atmosphere

Article Major Floods and Tropical Cyclones over : Clustering from ENSO Timescales

Md Wahiduzzaman

Institute for Climate and Application Research, Nanjing University of Information Science and Technology, Nanjing 210000, China; [email protected]

Abstract: The present study analyzed major floods and tropical cyclones (TCs) over Bangladesh on El Niño-Southern Oscillation (ENSO) timescales. The geographical location, low and almost flat topography have introduced Bangladesh as one of the most vulnerable countries of the world. Bangladesh is highly vulnerable to the extreme hazard events like floods and cyclones which are impacted by ENSO. ENSO is mainly a tropical event, but its impact is global. El Niño (La Niña) represents the warm (cold) phase of the ENSO cycle. Rainfall and cyclonic disturbances data have been used for the period of 70 years (1948–2017) and compared with the corresponding observations of the Southern Oscillation Index. Result shows that major flood events occurred during the period, and most of them are during the La Niña condition, consistent with the historical archives of flood events in Bangladesh. Synoptic conditions of these events are well matched during La Niña condition. On the other hand, the major TC cases are in the period of either pre-monsoon or post-monsoon season. The pre-monsoon cases are under neutral (developing La Niña) or El Niño and the post-monsoon cases are under La Niña, consistent with climatology studies that La Niña is  favorable to have more intense TCs over the . 

Citation: Wahiduzzaman, M. Major Keywords: flood; tropical cyclone; El Niño Southern Oscillation; Bangladesh Floods and Tropical Cyclones over Bangladesh: Clustering from ENSO Timescales. Atmosphere 2021, 12, 692. https://doi.org/10.3390/ 1. Introduction atmos12060692 Bangladesh is located at the interface of two contrasting settings with the Bay of Bengal (BoB) to the south and the to the north. This provides the country the life-giving Academic Editors: Corrado Camera monsoon, on the one hand, and the catastrophic disasters like tropical cyclone (TC), storm and Georgios Zittis surge, flood, drought, and , on the other [1]. The geographical vulnerability of Bangladesh lies in the fact of an exceedingly flat, low-lying, alluvial plain covered by over Received: 7 April 2021 230 rivers and rivulets with approximately 580 km of exposed coastline along the BoB. As Accepted: 26 May 2021 Published: 28 May 2021 a result of its geography, Bangladesh frequently suffers from devastating floods, TCs and storm surges, tornadoes, riverbank erosion, and drought etc. [2,3] which is highly impacted

Publisher’s Note: MDPI stays neutral by the El Niño-Southern Oscillation, a main source of year-to-year variability in weather with regard to jurisdictional claims in and climate for many areas of the world [4]. published maps and institutional affil- The El Niño-Southern Oscillation (ENSO) is a global scale ocean/atmosphere phe- iations. nomenon between the Pacific and Indo-Australian regions. In the Pacific Ocean, three basic states exist—El Niño, La Niña, or Neutral [5]. El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle and La Niña refers to the periodic cooling of sea-surface temperature across the east-central equatorial Pacific. It represents the cold Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. phase of the ENSO cycle, and is sometimes referred to as a Pacific cold episode [6]. El Niño This article is an open access article is an extensive warming of the upper ocean in the tropical eastern Pacific lasting more distributed under the terms and than 5 months. Usually the Niño3 index is defined as the mean sea surface temperature ◦ ◦ ◦ ◦ conditions of the Creative Commons anomaly to climatology over the region 5 N–5 S, 150 W–90 W to identify El Niño ◦ Attribution (CC BY) license (https:// events. Quantitative definition of an El Niño needs a Niño3 index to be larger than 0.5 for creativecommons.org/licenses/by/ 5 months at least [7]. In addition, the warmer than average waters were shown to be closely 4.0/). related to a global atmospheric pressure oscillation known as the Southern Oscillation.

