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Intense tropical cyclone activities in the

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Article in International Journal of Climatology · November 2012

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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 archived as being categories 2 or 3 had been re-analysed

and assigned as categories 4 or 5 in the northern Indian
The northern Indian Ocean is not considered to be one
Ocean. This has the effect of questioning the trend
of the world’s most active cyclonic basins. On average,
proposed by Webster et al. (2005). Kossin et al. (2007)
every year, five or six systems attain at least the stage
did not note any trend towards an increase in the number
of tropical storm with sustained winds over a 1-min
of categories 4 and 5 cyclones in the northern Indian
period of 35 knots or more (Singh et al., 2001). And,
Ocean for their period of analysis, which covered from
the current Joint Typhoon Warning Center (JTWC, 2009)
1983 to 2005. To provide other considerations, this article
database shows that on average one intense cyclone
treats the original elements of intense cyclones activity in
forms every 2 years in the northern Indian Ocean. In
the northern Indian Ocean from 1980 to 2009 on the basis
the current debate on global warming and the change
of a homogenous re-analysis of satellite imagery. Intense
in the number of intense cyclones, initial studies carried
cyclones are those that generate sustained winds over a
out have shown very different results for the northern
1-min period of 100 knots or more, being categories 3–5
Indian Ocean. Using the Saffir–Simpson scale (Simpson,
cyclones. This matches with a current intensity reaching
1974), Webster et al. (2005) found that there had been at least 5.5 in the 1984 Dvorak’s technique.
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
2. The indispensable re-analysis of databases
that databases were not sufficiently reliable as cyclones
INSAT series geostationary satellites were launched by
India above the northern Indian Ocean as from April
* Correspondence to: Karl Hoarau, University of Cergy-Pontoise,
M.R.T.E. Laboratory, 33 Boulevard du Port, 95011 Cergy Cedex, 1982 (Foley, 1995). However, other countries did not
France. E-mail: KHoarau@aol.com 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 whether there was a particular trend and that their work
a spatial resolution of 11 km before 1990 and of 8 km could not determine the real intensity of cyclones. In fact,
as from that year with INSAT 1D. Before the arrival only images with an 8-km resolution from geostationary
of the European Meteosat geostationary satellite (1998), satellites were used. In concrete terms, this meant that,
the northern Indian Ocean was at the western edge of for the northern Indian Ocean, the analysis by Kossin
the sector covered by Japanese GMS satellites on the et al. (2007) was made without resolving the angle of
140th East meridian and on the eastern edge of the view problem faced by GMS until now and Meteosat
sector covered by European satellites stationed above until 1998. The intensity of the cyclones was systemati-
the Gulf of Guinea in western Africa. Despite a 4-km cally underestimated as GMS and Meteosat (1–4) gave
infrared resolution, GMS and Meteosat (1–4) could not an eye temperature that was colder than in reality.
restore the ‘real’ temperature of the warmest pixel in Consequently, this research is the first re-analysis
the eye of tropical cyclones. This parameter is very concerning the intensity of intense cyclones using satellite
important for estimating the intensity of cyclones from data provided by a 4-km resolution. All the intense
infrared images in cyclonic basins where there is no cyclones in the northern Indian Ocean were analysed
aircraft reconnaissance. Consequently, it was necessary using the Dvorak (1984) technique for the period from
to use the 4-km infrared imagery resolution of orbiting 1980 to 2009. Prior studies had used the archives of the
satellites belonging to National Oceanic and Atmospheric Hawaii Joint Typhoon Warning Center (JTWC, 2009).
Administration (NOAA). The intensity stage of cyclones Tropical cyclone 03B provides an example of the need
is in fact obtained using the Dvorak (1984) analysis to re-analyse the intensity of cyclones and the method
that needs to know the highest temperature of the eye used (Figure 1).
