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Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone
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ScienceDirect Advances in Space Research xxx (2015) xxx–xxx www.elsevier.com/locate/asr
Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone K. Muni Krishna a,⇑, Guiting Song b a
Dept of Meteorology and Oceanography, Andhra University, Visakhapatnam 530003, India b Department of Marine Science, Zhejiang Ocean University, Zhou Shan 316022, China
Received 17 February 2015; received in revised form 12 July 2015; accepted 18 July 2015
Abstract Baaz cyclone is a slow moving weak cyclone in the south central Bay of Bengal (SCBoB) and it lingered for 3 days and caused a significant cooling (2.6 °C) and enhancement of chlorophyll-a (5.6 mg/m3) at the right side of the cyclone track. In this study multi-satellite observations are used to explore the bio-physical changes due to the cyclone. We found that the speed of the Ekman pumping velocity is ten times more and the mixed layer is deepened about 19 meters during the cyclone than pre cyclone period. The maximum sea surface cooling (1.2–2.6 °C) took place when the translation speed of cyclone is only 1.2–2.3 m/s. So the extent of the sea surface temperature drop is probably related to the moving speed of cyclone and the mixed layer depth. In addition, the area with large decline of the SSH can signify the location where the maximum cooling occurs. Ó 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Cyclone; Sea surface temperature; Mixed layer; Chlorophyll-a; Sea surface height; Ekman pumping
1. Introduction Tropical cyclones are one of the most destructive natural disasters known to mankind. Strong wind and heavy rain are associated with these weather systems which produce large waves in the ocean. The land falling of the tropical cyclones cause disastrous damage to the coastal infrastructure, the natural environment, extensive flooding due to storm surge and very often loss of life. They are also play a key role on upper ocean environment changes in the tropics and subtropics. It also alters the physical characteristics of the upper ocean (Price, 1981; Black, 1983; Gopala Krishna et al., 1993; Chinthalu et al., 2001; Babin et al., 2004; Gierach and Bulusu Subrahmanyam, 2008; Chang et al., 2008; Gierach et al., 2009; Lee and Park, 2010; Yang et al., 2010; Wanga et al., 2011). The intensity of ⇑ Corresponding author. Tel.: +91 891 2717663.
E-mail address: [email protected] (K. Muni Krishna).
severe storms is strongly influenced by the thermodynamic structure of the upper ocean, and an accurate prediction of the storm’s future intensity requires measurements of the ocean’s thermal structure ahead of the storm. Upper ocean biological and physical responses to cyclones are always one of the hottest issues on cyclone-ocean interaction (Walker et al., 2005; Tommy et al., 2008; Shang et al., 2008; Zheng et al., 2008; Zhao et al., 2009; Chen et al., 2012; Mcphaden et al., 2009; Shi and Wang, 2011; Byju and Prasanna Kumar, 2011). The measurement of biological and physical parameter onboard ship is very difficult because of the severe ocean conditions during cyclone period. The development of multi satellite remote sensing, especially microwave remote sensing, in combination with numerical simulation, provide powerful tools to further understand the upper ocean changes induced by cyclone. According to Price (1981) the passage of cyclone can stimulate ocean mixing and upwelling which brings the nutrient rich water from sub surface to the euphotic zone where
http://dx.doi.org/10.1016/j.asr.2015.07.025 0273-1177/Ó 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
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there is abundant light for photosynthesis but often lack of nutrients. Consequently it will contribute to the growth of phytoplankton and take a significant impact on the increase of ocean primary production. Stramma and Cornillon (1986) revealed that when the storm moves rapidly, the maximum cooling occurs well to the right of the track, whereas for slowly moving storms the maximum cooling occurs near or on the track. Suetsugu et al. (2000) described the cooling of the sea surface induced by the tropical cyclone as air-sea parameter dependence i.e. storm strength, storm translation speed and upper ocean thermal structure. They concluded that the strong and slow moving typhoon causes the maximum cooling of 5 °C in the cyclone wake. Black (1983) described the coolness as an complex system of internal waves extending through the thermocline and conducting colder water to the surface by an eddy mixing process. The SST can drop by as much as 5 °C due to this mixing. His study also revealed that the cooling is substantial when the hurricane moves at speeds less than the internal gravity waves (4–8 knots). Their result shows that the sea surface cooling biased towards the right of the storm track and sub surface intense upwelling. Such a significant response in the ocean thermal structure may weaken surface winds through the modulation of cold sea surface temperature on the stability of the atmosphere. Strong wind forcing exerted on the ocean surface by hurricane strength tropical cyclones produces turbulent mixing of the upper oceanic layer, mixed layer deepening and entrainment of colder water form below which is accompanied by an enhancement of primary production. This probably is the primary mechanism responsible for the SST decrease during the tropical cyclone passage. According to Hong and Sohn (2004) and Kim et al. (2007), the upwelling caused by Ekman pumping is active when a typhoon passes over a specific sea surface, and the cold water injection into the surface layer from a deeper layer continues to spread horizontally outwards. This horizontal spreading is considered as one of the main factors supporting vertical mixing, and, as a result, continuous changes in various factors may last for some time. Babin et al. (2004) has shown that surface chlorophyll concentrations change in post-storm surface usually continued for approximately two to three weeks. Hong (2008) also suggests that the sea surface cooling (SSC) continues for nearly 20 days. This continuity period depends relatively on the scale of the typhoon and the oceanic conditions. In the present study we wish to investigate the physical and biological changes due to the passage of Baaz cyclone in the Bay of Bengal. 2. Data and methodology 2.1. Cyclone data The tropical cyclone data used in this study is provided by the India Meteorological Department and Joint
Typhoon Warning Center, from 27th October to 1st December 2005, in the geographical range of 80°–90°E and 8°–15°N (Fig. 1). The data contain the name of tropical cyclone, observing time (about 6-h interval), location of cyclone center, maximum sustained wind speed, wind direction, and central pressure (www.imd.gov.in). Baaz cyclone is lingered in the south Bay of Bengal for 5 days with a relative slow moving speed of 1.2–2.3 m/s and made landfall off north Tamil Nadu on 2 December 2005 (Table 1). 2.2. Sea surface temperature A Group for High Resolution Sea Surface Temperature (GHRSST) global Level 4 sea surface temperature analysis produced daily on a 0.25 degree grid at the NOAA National Climatic Data Center. Merged SST daily products with a 25 km spatial resolution retrieved from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) and advanced high resolution radiometer (AVHRR). This product uses optimal interpolation (OI) using data from the Advanced Very High Resolution Radiometer (AVHRR) Pathfinder Version 5 time series (when available, otherwise operational NOAA AVHRR data are used), the Advanced Microwave Scanning Radiometer-EOS (AMSR-E), and in situ ship and buoy observations. The AMSR-E and AVHRR can provide continuous measurement of the SST even when there is typhoon with heavy clouds. MW data is likely to be available at a suitable quality for pixels where there is cloud affected in AVHRR because MW can penetrate the cloud and observe the sea surface (the cloud coverage in tropical areas typically exceeds 90% – Chelton et al., 2000; Wentz et al., 2000) http://www.ncdc.noaa.gov/oa/climate/research/sst/oi-daily.php. 2.3. Sea surface wind The Blended Sea Winds contain globally gridded, high resolution ocean surface vector winds and wind stresses on a global 0.25° grid. The wind speeds were generated by blending observations from multiple satellites (up to six satellites since June 2002). The wind directions came from two sources depending on the products: for the research products the source is the NCEP Reanalysis 2 (NRA-2) and for near-real-time products the source is the ECMWF NWP. The wind directions were interpolated onto the blended speed grids. The blending of multiple-satellite observations fill in the data gaps of the individual satellite samplings both temporally and spatially and reduce the subsampling aliases and random errors (http://www.ncdc.noaa.gov/oa/climate/research/wind). The spatial variation of wind force produces a consequent variation in the corresponding Ekman transport in the surface layer, causing convergence and divergence in different areas. This leads to “Ekman pumping” (Enriquez and Friehe, 1995). In this paper, Ekman