Atmosphere 2021, 12, 692. https://doi.org/10.3390/atmos12060692 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 692 2 of 15

ENSO has an influence on flood which is defined as a relatively high flow of water that overtops the natural or artificial banks of a stream [8]. Flood causes major damage to life and property (i.e., land and buildings) in Bangladesh [9]. For example, the flood in 1987 affected 39.9% area in Bangladesh which was devastating in nature compared to previous flood years i.e., 1955, 1963, 1969, 1970, and 1974 [10]. An increased amount of precipitation can cause flooding in Bangladesh [10]. An above normal monsoon downpour in the -Brahmaputra-Meghna drainage system is thought to be the primary cause of the 1988 flood in Bangladesh. The causes of floods are thoroughly discussed by Brammer [10]. Another natural hazard, for example, a devasting TC depends on its intensity. The BoB is a favorable breeding ground of TCs [10–21] and Bangladesh is the worst sufferer due to cyclonic casualties in the world [16]. About 5.5% of the TCs of the global total form in the BoB [11–22] and about 1% hit Bangladesh. On the other hand, if the TC disasters due to each of which the minimum death tolls were 5000 are considered, then it is found that a death toll of about 53% of the global total occurred in Bangladesh [1–3]. ENSO has also impact on TCs. During strong El Niño years, Bangladesh has not faced any catastrophic cyclone but experienced a high number when ENSO condition is neutral for example, during 1960–1970 period there was no strong El Niño but neutral and Bangladesh faced eighteen cyclones [23] during that time. Singh et al. [24] found a reduction rate of severe TC activity over the BoB in pre-monsoon and post-monsoon season during warm phases of ENSO. They also investigated the impact of Southern Oscillation on the cyclogenesis over the BoB during the summer monsoon and found more depressions formed in El Niño years. Two interesting results were reported by Mandal [25,26] regarding the recurving and landfall properties of the TCs of the BoB with respect to ENSO activity. In the North Indian Ocean, about 35% and 65% of the TCs are recurving and non-recurving types, respectively. TCs tracks analysis during severe El Niño years (1901 to 1990) showed 39% and 61% TCs have found to be recurving and non-recurving types, respectively and those TCs formed in the BoB, mostly cross the Indian coast (south of 20◦ N) in the El Niño years whereas in La Niña years, most of the cyclones crossed coastal regions (north of 20◦ N) of West Bengal and Bangladesh [25,26]. In the present study, the major floods and TCs are identified and categorized under El Niño, La Niña, and neutral ENSO. Moreover, the recurving properties of the TCs that hit Bangladesh are also categorized during ENSO time scales. This will be helpful to tackle the future climate change in Bangladesh. TCs in BoB and their relationship with ENSO have been investigated in many earlier studies [27–29]. The influence of ENSO on TC activities in the BoB during post-monsoon season is studied by Girishkumar and Ravichandan [19] and they found more TCs formed in La Niña years than El Niño years. During La Niña years, the existence of low-level cyclonic vorticity, enhanced convection, and high TC heat potential in the BoB provided favorable conditions for TC activities [27]. This paper categorizes the major floods and TCs that affected Bangladesh on the ENSO timescales and attempts to find out the mechanisms behind it during those events by considering the synoptic conditions. The structure of this paper is organized as follows. The rainfall, TCs, and synoptic data are described in Section2. Section3 presents the results of the major floods and TCs activity over the Bangladesh by considering El Niño, La Niña and Neutral phase. Mechanisms are discussed also in Section3 and conclusion is provided in Section4.

2. Data and Methods The El Niño and La Niña years are identified from the multivariate ENSO Index (Table1 ). The Southern Oscillation Index (SOI) is calculated from the monthly fluctuations in the air pressure difference between Tahiti and Darwin. This SOI uses the surface pressure normalized by a standard deviation. Sustained negative values of the SOI indicate El Niño episodes and positive values of the SOI are associated with La Niña episode [6,30]. These data (1948–2017) were collected from Bureau of Meteorology, Australia. The same period Atmosphere 2021, 12, 692 3 of 15

of quality controlled rainfall data in Bangladesh were collected from both Bangladesh Meteorological Department and Bangladesh Water Development Board, respectively. The data are then compared against the corresponding SOI data on a monthly basis. To find out the major inland floods of Bangladesh, 35% affected area are considered and therefore, 7 floods are identified. The events of the floods are collated in tabular form as function of time by considering El Niño and La Niña. Neutral considers the SOI value which is in between +0.5 and −0.5.

Table 1. Identified years under the phase of ENSO.