and the temperature of cloud tops within a radius of This cyclone, formed in November 1984 in the Bay
55 km around the centre. The technique is based on of Bengal, had been estimated at 85 knots (category 2)
cyclone operational procedures that allow the intensity to by JTWC. An examination of the satellite images reveals
be estimated through the intermediary of the maximum that 03B attained its maximum intensity on 13 November
sustained wind over a 1-min period. 1984 in the middle of the day as it skirted the south-east
Kossin et al. (2007) carried out a re-analysis of coast of India (Figure 2). NOAA 7 revealed a system
cyclones across the world for a period from 1983 to 2005 with a small central structure and a perfectly circular
that included the northern Indian Ocean. These authors eye. The latter corresponds to a warm point of +18.5 ° C
warned that the aim of their research was to determine 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

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
Figure 2. NOAA 7 satellite image (Basic Dvorak) of cyclone 03B taken
on 13 November 1984 at 10 : 14 UTC. Source: Images treated using to another. For this research, it was decided to assign
raw data from NCDC. all tropical systems to the basin in which the maximum
intensity had been observed.
The comparison between JTWC DT and those of
However, using Dvorak’s basic enhancement, it is the re-analysis reveals a considerable difference for the
the ‘black’ belt with cloud tops at −64/−69 ° C that decade from 1980 to 1989 (Figure 4). The Gay (1989)
measured a minimum of 55 km across. The warmer the and Forrest (1992) cyclones, formed in the western North
eye and the colder the cloud tops suggest the greater the Pacific should have been included in the northern Indian
cyclone intensity. The application of the Dvorak (1984) Ocean archives as they subsequently became intense
technique to 03B reveals a substantial difference between cyclones in the Bay of Bengal (JTWC, 2009). Unlike
the maximum intensity estimated at 85 knots (category 2) the JTWC best track, the re-analysis does not show a
by JTWC and that re-analysed at 120 knots, which ranks trend towards an increase in the number of categories
this as a category 4 cyclone (Figure 3). On the chart, the 3–5 cyclones over the last three decades and in fact the
satellite DT shows the convection intensity of the system, 1990s were twice as active as the other two periods.
characterized by the temperature of the eye and the cloud Furthermore, it is clear that 30 years of data are
tops. Despite the DT number is displayed every 3 h, the insufficient to update a natural activity variation cycle.
intensity has been estimated every 6 h by JTWC and for Despite the peak noted in the decade from 1990 to
the re-analysis. 1999, the information given in Figure 4 is insufficient to
A plausible explanation for the underestimation of judge whether there is a systematic alternation between
03B’s intensity lies in the fact that only Japanese GMS an active decade and another that is less so in terms of
satellite images with a resolution of 4 km were used. This intense cyclones. A further 30–50 years of reliable data
satellite gave the eye temperature as being −38 ° C on 13 would be needed to better appreciate the changes. In any
November at 08 : 30 UTC and −39 ° C at 11 : 30 UTC. As case, the results, valid here for categories 3–5 cyclones,
it was not positioned over the northern Indian Ocean, this do not confirm the conclusions of the study carried out by
geostationary satellite could not reconstruct the highest 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 in 1992, all show a level of activity at least twice as
the northern Indian Ocean over the period from 1975 to great as that of the northern Indian Ocean. The statistics
2004. mentioned here are official for the different basins.
There is another element that worth mentioning, being In the northern Indian Ocean, the connection between
that the northern Indian Ocean is the basin in the world the number of intense cyclones and a hydro-climatic
with the least intense cyclones (Mc Bride, 1995) with, phenomenon such as El Nino is not self-evident. The
on average, a single system every 2 years over the oceanic Nino index (ONI) is the standard that NOAA
decades from 1980 to 1989 and from 2000 to 2009 and uses to identify El Nino and La Nina events in the tropical
a phenomenon every year over the period from 1990 to Pacific (CPCM, 2010). It is the running 3-month mean
1999. Despite a lower activity level, extreme intensities sea surface temperature anomaly for the Nino 3.4 region
are quite comparable with those of other basins. During (5 ° N–5 ° S and 120° –170 ° W). Events are defined as five
the ‘record’ 1999 season, of the three intense systems consecutive months at or above the +0.5 ° C anomaly
formed (Figure 5), 05B generated sustained winds over for warm events (El Nino) and at or below the −0.5 ° C
a 1-min period estimated at 155 knots, being the peak of anomaly for cold events (La Nina). Weak, Moderate and
