ORI GIN AL PA PER

Impacts of ENSO and IOD on tropical cyclone activityin the Bay of Bengal

Biranchi Kumar Mahala • Birendra Kumar Nayak •

Pratap Kumar Mohanty

Received: 7 May 2013 / Accepted: 3 August 2014� Springer Science+Business Media Dordrecht 2014

Abstract The impacts of El Nino-Southern Oscillation (ENSO) and Indian Ocean

Dipole (IOD) on tropical cyclone (TC) activity (intensity, frequency, genesis location,

track and average lifetime) in the Bay of Bengal (BoB) are studied for the period

1891–2007 using cyclone e-Atlas of India Meteorological Department, Nino3.4 Index,

Oceanic Nino Index and Dipole Mode Index (DMI). TCs in the present study include

cyclones with maximum sustainable wind (MSW) C34 knots (referred as cyclonic storms)

and severe cyclonic storms with MSW C48 knots. The study shows a total of 502 TCs over

BoB during the 117-year study period at the rate of 4.29 TC per year. Seven-year running

mean of TCs for the period 1891–2007 shows a decreasing trend. Correlation between

Nino3.4 Index and DMI for the 117-year period is significant and positive and the sig-

nificance level is higher (lower) for the period with higher (lower) TC frequencies. One-

third monthly interval analysis for the 117-year period indicates first third (1–10) of

November as the most favoured period of TC formation over BoB. 117-year study period is

divided into years of ENSO (El Nino, La Nina and neutral ENSO) and IOD (?ve IOD, -ve

IOD and no IOD) categories. Maximum frequency of TC is observed during La Nina years,

-ve IOD years and also when La Nina co-occurred with -ve IOD. More severe cyclones

are formed during La Nina and ?ve IOD years. Genesis location of TCs indicates that

during La Nina (El Nino) years, the TCs are oriented in the south-east–north-west (south-

west–north-east) direction. TCs in no IOD and -ve (?ve) IOD years are more (less) in

northern BoB (north of 15� N), while in southern BoB (south of 15� N), TCs are more

(less) during no IOD and ?ve (-ve) IOD condition. BoB is divided into four quadrants,

and number of TCs in each quadrant is computed under different ENSO–IOD events. Peak

B. K. Mahala (&)Department of Mathematics, KIIT Polytechnic, Bhubaneswar 751024, Odisha, Indiae-mail: biranchimahala@gmail.com

B. K. NayakDepartment of Mathematics, Utkal University, Vani Vihar, Bhubaneswar 751004, Odisha, India

P. K. MohantyDepartment of Marine Sciences, Berhampur University, Berhampur 760007, Odisha, India

123

Nat HazardsDOI 10.1007/s11069-014-1360-8

direction of track movement is observed as north-east followed by north–north-west which

is corroborated from the dissipation of TCs in the specific quadrant. Total TC tracks in the

peak direction of track movement are maximum during El Nino and no IOD years. The

study reveals that TCs with shorter lifetime are observed during El Nino and -ve IOD

years, while TCs with longer lifetime are observed during La Nina, neutral ENSO and ?ve

IOD years. The decade with maximum TC formation is observed as 1921–1930, and the

impacts of ENSO and IOD on decadal variability are distinctly observed.

Keywords El Nino-Southern Oscillation � Indian Ocean Dipole � Tropical cyclone

1 Introduction

El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are two spectacular

and profound climate phenomena in the tropical ocean atmosphere-coupled system having

low-frequency variability. Impacts of ENSO and IOD reach far beyond the basins of their

evolution, i.e. Tropical Pacific and Indian Ocean, respectively. Understanding the influence

of ENSO and IOD on tropical cyclogenesis is a problem of great scientific and societal

importance.

The north Indian Ocean (NIO), including the Bay of Bengal (BoB) and Arabian Sea

(AS), is one of the important basins contributing about 7 % of total tropical cyclones (TCs)

over the world (WMO technical report 2008). The frequency of the TCs in general and

landfall of TCs in particular are more frequent over the BoB with the development of about

4 TCs per year and hence cause more disasters than the AS TCs (IMD 2008). TCs in BoB

occur more frequently in the months of October–December (primary TC peak season) and

April–June (secondary TC peak season). Due to the lack of observations, frequency,

intensity, genesis location, track and average lifetime of TCs and their inter-annual vari-

ability have not been studied in detail over BoB. Therefore, in the present study, we make

use of a long-term TC climatology data (1891–2007) set prepared by India Meteorological

Department (IMD) to study the impacts of ENSO and IOD on TC activity over BoB.

ENSO is a genuine ocean–atmospheric phenomenon born out of active interaction

between the two components of the climate system and is considered as the most dominant

inter-annual mode of tropical-coupled ocean–atmosphere system (McPhaden 2002). ENSO

events last approximately 12–18 months and occur every 2–7 years with large variation in

strengths (Chang et al. 2006). The extremes of ENSO, termed El Nino and La Nina,

encompass a wide range of climatic conditions. During ENSO events, the atmospheric

response to sea surface temperature (SST) anomalies in the equatorial Pacific perturbs the

Walker Circulation (Walker and Bliss 1932) and greatly influences the oceanic and

atmospheric condition across the globe (Trenberth 1997; McPhaden 2002). The influence

of ENSO on TC activity in the various basins has been studied by a large number of

investigators. TC activity over north-western Pacific (NWP) in relation to ENSO have been

well studied (Chan 2000; Wang and Chan 2002; Chia and Ropelewski 2002; Camargo and

Sobel 2005; Saunders et al. 2000; Wu et al. 2004; Webster et al. 2005; Camargo et al.