Phase of ENSO Years 1958, 1966, 1973, 1978, 1980, 1983, 1987, 1988, 1992, 1995, 1998 2003, El Niño 2007, 2010, 2014 1950, 1951, 1955, 1956, 1962, 1971, 1974, 1976, 1989, 1999, 2000 2008, La Niña 2011, 2012 1948–1949, 1952–1954, 1957, 1959–1961, 1963–1965, 1967–1970, 1972, Neutral 1975, 1977, 1979, 1981–1982, 1984–1986, 1990–1991, 1993–1994, 1996–1997, 2001–2002, 2004–2006, 2009, 2013–2014, 2015–2016.

BoB TCs data have been collected from Bangladesh Meteorological Department and India Meteorological Department. The data are again compared against the corresponding SOI data on monthly basis. Again, to find out the major tropical cyclones of Bangladesh, maximum sustained wind speeds is considered and therefore, 12 tropical cyclones are identified. The TC event is collated in tabular form as a function of time under El Niño and La Niña. Surface precipitable water, surface pressure, sea level pressure, precipitation and surface runoff anomalies are calculated from NOAA Physical Science Laboratory using NCEP/NCAR reanalysis data. The NCEP/NCAR reanalysis does not accurately depict the pressure and rainfall from tropical cyclones, but it is considered to be more representative of the large-scale composite synoptic environment. Monthly ENSO SST series are downloaded from NOAA physical sciences laboratory. The vertical wind shear (u and v as zonal and meridional velocity components respectively) is calculated according to the formula below q 2 2 (u200 − u850) + (v200 − v850) The low-level vorticity and relative humidity here are represented by 850-hPa pressure level vorticity and relative humidity. Wind, vorticity, and humidity data are downloaded from NOAA physical sciences laboratory.

3. Results 3.1. Categorizing of Major Floods on ENSO Timescales The monthly and seasonal percentage of the total annual rainfall from 1948 to 2017 have been shown in Figure1. The highest percentage of the total annual rainfall (21.1%) is observed in the month of July (Figure1a). Besides, the percentage of the total annual rainfall is relatively high in June (19.93%), August (17.14%), and September (13.05%). On the other hand, the percentage of the total annual rainfall (0.08%) is lowest in February (Figure1a). There are mainly four seasons in Bangladesh—winter (December–February), pre-monsoon (March–May), monsoon (June–August) and post-monsoon (October–November). Season- ally, the highest percentage of the total annual rainfall (70.6%) is observed in monsoon and lowest (1.4%) in winter (Figure1b). Pre-monsoon and post-monsoon contribute 20% and 8% of total respectively (Figure1b). Atmosphere 2021, 12, x FOR PEER REVIEW 4 of 15

pre-monsoon (March–May), monsoon (June–August) and post-monsoon (October–

Atmosphere 2021, 12, 692 November). Seasonally, the highest percentage of the total annual rainfall (70.6%)4 of 15is observed in monsoon and lowest (1.4%) in winter (Figure 1b). Pre-monsoon and post-monsoon contribute 20% and 8% of total respectively (Figure 1b).

25

) 80

% 20

) 70 (

% y

( 60

15 y 50 10 uenc 40 q uenc 30 5 q

Fre 20 0 Fre 10 0 n r r y n l t v c eb p Ju ug p c Ja F Ma A Ma Ju A Se O No De Winter Pre monsoon Monsoon Post monsson Month Season

(a) (b)

Figure 1. Monthly ( a)) and and seasonal seasonal ( b)) percentage of the total annual rainfall during 1948–2017.1948–2017.