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
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Fig. 1. Tropical cyclone track with intensities. Light blue color balloons indicate position of Argo floats. (Source: Google earth, IMD, JTWC). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Baaz Cyclone at different stages (Source: India Meteorological Department). Name
Date
Time (UTC)
Latitude
Longitude
Estimated central pressure (hPa)
Maximum sustained surface wind (kt)
Grade
BAAZ BAAZ BAAZ BAAZ BAAZ BAAZ
28/11/2005 28/11/2005 29/11/2005 30/11/2005 01/12/2005 02/12/2005
0300 0600 0600 0600 1200 0300
10.5 10.5 11.0 12.5 12.5 13.0
90.5 90.0 86.0 84.0 83.5 82.5
1006 1004 998 998 998 1004
25 30 45 35 30 25
D DD CS CS DD D
pumping velocity, which reflects the velocity of upwelling, is computed by the following equation given by Enriquez and Friehe (1995). WE ¼
1 ðr sÞ qf
ð1Þ
where q = density of sea water = 1028 kg/m3 f = coriolis force = 2 X sin/ $ s = wind stress curl 2.4. Sea surface height Weekly TOPEX (TOPography EXperiment for ocean circulation)/Poseidon SSH data with 0.25° spatial resolution are provided by Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO). http://www.aviso.altimetry.fr/en/data/products/sea-surfaceheight-products/global/ssha.html. 2.5. Chlorophyll-a concentration The launch of the Ocean Color Monitor (OCM) sensor onboard IRS-P4 satellite has been a boon to the scientific community involved in the analysis and characterization of the chlorophyll concentration in the oceans. The IRS-P4 OCM has eight spectral channels in visible and near-infrared wavelengths (412–865 nm) with a spatial resolution of 360 m and revisit cycles of 2 days. OCM have eight bands both in BIL and BSQ mode. Each band is stacked and exported as Generic Binary. These 8 bands Generic Binary data was processed using ERDAS 8.4 (with OCM-DAS module) software. Weekly data is available at INCOIS web site (www.incois.gov.in).
2.6. In-situ observations (Argo) Argo data used in this study and it is downloaded from the Indian National Center for Oceanic Services (www.incois.gov.in) and Coriolis Data Center (http://www.coriolis.eu.org/) from France during the cyclone period. Two sets of T-S profiles are obtained from the argo float with the ID2900106 and 2900107 for 26 Nov and 6 Dec 2005 and they are situated at (longitude and latitude data), about 50–100 km left side of the track in the study area encircling geographical co-ordinates bordered by 10– 15°N latitude and 80–85°E longitude within the environs of the cyclone track. For each profile, a real-time and a delayed-mode quality control were conducted. The calibration method developed by Wong et al. (2003) is employed to calibrate the sensor drift of salinity measurements in the Argo data. The distance between two Argo floats are within 200 km distance from both sides of the cyclone (Park et al., 2004). The isothermal layer depth (ILD) is determined from the temperature based criteria, where the temperature decreases to a value of 0.8 °C compared to the surface (shallowest level obtained from the ARGO profile). The intermediate layer which prevails between the mixed layer and thermocline is named as barrier layer, which forms over the salinity stratified regions (Lukas and Lindstrom, 1991). The implication of this barrier layer is to inhibit cooling from sub-surface waters, which can eventually energize tropical storms in the Bay of Bengal (McPhaden et al., 2009). A variable density criteria is chosen for the determination of mixed layer depth (MLD) as proposed by Kara et al. (2000) wherein MLD is constructed using density variability (rt ) determined from the corresponding
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temperature change DT (0.8 °C) in the equation of state. Difference between ILD and MLD is defined as barrier layer thickness (BLT). 2.7. Synoptic view of Baaz cyclone The system initially was seen as a depression and located at latitude 10.5°N and Longitude 90.5°E since 03:00 h on 28 November 2005, the system rapidly intensified into a deep depression and continued its westward movement, it further intensified into a cyclonic storm and located at latitude 10.5°N and longitude 88°E at 18:00 h on 28 November 2005 and reached its peak intensity of 45 kts and was located at about 425 nm east-southeast of Madras, India and named Baaz cyclonic Storm by 28/1800 UTC. Subsequently it concentrated on 29 November 2005 (Dvorak intensity estimates of T3.5), it moved northwestwards. The storm’s motion slowed down and became somewhat erratic on 30th November as it approached the Indian coastline. Baaz continued moving on awest–northwestward track and maintained strength until the morning (03:00 h) of 1 December, after that, it began to weaken into a deep depression on 2nd December 2005. 3. Results Warm sea surface temperature is observed (28.8 °C) on the right side of the cyclone track, it fuels to the Baaz cyclone and intensify it into a cyclonic storm over the southeast Bay of Bengal (Fig. 2a). It is very slow moving storm with a speed of 2.3 m/s. Two days after its passage the ocean water become cool (2.6 °C) in the central Bay of Bengal and also 1.5 °C near the Chennai coast
(Fig. 2b). Spatial distribution of the cool wake is observed over a large area at the right side of the track. The time-longitude cross section of sea surface temperature along the track clearly shows the duration of the cool wake in the open ocean is less compared with coastal area but 100 km left and right side of the track shows almost the same duration (three days) in both areas. Baaz cyclone was declared as a tropical storm on the Saffir–Simpson hurricane scale. It lingered with a slow speeds (1.2–2.3 m/s) in the south Bay of Bengal from 28 November to 2 December 2005 before moving rapidly westward thereafter (Fig. 1). The cyclone remained almost at rest during 2 December 2005. Satellite observations provided an opportunity to investigate the bio-physical changes over the ocean surface. Four satellite datasets mentioned on Section 2.2 were used in the present study. Before cyclone Baaz’s arrival, the south central Bay of Bengal was characterized by predominantly warm sea surface temperature (SST) which is greater than 28 °C. The warm sea surface temperature (28.8 °C) is situated on the right side of the track of Baaz (Fig. 2a). It provides a good demonstration of sea surface heating over the region. After two days of Baaz’s departure, a cold SST area with the temperature range of26.2–27.8 °C (along and right side) and a size comparable to Baaz’s 180 km RMW (Radius of Maximum Wind), is observed and co-located with the Baaz’s track. The minimum SST of 26.2 °C is found at the center (84°E, 11.8°N) of the cold pool (Fig. 2b). In comparison with pre-cyclone conditions, the SST had dropped by as much as 1.2–2.6 °C. This cold pool slowly decayed after the passage of the Baaz, but still maintained a low temperature of 27 °C. Daily variations in SST before and after the passage of the Baaz cyclone depends
Fig. 2. Spatial distribution of sea surface temperature before (a), and after (b) the Baaz cyclone and (c) cooling (after-before cyclone) .