2007a, b, c). ENSO phenomenon is considered as the prominent factor controlling the

inter-annual variability of TC activity in the NWP (Chan 2007). The study by Camargo and

Sobel (2005) over NWP showed that during El Nino (La Nina) years, typhoons are more

(less) intense and long-living (short-living) due to large (small) values of accumulated

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cyclone energy. Webster et al. (2005) used 35 years (1970–2004) TC statistics and showed

that there is an increase in intensity and decrease in frequency and lifetime of TCs in all

basins except the North Atlantic. Reviews by Landsea (2000) and Chu (2004) on this

subject indicate large number of studies in the Pacific and Atlantic basins, while studies in

the NIO, particularly in BoB, are limited. Singh et al. (2000) reported a reduction in TC

activity over the BoB in severe cyclone months of May and November during warm phases

of ENSO. Mohanty et al. (2012) used 120-year cyclone data (1891–2010) of IMD over

Indian seas to study the genesis and intensity of TCs in warming environment. Their study

revealed that there is a large increase in the number of SCS during post-monsoon season

over southern sector of BoB (south of 10�N) and significant reduction in the rate of

dissipation of SCS over BoB during the current warming period (period after 1950). It was

also reported that relatively high (28–30 �C) SST, thermodynamically unstable atmosphere

and weak tropospheric wind shears were favourable for the TC development during these

periods of severe cyclone months (McPhaden et al. 2009). Many studies have investigated

the influence of atmospheric and oceanic conditions on the modulation of TC activity in

the BoB (Liebmann et al. 1994; Goswami et al. 2003; Ali et al. 2007; Sengupta et al. 2007;

Kikuchi et al. 2009; Lin et al. 2009). Camargo et al. (2007c) used the genesis potential

index to study the effect of ENSO on TC genesis in the world ocean and reported the

occurrence of a shift in the genesis potential from the northern to southern part of the BoB

between La Nina and El Nino year. However, the data period for computation of genesis

potential index was 1950–2005. Girishkumar and Ravichandran (2012) studied the influ-

ence of ENSO on TC activity in the BoB and indicated that ENSO significantly influences

the frequency, genesis location and intensity of TC during October–December. TC activity

is more (less) during La Nina (El Nino), and the genesis location shifts to the east (west) of

87�E of BoB. However, the data period for the study was limited to 1993–2010, and the

study was confined to primary TC peak season only.

The IOD, also referred to as the Indian Ocean Zonal Mode (IOZM), is another major

mode of climate variability in the Indian Ocean on inter-annual time scales (Saji et al.

1999; Webster et al. 1999; Murtugudde et al. 2000). Normally an IOD event evolves in

spring (May/June), peaks in fall (October–November) and terminates in early winter

(December) (Saji et al. 1999). A positive (?ve) [negative (-ve)] IOD period is charac-

terized by cooler (warmer) than normal water in the eastern tropical Indian Ocean (ETIO)

and warmer (cooler) than normal water in the western tropical Indian Ocean (WTIO) (Saji

et al. 1999). This pattern of SST variability has substantial impacts on the modulation of

the oceanic and atmospheric conditions in the Indian Ocean region and over the globe (Saji

et al. 1999; Ashok et al. 2001; Saji and Yamagata 2003; Girishkumar et al. 2012).

The role of IOD in climate variability has gained attention in recent years. The impacts

of the spatial structure of SST anomaly on Indian summer monsoon rainfall (Terray et al.

2003) and east African rainfall (Black et al. 2003) have been studied. Singh (2008)

observed negative and significant correlation between Indian Ocean Dipole Mode (IODM)

Index from September to October and November TC frequency in the BoB, and suggested

that, with the lead time of one month, IODM could be a potential predictor of intense

cyclone of November over BoB. According to Girishkumar and Ravichandran (2012), a -

ve IOD event can also trigger extreme TC cases in the BoB just as La Nina events do. It is

also observed that one of the contributing factors of ?ve IOD during El Nino years could

be the anomalous easterlies and north easterlies, which induce ocean currents that advect

the warm mean mixed layer of the central Indian Ocean to NIO (Drbohlav et al. 2007). In

non–El Nino years, the cooling of the ETIO, rather than warming of the WTIO, dominates

the IODM, and hence, IODM may not grow into the self-sustained mode of the

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tropospheric biennial oscillation (TBO). Besides monsoon-type circulation, El Nino related

winds may induce IODM, however, with a difference in structural pattern notwithstanding

the controversial ENSO–IOD relationship (Allan et al. 2001; Saji and Yamagata 2003;

Yamagata et al. 2002; Ashok et al. 2003). It is clearly indicated that IOD is a part of ENSO

evolution (Xie et al. 2002; Meyers 1996; Shinoda et al. 2004), but is also triggered by other

mechanisms (Saji et al. 2006). The co-occurrence of ENSO and IOD events make it

difficult to reach a clear conclusion of complex IOD–ENSO interaction on TC activity in

the BoB. In the present study, we use long-term TC climatology (1891–2007) to study the

impacts of ENSO and IOD on intensity, frequency, genesis location, tracks, dissipation,

average lifetime, inter-annual and decadal variability of TC in BoB.

2 Data and methodology

2.1 Data

Cyclone eAtlas–IMD (Electronic Version 1.0/2008) prepared by IMD (2008) is used to

generate the TC climatology data for 117 years (1891–2007) and constitute the main data

source for the present study. The domain for the present study covers the entire BoB

region. Cyclone eAtlas provides information on cyclonic disturbances (CD) which includes

depressions (D), cyclonic storms (CS) and severe cyclonic storms (SCS). The definition

used to classify a CD is as per IMD (2003). In the present study, TC includes CS (with

maximum sustained wind (MSW) 34–47 knots) and SCS (with MSW C 48 knots). The

climatology of CS and SCS is termed as TC climatology. Other data for the present study

include Nino3.4 Index, Oceanic Nino Index (ONI) and the Dipole Mode Index (DMI). The

sources and methodology adopted for computation of these indices are discussed in the

following section.

