International Journal of Applied Environmental Sciences ISSN 0973-6077 Volume 12, Number 5 (2017), pp. 717-729 © Research India Publications http://www.ripublication.com
Influence of cyclone Phailin on the Upper Ocean over Bay of Bengal
C. Purna Chand1, M.V. Rao1, K.V.S.R. Prasad2 and K.H. Rao1 1National Remote Sensing Centre, Hyderabad, India 2 Dept. of Meteorology and Oceanography, Andhra University, Visakhapatnam, India
Abstract
The influence of cyclonic storm Phailin during October 2013 on the ocean surface over Bay of Bengal have been studied using Earth Observation (EO) Sensors data as well as in-situ observations including temperature and salinity profiles from Argo float data near vicinity of the Phailin track. The Phailin (8th to 13th October 2013) induced cooling along the cyclone track is clearly seen with substantial cooling observed before the landfall of the cyclone. This cooling is a result of vertical mixing induced by upwelling due to strong winds and Ekman current during the Phailin Cyclone. The significant wave height increase along the coast is observed in the SARAL-Altika and Jason-2 Altimeter data. The pressure drop and the increase in salinity are found during 12th October 2013 due to passage of Phailin cyclone. Keywords: Cyclone, Argo float, phailin, upper ocean, vertical mixing.
1. INTRODUCTION Meteorologists use the term "tropical cyclone" for a closed atmospheric circulation that forms over a tropical or subtropical ocean. Once maximum sustained wind speed exceeds 74 miles per hour these storms are called hurricanes in the Atlantic Ocean, typhoons in the Pacific Ocean, and very severe cyclones (super cyclones in recent terminology) in the Indian Ocean. The Tropical Cyclone damages the property and lot of casualties in coastal regions due to gale winds and heavy rains associated flooding. People realized that the oceans play a significant role in formation, development and strengthening of Tropical 718 C. Purna Chand, M.V. Rao, K.V.S.R. Prasad and K.H. Rao
Cyclone. Aftermath of Tropical Cyclone, variation in Sea Surface Temperature, gale winds, Sea Waves, Surface Pressure, currents and biological activity are seen. The changes of these parameters due to Tropical Cyclone are studied independently by several researchers. The Cyclone induced Sea Surface Temperature cooling has been investigated by different authors, using remotely sensed data sets [Stramma et al., 1986, Peter et al., 1995, Suetsugu et al., 2000, Rao et al., 2002, Sadhuram. Y., 2004, and Yablonsky et al., 2013]. Space-born wind measurements and significant response to the strong wind forcing during tropical cyclone have been noticed [Atlas et al., 1996, Atlas and Hoffman 2000, Adams et al., 2005, 2006, Liu and Xie 2008, Shuwen Zhang et al., 2014, C P Chand et al., 2014, 2015 and Sasamal et al., 2015]. The impact of Scatterometer wind on tropical cyclone forecasting has been studied [Isaksen and Stoffelen 2000]. The nutrients are efficiently replenished in euphotic layer and phytoplankton biomass rapidly enhanced due to tropical storm passage [Shiah et al., 2000, Subramanyam et al., 2002,Preethi et al, 2015 and Aneesh et al., 2014]. The upper ocean response to a moving storm has been an important and interesting topic in the physical and biological oceanographic studies. Keeping this in view, a comprehensive study during one Tropical Cyclone (Phailin) using Earth Observation Sensors data and in-situ observations during October 8-14, 2013 has been carried out.
1.1 Very Severe Cyclonic Storms ‘Phailin’ The cyclone Phailin during October 8-14, 2013 is the category four Hurricane, out of three very severe cyclonic storms occurred during 2013. As per India Meteorological Department (IMD) [www.imd.gov.in] reports on 8th October 2013 the Deep Depression upgraded to Cyclone and subsequently a cyclonic storm named it as Phailin. It is further strengthened and formed into a very severe cyclonic storm by 10th October 2013. The Phailin is further intensified with a central pressure of 994 mb and crossed the coast near Gopalpur, Odisha, East Coast of India on 12th October 2013. The cyclone ‘Phailin’ prompted India's biggest evacuation in last 23 years with more than 550,000 people moving up from the coastline in Odisha and Andhra Pradesh to safer places. Forty four deaths and 696 million USD damage due to the cyclone have been reported.