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
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on the distance from its center (Fig. 3). To quantify the SST and wind speed relation, the time-longitude cross section of sea surface temperature along the track (12.23°N), right side (13.58°N) and left side (10.9°N) clearly shows the duration of the cool pool in the open ocean is less compared with coastal area, but 100 km left and right side of the track shows almost the same duration (three days) in both areas. The sharp drop in the surface temperature can be explained in terms of several processes (Jordan, 1964): seasonal cooling, removal of warm surface water by wind force and upwelling of colder deep water, the deeper mixing by cyclone winds, cold rainfall, and the loss
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of heat energy to the storm. In this study the cooling is due to the strong vertical mixing caused by the cyclone winds. The Ekman layer depth is also deepened which coincides with the cool pool area. The maximum depth of Ekman layer (220 m) is observed along the track. Pre-cyclone Ekman layer depth (120 m) is comparable shallower that the post cyclone Ekman Layer depth (220 m). The wind speed inside the cold pool is between 10 and 15 m/s while the surrounding wind speed is between 6 and 9 m/s along the track section. But at the coastal area the wind speed is only 8 m/s. At the right side of the cyclone track the wind speed is more 13 m/s at open ocean and greater than 15 m/s
Fig. 3. Time-Longitudinal cross section of sea surface temperature (left panel), Wind speed (right panel, fill contours) and Ekman depth (right panel, contours) along the cyclone track (A, D), Left side of the track and (B, E) and right side of the track (C, F).
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Fig. 4. Same as Fig. 2 for Ekman pumping velocity (105 m/s contours) and Ekman depth (m) (filled contours).
at the coastal regions. Similarly the maximum depth of Ekman layer is also observed at the coastal regions. Fig. 4 shows the Ekman pumping velocity on 28 November and 1 December 2005, respectively. The upwelling was weak (0.3 104 m/s) and its distribution was relatively small on 28 November before the Baaz cyclone in the south central Bay of Bengal. The Ekman pumping velocity WE is negative, i.e. there is a divergence of Ekman transports that brings water upward from the ocean interior. On 1st December, the upwelling was apparently stronger with a wider area. The Ekman pumping velocity increased about eight times. The Ekman transport is outward and away from the center of the cyclone, so it produces an upward Ekman pumping velocity below and also deepens Ekman layer depth. Due to the upward pumping effect in the ocean the thermocline is raised and the pressure is reduced in the ocean surface layers. Just one day after Baaz’s landfall on 3 November, the Ekman pumping velocity returned to the level as 28 October. Therefore, the strong upwelling triggered by strong wind stress vectors contributed a lot to this event. On the one hand, the passage of Baaz cyclone resulted in a vertical mixing which induces strong upwelling, and brought the phytoplankton inform the subsurface to the upper layer
(euphotic zone) and caused a enhancement of Chl-a concentration which can be seen from Fig. 6. The physical response in the form of sea surface height (SSH) changes also can be observed during Baaz cyclone period (Fig. 5). The SSH shows higher values before the cyclone Baaz and it is around 50 cm in the open ocean and 30 cm off the Tamil Nadu coast. But two days after its passage SSH is decrease 10 cm in the open ocean and also its looks like a cold core eddy shape located at the coastal region. It is about half of the open ocean value. It clearly indicates that the upwelling phenomenon is stronger in the open ocean than in the coastal area. In general, cyclone-caused oceanic biological changes as well as physical environmental changes such as SST, SSH and the concentrations of Chl at the upper layers of the ocean happen over a relatively short period. Time series of spatial changes in SST and Chl-a concentration can be measured easily by satellite sensors. Before the cyclone along the Tamil Nadu and South coastal Andhra Pradesh coastal area shows a high Chl-a concentration but it is due to the river discharge. In this study IRS-P4 Ocean Color Monitor (OCM) with a spatial resolution of 4 km clearly indicates the biological environmental change in Chl a concentrations at the right side of the track
Fig. 5. Weekly sea surface height changes before and after the passage of Baaz cyclone.
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Fig. 6. Sea Surface chlorophyll-a concentration before and after the Baaz cyclone.
Fig. 7. Vertical distribution of temperature (red). and salinity (blue). Solid and dotted lines represents before and after the cyclone respectively (Panel A & B). Panel C and D represent the temperature and salinity difference (post-pre cyclone). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 2 Changes in upper dynamics during pre and post cyclone period at the left side of the Baaz track. Within brackets represent Levitus climatology. Parameter
Float ID-2900106 26112005
06122005
26112005
06122005
Isothermal layer depth (m) Mixed layer depth (m) Mixed layer temperature (°C) Mixed layer salinity (psu) Barrier layer thickness (m) Depth of 26 °C isotherm (m)
34.74 23.61 27.88 32.82 11.13 39.67
44.59 32.13 27.36 33.46 12.45 46.54
43.28 12.3 28.03 32.66 35.12 51.73
47.42 31.49 27.40 33.07 11.79 54.27
increased after passage of the Baaz cyclone. This increase continued nearly 5 days, but the duration period varied with the distance from the center of the cyclone. Change ranges in Chl a concentrations within a 60–150 km distance on both sides of the cyclone’s center was higher than that at other areas. Spatial changes in Chl a in the south central Bay of Bengal, before and after passage of the Baaz, is also clear (Fig. 6). The passage of the cyclone caused an increase in the Chl a concentration over a wide area of the south central Bay of Bengal. The enhancement in Chl-a due to the passage of cyclone is approximately 1.2–5.6 mg/m3 along Baaz’s track, but the area is very small. The role of mixing and upwelling is examined using the Argo profiles (Fig. 7) observations. Two Argo profiles are available only at the left side of the track (50 km, 100 km from the eye of the cyclone). The depth of the mixed layer and stability at the thermocline increased after passage of the cyclone (Fig. 7b). The well-mixed upper ocean would need stronger momentum flux to break stability and to increase entrainment and mixing. The vertical mixing phenomenon is also identified in the Argo observations in terms of migration of 26 °C isotherm. The SST cooling and strong wind stress curl directly responses to the expelling of the mixed layer and thermocline (20 °C isotherm) by upwelling process (Table 2). The deepening of 26 °C isotherm also indicates the entrainment process on left side of the track. Cyclone passage is associated with remarkable variation in mixed layer salinity (Fig. 7). Initially, after cyclone passage salinity increases (0.64 psu), which correlates well with entrainment and mixing of cold and salty water from beneath the mixed layer. In the following days a gradual freshening of the mixed layer originating at the ocean surface can be seen. This is due to the effect of high precipitation lasting longer than the increased wind forcing. Rainfall water forms a stable layer on the top of the mixed layer which slows downward transport of the fresh water. A weaker increase of salinity (0.41 psu) during passage of the cyclone agrees well with the weaker entrainment as documented by temperature observations. Presence of a stably stratified layer close to the ocean surface is not react in temperature profiles, since intensive mixing before its formation resulted in uniformization of temperature across the mixed layer. This means that the effect of water supply due to rainfall and the following stratification changes is only visible on salinity profiles.