the Saffir–Simpson (Simpson, 1974) rating system. Strong events are those with an anomaly of 0.5–0.9,
This intensity is similar to that of Katrina or Rita in 1.0–1.4 and 1.5 or above, respectively. In the northern
the North Atlantic in 2005 or that of Monica (2006) and Indian Ocean, among 21 intense cyclones, only 4 formed
Geralda (1994), respectively in the South Pacific and the during El Nino events, 8 during La Nina events, and 9
southern Indian Ocean. were associated with neutral conditions (ONI between
With only 30 years of reliable data for the northern −0.4 and +0.4 ° C). Also, it is interesting to notice that
Indian Ocean, it is very difficult to observe any significant 13 intense cyclones formed when the ONI was negative,
trend towards an increase in extreme intensity: Gay 6 when the ONI was positive and 2 when the ONI was at
(1989) 140 knots, 05B (1999) 155 knots, and Gonu zero. The intense cyclones formed more frequently during
(2007) 145 knots were respectively the most intense La Nina events or under neutral conditions associated
cyclones of the last three decades. However, there is with a negative ONI. The record of three intense cyclones
no reason not to believe that comparable or even more in 1999 took place during a strong and long La Nina
powerful cyclones may have taken place in the past. event. Chang Seng and Jury (2010a, 2010b), who studied
The inter-annual distribution reveals a high level of the intense cyclones (90 knots over 10 min or 100 knots
irregularity (Figure 5). Over a period of three decades, over 1 min) in the south-west Indian Ocean, found also
13 years did not have categories 3–5 cyclones. In the that La Nina event was a governing factor.
1990s, only 1993 did not have any intense cyclone. The Another indicator is not in accord with the conclusions
maximum of three intense cyclones in 1999 appears low found by Webster et al. (2005). In fact, the proportion
when compared with other basins across the world. The of categories 3–5 cyclones did not increase in a steady
fact is that the North Atlantic with 7 major hurricanes in manner in all cyclones having reached an intensity
2005, the western North Pacific with 12 in 1997, as well of 65 knots and more over the last three decades
as the southern Indian Ocean with 6 intense cyclones in (Figure 6).
1980, the South Pacific with 6 intense cyclones in 2003 The proportion of intense cyclones reached a peak
and the northern East Pacific with 10 major hurricanes 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 upper troposphere. In addition, from July to September,
potential global warming influence can be noted here the monsoon trough responsible for the tropical storms
as Webster et al. (2005) have demonstrated that the genesis is positioned near or on the landmass. Five of
northern Indian Ocean had been at its warmest during the 21 intense cyclones in the northern Indian Ocean are
the 2000s. formed in the Arabian Sea which has its greatest activity
Apart from the low annual number of cyclones when level in May and June. However, although it has a smaller
compared with other basins across the world (Mc Bride, ocean surface area, the Bay of Bengal generates three
1995), the characteristics of the northern Indian Ocean times more categories 3–5 cyclones. To attain the high
lies in the unique bimodal distribution of its activity intensity stage, 17 of the 21 cyclones in the northern
(Figure 7). Indian Ocean (81%) developed rapidly over a 24-h period
The first part of the season concerns the quarter from
(Figure 8). This means that an increase occurred in the
April to June, with a peak of seven intense cyclones
speed of the sustained wind over a 1-min period of at least
in May. The second seasonal cyclone activity occurs
in October to November with a maximum of eight 35 knots, being from 65 to 100 knots. This minimum
intense cyclones in November. No intense cyclones were threshold is considered as representative in the Dvorak
observed during the July to September quarter between (1984) method.
1980 and 2009. In fact, maps produced by the University To assess the intensification speed of tropical cyclones,
of Wisconsin show that this period, corresponding to DeMaria and Kaplan (1999) for the North Atlantic and
the middle of the monsoon season, is characterized by Holliday and Thompson (1979) for the western North
a strong vertical wind shear preventing storms from Pacific used the drop in the central atmospheric pressure
becoming intense cyclones (Krishna, 2009). The south- measured or estimated in the eye. However, the dropson-
west flows of the lower and middle troposphere are des used since 1997 for aerial reconnaissance in the North
crowned by winds with an easterly component in the Atlantic have permitted better wind measurements. These

Copyright  2011 Royal Meteorological Society Int. J. Climatol. 32: 1935–1945 (2012)
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- additional intensification but at a more moderate rate can
sure, two cyclones could have very different sustained continue up to landfall.