2.2 Methodology

2.2.1 Seven-year running mean

The concept of ‘‘running mean’’ was introduced by Panofsky and Brier (1958). It is an

averaging technique that allows for data, characterized by periodical oscillations on various

scales, to be filtered or smoothed. The principal utility of such an analysis, as applied in

this work, was to smooth out short-period fluctuations, allowing for potentially better

clarity with respect to longer-term climatological considerations (Schultz 2008). The time

period chosen for a yearly mean in this study corresponds to the calendar year (January–

December).

To compute the seven-year running mean, the total number of TC occurrences in a

calendar year was averaged over a period of 7 years. This value was then assigned to the

ending year of that time period. By repeating this process through advancing the calcu-

lations 1 year (both beginning and ending times), a 7-year running mean of the averages

was created. However, the climatology from April to December were considered for

determining the most favoured period of formation of TCs, as the number of TCs over BoB

during January–March was very less and the evolution periods of ENSO and IOD cover the

periods spanning from secondary TC peak season to primary TC peak season. Data from

April to December for the 117 years were divided into 10-day (or one-third monthly)

interval to focus the most favoured period of formation of TCs.

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2.2.2 ENSO and IOD dynamics: a basis for classifying the years

Equatorial Indian Ocean is warmer in the east, has a deeper thermocline and mixed layer,

and supports a more convective atmosphere than in the west. During certain years, the

eastern Indian Ocean becomes unusually cold, anomalous winds blow from east to west

along the equator and south-eastward off the coast of Sumatra, thermocline and mixed

layer lift up and the atmospheric convection gets suppressed. At the same time, western

Indian Ocean becomes warmer and enhances atmospheric convection. This coupled ocean–

atmospheric phenomenon in which convection, winds, SST and thermocline take part

actively is known as the IOD (Saji et al. 1999; Yamagata et al. 2003, 2004).

ENSO is a low-frequency mode of climate variability with strong coupling between the

ocean and the atmosphere in the Pacific equatorial cold tongue. The cold tongue is

maintained by upwelling of cooler water from the thermocline caused by the divergence of

directly wind-driven surface currents (McPhaden 2004). Enhanced upwelling is associated

with a shallow thermocline and stronger than normal surface divergence, and it results in

the cold extreme of ENSO, called La Nina. Diminished upwelling (deep thermocline and

weak surface divergence, or convergence in the case of westerly winds) results in the warm

extreme, the well-known El Nino. ENSO events last approximately 12–18 months and

occur every 2–7 years with large variation in strength. The long persistence of El Nino and

La Nina is a consequence of strong, two-way, positive feedback between the ocean and

atmosphere (upwelling and wind) in the cold tongue region, while the transition from El

Nino to La Nina (or vice versa) is controlled by a delayed negative feedback transmitted in

the depth of the thermocline (Kessler 2002; McPhaden 2004). Upwelling is the oceanic

process that links the slow physics of thermocline dynamics (Rossby and Kelvin waves) to

SST, giving long persistence and predictability to the climate system. The strength of

upwelling in the central and eastern Pacific is an essential controlling process in the ENSO

cycle, often represented by the so-called Nino3.4 Index (SST anomaly averaged in 5�N–

5�S, 120�W–170�W).

2.2.3 A method of classifying the years of ENSO and IOD

Different methods and numerous indices have been used to note the occurrence and

classification of ENSO events. Singh et al. (2011) and Meyers et al. (2007) in their paper

have explained the method for the classification of years statistically. Nino3.4 Index is one

of the several ENSO indicators based on SST. Nino3.4 is the average SST anomaly in the

region bounded by 5�N–5�S, from 120�W to 170�W. This region has large variability on El

Nino time scales, and is close to the region where changes in local SST are important for

shifting the large region of rainfall typically located in the far western Pacific. An El Nino

(La Nina) year is identified if the 5-month running average of the Nino3.4 Index exceeds

?0.4 �C (-0.4 �C) for at least 6 consecutive months (http://www.weatherzone.com.au). In

the present study, Nino3.4 Index (www.cgd.ucar.edu/cas/catalog/climind/TNI_N34/index.

html#) computed by Trenberth and Stepaniak (2001) is used to classify the year as El Nino/

La Nina/Neutral ENSO for the period from 1891 to 1949. In 2005, World Meteorological

Organization (WMO) adopted definitions of El Nino and La Nina events based on the

analysis of the ONI [3-month running mean of ERSST.v3b SST anomalies in the Nino3.4

region (5�N–5�S, 120�–170�W)], based on the 1971–2000 base period (http://www.cpc.

ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears1971-2000_climo.shtml).

WMO defined El Nino (La Nina) years, if ONI rose above 0.5 �C (fell below -0.5 �C), a

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minimum of 5 consecutive months and the same criterion has been used in the present

study to categorize El Nino/La Nina/Neutral ENSO years from 1950 to 2007.

Intensity of the IOD is commonly measured by an index that is anomalous SST gradient

between the Western Equatorial Indian Ocean (WEIO) (50�E–70�E and 10�S–10�N) and

the South-Eastern Equatorial Indian Ocean (SEEIO) (90�E–110�E and 10�S–0�). The

index is called the DMI. When the DMI is positive, then the phenomenon is referred as the

?ve IOD, and when it is negative, it is referred as -ve IOD (http://www.jamstec.go.jp/

frcgc/research/d1/iod/kaplan_sst_dmi_new.txt). In this study, we have computed the mean

of the DMI from June to November of every year and assigned the value to represent the

DMI of that particular year. Then, the mean and standard deviation [SD (r)] of DMI for

the climatology period (1891–2007) have been computed. We categorized the year as ?ve

IOD if the mean DMI (June–November) is greater than or equal to mean ? 1r and as -ve

IOD if the mean DMI (June–November) is less than or equal to mean - 1r (Fig. 1).

However, the years 1975, 1989, 1992 have been considered as -ve IOD years although the

mean DMI (June–November) is slightly greater than mean - 1 r as these years were also

considered as -ve IOD year by Meyers et al. (2007). The year 2007 has been considered as

a ?ve IOD year (http://www.bom.gov.au/climate/IOD/positive/).

