Impact of Changing Climate on North Indian Ocean Cyclonic Disturbances and Associated Meteorological Features

Abstract

Cyclonic disturbances (CDs) have a significant impact on human life, properties, and the environment. Therefore, it has been an interesting research area among researchers all over the world. Fundamental and advanced processes associated with CDs are studied by most of the researchers to understand better and thereby predict the genesis and evolution of CDs. However, the meteorological, climatological, and landfalling features associated with CDs over a particular ocean basin are quite significant to be emphasized too. This thesis provides an overview of the climatology of these fascinating storms formed over the North Indian Ocean (NIO) basin and associated rainfall, meteorological and landfalling characteristics, and other environmental features. For this purpose, TC best track data provided by India Meteorological Department (IMD) and Joint Typhoon Warning Center (JTWC), rainfall product from IMD, several parameters like sea surface temperature (SST), air temperature, surface-level relative humidity (RH), mid-tropospheric relative humidity (RH500), surface-level wind (SW), and potential evaporation factor (PEF) from International Comprehensive Ocean-Atmosphere Data Set (ICOADS), National Oceanic and Atmospheric Administration (NOAA), and Hadley centre and inter-annual oscillation indices from NOAA, the United States and Bureau of Meteorology, Australia are used.
Prior to using the IMD TC best track data, the reliability of the same, and improvement by the implementation of satellite technology is discussed. The analysis indicates an improvement in the IMD best track data over the years in terms of quality, availability, and the frequency of genesis, intensity, and landfall etc. The determination of location and proper track of the CDs over NIO has improved during the satellite era, and the information related to the frequency of looping, southward moving, and recurving CDs is improvised. The percentage of the systems crossed or grazed the coast in the NIO, Bay of Bengal (BOB), and Arabian Sea (AS) basin/sub-basin is ~100% for both pre-satellite and satellite era. There has been almost robust data availability from 1961 onward with the advent of satellite technology.
Based on the annual SST anomaly trend, the period of study (from 1891 onward) is divided into pre-warming (PWP; during 1880–1946) and current warming (CWP; 1947 onward) with negative and positive anomaly (trend) respectively. The Mann-Kendall test and Sen’s slope estimation indicates a decreasing trend in annual CD (total storms) and CS+SCS (cyclones and severe cyclones) frequency during CWP for NIO region and particularly BOB at 95% confidence level. However, the CD and CS+SCS frequencies were increasing during the PWP. CD activity over southern and northern BOB is decreasing sharply during CWP. The southern sector of BOB hosts mostly severe systems (intensity >48 kts) and middle sector, tropical cyclones (intensity ≥ 34 kts). CD activity over the eastern sector of AS shows considerable enhancement during CWP. Increasing SST, SW, RH500, and PEF are helpful in the formation of intensified storms during CWP. The activities during PWP were reversed compared to that of CWP. A significant temperature anomaly difference between atmosphere and ocean also perceived to play a key role in modulating the enhanced intensity of TCs during CWP. The SST range of 27.5 to 29.5 °C and the supportive flow field is helping to enhance the middle and upper tropospheric moisture content; eventually, resulting in increased SST, PEF, and RH through a possible feedback mechanism. The trend for vertical wind shear is decreasing, supporting a higher rate of intensification of depressions to severe ones.
The impact of the warming climate on landfall activity reveals that Bangladesh (BD), Andhra Pradesh (AP), and Tamil Nadu (TN) are more vulnerable to severe cyclones formed over BOB during the CWP. Among western coastal states, Gujarat (GJ) is prone to SCS, and Arabian Peninsula countries are vulnerable to CS formed over the AS during the current warming climate as well. During CWP, BD and Arakan are more vulnerable to CD landfall in the pre-monsoon season, whereas in post-monsoon months, AP, TN, and BD are more prone coastal areas of BOB. The enhanced genesis over the southern and middle sector of BOB is mainly responsible for more landfall over AP, TN, and BD. The seasonal analysis of change in genesis location of CDs during PWP and CWP over BOB and AS agrees well with the landfall point of CDs. Also, changes in wind direction from NW to N-NW and increased meridional SST over BOB found to be encouraging the landfall activity near AP and TN coasts. The W-SW and zonally distributed SST supports landfall over Gujarat. There is less impact of change in genesis location over AS landfalling CDs. The destruction potential of CDs in terms of accumulated cyclone energy (ACE) during recent years is observed to be increasing as the rate of intensification of CDs has increased over NIO.
Every year, CDs cause destruction along the coastal areas of the world basins by pouring heavy rainfall, which causes floods and landslides. By using high-quality daily rainfall data, the contribution of rainfall by NIO CDs over India was also investigated.
Among eastern coastal states, the accumulated rainfall is higher over AP, TN, Odisha (OD), and southern West Bengal (WB) during pre-monsoon season. Among western coastal states, Karnataka (KA) and Kerala (KL) suffer maximum rainfall from CDs. During the post-monsoon season, coastal AP, TN, OD, KA, and coastal KL received higher accumulated rainfall. Gujarat received ~70%, and both AP and TN received up to 20-30% of rainfall by CDs during pre-monsoon months. In most of the states, the overall rainfall contribution by CDs is observed to have a decreasing trend during both seasons. Owing to the stable rainfall trend along with decreasing CD frequency during the post-monsoon season, the results indicate an increased amount of rainfall contribution by CDs during the season. CDs contribute a considerable amount of rainfall to central and northern India during the post-monsoon season.
Further, the rainfall contribution by CDs during El-Nino southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Madden-Julian Oscillation (MJO) under the impact of the warming climate emphasized for both NIO TC sessions. The accumulated rainfall observed to be high over AP and OD during the events considered in pre-monsoon season except for the La-Nina event where rainfall is high over GJ. During the post-monsoon season, the accumulated rainfall is high over AP, OD, and TN. In terms of annual variations, the annual CD days are low; however, the annual average rainfall is high for MJO periods during pre-monsoon season. La-Nina periods contributed second highest annual average CD rainfall during pre-monsoon season. For the post-monsoon season, negative IOD and La-Nina contributed higher CD rainfall, and the maximum CD days also observed to be higher. A significant amount of CD attributed rainfall is observed during a positive IOD event, and the same is also confirmed by the principal component analysis. The WRF regional analyses showed that CD rainfall in both seasons is relatively lower than the observations, but the spatial distribution is reasonably well predicted.
From the study carried out in this thesis, it is quite evident that the warming climate has an adverse impact on CD genesis over NIO. The decrease in the number of CDs with an increased rate of intensification in the changing climate scenario poses a threat to society. And, the increased rainfall by CDs during post-monsoon is also causing massive destruction in recent years.