As shown inin FigureFigure1 b,1b, Bangladesh Bangladesh receives receives most most of theof the rainfall rainfall during during monsoon monsoon sea- seasonson when when the floodsthe floods generally generally occur dependingoccur depending on the rainfall on the intensity rainfall inintensity the catchments in the catchmentsof the rivers. of The the major rivers. flood The years major have flood been years categorized have been based categorized on the amount based ofon large the amountaffected of area large (at affected least one area third (at portion least one of Bangladesh).third portion Atof Bangladesh). least two-third At of least the two-third total area ofin Bangladeshthe total area was in affectedBangladesh by the was flood affected of 1988 by and the 1998 flood whereas, of 1988 at and least 1998 one-third whereas, part ofat leastBangladesh one-third was part affected of Bangladesh by the flood wasof affected 1955, 1974, by the 1987, flood 2004, of 1955, and 20071974, (Table1987, 22004,). These and 2007years (Table are then 2). consideredThese years under are then the phaseconsider ofed ENSO under to findthe phase which of years ENSO belong to find to which yearsENSO belong phase to (Table which2) soENSO that phase a conclusion (Table 2) can so be that made a conclusion on major can floods be made of Bangladesh on major floods(Figure 2ofa–g) Bangladesh under ENSO (Figure timescales. 2a–g) Theunder flood EN yearsSO timescales. of 1955 (Figure The 2flooda), 1974 years (Figure of 21955b), (Figure1988 (Figure 2a), 21974d), 1998 (Figure (Figure 2b),2 e)1988 belong (Figure to La 2d), Niña 1998 and (Figure 1987 (Figure 2e) belong2c), 2004 to La (Figure Niña2 andf) 1987in El (Figure Niño and 2c), 2007 2004 (Figure (Figure2g) 2f) belongs in El Niño to neutral and 2007 ENSO (Figure phase. 2g) belongs to neutral ENSO phase. Table 2. Categorizing the major floods in Bangladesh on ENSO timescales. Table 2. Categorizing the major floods in Bangladesh on ENSO timescales. Year Flood Affected Area (Sq km) Affected Area (%) SOI Phase of ENSO Year1955 Flood Affected 50,700 Area (Sq km) Affected 34.2 Area (%) 2.4 SOI Phase La of Niña ENSO 19551974 52,72050,700 36.6 34.2 1.42.4 La Niña 19741987 57,49152,720 39.9 36.6 −2.21.4 La El NiñoNiña 1988 89,970 63 2.8 La Niña 19871998 100,25057,491 68 39.9 1.9−2.2 LaEl Niño Niña 19882004 55,00089,970 38 63 −1.22.8 La El NiñoNiña 19982007 100 62,289,250 42.268 0.11.9 La Neutral Niña 2004 55,000 38 −1.2 El Niño 2007A flood occurred during62,289 July–August in 1955 which 42.2 inundated 34.20%0.1 area ofNeutral Bangladesh and SOI indicates positive value in this time, as a result La Niña was prominent in that period (Figure2a). La Niña (Figure2a,b,d,e) was also prominent in 1974 (inundated about 36.6%), 1988 (inundated about 60%), 1998 (maximum inundated ~68% area of Bangladesh) and floods occurred in monsoon period (July–August). On the other hand, El Niño (Figure2c,f ) was also prominent in 1987 (inundated about 40%) and 2004 (inundated about 38%), respectively. A flood in 2007 (inundated about 42%) was recorded under neutral phase of ENSO (Figure2g).

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SOI in 1955 Flood of 1955 SOI in 1974 Flood of 1974

4 5 3 4 2 3 1 2 SOI SOI 0 1 -1 0 -2 -1 123456789101112 123456789101112 Month Month

(a) (b)

SOI in 1987 Flood of 1987 SOI in 1988 Flood of 1988

0 4 -0.5 3 -1 2 -1.5 1 SOI SOI -2 0 -2.5 -3 -1 -3.5 -2 123456789101112 123456789101112 Month Month

(c) (d)

SOI in 1998 Flood of 1998 SOI in 2004 Flood of 2004

4 3 2 2 0 1 0 -2 SOI SOI -1 -4 -2 -6 -3 -8 -4 123456789101112 123456789101112 Month Month

(e) (f)

SOI in 2007 Flood of 2007

4 3 2 1

SOI 0 -1 -2 -3 123456789101112 Month

(g)

FigureFigure 2. Seven 2. Seven major major floods floods (a ()a 1955,) 1955, (b (b)) 1974, 1974, ( (cc)) 1987,1987, ((d) 1988, ( (ee)) 1998, 1998, (f ()f )2004, 2004, (g ()g 2007) 2007 over over Bangladesh Bangladesh during during ENSO ENSO phase.phase. Black Black line line represents represents the variabilitythe variability of SOI, of SOI, and greenand green line isline the is base the linebase where line where positive positive value value (>0.5) (>0.5) of SOI of indicates SOI El Niño, negative value (<−0.5) indicates La Niña, and rest (−0.5 to 0.5) shows the neutral ENSO. Magenta dot indicates the month of the year when flood started.

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indicates El Niño, negative value (<−0.5) indicates La Niña, and rest (−0.5 to 0.5) shows the neutral ENSO. Magenta dot indicates the month of the year when flood started.