3 Results and discussion

3.1 Trends in frequencies of tropical cyclones

A total of 502 TCs occurred during the 117-year study period (1891–2007) at the rate of

4.29 TCs per year. However, the study by Mohanty et al. (2012) using 120-year data

(1891–2010) depicted 606 CDs (which includes deep depression (DD), CS and SCS) over

BoB. The higher number of CDs reported by Mohanty et al. (2012) could be due to

difference in the data period and the inclusion of DD under CDs. The 7-year running mean

for the total yearly number of TCs is shown in Fig. 2.

This filtered data indicates that the number of TCs peaked around 1930. Figure 2 shows

a cyclic trend. The periods 1898–1905, 1916–1923, 1930–1938, 1945–1954 and

1972–2007 show a decreasing trend, while the periods 1905–1916, 1923–1930, 1938–1945

and 1954–1972 show an increasing trend. However, the results show an overall decreasing

trend in the frequency of TCs in the BoB for the entire period (1891–2007). The fre-

quencies of TCs were lower than the average frequency (4.29 per year) during 1901–1907,

1920–1924, 1949–1966, 1983–2006 and higher during 1897–1900, 1908–1919,

1925–1948, 1967–1982. In order to understand the impact of ENSO and IOD on the

frequency of TCs, we computed the correlation coefficient between Nino3.4 Index and

DMI for the 117 years. It is observed that except for the period 1897–1907, the correlation

is significant and positive indicating definite role of ENSO and IOD on the frequencies of

TCs over BoB. It is also observed that the level of significance is lower (higher) for the

periods with TC frequencies lower (higher) than the average frequency.

In order to get a clear picture of TC variability, we have computed the number of TCs

during a decade for the period from 1891 to 2007 (Fig. 3). Figure 3 suggests that the peaks

in the TC formation occurred in the decade 1921–1930 followed by 1931–1940. The period

corresponds to the period of missing data and data inhomogeneity during which the IOD–

ENSO relationship was inconsistent (Yuan and Yin 2008). A decline trend was observed

from the primary peak at 1921–1930 till 1951–1960. 1961–1970 showed a secondary peak

and thereafter the frequency showed a decline trend. Webster et al. (2005) analysis using

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35-year (1970–2004) TC statistics also yielded a decreasing trend in all ocean basins

including the Indian Ocean except the North Atlantic during the past decade.

For the decades 1891–1900, 1901–1910, 1941–1950, 1951–1960 and 1971–1980, the

frequency of TCs in La Nina years was more than that of El Nino years. A reverse pattern

was observed in the decades 1911–1920, 1921–1930, 1931–1940, 1961–1970, 1981–1990,

1991–2000 and 2001–2007. In the decades 1891–1900, 1901–1910, 1931–1940,

1941–1950, 1951–1960, 1981–1990 and 1991–2000, the frequency of TCs in a -ve IOD

event was more than that of a ?ve IOD event. Similarly a reverse pattern was observed in

the decades 1911–1920, 1921–1930, 1961–1970, 1971–1980 and 2001–2007. The study

suggests that the ENSO–IOD interaction is stronger (weaker) in the years with higher

(lower) frequencies of TC in the BoB and has been further examined in the succeeding

sections.

3.2 Favoured periods for tropical cyclone formation

As a means to investigate the most probable period for TC formation in the BoB, the TC

season from first one-third of April to December 31 was divided into 10-day or one-third

monthly intervals for the 117-year study period. Period from January to March is not

Fig. 1 Classification of IOD years based on DMI data and its standard deviation (r)

Fig. 2 Seven-year running mean for the total yearly number of TCs. The solid line represents the lineartrend

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considered as the number of TCs during these periods is very negligible. A 10-day interval

was chosen, rather than monthly, so that any favoured development periods might become

more apparent. Table 1 shows the TC season divided into 10-day intervals, the total

number of TC formations, and the average number of TC that occurred for the 10-day

intervals during the 117-year period of study. A close inspection of the data in the Table 1

for the months of April through December shows maximum peaks in TC development

(primary TC peak period) during first third of November and is considered as the most

favoured period of TC formation followed by during second one-third of November and

last one-third of October. Last one-third of April and entire May depict secondary peaks in

TC development. Besides depicting the primary and secondary TC peak periods over BoB,

the analysis also reveals peaks during last one-third of June and July which match with the

peaks of the secondary peak season (May). Thus, the results indicate that although mon-

soon season (June–September) is the most favoured period for D and DD, developments of

TCs matching with the secondary peak period do occur during monsoon season.

3.3 ENSO and IOD years

Nino3.4 and ONI values were employed for the period 1891–1949 and 1950–2007,

respectively, to determine the El Nino, La Nina and neutral ENSO years for the study

period. DMI was used to classify the years as ?ve IOD, -ve IOD and no IOD. Table 2

depicts the years classified as El Nino, La Nina, neutral ENSO, ?ve IOD, -ve IOD, no

IOD as well as the years with co-occurrence of ENSO–IOD events. Out of the 117 years,

33 years were identified as La Nina years, while El Nino and neutral ENSO years were 42

each. It may be noted that various authors have adopted different methodologies to classify

the ENSO and IOD years. As per WMO classification, a year is an El Nino year, if the ONI

is above 0.5 �C for a minimum of 5 consecutive months. In 1993, ONI was above 0.5 �C

for four consecutive months, yet it is considered as an El Nino year following some of the

recent studies (Kuleshov et al. 2008; Yuan and Yin 2008; Hendon et al. 2009; Singh et al.

2011; Girishkumar and Ravichandran 2012). It is, however, pertinent to mention that the

years identified as El Nino, La Nina and neutral ENSO closely match with other studies

Fig. 3 Decadal variability of TCs over BoB

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(Meyers et al. 2007; Yuan and Yin 2008; Girishkumar and Ravichandran 2012) although

the study periods were different for different authors.

