J. Earth Syst. Sci. (2021) 130:157 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-021-01668-1 (0123456789().,-volV)(0123456789().,-volV) Chlorophyll-a, SST and particulate organic carbon in response to the cyclone Amphan in the Bay of Bengal 1, 2 1 MD RONY GOLDER * ,MD SHAHIN HOSSAIN SHUVA ,MUHAMMAD ABDUR ROUF , 2 3 MOHAMMAD MUSLEM UDDIN ,SAYEDA KAMRUNNAHAR BRISTY and 1 JOYANTA BIR 1Fisheries and Marine Resource Technology Discipline, Khulna University, Khulna 9208, Bangladesh. 2Department of Oceanography, University of Chittagong, Chittagong 4331, Bangladesh. 3Development Studies Discipline, Khulna University, Khulna 9208, Bangladesh. *Corresponding author. e-mail: [email protected] MS received 11 November 2020; revised 20 April 2021; accepted 24 April 2021 This study aims to explore the variation of Chlorophyll-a (Chl-a), particulate organic carbon (POC) and sea surface temperature (SST) before (pre-cyclone) and after (post-cyclone) the cyclone Amphan in the Bay of Bengal (BoB). Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite level-3 data were used to assess the variability of the mentioned parameters. Chl-a concentration was observed to be significantly (t = À3.16, df & 18.03, p = 0.005) high (peak 2.30 mg/m3) during the post-cyclone period compared to the pre-cyclone (0.19 mg/m3). Similarly, POC concentration was significantly (t = 3.41, df & 18.06, p = 0.003) high (peak 464 mg/m3) during the post-cyclone compared to the pre-cyclone (59.40 mg/m3). Comparatively, high SST was observed during the pre-cyclone period and decreases drastically with a significant difference (t = 14, df = 33, p = 1.951e-15) after the post-cyclone period. The results indicated an increase in Chl-a (502%) and POC (240%) with a positive anomaly of 0.55 and 94.11 mg/m3, but a decrease in SST (8.52%) with a negative anomaly of 2.7°C in the BoB. Keywords. Super cyclone Amphan; Chl-a; SST; POC; Bay of Bengal; MODIS. 1. Introduction Ganges–Brahmaputra–Meghna river system (Curray et al. 2002) which make the BoB well The Bay of Bengal (BoB) suffers from tropical stratiBed and very sophisticated (Vidya et al. cyclones frequently because of its unique geo- 2017). Moreover, the characteristics of upper graphic location and hydrographic characteristics ocean water circulation are inCuenced by the (Prakash and Pant 2020). It is a semi-enclosed seasonal reversal of monsoon winds, the high U-shaped basin, which lies in the northeastern freshwater inCux from various rivers and rainfall, part of the Indian Ocean. On the western side of which make the BoB unique from any other parts the bay, the seabed dips to 2000 m after nar- of tropical oceans (Suzuki et al. 2018). rowing coastal strip along the East Indian sea Tropical cyclones are a large-scale system or coast (Kay et al. 2018). The northern boundary windstorm that rotates about a centre of low of the bay receives a large volume of fresh- atmospheric pressure over a vast area of warm water inCux from oceanic precipitation (Prasad tropical waters. It is also one of the deadliest nat- 1997) and runoA from the adjoining ural hazards, which deliver heavy rainfall and Cood 157 Page 2 of 9 J. Earth Syst. Sci. (2021) 130:157 resulting in a significant loss of life and damage to nutrient-rich water with the surface water (Lot- the property. The winds can extend outward to liker et al. 2014; Baliarsingh et al. 2015). It also hundreds of miles from the centre of the cyclone. plays an important role in cooling the surface The coastal area of both Bangladesh and India are layer during the cyclone period (Mandal et al. seriously vulnerable to these type of tropical 2018). Studies suggested that Chl-a acts as a cyclones (Venkateswrlu and Rao 2004). In general, proxy to indicate the phytoplankton biomass, in tropical cyclone arises in the BoB during spring association with a phytoplankton bloom, in suit- (April–May) and fall (October–November) inter- able conditions following the cyclone (Nayak monsoons (Hoque et al. 2020). The BoB have et al. 2001; Subrahmanyam et al. 2002; Sarangi experienced 26 out of 35, the deadliest storms in 2011). Besides, POC concentration in the ocean the world (Masters 2020). More recent cyclones is aAected by several factors, e.g., phytoplankton, have included Roanu (14–22 May 2016), Mora Chl-a, salinity, temperature, river discharge, (28–31 May 2017), Bulbul (5–11 November 2019) current and wind vector which used to alter and the last super cyclonic storm Amphan (16–21 during the cyclone (Golder 2020). So, the study May 2020) approached towards the coastal regions on Chl-a in addition to POC and SST has of India and Bangladesh (Balasubramanian and enormous importance to understand the vari- Chalamalla 2020). As the origin