Upper Ocean and Subsurface Variability in the Bay of Bengal During Cyclone ROANU: a Synergistic View Using in Situ and Satellite Observations

Pure Appl. Geophys. Ó 2018 Springer International Publishing AG, part of Springer Nature https://doi.org/10.1007/s00024-018-1932-8 Pure and Applied Geophysics Upper Ocean and Subsurface Variability in the Bay of Bengal During Cyclone ROANU: A Synergistic View Using In Situ and Satellite Observations 1 1 1 2 SAMIRAN MANDAL, SOURAV SIL, ABHIJIT SHEE, and R. VENKATESAN Abstract—In this study, the upper ocean and subsurface vari- 1. Introduction ability during the different phases of the cyclonic storm ROANU along the western Bay of Bengal (BoB) in May 2016 are investi- gated by using the moored buoys, Argos, HF radar and satellite The Bay of Bengal (BoB) is one of the tropical datasets in the proximity of the cyclone track. The moored buoy basins in the world, which is prone to cyclonic storms observations recorded a decrease of sea surface temperature (SST) (CS) due to its unique oceanic and meteorological by * 1 °C all over the track, whereas increase in salinity by * 1.5 PSU was detected along with the highest wind speed of conditions. During the pre-monsoon (April–May) and 16 m s-1, pressure drop of 14 hPa and air temperature drop of the post-monsoon months (October–November), the 4 °C. The cooling at the cyclone centers from the satellite data BoB experiences intense tropical cyclones, with indicated higher (lower) SST drops when translation speed of the cyclone was low (high) and took more (less) time to recover to its considerable inter-annual variability of the cyclone pre-cyclone state in southern (northern) BoB. Mostly, higher SST intensities (Obasi 1997; McPhaden and Vialard 2009; drop was observed along the right side of the cyclone track. Maneesha et al. 2012; Vissa et al. 2012). Numerous Interestingly, the opposite phenomenon occurred before landfall, studies have reported remarkable changes in the where SST drop was higher on the left due to upwelling in the head bay as observed both from wave rider buoy (WRB) at Digha and upper ocean in terms of sea surface temperature satellite SST. The WRB near Vizag showed the maximum increase (SST), sea surface salinity (SSS), circulation as well in significant wave heights by * 2.4 m during the passage of as the thermohaline structure of the ocean mostly cyclone. Argos also captured cyclone-induced drop in temperature due to upwelling and entrainment reasonably well. In the south- using satellite, few in situ observations and models western bay, significant upwelling was observed from the Argos (Ali et al. 2007; Sengupta et al. 2008; Vissa et al. with drop in temperature and increase in salinity in the upper 2012; Wang et al. 2012; Girishkumar et al. 2014; layers. However, a strong stratification was observed from Argos in the northwestern BoB due to lesser salinity and higher precipita- Prakash and Pant 2017). Due to efficient availability tion. The currents from in situ as well as HF radar datasets of satellite datasets, most of the studies are confined measured the increase in current magnitude during the passage of to the upper ocean to analyze the factors involved in ROANU. Rotary spectral analysis showed strong inertial currents cyclone formation and intensification. The quantity of with frequency * 2.1 days at BD11 location, with higher ampli- tudes of the clockwise component during the cyclone period. change in SST and related oceanic processes depend on the speed of the cyclones. It is reported that Key words: Bay of Bengal, ROANU cyclone, Argo, OMNI upwelling has a negligible relationship with the buoys, Inertial currents, HF radar. lowering of SST for fast-moving cyclones, whereas for slow-moving cyclones it plays a vital role (Price 1981; Black 1983; Bender et al. 1993; Behera et al. 1998; Yablonsky and Ginis 2013). Previous studies over the BoB have reported decline in SST by 0.3–6.0 °C, predominantly due to the cyclones, depending on their path, strength, location and time of occurrence (Rao 1987; 1 School of Earth, Ocean and Climate Sciences, Indian Gopalakrishna et al. 1993; Chinthalu et al. 2001; Institute of Technology Bhubaneswar, Jatni, Odisha 752050, India. Subrahmanyam et al. 2005; Sengupta et al. 2008; E-mail: [email protected] 2 Ocean Observation Systems Group, National Institute of Prasad et al. 2009; Pothapakula et al. 2017). The Ocean Technology, Chennai, India. S. Mandal et al. Pure Appl. Geophys. vertical mixing plays an important role in the surface Sect. 3, and the results from various observational layer cooling during the cyclone period (Maneesha datasets are described in Sect. 4, followed by dis- et al. 2012; Vissa et al. 2012, 2013). In case of pre- cussions in Sect. 5. Finally, the conclusions are monsoon