Float ID-2900107
4. Conclusions In this study, we investigated the ocean physical and biological responses to Baaz cyclone using multi-satellites data and the Argo Profiles. The cool water wake area (coastal and open ocean waters) is shifted more to the right of the Baaz track shown in Fig. 2. Because of the translational movement of the Baaz, the maximum horizontal wind is found to the right of the track of the cyclone eye. Before and after the cyclone passage, the spatial change of the SST showed that there are warm and cold water areas on the right side of the track. The spatial distribution of pre cyclone SST provides a good demonstration of sea-surface heating over the whole of the SCBoB. Nonetheless, the periphery between cold and warm water regions after the passage of Baaz is clear. This is one of the reasons why sea surface cooling effects in the cold-water region were remarkable compared to the other regions, despite stronger winds on the right side of Baaz track. The maximum temperature drop existed in the SCBoB mainly due to the slow translation speed, the shallow mixed layer depth and D26 which enabled the cold water to be easily transported from subsurface layer to surface. Another important physical aspect is the surface temperature changes lagged behind the sea surface height (SSH). So the region with the largest increase of the sea surface height can indicate the location where the maximum cooling occurs. SSH increase is more in the coastal region compared with open ocean water. This probably is because the coastal current system also helps increase the surface in addition to cyclone Baaz. In general, the magnitude of wind on the right side of the cyclone track is stronger than the left side, and therefore environmental changes such as the sea surface cooling on the right side from the Baaz’s center could be greater than those on the left side. However, this stipulation varies with oceanic conditions. In the present study, time series changes in physical and biological factors on the right side was greater compared to those on the left side. The results shows that the strong upwelling was induced by Baaz cyclone and its velocity (also Ekman Layer depth) increased an order of magnitude, which provided perfect conditions for significant enhancement of chlorophyll-a at the right side of the track in the SCBoB. The SSC effect, chlorophyll-a increase (ten times higher), and nutrient concentration are clearly observed; these phenomena
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
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persevered after the passage of Baaz cyclone. It entails correlative changes in surface physical, biological, and chemical properties, these changes usually continued for a period of 4–10 days (post-cyclone) before reaching its previous position due to inertial oscillation. The enhancement in chlorophyll-a is more in the open waters than coastal regions. This is well correlated with SSC and it is 10 times more to climatological value over the same period. Acknowledgments The present research work is funded by the Department of Science and Technology, Govt. of India under the Fast Track Young Scientist Project (SR/FTP/ES-09/2008) which is gratefully acknowledged. I thank the Director, Indian National Centre for Ocean Information Service, Hyderabad, the Director, India Meteorological Department, New Delhi and Joint Typhoon Warning Centre for providing data products. References Babin, S.M., Carton, J.A., Dickey, T.D., Wiggert, J.D., 2004. Satellite evidence of hurricane-induced phytoplankton blooms in an oceanic desert. J. Geophys. Res. 109, C03043. Black, P.G., 1983. Ocean temperature changes induced by tropical cyclones (Ph.D. dissertation). Pennsylvania State University, 278. Byju, P., Prasanna Kumar, S., 2011. Physical and biological response of the Arabian Sea to tropical cyclone Phyan and its implications. Mar. Environ. Res. 71, 325–330. Chang, Y., Liao, H.-T., Lee, M.-A., Chan, J.-W., Shieh, W.-J., Lee, K.-T., Wang, G.-H., Lan, Y.-C., 2008. Multisatellite observation on upwelling after the passage of Typhoon Hai-Tang in the southern East China Sea. Geophys. Res. Lett. 35, L03612. http://dx.doi.org/ 10.1029/2007GL032858. Chelton, D., Wentz, F.J., de Gentemann, C., Szoeke, R., Schlax, M., 2000. Satellite microwave SST observations of trans-equatorial tropical instability waves. Geophys. Res. Lett. 27, 1239–1242. Chen, Xiaoyan, Pan, Delu, He, Xianqiang, Bai, Yan, Wang, Difeng, 2012. Upper ocean responses to category 5 Typhoon Megi in the western north Pacific. Acta Oceanol. Sin. 31 (1), 51–58. Chinthalu, G.R., Seetaramayya, P., Ravichandran, M., Mahajan, P.N., 2001. Response of the Bay of Bengal to Gopalpur and Paradip super cyclone during 15–31 October 1999. Curr. Sci. 81 (3), 283–291. Enriquez, A.G., Friehe, C.A., 1995. Effects of wind stress and wind stress curl variability on coastal upwelling. J. Phys. Oceanogr. 25 (7), 1651– 1671. Gierach, M.M., Subrahmanyam, Bulusu, 2008. Biophysical responses of the upper ocean to major Gulf of Mexico hurricanes in 2005. J. Geophys. Res. 113, 1–11. Gierach, M.M., Subrahmanyam, Bulusu, Prasad, T.G., 2009. Physical and biological responses to Hurricane Katrina (2005) in a 1/25° nested Gulf of Mexico HYCOM. J. Mar. Syst. 78, 168–179. Gopala Krishna, V.V., Murty, V.S.N., Sarma, M.S.S., Sastry, J.S., 1993. Thermal response of upper layers of Bay of Bengal to forcing of a severe cyclonic storm: a case study. Indian J. Mar. Sci. 22, 8–11. Hong, C.H., 2008. A numerical study of sea surface cooling with the passage of Typhoon Abby in the northwestern Pacific. J. Korean Fish Soc. 41 (6), 518–524. Hong, C.H., Sohn, I.S., 2004. Sea surface in the East Sea with the passage of typhoons. J. Korean Fish Soc. 37 (2), 137–147.