A flood occurred during July–August in 1955 which inundated 34.20% area of Bangladesh and SOI indicates positive value in this time, as a result La Niña was prominent in that period (Figure 2a). La Niña (Figure 2a,b,d,e) was also prominent in 1974 (inundated about 36.6%), 1988 (inundated about 60%), 1998 (maximum inundated ~68% area of Bangladesh) and floods occurred in monsoon period (July–August). On the other hand, El Niño (Figure 2c,f) was also prominent in 1987 (inundated about 40%) and 2004 (inundated about 38%), respectively. A flood in 2007 (inundated about 42%) was Atmosphere 2021, 12, 692 recorded under neutral phase of ENSO (Figure 2g). 6 of 15

3.2. Categorizing of Major TCs on ENSO Timescales 3.2. CategorizingTwelve severe of Major cyclonic TCs stor on ENSOms with Timescales hurricane speed (on the basis of maximum wind speeds) were identified (Table 3). La Niña (Figure 3a,e,h,l) was prominent in 1960 (wind Twelve severe cyclonic storms with hurricane speed (on the basis of maximum wind speed 193 km/h), 1970 (wind speed 193 km/h), 1988 (wind speed 193 km/h), and 2007 speeds) were identified (Table3). La Niña (Figure3a,e,h,l) was prominent in 1960 (wind (wind speed 193 km/h). On the other hand, El Niño (Figure 3i,j,k) was also prominent in speed 193 km/h), 1970 (wind speed 193 km/h), 1988 (wind speed 193 km/h), and 2007 1991 (wind speed 225 km/h), 1994 (wind speed 200 km/h), and 1997 (wind speed 230 (wind speed 193 km/h). On the other hand, El Niño (Figure3i,j,k) was also prominent km/h), respectively. Rest five TCs 1961 (wind speed 161 km/h), 1963 (wind speed 202 in 1991 (wind speed 225 km/h), 1994 (wind speed 200 km/h), and 1997 (wind speed km/h), 1965 (wind speed 184 km/h), 1974 (wind speed 161 km/h), 1985 (wind speed 154 230 km/h), respectively. Rest five TCs 1961 (wind speed 161 km/h), 1963 (wind speed km/h) were under neutral phase of ENSO (Figure 3b–d,f,g). 202 km/h), 1965 (wind speed 184 km/h), 1974 (wind speed 161 km/h), 1985 (wind speed 154 km/h) were under neutral phase of ENSO (Figure3b–d,f,g). Table 3. Categorizing of major TCs in Bangladesh.

TableYear 3. Categorizing Month of major TCs in Bangladesh. Crossing Area SOI Phase of ENSO 1960 30–31 October North Part of Chittagong 0.8 La Niña Year Month Crossing Area SOI Phase of ENSO 1961 5–9 May Meghna Estuary 0.2 Neutral 19631960 28 30–31 May October North North Part Part of Chittagong of Chittagong 0.2 0.8 Neutral La Niña 1961 5–9 May Meghna Estuary 0.2 Neutral 19651963 7–15 December 28 May North Cox’s Part Bazar of Chittagong 0 0.2 Neutral Neutral 19701965 13 November 7–15 December North Part Cox’s of Chittagong Bazar 2.8 0 La Neutral Niña 19741970 24–28 13November November North Chittagong Part of Chittagong −0.5 2.8 Neutral La Niña 19851974 22–25 24–28 May November Chittagong Chittagong 0.3−0.5 Neutral Neutral 1985 22–25 May Chittagong 0.3 Neutral 19881988 24–30 24–30 November November Khulna Khulna 3 3 La La Niña 19911991 25–30 25–30 April April North North Part Part of Chittagong of Chittagong −−2 2El El Niño Niño 19941994 24 April–2 24 April–2 May May Teknaf Teknaf −−1.71.7 El El Niño Niño 19971997 15–19 15–19 May May Teknaf Teknaf −−3 3El El Niño Niño 2007 11–15 November Khulna Barisal 1.4 La Niña 2007 11–15 November Khulna Barisal 1.4 La Niña

SOI in 1961 Cyclone of 1961

4 2 0

SOI -2 -4 -6 123456789101112 Month

(a) (b)

Figure 3. Cont.

