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Inter-annual Variation and Trends in Tropical Cyclones and Monsoon Depressions Over the North Indian Ocean

M. Mohapatra, A.K. Srivastava, S. Balachandran and B. Geetha

Introduction

Tropical cyclones (TCs) are large synoptic scale weather systems which originate over the warm oceans of the world and develop into massive vortices composed of swirling winds, intense clouds and torrential rains by drawing energy from the oceans. While moving over land, they cause large-scale destruction to life and property over the coastal areas of the world. The east and west coasts of India are prone to the destructive features of TC activity (Mohapatra et al. 2012a) over the North Indian Ocean (NIO) comprising of the Bay of Bengal (BOB) and the Arabian Sea (AS).

The climatological features of the cyclonic disturbances (CDs) forming over the NIO, BOB and AS are given in Tables 1 and 2. On an average, 11 CDs with maximum sustained wind speed (MSW) of 17 knots or more including depressions and TCs develop over the NIO every year (based on data of 1961-2010). Over the BOB and AS, 9 and 2 CDs, respectively, form as per the data of satellite period (1961-2010) (Mohapatra et al. 2014). Out of these, about five CDs intensify into TCs (MSW > 34 knots), including about 4 over the BOB and 1 over the AS. About 3 severe TCs (MSW > 48 knots) form over the NIO (2 over BOB and 1 over the AS). The frequency of very severe TCs (64 knots or more) over the NIO is about 2. [1] [2]

Table 1 Mean, standard deviation and linear trend in frequency of CDs, TCs and severe TCs over the NIO based on data of 1901-2010

Basin

Annual/season

CDs

TCs

Severe TCs

Mean

SD

Trend/decade

Mean

SD

Trend/decade

Mean

SD

Trend/decade

NIO

Annual

12.6

3.8

-0.188

5.1

1.9

-0.193

2.5

1.6

0.068

Pre-monsoon

1.4

0.9

0.003

1.0

0.8

0.001

0.7

0.7

0.002

Monsoon

6.4

2.8

-0.254

1.5

1.3

-0.173

0.4

0.7

-0.009

Post-monsoon

4.5

1.7

0.062

2.6

1.2

-0.02

1.3

1.2

0.074

BOB

Annual

9.8

3.2

-0.299

4.1

1.7

-0.196

1.9

1.3

0.057

Pre-monsoon

1.1

0.8

-0.002

0.8

0.7

0

0.5

0.6

0.014

Monsoon

4.9

2.3

-0.299

1.1

1.1

-0.176

0.2

0.5

-0.021

Post-monsoon

3.7

1.5

0.02

2.2

1.2

-0.019

1.1

1.1

0.064

AS

Annual

1.8

1.4

0.117

1.0

1.1

0.011

0.6

0.8

0.017

Pre-monsoon

0.4

0.6

0.004

0.3

0.5

0.002

0.2

0.4

-0.012

Monsoon

0.6

0.7

0.054

0.3

0.6

0.009

0.2

0.4

0.011

Post-monsoon

0.8

0.9

0.060

0.4

0.6

0.003

0.2

0.5

0.018

Trends significant at the 95 % confidence level are emphasised in bold

Table 2 Linear trend in frequency of CDs, TCs and severe TCs over NIO during the satellite era (1961-2010)

Basin

Annual/season

CDs (1961-2010)

TCs (1961-2010)

Severe TCs (1961-2010)