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Jordan, C.L., 1964. On the influence of tropical cyclones on the sea surface temperature. In: Proceedings of the Symposium on Tropical Meteorology. New Zealand Meteorological Service, Wellington, pp. 614–622. Kara, A.B., Rochford, P.A., Hurlburt, H.E., 2000. An optimal definition for ocean mixed layer depth. Geophys. Res. Lett. 105 (16803–1682), 1. Kim, S.W., Yamad, K., Jang, L.H., Hong, C.H., Go, W.J., Suh, Y.S., Lee, C., Lee, G.H., 2007. Short-term variation of sea surface temperature caused by Typhoon Nabi in the Eastern Sea of Korean peninsula derived from satellite data. J. Korean Fish Soc. 40 (2), 102–107. Lee, Chung I, Park, Mi-Ok, 2010. Time series changes in sea-surface temperature, Chlorophyll a, nutrients, and sea-wind in the East/Japan Sea on the left- and right-hand sides of Typhoon Shanshan’s track. Ocean Sci. J. 45 (4), 253–265. Lukas, R., Lindstrom, E., 1991. The mixed layer of the western equatorial Pacific Ocean. J. Geophys. Res. 96, 3343–3357. McPhaden, M.J., Foltz, G.R., Lee, T., Murty, V.S.N., Ravichandran, M., Vecchi, G.A., Vialard, J., Wiggert, J.D., Yu, L., 2009. Ocean– atmosphere interactions during cyclone Nargis. EOS Trans. Am. Geophys. 90, 53–54. Park, J.J., Park, K.A., Kim, K., 2004. Upper ocean response to typhoons and tropical storms in the North Pacific. A joint Korea-China-Japan Regional ARGO Workshop, Jeju Island, Korea, 6–8 September. Price, J.F., 1981. Upper ocean response to a hurricane. J. Phys. Oceanogr. 11, 153–175. Shang, S., Li, L., Sun, F., Wu, J., Hu, C., Chen, D., Ning, X., Qiu, Y., Zhang, C., Shang, S., 2008. Changes of temperature and bio-optical properties in the South China Sea in response to Typhoon Lingling, 2001. Geophys. Res. Lett. 35, L10602. http://dx.doi.org/10.1029/ 2008GL033502. Shi, W., Wang, M., 2011. Satellite observations of asymmetrical physical and biological responses to Hurricane Earl. Geophys. Res. Lett. 38, L04607. http://dx.doi.org/10.1029/2010GL046574. Stramma, L., Cornillon, P., 1986. Satellite observation of sea surface cooling by hurricanes. J. Geophys. Res. 91, 5031–5035. Suetsugu, M., Kawamura, H., Nishihama, S., 2000. Sea surface cooling caused by typhoons in the western North Pacific Ocean. In: PORSEC proceedings, vol. 1. Goa, India, pp. 258–262. Tommy, D.D., Francesco, Nenciolia, Victor, S.K., Carrie, Leonardb, Wil, Blacka, Yoshimi, M., Robert, R. Bidigarec, Qin, Zhanga, 2008. Physical and bio-optical observations of oceanic cyclones west of the island of Hawai’i. Deep-Sea Res. II 55, 1195–1217. Walker, N.D., Leben, R.R., Balasubramanian, S., 2005. Hurricane-forced upwelling and chlorophyll a enhancement within cold-core cyclones in the Gulf of Mexico. Geophys. Res. Lett. 32 (18). Wang, XiDong, Han, GuiJun, Qi, YiQuan, Li, Wei, 2011. Impact of barrier layer on typhoon-induced sea surface cooling. Dyn. Atmos. Oceans 52, 367–385. Wentz, F.J., Gentemann, C., Smith, D., Chelton, D., 2000. Satellite measurements of sea surface temperature through clouds. Science 288 (847–850), 2000. Wong, A.P.S., Johnson, G.C., Owens, W.B., 2003. Delayed-mode calibration of autonomous CTD profiling float salinity data by h-S climatology. J. Atmos. Ocean. Technol. 20, 308–318. Yang, Yuan-Jian, Sun, Liang, Liu, Qi, Xian, Tao, Fu, Yun-Fei, 2010. The biophysical responses of the upper ocean to the Typhoons Namtheun and Malou in 2004. Int. J. Remote Sens. 31 (17), 4559–4568. Zhao, H., Tang, D.-L., Wang, D.-X., 2009. Phytoplankton blooms near the Pearl River Estuary induced by Typhoon Nuri. J. Geophys. Res. 114, C12027. Zheng, Z.-W., Ho, C.-R., Kuo, N.-J., 2008. Importance of pre-existing oceanic conditions to upper ocean response induced by Super Typhoon Hai-Tang. Geophys. Res. Lett. 35.
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
ScienceDirect Advances in Space Research xxx (2015) xxx–xxx www.elsevier.com/locate/asr
Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone K. Muni Krishna a,⇑, Guiting Song b a
Dept of Meteorology and Oceanography, Andhra University, Visakhapatnam 530003, India b Department of Marine Science, Zhejiang Ocean University, Zhou Shan 316022, China
Received 17 February 2015; received in revised form 12 July 2015; accepted 18 July 2015
Abstract Baaz cyclone is a slow moving weak cyclone in the south central Bay of Bengal (SCBoB) and it lingered for 3 days and caused a significant cooling (2.6 °C) and enhancement of chlorophyll-a (5.6 mg/m3) at the right side of the cyclone track. In this study multi-satellite observations are used to explore the bio-physical changes due to the cyclone. We found that the speed of the Ekman pumping velocity is ten times more and the mixed layer is deepened about 19 meters during the cyclone than pre cyclone period. The maximum sea surface cooling (1.2–2.6 °C) took place when the translation speed of cyclone is only 1.2–2.3 m/s. So the extent of the sea surface temperature drop is probably related to the moving speed of cyclone and the mixed layer depth. In addition, the area with large decline of the SSH can signify the location where the maximum cooling occurs. Ó 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Cyclone; Sea surface temperature; Mixed layer; Chlorophyll-a; Sea surface height; Ekman pumping
1. Introduction Tropical cyclones are one of the most destructive natural disasters known to mankind. Strong wind and heavy rain are associated with these weather systems which produce large waves in the ocean. The land falling of the tropical cyclones cause disastrous damage to the coastal infrastructure, the natural environment, extensive flooding due to storm surge and very often loss of life. They are also play a key role on upper ocean environment changes in the tropics and subtropics. It also alters the physical characteristics of the upper ocean (Price, 1981; Black, 1983; Gopala Krishna et al., 1993; Chinthalu et al., 2001; Babin et al., 2004; Gierach and Bulusu Subrahmanyam, 2008; Chang et al., 2008; Gierach et al., 2009; Lee and Park, 2010; Yang et al., 2010; Wanga et al., 2011). The intensity of ⇑ Corresponding author. Tel.: +91 891 2717663.
E-mail address: [email protected] (K. Muni Krishna).
severe storms is strongly influenced by the thermodynamic structure of the upper ocean, and an accurate prediction of the storm’s future intensity requires measurements of the ocean’s thermal structure ahead of the storm. Upper ocean biological and physical responses to cyclones are always one of the hottest issues on cyclone-ocean interaction (Walker et al., 2005; Tommy et al., 2008; Shang et al., 2008; Zheng et al., 2008; Zhao et al., 2009; Chen et al., 2012; Mcphaden et al., 2009; Shi and Wang, 2011; Byju and Prasanna Kumar, 2011). The measurement of biological and physical parameter onboard ship is very difficult because of the severe ocean conditions during cyclone period. The development of multi satellite remote sensing, especially microwave remote sensing, in combination with numerical simulation, provide powerful tools to further understand the upper ocean changes induced by cyclone. According to Price (1981) the passage of cyclone can stimulate ocean mixing and upwelling which brings the nutrient rich water from sub surface to the euphotic zone where
http://dx.doi.org/10.1016/j.asr.2015.07.025 0273-1177/Ó 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.