2. DATA USED AND METHODOLOGY In this study Level 2B wind field data from Ocean Sat-2 Scatterometer (OSCAT) obtained during Phailin Cyclone (both descending and ascending passes, equatorial crossing time nearly at 6:30 and 18:30 GMT respectively) from NRSC, Hyderabd. The wind field data (both speed and direction) is available in 25 Km and 50 Km grid size. The 25 Km wind field data has been used to estimate wind stress and wind Influence of cyclone Phailin on the Upper Ocean over Bay of Bengal 719 stress curl using standard formula [Sourabh et al., 2014]. The 50 Km wind field data is used to compute pressure fields using University of Washington Planetary Boundary Layer (UWPBL) Model as described by Patoux et al., [2003]. The UWPBL model compute surface wind from the knowledge of geostrophic wind ‘Farward’ and ‘Inverse’ model calculate geostrophic wind at top of boundary layer from surface wind. The boundary layer wind in two different models viz; two layer similarity model for mid-latitude (60o to 10oS and 10o to 60oN) and Mixed layer model for tropical region (20oS to 20oN) yield the pressure gradients. The absolute pressure values can be obtained through least square fit from a priori pressures from observations/model values and here we used buoy data from Indian National Centre for Ocean Information Services (INCOIS) [www.incois.gov.in] as a priori pressures. The model required input data other than wind fields on air temperature, sea surface temperature, humidity, and horizontal temperature gradient. These inputs are taken from the climatological data. The Sea Surface Temperatures (SSTs) both daily and weekly were extracted from Tropical Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI) [www.remss.com] during the period of cyclone Phailin to study the cooling effect. The ocean surface currents are estimated from Ekman surface current through OSCAT wind stress component and geostrophic current through SARAL Altika derived sea surface heights [Saurabh et al., 2014]. The chlorophyll-a estimates are obtained from OCM-2 data during Phailin. The significant wave heights were estimated using the Saral-Altika and Jason-2 data during Phailin Cyclone. In addition temperature and salinity profile Argo float data near vicinity of the Phailin track were obtained from coriolis project and programs (http://www.coriolis.eu.org) during cyclone period. These profile data are used to compute Mixed Layer Depth (MLD).
3. RESULTS AND DISCUSSIONS The Tropical Cyclone ‘Phailin’ induced changes over Bay of Bengal were studied with the help of multi sensor derived ocean parameters. Normally during the Tropical Cyclone, the heat transfer from ocean to the atmosphere through evaporation is enhanced by cyclone induced surface winds, which transport water vapor to the troposphere. Other way the cyclone induced surface winds do generate upper-ocean currents and this process of current generation does not happen directly. The winds build up waves at the sea surface, and some of the energy from the winds goes into growing and propagating these waves. When the waves break, energy is transferred downward into the ocean currents. 720 C. Purna Chand, M.V. Rao, K.V.S.R. Prasad and K.H. Rao
The cyclone-induced chl-a increase is another process accounting for a few percent of the total chl-a increase in some areas. In oligotrophic regions, the increase in chl-a tended to become larger as the corresponding SST decrease became larger, although the relationship between them is opposite in mesotrophic and eutrophic regions.
3.1 Sea Surface Temperature To compare the sea surface temperatures during cyclone period with that prior to cyclone, SSTs from first week of October 2013 (i.e. week ending on 5th October 2013) have been chosen as reference. The SST anomaly’s estimated from the individual day SST from 8th to 13th October, 2013 by subtracting daily SST from weekly (week ending on 5th October 2013) SSTs. Hence the positive value indicates cooling and negative represents warming. The colour coded SST anomaly images are generated, have been presented in figure 1(a). The temperature anomalies during the cyclone period along the track from 8th to 13th have been presented in figure 1(b). It is seen from figure 1, that the cooling is observed 1.0 to 2.0 0 C from 8th to 11th and cooling increases as it approaches to the coast, attains maximum (≈ 6.0 0C) on 13th October 2013 near land fall point.
Figure 1. (a) Colour coded images depicting SST anomaly (weekly average for the week ending on 5th October 2013 minus SSTs of daily from 8th to 13th October 2013), high positive anomaly along coast indicate maximum cooling on 13th October 20113. (b) SST anomaly all along the Phailin cyclone track is shown from 8th to 13th October 2013(Maximum cooling observed).