Mean

SD

Trend/decade

Mean

SD

Trend/decade

Mean

SD

Trend/decade

NIO

Annual

11.9

3.9

-1.818

4.8

1.8

-0.683

3.0

1.6

-0.524

Pre-monsoon

1.4

0.9

-0.085

1.1

0.8

-0.078

0.8

0.8

-0.065

Monsoon

5.5

2.8

-1.183

0.9

0.9

-0.243

0.4

0.7

-0.090

Post-monsoon

4.8

1.8

-0.507

2.7

1.3

-0.377

1.8

1.2

-0.370

BOB

Annual

8.6

3.1

-1.467

3.6

1.5

-0.627

2.3

1.3

-0.513

Pre-monsoon

1.0

0.7

-0.065

0.8

0.6

-0.084

0.6

0.6

-0.070

Monsoon

3.7

2.2

-0.931

0.5

0.7

-0.188

0.2

0.4

-0.090

Post-monsoon

3.7

1.5

-0.429

2.3

1.2

-0.369

1.5

1.1

-0.355

AS

Annual

2.2

1.5

-0.118

1.1

1.0

-0.020

0.7

0.8

0.001

Pre-monsoon

0.4

0.6

-0.025

0.3

0.5

0.007

0.2

0.4

0.005

Monsoon

0.8

0.8

-0.001

0.4

0.6

0.003

0.2

0.5

0.012

Post-monsoon

1.0

1.0

-0.091

0.5

0.6

-0.028

0.3

0.5

-0.016

Trends significant at the 95 % confidence level are emphasised in bold

Development of TCs is in general seasonal in nature, with most tropical ocean basins having maximum frequency of formation during the late summer-to-early autumn period. This is associated with the period of maximum sea surface temperature (SST), although other factors, such as the seasonal variation in the Inter-Tropical Convergence Zone (ITCZ)/monsoon trough location, are also important (Gray 1968). However, unlike other ocean basins, the TCs frequency over the NIO shows bimodal character with primary peak during October to December (OND, post-monsoon season) followed by the secondary peak during the pre-monsoon season (March-May, MAM) (Li et al. 2013). During the south-west monsoon season (SWM) of June to September (JJAS), intense low-pressure systems usually do not develop due to the northward shift of the convergence zone (monsoon trough) over the land and high vertical wind shear over the region (Rao 1976). About 5-6 CDs including about one TCs form over the NIO during the monsoon season (Tables 1 and 2). The TCs in monsoon season usually occurs during the onset phase (month of June) and withdrawal phase (month of September) of south-west monsoon (Rao 1976).

India Meteorological Department (IMD), the official operational weather forecaster of India, is responsible for the detection, tracking and forecasting the movement and intensity of TCs over the NIO and has meticulously archived records of CDs over the NIO for more than a century. The data have been initially archived and presented month-wise in chart/map form from 1877 to 1990 [IMD (1979) and IMD (1996)]. In the present form, this archive is available as a more versatile electronic tool, Cyclone eAtlas—IMD (IMD 2008, 2011), which could generate tracks and statistics of CDs over the NIO (www.rmcchennaieatlas.tn.nic.in). An extensive climatology of TCs over the NIO covering statistical aspects of CDs formation, intensification, movement, landfall, dissipation, etc. has been documented by IMD using these data and products (Raj 2011).

Regarding the trends in CDs/TCs activity over the NIO, detailed review is available in Sikka (2006), Mohapatra et al. (2012b) and Niyas et al. (2009). Sikka (2006) has noted a drastic decrease in the number of total CDs in the decade of 1991-2000 to less than even 50 % of the average of earlier 10 decades (1891-1990). Not only the total number of CDs has drastically decreased, but the number of TCs and severe TCs on the decadal basis has also shown significant decrease in the last two decades. Considering all CDs over the NIO (excluding short-lived CDs with life period of less than a day), Mohapatra et al. (2012b) noted that there is no significant trend in frequency of CDs over the BOB and AS during the period 1891-2010. Tyagi et al. (2010) have shown that there is no significant trend in frequency of land-falling CDs over the east and west coasts of India during 1891-2007.

Considering only TCs, based on data of 1891-2008, Niyas et al. (2009) have observed a significant decreasing trend in the frequency of TCs over the NIO, and the rate of decrease in frequency is maximum for the monsoon season. Considering the frequency of (i) TCs, (ii) severe TCs and (iii) very severe TCs during satellite era (1961-2010), there are significant decreasing trends in all these frequencies over the BOB and NIO as a whole (Mohapatra et al. 2014). The frequency of TCs has decreased at the rate of about 0.7 and 0.6 per decade, respectively, over the NIO and BOB.