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there is abundant light for photosynthesis but often lack of nutrients. Consequently it will contribute to the growth of phytoplankton and take a significant impact on the increase of ocean primary production. Stramma and Cornillon (1986) revealed that when the storm moves rapidly, the maximum cooling occurs well to the right of the track, whereas for slowly moving storms the maximum cooling occurs near or on the track. Suetsugu et al. (2000) described the cooling of the sea surface induced by the tropical cyclone as air-sea parameter dependence i.e. storm strength, storm translation speed and upper ocean thermal structure. They concluded that the strong and slow moving typhoon causes the maximum cooling of 5 °C in the cyclone wake. Black (1983) described the coolness as an complex system of internal waves extending through the thermocline and conducting colder water to the surface by an eddy mixing process. The SST can drop by as much as 5 °C due to this mixing. His study also revealed that the cooling is substantial when the hurricane moves at speeds less than the internal gravity waves (4–8 knots). Their result shows that the sea surface cooling biased towards the right of the storm track and sub surface intense upwelling. Such a significant response in the ocean thermal structure may weaken surface winds through the modulation of cold sea surface temperature on the stability of the atmosphere. Strong wind forcing exerted on the ocean surface by hurricane strength tropical cyclones produces turbulent mixing of the upper oceanic layer, mixed layer deepening and entrainment of colder water form below which is accompanied by an enhancement of primary production. This probably is the primary mechanism responsible for the SST decrease during the tropical cyclone passage. According to Hong and Sohn (2004) and Kim et al. (2007), the upwelling caused by Ekman pumping is active when a typhoon passes over a specific sea surface, and the cold water injection into the surface layer from a deeper layer continues to spread horizontally outwards. This horizontal spreading is considered as one of the main factors supporting vertical mixing, and, as a result, continuous changes in various factors may last for some time. Babin et al. (2004) has shown that surface chlorophyll concentrations change in post-storm surface usually continued for approximately two to three weeks. Hong (2008) also suggests that the sea surface cooling (SSC) continues for nearly 20 days. This continuity period depends relatively on the scale of the typhoon and the oceanic conditions. In the present study we wish to investigate the physical and biological changes due to the passage of Baaz cyclone in the Bay of Bengal. 2. Data and methodology 2.1. Cyclone data The tropical cyclone data used in this study is provided by the India Meteorological Department and Joint
Typhoon Warning Center, from 27th October to 1st December 2005, in the geographical range of 80°–90°E and 8°–15°N (Fig. 1). The data contain the name of tropical cyclone, observing time (about 6-h interval), location of cyclone center, maximum sustained wind speed, wind direction, and central pressure (www.imd.gov.in). Baaz cyclone is lingered in the south Bay of Bengal for 5 days with a relative slow moving speed of 1.2–2.3 m/s and made landfall off north Tamil Nadu on 2 December 2005 (Table 1). 2.2. Sea surface temperature A Group for High Resolution Sea Surface Temperature (GHRSST) global Level 4 sea surface temperature analysis produced daily on a 0.25 degree grid at the NOAA National Climatic Data Center. Merged SST daily products with a 25 km spatial resolution retrieved from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) and advanced high resolution radiometer (AVHRR). This product uses optimal interpolation (OI) using data from the Advanced Very High Resolution Radiometer (AVHRR) Pathfinder Version 5 time series (when available, otherwise operational NOAA AVHRR data are used), the Advanced Microwave Scanning Radiometer-EOS (AMSR-E), and in situ ship and buoy observations. The AMSR-E and AVHRR can provide continuous measurement of the SST even when there is typhoon with heavy clouds. MW data is likely to be available at a suitable quality for pixels where there is cloud affected in AVHRR because MW can penetrate the cloud and observe the sea surface (the cloud coverage in tropical areas typically exceeds 90% – Chelton et al., 2000; Wentz et al., 2000) http://www.ncdc.noaa.gov/oa/climate/research/sst/oi-daily.php. 2.3. Sea surface wind The Blended Sea Winds contain globally gridded, high resolution ocean surface vector winds and wind stresses on a global 0.25° grid. The wind speeds were generated by blending observations from multiple satellites (up to six satellites since June 2002). The wind directions came from two sources depending on the products: for the research products the source is the NCEP Reanalysis 2 (NRA-2) and for near-real-time products the source is the ECMWF NWP. The wind directions were interpolated onto the blended speed grids. The blending of multiple-satellite observations fill in the data gaps of the individual satellite samplings both temporally and spatially and reduce the subsampling aliases and random errors (http://www.ncdc.noaa.gov/oa/climate/research/wind). The spatial variation of wind force produces a consequent variation in the corresponding Ekman transport in the surface layer, causing convergence and divergence in different areas. This leads to “Ekman pumping” (Enriquez and Friehe, 1995). In this paper, Ekman
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
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Fig. 1. Tropical cyclone track with intensities. Light blue color balloons indicate position of Argo floats. (Source: Google earth, IMD, JTWC). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Baaz Cyclone at different stages (Source: India Meteorological Department). Name
Date
Time (UTC)
Latitude
Longitude
Estimated central pressure (hPa)
Maximum sustained surface wind (kt)
Grade
BAAZ BAAZ BAAZ BAAZ BAAZ BAAZ
28/11/2005 28/11/2005 29/11/2005 30/11/2005 01/12/2005 02/12/2005
0300 0600 0600 0600 1200 0300
10.5 10.5 11.0 12.5 12.5 13.0
90.5 90.0 86.0 84.0 83.5 82.5
1006 1004 998 998 998 1004
25 30 45 35 30 25
D DD CS CS DD D
pumping velocity, which reflects the velocity of upwelling, is computed by the following equation given by Enriquez and Friehe (1995). WE ¼
1 ðr sÞ qf
ð1Þ
where q = density of sea water = 1028 kg/m3 f = coriolis force = 2 X sin/ $ s = wind stress curl 2.4. Sea surface height Weekly TOPEX (TOPography EXperiment for ocean circulation)/Poseidon SSH data with 0.25° spatial resolution are provided by Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO). http://www.aviso.altimetry.fr/en/data/products/sea-surfaceheight-products/global/ssha.html. 2.5. Chlorophyll-a concentration The launch of the Ocean Color Monitor (OCM) sensor onboard IRS-P4 satellite has been a boon to the scientific community involved in the analysis and characterization of the chlorophyll concentration in the oceans. The IRS-P4 OCM has eight spectral channels in visible and near-infrared wavelengths (412–865 nm) with a spatial resolution of 360 m and revisit cycles of 2 days. OCM have eight bands both in BIL and BSQ mode. Each band is stacked and exported as Generic Binary. These 8 bands Generic Binary data was processed using ERDAS 8.4 (with OCM-DAS module) software. Weekly data is available at INCOIS web site (www.incois.gov.in).
