**3. Materials and methodology**

The SST data analysis for the present study was obtained from National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch's (CRW) (known as 'CoralTemp') high-resolution time-series data product (available from NOAA Coral Reef Watch 2019 https://coralreefwatch.noaa.gov). This data set has a high spatial resolution of 5 km (0.05 × 0.05o C exactly) and a temporal resolution of one day. NOAA CRW global data product provides near real-time SST data from 1985 to the present. This datasets product uses advanced very high-resolution radiometer (AVHRR) satellite data from NOAA Pathfinder SST and has been found in good agreement with *in situ* data from ships and buoys. It also includes a large-scale adjustment of satellite biases with respect to the *in situ*. Bleaching Threshold (BT), Positive SST Anomaly (PA), and Degree Heating Weeks (DHW) are commonly used indices for calculating thermal stress on coral reefs. BT is based on the concept of Thermal Threshold (also known as long-term climatological mean), and the thermal threshold for the Gulf of Kachch & Malvan region was computed using the mean of warmest month SST based on NOAA Optimum Interpolated Sea Surface Temperature (OISST) from 1982 to 2016 (35 years period). SST Anomaly was derived by subtracting the thermal threshold from daily SST values. SST Anomaly provided the information of magnitude of thermal stress and was computed for the 11 years during the warmest period (from 2010 to 2020). DHW provides information on the intensity and duration of thermal stress experienced by coral reefs. DHW product is a cumulative measure of thermal stress over an area over three months [33]. The DHW product indicates the reefs around the world which are at risk of bleaching. Coral bleaching generally begins for corals exposed to a DHW value of 0.5 or more [34]. The categories which are used to describe the severity of bleaching for Indian regions are no stress (0° C<DHW≤2° C), bleach watch (2° C<DHW≤4° C), warning (4° C<DHW≤6° C), alert level-1 (6° C<DHW≤8° C) and alert level-2 (DHW>8° C) based on DHW [8, 15].

### **4. Results and discussion**

The SST variations during the warmest period from 2010 to 2020 for both regions provide information on magnitude, intensity, and duration of thermal stress. The warmest month, warmest quarter, thermal threshold, and bleaching threshold for both regions were computed from NOAA CRW datasets. Based on the maximum frequency of the warmest month recording the maximum monthly mean SST in the year, the climatologically warmest months were identified for both coral reef regions. The warmest month, warmest quarter, Thermal Threshold, and Bleaching

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

#### **Figure 2.**

*(Top) Sea surface temperature; (middle) sea surface temperature anomaly; (lower) degree heating weeks variations over Gulf of Kachchh region during warmest quarter (May to July) period from 2010 to 2020.*

Threshold for both regions were found to be different. The climatologically warmest month for the Gulf of Kachchh region was June, and the warmest quarter was May to July. Similarly, the climatologically warmest month for the Malvan region was May, and the warmest quarter was April to June. The Gulf of Kachchh region recorded a maximum thermal threshold of 29.35° C(±0.45° C), and the Malvan region recorded

#### **Figure 3.**

*(Top) Sea surface temperature; (middle) sea surface temperature anomaly; (lower) degree heating weeks variations over Malvan region during warmest quarter (April to June) period from 2010 to 2020.*

maximum threshold of 29.39° C(±0.49° C) (**Figures 2** and **3**). In both figures, the orange dotted line shows the thermal threshold, and the red dotted line shows the bleaching threshold for the corals become stressed when SSTs crossed thermal threshold and get bleached if the elevated SST regime is prolonged. BT and daily SST anomalies have been computed on the basis of thermal threshold (sometimes *Climate Change Induced Thermal Stress Caused Recurrent Coral Bleaching over Gulf… DOI: http://dx.doi.org/10.5772/intechopen.96806*

referred as climatologically mean warmest month). Once the climatologically warmest month was identified, the climatologically warmest quarters were identified comparing SST values averaged over three summer months period from a combination of the warmest month and two adjacent months (i.e., the pre and post month). SST anomalies for the warmest quarter were calculated as the absolute difference between the daily SST and the thermal threshold. The warmest quarter anomalies were plotted from 2010 to 2020 for both regions, and positive anomalies were represented in red color while negative anomalies were represented in blue color. It was found that a stark rise in the number of days recording the positive anomalies. Another thermal stress index termed as coral bleaching DHW was calculated in order to assess the accumulative thermal stress. The DHW was generated using the warmest quarter daily SST data. The DHW was also represented in color from blue to green based on their intensity and duration of thermal stress.
