**1. Introduction**

Coral reefs are one of the most ancient, dynamic, highly sensitive, complex, biologically diverse, highly productive ecosystems, found in the tropical coastal environment between 30° N and 30° S latitudes. Coral reefs provide an conducive environment where one-third of all marine fish species and many thousands of other species are found and offer substantial ecological and economic services to millions of people through fishery and tourism worldwide [1]. Coral reef ecosystems are degrading in rapid pace, and some facing extinction risk due to the synergistic impact of global climate change and chronic human activities including overfishing, pollution, eutrophication, sedimentation, coastal development [2–4]. Rapid decline of coral reef ecosystem health is now the most pressing challenge to the reef managers [1, 5]. Most of the tropical coral reefs are found only within a narrow range of environmental conditions, making them vulnerable to abrupt change in seawater physico-chemical parameters like temperature, and salinity [6–10]. Coral bleaching events are associated with thermal stress are acute disturbances recognized as the primary global challenge to the persistence of coral reefs, which disrupts the mutualistic relationship of corals with the thermo-sensitive endosymbiotic dinoflagellates of the family *Symbiodinaceae* by photoinhibition and their expulsion. Coral bleaching can be divided into two parts: (1) The initial response where corals expel *Symbiodinium*, and (2) The longer-term effect, which may be either coral tissue recovery or mortality [11]. Coral bleaching events occur when sea surface water becomes so warm and remains high for more than 28 days [6, 8, 12]. Coral mortality after bleaching depends on the extent of heat stress, its severity, and duration of bleaching [13, 14]. Coral bleaching prevalence and the extent of subsequent coral mortality patterns are commonly associated with natural and anthropogenic disturbances, which is highly venerable both within and across the region.

The ENSO event is one of the significant climatic events that trigger the rapid warming of the water column of the seas, altering biogeochemical processes and marine life. Thermal stresses associated with the El Niño Southern Oscillation (ENSO) are occurring with increasing frequency and severity [10]. The global SSTs have risen gradually since the 1980s, which have caused mass coral bleaching (MCB) and mortality in more than 90% of reefs since 1997–1998. Many researchers have reported four significant MCB events (i.e., 1982–1983, 1997–1998, 2010, 2015–2016) all over the world over the past four decades due to global warminginduced by the ENSO event [8, 15–17]. The 2015–2016 ENSO event emerged as the most extreme event in terms of ocean warming intensity and extent across the tropical oceans [18–21], which caused one of the most severe and widespread MCB events across the Indo-Pacific [15, 20]. The MHW caused by the 2015–2016 ENSO was unprecedented over the period of two centuries resulting in ecological and economic consequences worldwide [10]. More than 75% of global coral reefs have witnessed MCB and mortality back-to-back from 2014 to 2016 [19, 20]. Corals can re-establish themselves after mass bleaching in some cases; it takes one to two decades for the ecosystem to return to the pre-bleaching state [22]. However, the increasing thermal stress left no window of recovery for corals from the previous bleaching events, leading to mass mortality [19, 20, 23]. Mass coral bleaching events can cause long-term ecological, economic, and social impacts [1, 24]. As increase in the frequency and severity of MCB could overwhelm the ability of coral reefs to recover between events. Consecutive mass coral bleaching episodes and associated coral mortalities could shift coral reefs from coral dominated state to Cyanobacteria and algae dominated state [25, 26].

The objective of this study is to examine the thermal stress that causes coral bleaching over the coral reef regions on the Eastern Arabian Sea in the Indian Ocean using long-term NOAA CRW SST data. We computed the long-term climatologically mean and trend of SST for these coral reef regions and computed coral bleaching thermal indices: BT, PA, and DHW based on the SST analysis. Further, to ground-truth the accuracy of the computed coral bleaching indices, we visit at field sites for coral monitoring and analyzed the bleaching percentage.

*Climate Change Induced Thermal Stress Caused Recurrent Coral Bleaching over Gulf… DOI: http://dx.doi.org/10.5772/intechopen.96806*