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SOI in 1963 Cyclone of 1963 SOI in 1965 Cyclone of 1965 3 2 1 1 0 0 -1 SOI -1 SOI -2 -2 -3 -3 123456789101112 -4 Month 123456789101112 Month

(c) (d)

SOI in 1970 Cyclone of 1970 SOI in 1974 Cyclone of 1974

4 5 3 4 2 1 3 0 2 SOI SOI -1 1 -2 -3 0 -4 -1 123456789101112 123456789101112 Month Month

(e) (f)

SOI in 1985 Cyclone of 1985 SOI in 1988 Cyclone of 1988

2 4 3 1 2 0 1 SOI SOI 0 -1 -1 -2 -2 123456789101112 123456789101112 Month Month

(g) (h)

SOI in 1991 Cyclone of 1991 SOI in 1994 Cyclone of 1994

2 0.5 0 1 -0.5 0 -1 -1 -1.5 SOI SOI -2 -2 -2.5 -3 -3 -4 -3.5 123456789101112 123456789101112 Month Month

(i) (j)

Figure 3. Cont.

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SOI in 1997 Cyclone of 1997 SOI in 2007 Cyclone of 2007 SOI in 1997 Cyclone of 1997 SOI in 2007 Cyclone of 2007

3 4 3 4 2 3 2 3 1 2 1 2 1 00 1 SOI SOI -1 0 SOI SOI -1 0 -1 -2-2 -1 -2 -3 -2 -3 -3 -4-4 -3 123456789101112 11 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 123456789101112 Month Month Month Month

((kk)) ((ll))

FigureFigure 3. 3. Twelve Twelve major major cyclones cyclones ( a( a) ))1960, 1960, 1960, ( b( (b) ))1961, 1961, 1961, ( c( (c)c ))1963, 1963, 1963, ( d( (dd) ))1965, 1965, 1965, ( e( (ee) ))1970, 1970, 1970, ( f( (f)f ))1974, 1974, 1974, ( g( (gg) ))1985, 1985, 1985, ( h( (hh) ))1988, 1988, 1988, ( i( ()ii ))1991, 1991, 1991, ( j( ()jj ))1994, 1994, 1994, (k(k) )1997, 1997, ( (l(l)l) )2007 20072007 over over Bangladesh Bangladesh underunder under ENSOENSO ENSO phase. phase. phase. Black Black Black line line line represents re representspresents the the the variability variability variability of SOI,ofof SOI, SOI, and and and green green green line line isline the is is basethe the basebaseline line whereline where where positive positive positive value value value (>0.5) (>0.5) (>0.5) of SOI of of SOI indicatesSOI indicates indicates El Niño, El El Niño, Niño, negative negative negative value value value (<− 0.5)(< (<−−0.5)0.5) indicates indicates indicates La Niña,La La Niña, Niña, and and and rest rest (rest−0.5 ( −(−0.5 to0.5 0.5)to to 0.5)0.5)shows shows shows the the neutralthe neutral neutral ENSO ENSO ENSO phase. phase. phase. Magenta Magenta Magenta dot dot indicatesdot indi indicatescates the the monththe month month of theof of the yearthe year year when when when cyclone cyclone cyclone started. started. started.

AsAsAs aa a casecase case studystudy amongamong tenten cyclones,cyclones, thethe the rere re-curvature-curvature-curvature ofof of cyclonecyclone trackstracks hashas beenbeen studiedstudiedstudied (Figure(Figure (Figure 4a–j).44a–j).a–j). Ten Ten TCs TCsTCs tracks trackstracks analys analysisanalysisis over overover the thethe BoB BoBBoB (affected (affected Bangladesh)Bangladesh) show showshow ◦ thatthatthat thethe the re-curvaturere-curvature re-curvature takestakes takes placeplace place betweenbetween between 14–19°14–19° 14–19 NNN forfor for thosethose those developeddeveloped developed inin in ElEl NiñoNiño andand ◦ 12–16°12–16°12–16 N NN for for for those those those developed developed developed in in in La La La Niña Niña Niña with with with one one one exception exception exception of of of 29 29 29 October, October, October, 1999 1999 cyclone cyclone whichwhichwhich had had had no no no re-curvature re-curvature (Table (Table 4 4 and and FigureFigure4 4a–j). a–j).4a–j).

((aa)) ((bb))

Figure 4. Cont.

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(c) (d)

(e) (f)

Figure 4. Cont.