of so many ability in primary productivity and marine food cyclones are from here, the BoB is considered as a web and food chain during and after the cyclone hotspot for the formation of tropical cyclones (Shen et al. 2020). (Kikuchi and Wang 2010). Direct ship-borne measurements are not possible Super cyclone Amphan intensiBed from a cyclo- owing to the violent weather throughout the nic storm (CS) to a super cyclone (SuCS) within 16 cyclone periods. The best way to study the oceanic May UTC 18:00 to 18 May UTC 06:00 in the BoB variations is the satellite-based measurement after 21 yrs. According to India Meteorological (Vidya et al. 2017) during this kind of episodic Department (IMD), a tropical depression was events. Ocean colour remote sensing satellites have observed on 15 May 2020 due to the disturbance brought a new revolution to understand the inCu- formed over extremely warm sea surface tempera- ence of tropical cyclone on chlorophyll distribution tures. Cyclone Amphan formed on 16 May 2020 (Sarangi et al. 2015), phytoplankton bloom over the Indian Ocean and started moving north (Chacko 2017) and other oceanic physical pro- over the BoB (Bgure 1). Amphan undertook rapid cesses. Various sensors are currently available to intensiBcation and became an extremely severe detect cyclonic activity. But MODIS is the most cyclonic storm (ESCS) on 17 May 2020. It reached reliable one and it has been widely used to check its peak intensity with 3-min sustained wind speeds the impact of different cyclone such as Bulbul of 240 km/h (150 mph), 1-min sustained wind (Hoque et al. 2020), Hudhud (Baliarsingh et al. speeds of 260 km/h (160 mph) and became the 2015; Sarangi et al. 2015; Chacko 2017) and Phailin super cyclonic storm (SuCS) on 18 May 2020, at (Lotliker et al. 2014; Sarangi et al. 2015; Vidya approximately UTC 12:00. On 20 May 2020, it et al. 2017) on phytoplankton and associated bio- slammed into the coastal districts of West Bengal, logical and physical facts. India and then it entered Bangladesh in the eve- A large number of research works have been ning with a wind speed of 150 km/h and led to its conducted to know the eAect of earlier cyclones destruction among 26 districts throughout the (Lotliker et al. 2014; Baliarsingh et al. 2015; country. Sarangi et al. 2015; Hoque et al. 2020). However, Previous studies over the BoB have reported the eAect of super cyclone Amphan on the BoB remarkable change in Chlorophyll-a (Chl-a), sea has not yet been addressed. This study, there- surface temperature (SST) and particulate fore, aims to examine the variation of Chloro- organic carbon (POC) predominantly due to phyll-a (Chl-a), particulate organic carbon cyclones, depending on their path, strength, (POC) and sea surface temperature (SST) before location and time of occurrence (Lotliker et al. (pre-cyclone) and after (post-cyclone) the super 2014; Baliarsingh et al. 2015; Sarangi et al. 2015; cyclone Amphan in the BoB to explore the eAect Chacko 2017; Mandal et al. 2018; Bhowmick of super cyclone Amphan for understanding the et al. 2020; Ganguly et al. 2021). Cyclones ocean ecosystem and health. The study has been accelerate upwelling, which ultimately increases carried out with the ocean colour datasets using Chl-a and POC by mixing the subsurface MODIS-Aqua datasets. J. Earth Syst. Sci. (2021) 130:157 Page 3 of 9 157 Figure 1. Pathway of super cyclone Amphan in the BoB. 2. Materials and methods Wind data are freely available at INCOIS LAS (https://las.incois.gov.in/ASCAT). Daily outgo- Daily wind data on 0.2590.25-degree grids were ing longwave radiation (OLR) data with a spatial obtained from the Advanced Scatterometer resolution of 191-degree were obtained during the (ASCAT) to check the air–sea interaction during period of cyclone Amphan from National Centers for the period of Amphan on 17, 18, 19, 20 May 2020. Environmental Information (NCEI) of the National 157 Page 4 of 9 J. Earth Syst. Sci. (2021) 130:157 Oceanic and Atmospheric Administration (NOAA) increased wind speed (maximum 31 m/s) (https://www.ncei.noaa.gov/data/outgoing-long (Bgure 2C) compared to that on 18 May 2020. wave-radiation-daily). Eventually, the cyclone made a landfall on 20 May The three-hourly track data with latitude and (Bgure 2D). longitude, cyclone intensities, and wind speed were OLR is an essential component of the Earth’s obtained from the India Meteorological Depart- energy budget and represents the total radiation ment (www.imd.gov.in). Composite maps of eight going to the space emitted by the earth–atmo- days level-3 MODIS Aqua Chl-a, SST and POC sphere system. It is mainly aAected by clouds with a spatial resolution of 4 km were retrieved and dust in the atmosphere (Susskind et al.