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The upwelling caused by the cyclonic storm has to be balanced by the downwelling at the adjoining place. When the storm is closer to the coast, the downwelling might have probably taken place to the west of the storm, which is evident from the positive SST anomalies towards the west of the cyclonic track. The storm also has generated coastally trapped Kelvin waves with cooler waters that propagated towards the east coast of India. The extent of these cooled waters near the east coast of India is clearly seen during the landfall of cyclone Phailin (figure 1(a)). The Sea Surface Temperatures are cooled significantly along the storm track in the open ocean, but the cooling is more prominent along the coast (figure 1(b)). There are two possible explanations for surface cooling due to cyclones: extraction of thermal energy from the surface due to evaporation, or wind induced vertical mixing of the water column. This argument is based upon viewing the ocean as a two-layer fluid; a warm upper layer, and a cold deeper layer. (The interface is referred to as the main thermocline). The upper layer is relatively thick in October, so there must be a great deal of vertical mixing to draw deep cold water to the surface. Along the shelf, however, which the storm encounters as it approaches to the coast, the upper layer is relatively thin. Therefore, with the same amount of vertical mixing, more cold water can be drawn to the surface, and cooling is more substantial. To substantiate the observed cooling in satellite derived SST, we have also collected temperature and salinity profiles from Argo float data near vicinity of the Phailin track (figure 2). There are three Argo Floats data available near vicinity of the Phailin track with float ID Nos. 2901334, 2901335 and 2901327. Of which 2901335 is available daily data, whereas 2901334 is once in five days starting from 07th October 2013 and 2901327 is available on 8th, 12th and 15th October 2013. The Mixed Layer Depth (MLD) is computed based density criteria, the depth at which density is 0.125 at all three float locations and is presented in figure 2. For all the three floats, the salinity and temperature profiles are presented in figure 3. It is very clearly seen from all the profiles, that the surface cooling is observed on 12th October, 2013 and at the same time the increase in surface salinity is seen on 12th, indicating sub-surface cooled high saline waters appearing on the surface due to high vertical mixing. It is also observed deepening of MLD on 12th October, 2013. It is interesting to note that the surface salinity increase is quite high compared to surface temperature cooling. Surface cooling observed is more than 2oC at Argo id 2901335 location after the passage of the storm (i.e. from 9th to 12th October 2013). The salinity increase from 3.5 ppm, 4 ppm and 5.5 ppm at Argo id 2901334, 2901335 and 2901327 respectively during cyclone passage. From these observations (figure 3), it is clear that the vertical mixing is dominant and observed stable mixed layer during 12th October 2013. There are fluctuation in these Argo float data due their locations (2901335 on the track, 2901327 close to the track and 2901334 little away from the track), as seen in figure 2, as well as coverage of data (one float, once in 5 days, one float daily and the other on 8th, 12th and 15th October 2013). 722 C. Purna Chand, M.V. Rao, K.V.S.R. Prasad and K.H. Rao
Figure 2. The Phailin track with Argo floats Position; along with the Mixed Layer Depth plotted against the dates for the profiles from Argo floats.
3.2 Wind, Wind Stress, Wind Stress Curl and Ekman Current The wind, Wind Stress (WS) and Wind Stress Curl (WSC) products of the OSCAT are studied with reference to the impact of cyclone Phailin. The wind vector and WS are critically important for determining the large scale ocean circulation and transport. Vector winds are needed to study the ocean and atmospheric process, like Ekman transport, upwelling and mixed layer dynamics etc. Influence of cyclone Phailin on the Upper Ocean over Bay of Bengal 723
Figure 3. The salinity and temperature profiles for all the three Argo floats during the cyclone Phailin period. 724 C. Purna Chand, M.V. Rao, K.V.S.R. Prasad and K.H. Rao
The high resolution (25 Km) wind velocities from OSCAT during the phailin cyclone are studied. Though the wind speed measurement range of OSCAT is 4-24 m/s, the wind speeds retrieved show as high as around 107 km/hr (29.6 m/s) on 12th during landfall of Phailin. The estimated wind speeds reported by IMD are as high as 215 km/hr during landfall, which is double than the satellite estimation, and is mainly due to saturation of the sensor. WS and WSC also increased all along the track from 8th to 12th and reached to maximum, 2.58 N/m2 and 9.86 N/m3 respectively at landfall location on 12th October. The Tropical cyclone induced strong winds transfer the heat from ocean to atmosphere through evaporation, which transport water vapour into the troposphere. However, expected high evaporation may not be observed during the cyclone due to intense cooling of surface, though strong winds are present. The surface wind exerted stress lead to vertical mixing due to friction and generate currents in the oceanic mixed layer. It is also known that the wind stress during cyclone leads to move the waters away from the storm center through currents generated over there and to maintain equilibrium cooled sub-surface waters comes up. This process is known as storm generated ‘upwelling’. All these processes result in cooling the sea surface all along the cyclone track and are discussed in the previous section. The Phailin surface winds exert a stress on the ocean surface due to friction, generating ocean currents in the oceanic mixed layer. Under the assumption that the water currents are driven only by the transfer of momentum from the wind, Ekman theory explains the theoretical state of circulation and currents generated are known as Ekman Currents. Ekman motion describes the wind-driven portion of circulation seen in the surface layer. Hence the Ekman Currents are estimated using OSCAT wind during the Phailin cyclone. The maximum Ekman current velocity observed was around 170 cm/s on 12th October 2013. It is clearly seen that the waters moving away from the storm center and cooled sub- surface waters comes up there, leading to sea surface cooling all along the cyclone track.