winds (Hock et al., 1999). This very clearly signifies that
the wind is the most significant parameter for evaluating
cyclone intensity variation. 4. Intense cyclones considerably influenced by
By increasing from 70 to 145 knots in 24 h, being a 75 nearby land masses
knots wind increase, Gonu (2A in June 2007) intensified Intense cyclones considerably influenced by nearby land
in a remarkable manner in the Arabian Sea. This value, masses 13 intense cyclones attained their intensity peak at
which represents a record for the northern Indian Ocean less than 200 km from the coast (Figure 10). This repre-
over the period from 1980 to 2009, is comparable with sents a majority of 62% for which it is possible to believe
the values estimated in cyclones having taken place in the that the proximity of land could have impeded intensifi-
southern Indian Ocean, South Pacific and northern East cation. Chang Seng and Jury (2010a, 2010b) found that
Pacific. Only the North Atlantic and the western Pacific Madagascar could also block the flow converging towards
had greater values with a 95 knots increase in 24 h for the cyclone in the south-west Indian Ocean. The fact is
Hurricane Wilma (October 2005) and Typhoon Forrest that when the cyclonic circulation interacts with a land-
(September 1983). Fourteen systems developed rapidly mass, there is less humidity and therefore less energy
in the Bay of Bengal and three in the eastern Arabian in the central part of a tropical system. However, rela-
Sea, including Gonu (Figure 9). tively small cyclones can continue to intensify as they
Nine of the 17 cyclones completed the rapid intensi- approach coastlines. This was the case for cyclone 03B
fication process at less than 200 km from the coast. An 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 depended mainly on the warm sea surface temperature
Chennai (Figures 1 and 2). The very cold cloud tops and and the favourable vertical wind shear.
the excellent organization of the structure in the upper Working on the North Atlantic, DeMaria and Kaplan
troposphere lead one to suppose that 03B would have (1994) found that only 20% of hurricanes (65 knots
attained category 5 had the distance from the coast been and more) attained at least 80% of their maximum
any greater. This is why it is always difficult to make potential intensity. For the western North Pacific, Baik
significant comparisons, in this case between the extreme and Paek (1998) advanced a proportion of 37%. This
cyclones to be found in the northern Indian Ocean. What clearly signifies that factors such as a strong vertical wind
can be stated is that two category 5 systems were formed shear between the lower and the upper troposphere, the
in each of the three decades. intrusion of dry air or the weakness of the divergence
Despite a fundamental role in certain cases, the sea in the upper troposphere have a considerable influence
in limiting the intensification of cyclones (Merril, 1988).
surface temperature is not always a determining factor
In the northern Indian Ocean, if one simply considers
governing intensity. In fact, the correlation coefficient at
cyclones having reached a minimum of 65 knots, only
0.018 does not indicate any link between the maximum
5.66% (3 of 53) approached or attained their maximum
intensity of cyclones and the sea surface temperature in
potential intensity over the last three decades. In addition
the northern Indian Ocean for the period between 1980 to the limiting factors mentioned above, it is clear that
and 2009 (Figure 11). Only 3 of the 21 intense cyclones, the influence of landmasses represents a considerable
being 14.3%, approached their maximum potential inten- element for explaining this very small proportion. More
sity defined by the energy available in a given oceanic than elsewhere, the considerable presence of landmasses
space (Emanuel, 1988): the three cyclones had an inten- largely explains variations in the sea surface temperature
sity of 125, 140 and 155 knots, over a sea surface tem- over the months (Figure 12). The northern Indian Ocean
perature, respectively of 27.7, 28.2 and 28.7 ° C. This is closed off from subtropical latitudes by the southern
concept assumes that there are no thermodynamic con- tip of the Asian continent. As a result, the sea surface
straints in the atmosphere and that conditions are almost reaches a very high temperature much earlier than in
ideal. Chang Seng and Jury (2010a, 2010b) found that most of the other basins. Along with the western North
the intense cyclones in the south-west Indian Ocean 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 This very powerful system was presented by the media as
no equivalent in October for the southern hemisphere). being the result of global warming, but the fact is that on
On average, the northern Indian Ocean is at its warmest average (1971–2000), the central and eastern parts of the
in May when the water temperature exceeds 29 ° C over Arabian Sea a sea surface temperature greater than 29 ° C
large areas (Figure 12). It is the month of the first peak in May and June (Figure 12). This high threshold should
for intense cyclones. As from June, the monsoon flows have resulted in a larger number of intense cyclones
begins to considerably cool the ocean and, in August, than the five that developed between 1980 and 2009.