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(g) (h)

(i) (j)

Figure 4. Ten major TC TC tracks tracks in in ( (aa)) 1981, 1981, (b (b) )1988, 1988, (c ()c )1991, 1991, (d (d) )1992, 1992, (e ()e 1995,) 1995, (f) ( f1997,) 1997, (g ()g 1998,) 1998, (h) ( h1999,) 1999, (i) (2006,i) 2006, and and (j) (2007j) 2007 over over Bangladesh Bangladesh during during ENSO. ENSO. Black Black line line and and dot dot represent represent the the track track and and movement movement over over time. time.

Table 4. Re-curvature analysis of the major ten cyclones over Bangladesh during ENSO.

Point of Lat. of Re-Curvature Point of Cyclone Month SOI ENSO Phase Formation (N) (N) Landfall (N) 1981 December 0.7 La Niña 11◦ 12◦–16◦ 21.2◦ 1988 November 3 La Niña 10◦ 14◦ 20.5◦ 1991 April −2 El Niño 11.5◦ 16◦ 22.3◦ 1992 November −1.4 El Niño 9.5◦ 16◦ 21◦ 1995 November −0.1 El Niño 7.5◦ 14◦ 20◦ 1997 May −3 El Niño 7.5◦ 19◦ 21.8◦ 1998 November 1.7 La Niña 12◦ 16◦ 21◦ 1999 October 1.5 La Niña 12◦ Straight 19.8◦ 2006 April 1.5 La Niña 9.5◦ 12◦ 16.5◦ 2007 November 1.4 La Niña 9.5◦ 12◦ 21◦

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3.3. Mechanisms The ENSO affects Indian monsoon through an interaction between the equatorial Walker circulation and the regional Hadley circulation, which are considered as the world’s two prominent and important atmospheric zonal and meridional circulations, respectively. There is a reduction of rainfall in Bangladesh for all seasons during El Niño events and Bangladesh did not face any catastrophic flood during the strong El Niño events. However, the weak El Niño and the transitional time from El Niño to La Niña are favourable for having floods and TCs in Bangladesh. There is a strong correlation between monsoon years and strong La Niña events. During El Niño years, Bangladesh is not struck by any catastrophic cyclone but when the ENSO index is small (either negative or positive) and when 28.5 °C isotherm stays left of 165◦ E longitude then Bangladesh is likely being hit by a cyclone. This is the case for seeing cyclones in the pre-monsoon season in El Niño events. When the ENSO index is positive, Walker circulation is strong, atmospheric pressure in pacific is high and wind is easterly. During the negative ENSO index phase, the Walker circulation is weakened, and the easterly wind is reversed. When the Walker circulation is weakened then Hadley circulation gets stronger. The strength of Bangladesh monsoon is governed by the movements of cyclonic disturbances formed in the Pacific. During the weak or moderate El Niño events, the Hadley circulation may not be very strong and allows some of the tropical disturbances to cross into the BoB and enter up to Bangladesh region causing flood and cyclone, as Bangladesh falls between 22 and 26◦ N of the equator [22]. As shown in Sections 3.1 and 3.2, Bangladesh faces more floods and TCs during La Niña. Figure5 shows the composite anomalies of precipitable water, surface pressure, sea level pressure, surface runoff, and precipitation during La Niña events. We can see that these synoptic conditions are favourable in La Niña. For example, the surface precipitable water is positively correlated in the northern area of Bangladesh (Figure5a) where the major river systems in Bangladesh are located and Himalaya is also located in the north of Bangladesh. Other synoptic variables (surface pressure, sea level pressure, surface runoff, and precipitation climatology) are favorable for flood events in La Niña (Figure5b–e ). Figure6 shows the annual average anomalies of relative humidity, sea surface temperature, vorticities, and vertical wind shear in La Niña phase. During La Niña period, the low verti- cal wind shear and enhanced low level vorticity, relative humidity, sea surface temperature are more favorable environment for TC development in the BoB. This is consistent with the study by Wahiduzzaman et al. [29]. Atmosphere 20212021,, 1212,, 692x FOR PEER REVIEW 1212 of of 15

(a) (b)

(c) (d)

(e)

Figure 5. Synoptic conditions ( a) surface precipitable water, water, ( b) surface pressure, ( c) sea level pressure, ( d) surface runoff, runoff, and (e) precipitation anomaly)anomaly) duringduring LaLa Niña.Niña.