cyclones, a decrease in SST by 2–3 °C and reported in Sect. 6. mixed layer depth (MLD) deepening up to 80 m is observed which is due to comparatively less salinity stratification in the western and southern BoB (Rao 2. ROANU Cyclone 1987; Gopalakrishna et al. 1993; Behera et al. 1998). On the contrary, along the northern and western BoB Cyclonic storm ROANU is a relatively weaker during post-monsoon seasons, less SST cooling tropical cyclone in terms of intensity, which matured (* 0.3 °C) is observed due to intensive salinity over the BoB during 14–22 May 2016 and followed a stratification and temperature inversion layer in the unique track (source: IMD), moving very close to Sri upper ocean. The cyclones-induced vertical mixing Lanka and along the western BoB denoted by A to I leads to the entrainment of warm water, followed by (Fig. 1). It initially developed at 0300 UTC of 14 less cooling after the passage of cyclone (Sengupta May 2016 as a low-pressure system with very low et al. 2008). The increased buoyancy flux due to cyclone intensity (CI) over the southwestern BoB (A rainfall can lower down the cyclone-induced SST and B). Moving northward along the western BoB, it drop by making an increase in the near surface lay as a well-marked low-pressure area adjoining Sri stratification (Jourdain et al. 2013). However, the Lanka at 0300 UTC of 15 May 2016 near location C. same study reported that SST drop in BoB is less It concentrated into a depression (CI: 1.5) and lay influenced by rainfall stratification, since the subsur- centered over southwest BoB off Tamil Nadu coast face saline water entrainment into the ML overcomes on 17 May 2016 (E). The system continued to skirt the attenuation consequence of rainfall. Previous along the western boundary of the BoB. While studies in BoB have also reported an overall rise of moving northward it intensified (CI: 2) to a deep SSS by 1 PSU (McPhaden and Vialard 2009; depression (DD) due to the favorable environmental Maneesha et al. 2012). conditions near Andhra Pradesh on 18 May 2016 (F). Since the last decade, a remarkable development It intensified into a CS over west-central BoB and lay in ocean observing systems (OOS Programme), viz., centered near position G with a maximum wind speed more numbers of moored buoys in the Indian Ocean, of * 20 m s-1 at 0600 UTC of 19 May 2016 (CI: 2) make us capable of observing and analyzing the and to 24 m s-1 at 1800 UTC of 20 May 2016 (CI: 3) extreme weather events and associated dynamical (H) (IMD 2016). The system maintained its intensity processes in the BoB (Venkatesan et al. 2016). The of 24 m s-1 and crossed Bangladesh coast near advancements are primarily because of persistency location I (91.60°E, 22.60°N), to the north of Chit- and higher accuracy of the observations using Ocean tagong around 1000 UTC of 21 May 2016 as a CS. A Moored Buoy Network for northern Indian Ocean synoptic situation of intensification to CS on 19 May (OMNI) buoys, tide gauges and acoustic doppler 2016 has been well captured from KALPANA-1 current profilers (ADCPs). In this paper, we have satellite showing higher cloud coverage along the highlighted the development of an observation sys- cyclone track with associated lower outgoing long- tem over the BoB toward the analysis of extreme wave radiation (OLR) magnitude of * 110 W m-2 events like cyclone. It focuses not only on the vari- (Fig. 2) (Subrahmanyam et al. 2005). The synoptic ation of the thermal structure of the upper ocean, but OLR distribution and associated wind pattern show also describes the variability of the subsurface ocea- the influence region of the cyclone on the nearby nic parameters in the ocean before, during and after buoys. After landfall, the system started to weaken the passage of the cyclonic storm ROANU. due to land interactions. Continuing its northeastward The structure of the paper is as follows; the brief journey, the CS gradually degraded into a DD over description of ROANU cyclone passage is presented Mizoram at 1800 UTC of 21 May 2016, into a in Sect. 2, the data and methodology are discussed in depression over Myanmar and adjoining Manipur at Upper Ocean and Subsurface Variability in the Bay of Bengal During Cyclone ROANU Figure 1 Best estimated track (source: IMD) of the ROANU cyclone (black line) with centers at every 1200 UTC from 13 to 21 May 2016. Pink, green and blue squares indicate depression, deep depression and cyclonic storm stages of the cyclone, respectively. Observational data were taken from wave rider buoys (WRB) (blue diamonds) at Pondicherry (WRBPY), Vizag (WRBVG) and Digha (WRBDG), OMNI buoys (black triangles) and Argos of platform numbers 5904334 (black dots), 6901557 (pink dots), 2902194 (blue dots), 2902087 (red dots), 6901562 (green) and 2902196 (cyan).

Load more