Out of the 117 years of study period, number of ?ve IOD, -ve IOD and no IOD years

were identified as 18, 17 and 82, respectively. The identified types of IOD years match with

Meyers et al. (2007) and Girishkumar and Ravichandran (2012). Yuan and Yin (2008)

considered 11-year running mean correlation between seasonal mean IOD (JASON) and

Nino 3 indices and showed significant correlation from 1900 to 1920, low correlation from

1940 to 1970 and high correlation after 1970. They attributed low correlation before 1970

to missing data and data inhomogeneity. Similar observations were also made by Knutson

et al. (2010a, b), who indicated that the low-frequency variability of TC has an important

limitation due to data inhomogeneity resulting from changing observational systems with

time. Yuan and Yin (2008) concluded that after 1970, most El Nino events co-occur with

the IOD events and almost every ?ve IOD happens in an El Nino year. Girishkumar and

Ravichandran (2012) consider 1993–2010 as their study period and showed that except

1993 and 1996, all other ?ve (-ve) IOD years co-occurred with El Nino (La Nina) years.

Table 1 TCs formed over BoBduring 1891–2007 for one-thirdmonthly intervals

Month Date(one-third of amonth)

Total Averageper year

April 1–10 2 0.017

April 11–20 6 0.051

April 21–30 16 0.136

May 1–10 18 0.153

May 11–20 20 0.170

May 21–31 17 0.145

June 1–10 14 0.119

June 11–20 10 0.085

June 21–30 17 0.145

July 1–10 10 0.085

July 11–20 13 0.111

July 21–31 20 0.170

August 1–10 12 0.102

August 11–20 12 0.102

August 21–31 7 0.059

September 1–10 8 0.068

September 11–20 14 0.119

September 21–30 22 0.188

October 1–10 16 0.136

October 11–20 32 0.273

October 21–31 38 0.324

November 1–10 46 0.393

November 11–20 42 0.358

November 21–30 28 0.239

December 1–10 19 0.162

December 11–20 12 0.102

December 21–31 16 0.136

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In the present study, we observed 29 El Nino events and 10 ?ve IOD events before 1970,

out of which six events co-occurred. After 1970, there were 13 El Nino events and 8 ?ve

IOD events out of which five events co-occurred. Co-occurrence of ?ve (-ve) IOD years

with El Nino (La Nina) years also closely match with other studies (Meyers et al. 2007;

Yuan and Yin 2008; Girishkumar and Ravichandran 2012).

Our basic aim to classify the years as ENSO and IOD years is to assess the impact of

these events on the frequency, intensity, genesis location, life cycle and track position of

TCs over BoB. Table 3 depicts the statistics of TCs under different conditions. Out of 502

TCs over BoB during 117-year study period (1891–2007), 178,169 and 155 TCs were

observed during El Nino, neutral ENSO and La Nina years. The average frequency of TC

per year during El Nino, neutral ENSO and La Nina years are computed as 4.23, 4.02 and

4.69, respectively. On the other hand, the average frequency of TC per year during ?ve

IOD, no IOD and -ve IOD are computed as 4.0, 4.25 and 4.76, respectively. Considering

the years of co-occurrence, the frequency of TCs are computed as 3.81 for El Nino with

positive IOD, 4.5 for El Nino with -ve IOD, 4.37 for El Nino with no IOD, 5.25 for La

Nina with -ve IOD, 4.2 for La Nina with no IOD, 3.66 for neutral ENSO with ?ve IOD,

3.0 for neutral ENSO with -ve IOD and 4.18 for neutral ENSO with no IOD. We have not

considered the frequency for La Nina with ?ve IOD, as it was a single event during

117-year study period. Considering the impact of ENSO, it is apparent that the frequency

of TCs over BoB is highest during La Nina years followed by El Nino years. Similarly

considering the impact of IOD, the frequency of TCs is highest during -ve IOD years

followed by no IOD years. However, considering the co-occurrence of events, highest

frequency (5.25) is observed in case of La Nina with -ve IOD, which is even higher than

the highest frequency observed in case of individual ENSO or IOD events. Study by

Girishkumar and Ravichandran (2012) which examined the impact of ENSO on TC

activity in BoB, indicated that during La Nina regime both the thermodynamics and

dynamics conditions prevailing over BoB, favours most for the genesis of TCs which also

matches with our results showing the maximum frequency of TCs during La Nina

conditions.

However, the reasons of highest frequency during the period of co-occurrence of La

Nina with -ve IOD need to be investigated in detail, examining both the dynamics and

thermodynamics conditions prevailing then. It would be worth mentioning that during co-

occurrence of La Nina with -ve IOD, the SST anomaly pattern in the tropical Indian

Ocean is negative over WEIO and positive over SEEIO, while it is positive over Western

Equatorial Pacific Ocean and negative over South-Eastern Equatorial Pacific Ocean, which

forms a ‘‘- ? ? -’’ tropical Indian Pacific SST anomaly mode. As a result most of the

tropical Indian Ocean, Maritime Continent and Western Pacific exhibit a positive SST

anomaly. This condition leads to a strong convection over the Maritime Continent and

tropical Indian Ocean, while subsidence prevails over the relatively cold regions of WTIO

Table 3 Number of TCs over BoB in different ENSO–IOD events

El Nino Neutral ENSO La Nina Total number of TCs

-ve IOD 9 9 63 81

No IOD 127 138 84 349

?ve IOD 42 22 8 72

Total number of TCs 178 169 155 502

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and eastern Pacific. This forms two Walker Circulations over the tropical Indo-Pacific

sector and is well known as ‘‘atmospheric bridge’’ (Klein et al. 1999). Under the prevailing

condition of low-level convergence and upper-level divergence over the Maritime Con-

tinent, WTIO cyclogenesis is favoured. Cyclonic circulation is associated with southerlies

and south-easterlies over the south-eastern Indian Ocean (Drbohlav et al. 2007), and helps

the movement of the system in the north or north–westward direction favouring maximum

number of TCs over BoB. However, a critical examination of the thermodynamic and

dynamic variables under the La Nina with -ve IOD conditions would confirm our

hypothesis.