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Figure 6. Relative humidity, sea surface temperature, vorticities, and vertical wind shear anomalies during ENSO pos- Figureitive/La 6. NiñaRelative phase. humidity, Blue (Yellow, sea surface Red, andtemperature, Pink) shade vorticitie representss, and negative vertical (positive) wind shea units.r anomalies Dot points during indicate ENSO >5% positive/Lasignificance Niña level. phase. Blue (Yellow, Red, and Pink) shade represents negative (positive) units. Dot points indicate >5% significance level. 4. Conclusions 4. Conclusions The geographical location, land characteristics, multiplicity of rivers, and the monsoon climateThe render geographical Bangladesh location, highly vulnerableland characteristics, to natural hazardsmultiplicity like TC, of stormrivers, surge, and flood, the monsoondrought, climate tornado, render riverbank Bangladesh erosion highly etc. Flood vulnerable and TC to arenatural not hazards a new phenomenon like TC, storm in surge,Bangladesh. flood, Adrought, total of 7torn floodsado, and riverbank 14 TCs were erosion more etc. devasting Flood and catastrophicTC are not ina termsnew phenomenonof their intensity in Bangladesh. and percentage A total of total of 7 affected floods area.and 14 TCs were more devasting and catastrophicIn an El in Niño terms event, of their excess intensity rainfall and is percentage seen in July of and total deficit affected is observed area. in August– SeptemberIn an El and Niño during event, the excess La Niña rainfall event, is theseen excess in July rainfall and deficit is observed is observed in July–September in August– September(Figure7). Floodsand during occurred the duringLa Niña July–September event, the excess and the rainfall flooding is yearsobserved of 1955, in July– 1974, September1988, 1998, (Figure and 2007 7). wereFloods found occurred under during La Niña July–September and 1987, 2004 and were the inflooding El Niño. years About of 1955,5 floods 1974, were 1988, interlinked 1998, and with 2007 excess were rainfallfound under and La La Niña Niña event and whereas1987, 2004 2 floodswere in were El Niño.under About the deficit 5 floods rainfall were and interlinked El Niño event.with ex So,cess the rainfall excess and and La deficit Niña ofevent rainfall whereas are not 2 floodsalways were responsible under the for adeficit flood occurrencerainfall and in El Bangladesh. Niño event. So, the excess and deficit of rainfallFour are cyclonesnot always belong responsible to La Niña for a conditionflood occurrence which hitin Bangladesh. northern and north-western part of Chittagong and three cyclones belong to El Niño condition which hit southern part of Chittagong with exception of 1991 and the rest five cyclones belong to neutral ENSO. Among these five cyclones, four cyclones are found under weak La Niña and if the neutral condition is not considered then Bangladesh faces more TCs during La Niña. The re-curvature takes place in higher latitudes for those developed in El Niño than La Niña phase. Synoptic variables support the occurrence of major floods and TCs in Bangladesh under La Niña phase.

AtmosphereAtmosphere 20212021,, 1212,, x 692 FOR PEER REVIEW 1414 of of 15 15

June July August September

20

15

10 5 Frequency

0 Excess Deficit Excess Deficit

El Nino La Nina Month

FigureFigure 7. 7. FrequencyFrequency of of excess excess and and deficit deficit of of rainfall rainfall during during monsoon monsoon under under El El Niño Niño and and La La Niña. Niña.

Funding:Four Thiscyclones work belong is supported to La by Niña Special condit Grantion from which China hit Postdoctoral northern and Science north-western Foundation part(No.2020T130311). of Chittagong and three cyclones belong to El Niño condition which hit southern partInstitutional of Chittagong Review with Board exception Statement: ofNot 1991 applicable. and the rest five cyclones belong to neutral ENSO. Among these five cyclones, four cyclones are found under weak La Niña and if theInformed neutral Consent condition Statement: is not consideredNot applicable. then Bangladesh faces more TCs during La Niña. TheData re-curvature Availability Statement:takes placeData in higher will be availablelatitudes upon for those request. developed in El Niño than La Niña phase. Synoptic variables support the occurrence of major floods and TCs in Conflicts of Interest: The author declares no conflict of interest. Bangladesh under La Niña phase. 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