there exists an upwelling accompanied by masses of cool Incidentally, Hoarau (2001) cites the case of the Daniela
water in the western part of the Arabian Sea (Benestad, cyclone which reinforced explosively up to 125 knots in
2009). As soon as the monsoon ‘ends’ around the end of the south-west of the Indian Ocean in December 1996
September, the ocean begins to slowly reheat, although it above oceanic masses having a temperature profile of
does not reach the sea surface temperatures to be found in 27 ° C. The case of the Arabian Sea confirms that the sea
the month of May and the vertical wind shear diminishes. surface temperature alone is insufficient to comprehend
Chronologically, November corresponds to the second the intensity of cyclones (Lal, 2001).
peak of intense cyclones, although quantitatively to the The considerable presence of landmass in the northern
first peak, with eight intense systems as opposed to seven Indian Ocean is also translated by a considerable number
in May. Globally, the sea surface temperature remains far of landfalls by intense cyclones. Among the 21 systems
more favourable in the Bay of Bengal than in the Arabian formed, 16 made a landfall at an intensity at least equal
Sea. In addition, in the latter, the intrusion of dry air to 100 knots (Figure 13).
limiting convection is more frequent than in the Bay of This decadal distribution, true to that of the number of
Bengal which is better ‘protected’ from the continental air intense systems (Figure 4), did not increase on a regular
mass by the Himalayan barrier. This somewhat explains basis between 1980 and 2009. A reduction in the number
why there were three times more intense cyclones in of cyclones landfalls’ was obvious in the 2000s.
the Bay of Bengal over the last three decades. However, This was accompanied by a reduction in the num-
this does not exclude the intensification of a cyclone to ber of intense cyclones when compared with the decade
category 5, as was the case with Gonu on 4 June 2007. 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 2000–2009, India suffered three intense cyclones ‘land-
three decades: India, Bangladesh, Burma and Pakistan falls’ in the 1980s. It is also worth mentioning the case of
(Figure 14). While India is the country most frequently Burma as the Nargis cyclone, which killed over 135 000
affected due to the length of its coast, it is remarkable people.
that it has been spared in the 2000s. Despite the same The fact is that this catastrophe took place in a highly
number of five intense cyclones formed in the northern vulnerable and densely populated region and the archives
Indian Ocean during the two decades 1980–1989 and 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 activity in the 1990s when compared with the 1980s and
(2000–2009) characterized by a reduction in the number 2000s which were wholly comparable. Nor does new data
of categories 3–5 cyclones when compared with the reveal a continuous increase in the proportion of intense
previous decade. cyclones among all cyclones (categories 1–5) over the
While countries such as the Sultanate of Oman, Soma- three decades studied. The multi-year analysis shows that
lia or Yemen have not been concerned by intense 13 intense cyclones formed when the ONI was negative.
cyclones since 1980, this possibility cannot be ruled out Eight intense cyclones formed during La Nina events,
despite the relatively unfavourable thermodynamic con- whereas only four intense cyclones formed during El
ditions generally to be found along the north and west Nino events. Associated with a weak vertical wind shear,
coasts of the Arabian Sea. 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
5. Conclusion cyclones having attained at least the category 3. With 155
This study highlights the particularities of categories 3–5 knots in October 1999, 05B reached the highest intensity
intense tropical cyclones in the northern Indian Ocean estimated for a cyclone of the northern Indian Ocean in
over the last three decades. The re-analysis of intensity the 1980–2009 period.
using satellite images indicates that 21 intense cyclones Apart from the small annual number of cyclones when
were formed over a 30-year period. The decadal distribu- compared with the number in other basins across the
tion does not reveal a regular trend towards an increase world, the particular uniqueness of the northern Indian
in the number of these cyclones despite a doubling of 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 References
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I would like to thank the reviewers. Their comments and Webster PJ, Holland GJ, Curry JA, Chang HR. 2005. Changes in
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