We examined the impacts of ENSO and IOD on the intensity of TC over BoB. TC

climatology used in the present study includes CS and SCS. Out of 178 TCs during El Nino

years (Table 3), 104 are CS and 74 are SCS. During La Nina years, numbers of CS and

SCS formed are, respectively, 88 and 67, while during neutral ENSO years, numbers of CS

and SCS are, respectively, 90 and 79. Considering the average number of SCS in a year, it

is revealed that more severe cyclones are formed during La Nina years followed by neutral

ENSO and El Nino years. The results are in agreement with Girishkumar and Ravichan-

dran (2012) which indicated that extreme TC cases are more prominent in BoB during La

Nina years due to favourable thermodynamic and dynamic condition prevailing over BoB.

Under different IOD regimes, numbers of CS and SCS formed are, respectively, 28 and 72

during ?ve IOD, 42 and 39 during -ve IOD, 212 and 137 during No IOD years. The

results suggest that extreme (SCS) TC cases are more during ?ve IOD years followed by

-ve IOD and no IOD years.

3.4 Genesis, lifetime, track propagation, dissipation of tropical cyclones

Girishkumar and Ravichandran (2012) observed distinct difference in genesis location and

track position of TCs in the BoB between La Nina and El Nino regimes. Further, their

analysis indicated that the difference in mean genesis location is the primary factor that

determines the track of cyclones in the La Nina and El Nino regimes. Therefore, in the

present study we laid emphasis on the genesis location, track and lifetime of TCs during

different ENSO and IOD regimes.

3.4.1 Genesis

Genesis location decides the track length which plays an important role on lifetime and

intensity of TC (Camargo et al. 2007a). Therefore, to get detailed information on the

genesis location and its consequent role on TC cycle, we divided the BoB into four

quadrants: R1 (15–26�N, 88.3�–100�E), R2 (15–26�N, 76.3–88.3�E), R3 (5–15�N,

76.3–88.3�E) and R4 (5–15�N, 88.3–100�E), and computed the TCs formed in each

quadrant (Table 4). Figure 4 depicts the genesis location of TCs for different ENSO and

IOD years. During El Nino years, total number of TCs formed are more than those during

La Nina and neutral ENSO years. During El Nino years, maximum (minimum) numbers of

TCs are formed in R3 (R2), while during La Nina and neutral ENSO years, maximum

(minimum) numbers of TCs are formed in R4 (R2). TC frequency in each quadrant was

computed by dividing the number of TCs formed in each quadrant (Table 4) by the total no

of ENSO/IOD events (Table 2). Comparison of frequency of formation of TCs in a par-

ticular quadrant under different ENSO conditions reveals that maximum frequency of

formation is observed in R1, R3 during El Nino years and in R2, R4 during La Nina years.

The results suggest that during La Nina (El Nino) years, the genesis of TCs is oriented in

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the south-east–north-west (south-west–north-east) direction. Under different ENSO

regime, peak density in genesis position is observed in R1 as compared to other quadrants.

Genesis of TCs during IOD years indicates that in the northern BoB (north of 15�N), TCs

during no IOD and -ve (?ve) IOD are more (less), while in southern BoB (south of 15�N),

TCs during no IOD and ?ve (-ve) IOD are more (less). Although IOD is an event having

dominant spatial structure in the zonal mode, its impact on the genesis of TCs shows a

meridional distribution which needs further investigation. During ?ve IOD, -ve IOD and

no IOD years maximum (minimum) TCs are formed in R4 (R2). Comparison of frequency

of formation of TCs in a particular quadrant under different IOD conditions reveals that

maximum TCs are formed in R1 and R2 during -ve IOD years and in R3 and R4 during ?ve

IOD years.

Lifetime of TCs has important implications on the intensity of TC and damage caused

due to it. During El Nino years, average lifetime of TCs is relatively shorter as compared to

those during La Nina and neutral ENSO years (Table 5). The results corroborate with our

Table 4 Number of genesis location points (average lifetime in days) of TCs in different regions duringENSO/IOD events

Event R1 R2 R3 R4 Total

(a) El Nino 48 (5.22) 24 (4.00) 58 (4.67) 48 (5.27) 178

(b) La Nina 37 (5.48) 23 (5.43) 37 (4.62) 58 (5.05) 155

(c) Neutral ENSO 38 (5.65) 14 (5.64) 47 (5.36) 70 (5.35) 169

(a) ?ve IOD 10 (6.40) 05 (4.80) 24 (4.50) 33 (5.78) 72

(b) -ve IOD 20 (5.40) 13 (4.23) 21 (5.38) 27 (5.33) 81

(c) No IOD 93 (5.38) 43 (5.18) 97 (4.87) 116 (5.04) 349

Total (a ? b ? c) 123 61 142 176 502

Fig. 4 Genesis location of TCs during the ENSO/IOD events during 1891–2007 a El Nino years, b La Ninayears, c neutral ENSO years, d ?ve IOD years, e -ve IOD years, f No IOD years. R1 (15�–26�N, 88.3�–100�E), R2 (15�–26�N, 76.3�–88.3�E), R3 (5�–15�N, 76.3�–88.3�E), R4 (5�–15�N, 88.3�–100�E) are fourquadrants. The number without (with) bracket indicates the number of TCs formed (average lifetime of TCsin days)

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observation made earlier regarding the genesis location of TCs; i.e. during El Nino years,

TCs mostly form in R3 followed by R1 and hence have shorter lifetime than the TCs in R4

during La Nina and neutral ENSO years which are away from the coast. The results also

agree with the observations of Girishkumar and Ravichandran (2012), albeit with a longer

lifetime. Further, it is observed that TCs formed over R4 have longer lifetime followed by

over R1 during El Nino years, while during La Nina and neutral ENSO years, TCs formed

over R1 have longer lifetime followed by over R2. Lifetime of TCs formed over R3 is,

however, shorter in all types of ENSO events. Average lifetime of TCs is relatively longer

(shorter) during ?ve (-ve and no IOD) IOD years. The study shows that TCs formed over

R1 have longer lifetime in all types of IOD events. TCs with shorter lifetime are observed

over R2 during ?ve and -ve IOD events and over R3 during no IOD events.