2.6. In-situ observations (Argo) Argo data used in this study and it is downloaded from the Indian National Center for Oceanic Services (www.incois.gov.in) and Coriolis Data Center (http://www.coriolis.eu.org/) from France during the cyclone period. Two sets of T-S profiles are obtained from the argo float with the ID2900106 and 2900107 for 26 Nov and 6 Dec 2005 and they are situated at (longitude and latitude data), about 50–100 km left side of the track in the study area encircling geographical co-ordinates bordered by 10– 15°N latitude and 80–85°E longitude within the environs of the cyclone track. For each profile, a real-time and a delayed-mode quality control were conducted. The calibration method developed by Wong et al. (2003) is employed to calibrate the sensor drift of salinity measurements in the Argo data. The distance between two Argo floats are within 200 km distance from both sides of the cyclone (Park et al., 2004). The isothermal layer depth (ILD) is determined from the temperature based criteria, where the temperature decreases to a value of 0.8 °C compared to the surface (shallowest level obtained from the ARGO profile). The intermediate layer which prevails between the mixed layer and thermocline is named as barrier layer, which forms over the salinity stratified regions (Lukas and Lindstrom, 1991). The implication of this barrier layer is to inhibit cooling from sub-surface waters, which can eventually energize tropical storms in the Bay of Bengal (McPhaden et al., 2009). A variable density criteria is chosen for the determination of mixed layer depth (MLD) as proposed by Kara et al. (2000) wherein MLD is constructed using density variability (rt ) determined from the corresponding
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temperature change DT (0.8 °C) in the equation of state. Difference between ILD and MLD is defined as barrier layer thickness (BLT). 2.7. Synoptic view of Baaz cyclone The system initially was seen as a depression and located at latitude 10.5°N and Longitude 90.5°E since 03:00 h on 28 November 2005, the system rapidly intensified into a deep depression and continued its westward movement, it further intensified into a cyclonic storm and located at latitude 10.5°N and longitude 88°E at 18:00 h on 28 November 2005 and reached its peak intensity of 45 kts and was located at about 425 nm east-southeast of Madras, India and named Baaz cyclonic Storm by 28/1800 UTC. Subsequently it concentrated on 29 November 2005 (Dvorak intensity estimates of T3.5), it moved northwestwards. The storm’s motion slowed down and became somewhat erratic on 30th November as it approached the Indian coastline. Baaz continued moving on awest–northwestward track and maintained strength until the morning (03:00 h) of 1 December, after that, it began to weaken into a deep depression on 2nd December 2005. 3. Results Warm sea surface temperature is observed (28.8 °C) on the right side of the cyclone track, it fuels to the Baaz cyclone and intensify it into a cyclonic storm over the southeast Bay of Bengal (Fig. 2a). It is very slow moving storm with a speed of 2.3 m/s. Two days after its passage the ocean water become cool (2.6 °C) in the central Bay of Bengal and also 1.5 °C near the Chennai coast
(Fig. 2b). Spatial distribution of the cool wake is observed over a large area at the right side of the track. The time-longitude cross section of sea surface temperature along the track clearly shows the duration of the cool wake in the open ocean is less compared with coastal area but 100 km left and right side of the track shows almost the same duration (three days) in both areas. Baaz cyclone was declared as a tropical storm on the Saffir–Simpson hurricane scale. It lingered with a slow speeds (1.2–2.3 m/s) in the south Bay of Bengal from 28 November to 2 December 2005 before moving rapidly westward thereafter (Fig. 1). The cyclone remained almost at rest during 2 December 2005. Satellite observations provided an opportunity to investigate the bio-physical changes over the ocean surface. Four satellite datasets mentioned on Section 2.2 were used in the present study. Before cyclone Baaz’s arrival, the south central Bay of Bengal was characterized by predominantly warm sea surface temperature (SST) which is greater than 28 °C. The warm sea surface temperature (28.8 °C) is situated on the right side of the track of Baaz (Fig. 2a). It provides a good demonstration of sea surface heating over the region. After two days of Baaz’s departure, a cold SST area with the temperature range of26.2–27.8 °C (along and right side) and a size comparable to Baaz’s 180 km RMW (Radius of Maximum Wind), is observed and co-located with the Baaz’s track. The minimum SST of 26.2 °C is found at the center (84°E, 11.8°N) of the cold pool (Fig. 2b). In comparison with pre-cyclone conditions, the SST had dropped by as much as 1.2–2.6 °C. This cold pool slowly decayed after the passage of the Baaz, but still maintained a low temperature of 27 °C. Daily variations in SST before and after the passage of the Baaz cyclone depends
Fig. 2. Spatial distribution of sea surface temperature before (a), and after (b) the Baaz cyclone and (c) cooling (after-before cyclone) .
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on the distance from its center (Fig. 3). To quantify the SST and wind speed relation, the time-longitude cross section of sea surface temperature along the track (12.23°N), right side (13.58°N) and left side (10.9°N) clearly shows the duration of the cool pool in the open ocean is less compared with coastal area, but 100 km left and right side of the track shows almost the same duration (three days) in both areas. The sharp drop in the surface temperature can be explained in terms of several processes (Jordan, 1964): seasonal cooling, removal of warm surface water by wind force and upwelling of colder deep water, the deeper mixing by cyclone winds, cold rainfall, and the loss
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of heat energy to the storm. In this study the cooling is due to the strong vertical mixing caused by the cyclone winds. The Ekman layer depth is also deepened which coincides with the cool pool area. The maximum depth of Ekman layer (220 m) is observed along the track. Pre-cyclone Ekman layer depth (120 m) is comparable shallower that the post cyclone Ekman Layer depth (220 m). The wind speed inside the cold pool is between 10 and 15 m/s while the surrounding wind speed is between 6 and 9 m/s along the track section. But at the coastal area the wind speed is only 8 m/s. At the right side of the cyclone track the wind speed is more 13 m/s at open ocean and greater than 15 m/s
Fig. 3. Time-Longitudinal cross section of sea surface temperature (left panel), Wind speed (right panel, fill contours) and Ekman depth (right panel, contours) along the cyclone track (A, D), Left side of the track and (B, E) and right side of the track (C, F).