It has been documented by Webster et al. (2005) that TCs frequency has decreased over all the oceanic basins of the world except the North Atlantic, but the number of severe TCs has increased over all the oceanic basins. Knutson et al. (2010) examined the impact of climate change on TCs forming over different oceanic basins. The study concluded that it was uncertain whether past changes in TCs activity have exceeded the variability expected from natural causes. A review of multi-decadal scale TCs variations and possible “greenhouse warming” effects has been covered by Landsea (1998). According to Landsea (1998) and Chan (2006), the recent trend in TCs genesis frequency and intensity cannot be attributed to trends in SST, rather, to large-scale oscillations such as ENSO. However, TCs over various oceanic basins do not respond identically to ENSO. Some show changes in frequency of genesis, while others show shifts in the genesis locations or in intensity due to coupled interaction of local parameters and large-scale interaction of oceanic and atmospheric circulation features, including ocean thermal energy, vertical wind shear, relative vorticity embedded in large-scale oscillations such as ENSO, Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), Atlantic Multi-decadal Oscillation (AMO). Detailed review is available in Girish Kumar and Ravichandran (2012), Rajeevan et al. (2013) and Girish Kumar et al. (2014).

Patnaik (2005) examined variability of storm activity over the NIO during the pre-monsoon, post-monsoon and monsoon seasons for a period of 113 years (1891-2003) and found that large-scale atmospheric circulation is the main cause of the observed inter-decadal variability of the storm activity over the Indian region rather than the variability of SSTs over the region. Ng and Chan (2011) have examined the inter-annual variations in TCs activity over the NIO during 19832008 and found an evidence of the influence of ENSO and have shown that instead of local SSTs, such variations, at least over the BOB during the post-monsoon season, can be attributed to similar variations in the atmospheric flow patterns and moist static energy that are apparently forced largely by the ENSO.

Monsoon lows and depressions are the major synoptic scale systems that contribute a large quantum of south-west monsoon rainfall (Mooley and Shukla 1989). As the frequency of monsoon depression and depression days has decreased, the SW monsoon rainfall has been compensated by an increase in the number of monsoon lows and low days in the recent years, and therefore the all-India monsoon rainfall did not show any significant trend. However, the relationship between monsoon rainfall and frequency of CDs shows secular changes (Mooley and Shukla 1989; Mohapatra and Mohanty 2004, 2007). Similarly, the north-east monsoon rainfall over southern peninsular India during the post-monsoon season also shows secular variations with frequency of CDs. Land-falling CDs during OND contribute about 10 % of the seasonal rainfall over the peninsular India (Geetha and Raj

2014).

Considering all the above, trends in seasonal and annual frequency of CDs, TCs and severe TCs over NIO in inter-annual and decadal scales are presented in this chapter based on long-period data of 1901-2010 as well as for the period of 1961— 2010, corresponding to the satellite era. Impacts of these trends in the monsoon and post-monsoon seasonal rainfall over various meteorological sub-divisions are also brought out. The trends are also analysed from the point of view of climate forcing such as the ENSO.

  • [1] M. Mohapatra (H) India Meteorological Department, Earth System Science Organization, New Delhi, Indiae-mail: This email address is being protected from spam bots, you need Javascript enabled to view it A.K. Srivastava India Meteorological Department, Earth System Science Organization, Pune, India
  • [2] Balachandran • B. Geetha India Meteorological Department, Earth System Science Organization, Chennai, India © Springer Science+Business Media Singapore 2017 89 M.N. Rajeevan and S. Nayak (eds.), Observed Climate Variability and ChangeOver the Indian Region, Springer Geology, DOI 10.1007/978-981-10-2531-0_6
 
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