3.4.2 Track propagation

The risk at landfall depends not only on track but also on intensity and structure. In order to

isolate potentially predictable aspects of landfall, knowledge on the probabilistic behaviour

of track movement/propagation is essential. However, the track that a TC describes is

governed by a complex interaction of a number of internal and external influences. In the

absence of environmental steering, the northern hemisphere (NH) TCs move towards

northwest due to the equatorial beta effect (Chan and Williams 1987; Williams and Chan

1994). The upper (200 hPa) air flow pattern prevailing in the cyclone period is one of the

dominant factors influencing the TC movement (Neumann 1992) and also true for TCs

over BoB (Pattanaik and Rama Rao 2009). To determine the track propagation in BoB

under different ENSO–IOD conditions, we reconstructed the tracks of CS and SCS over

BoB as well as in three boxes, viz Box-A (18–30�N, 75–85�E), Box-B (18–25�N,

85–95�E), Box-C (10–18�N, 75–80�E) representing the north–north-westerly (NNW),

north-easterly (NE) and westerly moving tracks (Figs. 5, 6), respectively. Figure 5 depicts

the track propagation under ENSO events, while Fig. 6 depicts the track propagation under

IOD events.

The results reveal that more TCs move in NE followed by NNW, while less TCs move

in westward direction. In Box-A, total TC tracks as well as tracks of SCS are more during

El Nino years compared to La Nina and neutral ENSO years (Fig. 5). In Box-B, total TC

tracks are more in El Nino years compared to La Nina and neutral ENSO years, while the

tracks of SCS are more or less same under different ENSO types (Fig. 5). In Box-C, total

TC tracks as well as tracks of SCS are more in neutral ENSO years compared to El Nino

and La Nina years (Fig. 5). Excluding the number of tracks in Box-A, Box-B and Box-C,

there are about 25 tracks in BoB under El Nino condition, out of which, tracks of TCs

entering into AS are 11 including 7 SCS tracks. Similarly, TC tracks moving into AS

during La Nina are 9 and in neutral ENSO years are 21, out of which SCS tracks are 4 and

12, respectively.

The analysis shows that relatively less (more) number of TC tracks move in the

westward direction and further into AS during La Nina (El Nino) years. Girishkumar and

Table 5 Lifetime (in days) of TCs during ENSO/IOD events

Event El Nino La Nina Neutral ENSO ?ve IOD -ve IOD No IOD

Days 4.89 5.13 5.44 5.37 5.18 5.1

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Ravichandran (2012) indicated that eastward shifting of the mean genesis position, south-

easterly winds at 200 hPa over BoB and cyclonic vorticity over BoB induce TCs to steer

north-westward direction in a La Nina regime, while in El Nino regime the westerly

genesis position and the anticyclonic vorticity over BoB do not allow the TCs to be

significantly steered before their landfall. Thus, the genesis position of TCs that are

maximum in R3 and R4 during El Nino and La Nina years (Fig. 4), respectively, could be

attributed to the observed difference in westward moving tracks and the tracks of SCS

entering into AS. However, during neutral ENSO years maximum TCs are formed over R4

and also maximum TC tracks enter into AS, which is a matter of further investigation to

understand the specific track movement pattern.

Considering the IOD events, in Box-A (Fig. 6), total TC tracks as well as tracks of SCS

are more in no IOD years as compared to ?ve and -ve IOD years. Least number of track

propagation into Box-A occurs in the ?ve IOD years. In Box-B and Box-C also, total TC

tracks and SCS tracks are maximum in no IOD years and minimum in ?ve IOD years

Fig. 5 Tracks of TCs in specific boxes (left column) during ENSO events and all TC tracks (right column)in a specific ENSO event in BoB

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(Fig. 6). Total number of TC tracks and SCS tracks entering into AS is also maximum

during no IOD years.

The analysis shows that the pattern of SST variability, cooler (warmer) than normal in

the ETIO and warmer (cooler) than normal in the WTIO during ?ve (-ve) IOD years,

significantly modulates the TC activity over BoB and reduces the number to the minimum,

while no IOD condition favours TC activity. Minimum number of TC tracks in the NE,

NNW and westward direction during ?ve IOD condition is also attributed to the warm

(cold) SST anomaly in the west (east) Indian Ocean. The SST anomaly pattern creates

atmospheric convergence (divergence) in the west (east) Indian Ocean at low levels which

in turn drives anomalously ascending (descending) Walker Circulation in the west (east)

and strengthen (weaken) the convection in the western (eastern) Indian Ocean and induce

warm (cold) atmospheric Rossby waves that produce cyclonic (anticyclonic) circulation

anomaly at low level over BoB. The westerly (easterly) wind anomalies caused by anti-

cyclonic (cyclonic) circulation can strengthen (weaken) the westerly steering flow over

Fig. 6 Tracks of TCs in specific boxes (left column) during IOD events and all TC tracks (right column) ina specific IOD event in BoB

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BoB, which is unfavourable (favourable) for TCs moving westerly to the west of BoB.

Thus, in addition to the modulation of TC genesis conditions, IOD events also modify the

steering flow and subsequent track propagation of TCs (Yuan and Cao 2013).

3.4.3 Dissipation

Statistics of dissipation of TCs are generated based on point of dissipation in Cyclone

e-Atlas (IMD 2008). To distinguish the dissipation location, we divided the regions sur-

rounding BoB and AS into four sectors, viz S1 (18.5–32�N, 74.5–100�E), S2 (18.5–32�N,

49–74.5�E), S3 (5–18.5�N, 49–74.5�E) and S4 (5–18.5�N, 74.5–100�E) (figure not shown).

