimodal distribution of its activity. The
Copyright 2011 Royal Meteorological Society Int. J. Climatol. 32: 1935 –1945 (2012)
INTENSE TROPICAL CYCLONES ACTIVITY IN THE NORTHERN INDIAN OCEAN 1945
first part of the season concerns the quarter running from
April to June with a peak of seven intense cyclones in
May and a second occurring in OctoberNovember with
a maximum of eight intense cyclones observed in Novem-
ber. No cyclones were noted during the July to September
quarter between 1980 and 2009. This is because this
period, which corresponds to the middle of the monsoon,
is characterized by a strong vertical wind shear preventing
storms from becoming intense cyclones. Four of the five
cyclones formed in the Arabian Sea developed in May
and June. The subsequent monsoon winds coming from
a south-westerly direction cause a notable cooling in the
sea’s temperature in the north-west part of this region.
Seventeen of the 21 cyclones reached a minimum of cat-
egory 3 following a rapid intensification phase of at least
35 knots in 24 h. As in other basins, favourable tropo-
spheric conditions can facilitate the accelerated develop-
ment of storms. The small size of the northern Indian
Ocean provides an understanding as to why 13 of the
21 intense cyclones (being 62%) reach their maximum
intensity at less than 200 km from the coast. This inter-
action with the landmass explains why the large majority
of cyclones do not attain their maximum potential inten-
sity despite the presence of particularly warm oceanic
masses. It is also worth underlining another remarkable
aspect, being that 16 of the 21 intense cyclones retained
the 100 knots stage at the moment they penetrated the
landmass. However, there has not been a regular increase
in the number of cyclones ‘landfalls’ over the last three
decades (1980–2009). While India and Bangladesh are
the countries most affected, relatively spared countries,
such as the Sultanate of Oman, Somalia and Yemen,
could suffer the assault of an intense yet small cyclone in
May or June if the thermodynamic conditions temporarily
became favourable in the western part of the Arabian Sea.
The current period of 30 years for which we have reli-
able data is too short to pick out natural cycles in the
decadal variations of intense cyclones activity. It appears
essential that a few additional decades of observation are
needed for these cycles to be understood.
Acknowledgements
I would like to thank the reviewers. Their comments and
suggestions have been a precious help to improve this
article.
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... Regardless, it indicates that the correlation is spatially nonuniform. Hoarau et al. (2012) indicate that the correlation between the intensity of the cyclone and the SSTs is insignificant (only 0.018) for high intensity cyclones (wind speed higher than 100 knots) if we consider the entire north Indian Ocean. Based on the data for 1998-2011, Ali et al. (2013) showed that the intensity of more than 50% of the cyclones in the north Indian Ocean had no correlation with the SSTs. ...
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Technical Report
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