3.3 Sea Level Pressure fields The sea level pressure retrieved from OSCAT winds using UWPBL model during Phailin Cyclone have been studied. Here we observed 10 mb pressure drop from 8th to 13th October 2013. As per IMD, the pressure drop during the same period reported to be 63-64 mb. Deep Depression is upgraded to Cyclone and subsequently to a cyclonic storm named as Phailin. It is further strengthened and formed into a very severe cyclonic storm by 10th and is further intensified with a central pressure of 994 mb and crossed the coast near Gopalpur on 12th October 2013. By nomenclature a pressure drop of 5 to 9 mb with maximum sustained 3 minutes surface winds of 34 knots or more is called as cyclonic storm, which is clearly observed in our satellite based pressure estimations from 8th to 10th October 2013. Though the IMD estimated pressure values differ from satellite derived ones, the isobars maps generated are closely matching with IMD reports during this cyclone (Figure 4). The pressure drop at the cyclone centre is noticed to be low compared to estimated pressure drop Influence of cyclone Phailin on the Upper Ocean over Bay of Bengal 725 reported by IMD mainly due to saturation of winds (4-24 m/sec) measured by the Oceansat Scatterometer sensor, which is input for the retrieval of pressure fields using UWPBL model. The low pressure centers from UWPBL model derivatives are drawn to built the cyclone track and found it is closer to the IMD track. Though the pressure values differ from the IMD reported values, the increase in closed isobars from 8th to 12th October 2013 clearly indicate the intensity of cyclone.
Figure 4. Estimated pressures using UWPBL model from 8th to 13th October 2013 are presented here. A low pressure system is present in north Andaman Sea with 1003 mb pressure and for 12th October 2013, with a steep drop in pressure from surroundings was observed and is as low as 993 mb on 12th October 2013.
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3.4 Chlorophyll Concentration The Ocean Colour Monitor (OCM) – 2 is one of the sensor onboard Oceansat-2 intended to estimate Chlorophyll-a as one of the parameters. The Chlorophyll distribution along the cyclone track (according to IMD) has been computed before and after the cyclone Phailin. From the analysis, it is observed that the chlorophyll-a concentration has increased from 1.08 (before) to 7.06 mg/m3 after the cyclone [Preethi et al., 2015].
3.5 Significant Wave Height The significant wave heights were estimated using the SARAL-Altika and Jason-2 data during Phailin Cyclone i.e. on 12th October 2013. The Significant Wave Height (SWH) variations against latitude during the pass of Saral-Altika-051 reveal high SWH at 20o N latitude on 12th October, 2013 [Sasamal et al., 2015].
4. CONCLUSIONS The cyclone induced cooling along the cyclone track is clearly seen in the satellite derived Sea Surface Temperatures during the cyclone Phailin. Substantial cooling is observed before the landfall of the cyclone. The cooling is a result of all the three processes; namely evaporation, vertical mixing and upwelling. However, we observed that the vertical mixing induced by upwelling is the dominant one. The surface salinity increase is quite high compared to surface temperature cooling as evident in the vertical profiles of Argo float temperature and salinity data. The OSCAT winds, wind stress and wind stress curl impact is reflected in the ocean surface by indicating intensive mixing to deeper waters. It is clearly seen from the Ekman current that the waters are moving away from the storm center leading upwelling to happen at the storm center. The UWPBL model derived pressure fields are found useful in the construction of cyclone track and intensity, through numerically they are underestimated due to saturation of winds during cyclone. Though, the evaporation rates are expected to be high during cyclone due to high surface winds, but it is not as high as expected due to sudden surface cooling. Finally, the winds are the major cause for upper ocean response to a moving storm. Vertical mixing occurs because the cyclone’s surface winds exert a stress on the ocean surface due to friction, generating ocean currents in the oceanic mixed layer which is reflected in surface cooling, increase in surface salinity and high chlorophyll concentration observed in passage of Phailin cyclone.
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5. ACKNOWLEDGEMENTS We express our gratitude to J. Patoux, PBL Research Group, Department of Atmospheric Sciences, University of Washington, USA for his support rendered during the execution of UWPBL model at NRSC and also his valuable suggestions. Our sincere thanks to Saurabh Bansal former Scientist in OSG/ECSA/NRSC for his continuous support. This work was carried out at National Remote Sensing Centre (NRSC), as a part of Oceansat-II utilisation and NICES program. The Argo data were collected and made freely available by the Coriolis project and programmes that contribute to this study. TMI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Sciences Program is used in this study.
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