Note that these divisions are different from those mentioned in Table 4 about genesis

location. Irrespective of different ENSO and IOD conditions, maximum dissipation of TCs

occurs in S1 (18.5–32�N, 74.5–100�E) followed by S4 (5–18.5�N, 74.5–100�E) (Table 6).

This suggests that maximum TCs move in the NE and NNW direction and dissipate.

Dissipation of TCs also occur in large numbers in southern part of Indian landmass

(south of 18.5�N) and also in BoB. Corroborating the results for TC tracks, relatively more

(less) TCs dissipate over AS during El Nino (La Nina) years. As in case of TC tracks,

maximum dissipation in the AS occurs in neutral ENSO years. Like ENSO events, max-

imum dissipation of TCs during IOD events also occurs in S1 followed by in S4. Moving in

westward, TCs dissipate after making landfall over east coast of India, while some dis-

sipate over the BoB itself without landfall. TCs moving into S2 (18.5–32�N, 49–74.5�E)

and S3 (5–18.5�N, 49–74.5�E) and dissipating there during ?ve and -ve IOD years are

very less, while during no IOD years relatively large number of TCs move into S2 and S3

and dissipate there.

4 Conclusion

The influence of ENSO and IOD on TC activity in the BoB is studied for the period

1891–2007. The study shows that ENSO and IOD and their co-occurrence have significant

influence on the frequency, intensity, genesis location, track propagation, dissipation and

lifetime period of TC in the BoB. The study reveals a total of 502 TCs (CS with MSW

34–47 knots) and SCS (with MSW C 48 knots) in the BoB during the 117-year period at

the rate of 4.29 per year. Correlation between ONI and DMI is found positive and sig-

nificant for the 117-year period and suggests definite ENSO–IOD interaction on TC

activity over BoB. The decade with maximum TC formations is observed as 1921–1930

followed by 1931–1940, and the impacts of ENSO and IOD on decadal variability are

distinctly observed. Seven-year running mean shows a decreasing trend. 1–10 November is

Table 6 Dissipation of TCs indifferent regions during ENSO/IOD events

Event S1 S2 S3 S4 Total

(a) El Nino 109 17 07 45 178

(b) La Nina 103 13 03 36 155

(c) Neutral ENSO 95 19 09 46 169

(a) ?ve IOD 41 05 02 24 72

(b) -ve IOD 52 06 03 20 81

(c) No IOD 214 38 14 83 349

Total (a ? b ? c) 307 49 19 127 502

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found as the most favoured period of TCs coinciding with the primary TC peak season in

the BoB. Out of 117-year study period, 42 El Nino, 33 La Nina and 42 neutral ENSO years

are identified based on Nino3.4 Index and ONI, while DMI is used to identify 18, 17 and 82

?ve IOD, -ve IOD and no IOD years, respectively. TC frequency is highest (4.69 per

year) during La Nina years under ENSO type, during -ve IOD years (4.76 per year) under

IOD type and during La Nina with -ve IOD (5.25 per year) as co-occurrence event. The

highest TC frequency during the co-occurrence event is attributed to the SST anomaly

mode which is ‘‘- ? ?-’’ and the atmospheric bridge (Klein et al. 1999) over the tropical

Indo-Pacific sector which favours strong convection and cyclogenesis over the maritime

continent and tropical Indian Ocean.

The study reveals that more severe cyclones are formed during La Nina years under

ENSO type and during ?ve IOD under IOD type. The analysis shows that total number of

TCs formed during El Nino years are more than those during La Nina and neutral ENSO

years. Division of BoB into four quadrants: R1 (15–26�N, 88.3�–100�E), R2 (15–26�N,

76.3–88.3�E), R3 (5–15�N, 76.3–88.3�E) and R4 (5–15�N, 88.3–100�E) and computation of

TCs in each quadrant indicates that during El Nino years, maximum (minimum) TCs are

formed in R3 (R2), while during La Nina and neutral ENSO years maximum (minimum)

TCs are formed in R4(R2). The frequency of formation of TCs is maximum in R1 and R3

during El Nino years and in R2 and R4 during La Nina years. Genesis of TCs is mostly

favoured in the southern BoB (south of 15�N) than in northern BoB (north of 15�N) and

with more (less) TCs during no IOD and ?ve (-ve) IOD years. Frequency of formation of

TCs in northern (southern) BoB is maximum during –ve (?ve) IOD years. The study

reveals that TCs with shorter lifetime are observed during El Nino and -ve IOD years,

while TCs with relatively longer lifetime are observed during La Nina/neutral ENSO and

?ve IOD years.

Track propagation of TCs in BoB under different ENSO–IOD conditions is examined,

and it is revealed that more TCs move in NE followed by NNW, while less TCs move in

westward direction. Total TC tracks as well as tracks of SCS in the NE and NNW

directions are more during El Nino years compared to those during La Nina and neutral

ENSO years. However, westward moving TC tracks and SCS are more in neutral ENSO

years. The analysis shows that relatively less (more) number of TC tracks move in the

westward direction and further into AS during La Nina (El Nino) years. Total TC tracks as

well as tracks of SCS in the NE, NNW, westward direction and further into AS are

maximum (minimum) during no IOD (?ve IOD) years. The study shows that irrespective

of different ENSO–IOD conditions, dissipation of TCs mostly occur in S1 (18.5–32�N,

74.5–100�E) and S4 (5–18.5�N, 74.5–100�E) confirming the two distinct direction of track

movement, i.e. NE and NNW. The study clearly shows that the TC activities over BoB are

influenced by ENSO and IOD, and provides scope for future investigations on the dynamic

and thermodynamic conditions including numerical simulations to testify our results.

Acknowledgments The encouragement and facilities provided by Kalinga Institute of Industrial Tech-nology (KIIT) are gratefully acknowledged. The India Meteorological Department (IMD) provided cyclonee-Atlas.

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