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Fig. 4. Same as Fig. 2 for Ekman pumping velocity (105 m/s contours) and Ekman depth (m) (filled contours).
at the coastal regions. Similarly the maximum depth of Ekman layer is also observed at the coastal regions. Fig. 4 shows the Ekman pumping velocity on 28 November and 1 December 2005, respectively. The upwelling was weak (0.3 104 m/s) and its distribution was relatively small on 28 November before the Baaz cyclone in the south central Bay of Bengal. The Ekman pumping velocity WE is negative, i.e. there is a divergence of Ekman transports that brings water upward from the ocean interior. On 1st December, the upwelling was apparently stronger with a wider area. The Ekman pumping velocity increased about eight times. The Ekman transport is outward and away from the center of the cyclone, so it produces an upward Ekman pumping velocity below and also deepens Ekman layer depth. Due to the upward pumping effect in the ocean the thermocline is raised and the pressure is reduced in the ocean surface layers. Just one day after Baaz’s landfall on 3 November, the Ekman pumping velocity returned to the level as 28 October. Therefore, the strong upwelling triggered by strong wind stress vectors contributed a lot to this event. On the one hand, the passage of Baaz cyclone resulted in a vertical mixing which induces strong upwelling, and brought the phytoplankton inform the subsurface to the upper layer
(euphotic zone) and caused a enhancement of Chl-a concentration which can be seen from Fig. 6. The physical response in the form of sea surface height (SSH) changes also can be observed during Baaz cyclone period (Fig. 5). The SSH shows higher values before the cyclone Baaz and it is around 50 cm in the open ocean and 30 cm off the Tamil Nadu coast. But two days after its passage SSH is decrease 10 cm in the open ocean and also its looks like a cold core eddy shape located at the coastal region. It is about half of the open ocean value. It clearly indicates that the upwelling phenomenon is stronger in the open ocean than in the coastal area. In general, cyclone-caused oceanic biological changes as well as physical environmental changes such as SST, SSH and the concentrations of Chl at the upper layers of the ocean happen over a relatively short period. Time series of spatial changes in SST and Chl-a concentration can be measured easily by satellite sensors. Before the cyclone along the Tamil Nadu and South coastal Andhra Pradesh coastal area shows a high Chl-a concentration but it is due to the river discharge. In this study IRS-P4 Ocean Color Monitor (OCM) with a spatial resolution of 4 km clearly indicates the biological environmental change in Chl a concentrations at the right side of the track
Fig. 5. Weekly sea surface height changes before and after the passage of Baaz cyclone.
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Fig. 6. Sea Surface chlorophyll-a concentration before and after the Baaz cyclone.
Fig. 7. Vertical distribution of temperature (red). and salinity (blue). Solid and dotted lines represents before and after the cyclone respectively (Panel A & B). Panel C and D represent the temperature and salinity difference (post-pre cyclone). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 2 Changes in upper dynamics during pre and post cyclone period at the left side of the Baaz track. Within brackets represent Levitus climatology. Parameter
Float ID-2900106 26112005
06122005
26112005
06122005
Isothermal layer depth (m) Mixed layer depth (m) Mixed layer temperature (°C) Mixed layer salinity (psu) Barrier layer thickness (m) Depth of 26 °C isotherm (m)
34.74 23.61 27.88 32.82 11.13 39.67
44.59 32.13 27.36 33.46 12.45 46.54
43.28 12.3 28.03 32.66 35.12 51.73
47.42 31.49 27.40 33.07 11.79 54.27
increased after passage of the Baaz cyclone. This increase continued nearly 5 days, but the duration period varied with the distance from the center of the cyclone. Change ranges in Chl a concentrations within a 60–150 km distance on both sides of the cyclone’s center was higher than that at other areas. Spatial changes in Chl a in the south central Bay of Bengal, before and after passage of the Baaz, is also clear (Fig. 6). The passage of the cyclone caused an increase in the Chl a concentration over a wide area of the south central Bay of Bengal. The enhancement in Chl-a due to the passage of cyclone is approximately 1.2–5.6 mg/m3 along Baaz’s track, but the area is very small. The role of mixing and upwelling is examined using the Argo profiles (Fig. 7) observations. Two Argo profiles are available only at the left side of the track (50 km, 100 km from the eye of the cyclone). The depth of the mixed layer and stability at the thermocline increased after passage of the cyclone (Fig. 7b). The well-mixed upper ocean would need stronger momentum flux to break stability and to increase entrainment and mixing. The vertical mixing phenomenon is also identified in the Argo observations in terms of migration of 26 °C isotherm. The SST cooling and strong wind stress curl directly responses to the expelling of the mixed layer and thermocline (20 °C isotherm) by upwelling process (Table 2). The deepening of 26 °C isotherm also indicates the entrainment process on left side of the track. Cyclone passage is associated with remarkable variation in mixed layer salinity (Fig. 7). Initially, after cyclone passage salinity increases (0.64 psu), which correlates well with entrainment and mixing of cold and salty water from beneath the mixed layer. In the following days a gradual freshening of the mixed layer originating at the ocean surface can be seen. This is due to the effect of high precipitation lasting longer than the increased wind forcing. Rainfall water forms a stable layer on the top of the mixed layer which slows downward transport of the fresh water. A weaker increase of salinity (0.41 psu) during passage of the cyclone agrees well with the weaker entrainment as documented by temperature observations. Presence of a stably stratified layer close to the ocean surface is not react in temperature profiles, since intensive mixing before its formation resulted in uniformization of temperature across the mixed layer. This means that the effect of water supply due to rainfall and the following stratification changes is only visible on salinity profiles.
Float ID-2900107
4. Conclusions In this study, we investigated the ocean physical and biological responses to Baaz cyclone using multi-satellites data and the Argo Profiles. The cool water wake area (coastal and open ocean waters) is shifted more to the right of the Baaz track shown in Fig. 2. Because of the translational movement of the Baaz, the maximum horizontal wind is found to the right of the track of the cyclone eye. Before and after the cyclone passage, the spatial change of the SST showed that there are warm and cold water areas on the right side of the track. The spatial distribution of pre cyclone SST provides a good demonstration of sea-surface heating over the whole of the SCBoB. Nonetheless, the periphery between cold and warm water regions after the passage of Baaz is clear. This is one of the reasons why sea surface cooling effects in the cold-water region were remarkable compared to the other regions, despite stronger winds on the right side of Baaz track. The maximum temperature drop existed in the SCBoB mainly due to the slow translation speed, the shallow mixed layer depth and D26 which enabled the cold water to be easily transported from subsurface layer to surface. Another important physical aspect is the surface temperature changes lagged behind the sea surface height (SSH). So the region with the largest increase of the sea surface height can indicate the location where the maximum cooling occurs. SSH increase is more in the coastal region compared with open ocean water. This probably is because the coastal current system also helps increase the surface in addition to cyclone Baaz. In general, the magnitude of wind on the right side of the cyclone track is stronger than the left side, and therefore environmental changes such as the sea surface cooling on the right side from the Baaz’s center could be greater than those on the left side. However, this stipulation varies with oceanic conditions. In the present study, time series changes in physical and biological factors on the right side was greater compared to those on the left side. The results shows that the strong upwelling was induced by Baaz cyclone and its velocity (also Ekman Layer depth) increased an order of magnitude, which provided perfect conditions for significant enhancement of chlorophyll-a at the right side of the track in the SCBoB. The SSC effect, chlorophyll-a increase (ten times higher), and nutrient concentration are clearly observed; these phenomena
Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025
K. Muni Krishna, G. Song / Advances in Space Research xxx (2015) xxx–xxx
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Please cite this article in press as: Muni Krishna, K., Song, G. Physical and biological changes in the south Bay of Bengal due to the Baaz cyclone. Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.07.025