Seasonal Currents in the Bay of Bengal and Andaman Sea Revealed by Reprocessed Observations
Sakharin Suwannathatsa1, 2, +, Prungchan Wongwises1, 2 and Worachat Wannawong2, 3 1 The Joint Graduate of Energy and Environment, King Mongkut’s University of Technology Thonburi,
Bangkok, Thailand. 2 Center of Excellence on Energy Technology and Environment, S&T Postgraduate Education and
Development Office, Office of the Higher Education Commission, Ministry of Education, Thailand 3 Earth System Sciences Cluster, the Joint Graduate School of Energy and Environment,
King Mongkut’s University of Technology Thonburi, Bangkok, Thailand
Abstract. The currents in the Bay of Bengal (BOB) and Andaman Sea have been observed by many studies. The cycle of seasonal currents is well understood that the monsoonal winds are the main contributor. However recent observations from satellites reveal a character inside the main seasonal current. The approach of this study is to reprocess the data from satellite altimetry and create the oceanic model that is verified by the field measurement of Acoustic Doppler Currents Profilers. This gives the clearer picture of currents induced by the local wind along the coast and Rossby waves that both triggered by Ekman transport and radiated by Equatorial Kelvin waves that contribute to the intra-seasonal currents. The simulation also shows that coastal currents flow in the opposite direction to the offshore currents. The study reaches the conclusion that the local wind and meso-scale eddies caused by Ekman transport, Kelvin and Rossby waves play an important role in inducing the intra seasonal currents in the Bay of Bengal and Andaman Sea. The simulation can be used to implicitly explain the character of this intra-seasonal cycle.
Keywords: the Andaman Sea, Bay of Bengal, Seasonal Coastal Current, Reprocessed Satellite Data, Oceanic model
1. Introduction The domain of study is the Bay of Bengal and Andaman Sea [Fig.1]. In which the later has been the
famous tourist attraction for 30 years. The high currents during monsoonal seasons stop the tourism during a year. To understand these currents is the way to open the windows for tourism and the fishery. The Andaman Sea has been understood that mainly driven by the monsoonal winds but the detail circulation not. Recent reprocessed data from satellites and the created oceanic models in this study draw the new pictures of these intra-seasonal currents.
2. Materials and Methods The past observations from hydrographic ships and satellite-tracked drifter buoys for the last 20 years are
retrieved and analyzed to draw the picture of seasonal currents in the domain. Reprocessing these data by the computer of today technology that gives the faster computation and can manage enormous data points can create the oceanic model showing the currents in details. This implicitly describes the intra-seasonal current inside the seasonal current pattern which is different from the past study. In order to qualitatively explain these intra-seasonal currents, the oceanic model is built based on the reprocessed data of bathymetry, water temperature and salinity data. The model is driven by the winds during non-storm weather.
2.1. Past Observations In the past, scientists observed the currents by hydrographical ships and satellite-tracked drifter buoys
(ADCP) and noticed the dramatically reversal winds and currents in this region induced by seasonal monsoons. The North-East monsoon is in December through April and South-West monsoon is in July through October. The transitional seasons are between these monsoonal seasons. However the time line varies geographically between years (Laviolette, 1967).
Fig. 1: Map of the Bay of Bengal and Andaman Sea (colours represent depth in meter unit).
2.2. Reprocessed Observation Data ADCP ships, hydrographical observations and the satellite altimeter are reprocessed by the current
technology computer and carefully reviewed. Reprocessing the altimetry data gives the higher resolution of bathymetry data on coastal area and the better understanding in coastal currents than the past (Durand et al., 2009). They are used to create the oceanic model (Wannawong et al., 2010). 2.3. Model Configuration
The model has set to cover the Bay of Bengal and Andaman Sea (78 º E - 102 º E and 0 º - 26 º N). The size of model is 96 by 104 cells on horizon (resolution 1/4 º x 1/4º or about 28 km) and 16 layers on vertical plane. This study expresses a top layer from the surface to 5 meters depth which is the layer that represents the coastal currents near beaches and islands. The input data such as the bathymetry is obtained from the Global Relief Model from NOAA released March 2009, ETOPO1, (Amante and Eakins, 2008). The water temperature and salinity data are extracted from World Ocean Atlas 2005, WOA5, (Locarnini et al., 2005). The wind data is retrieved from the European Centre for Medium-Range Weather Forecasts, ECMWF, (Viberto and Betts, 1999). The simulation is created and run on January, May and September which are representing the monsoonal seasons and transitional windows in the domain.
Fig. 2: The 3-D visualization of a cut sector for QC
41
2.4. Input Data for Model Bathymetry data is obtained from the Global Relief Model from NOAA released March 2009 (ETOPO1)
is a 1 arc-minute global model of Earth's surface integrating land topography and ocean bathymetry. It is reprocessed and interpolated for the resolution of 1/4º lat and long or about 28 km [Fig.1, 2]. This creates a rectangular gridded model with 96 x 104 cells on horizon and 16 vertical layers.
Temperature and Salinity data is retrieved from World Ocean Atlas 2005 (WOA2005) that consists of an annual, seasonal, and monthly climatological distribution fields of temperature and salinity on a one-degree latitude-longitude grid. It is reprocessed and interpolated into the gridded model. In the domain, the temperature and salinity are between 24-32˚C and 1,018-1,024 psu.
Wind data [Fig. 3] is retrieved from the European Centre for Medium-Range Weather Forecasts (ECMWF) that create the atmospheric model in the global scale giving the data of wind (speed and direction), temperature, humidity and the pressure at the surface and the resolution is about 1 degree lat/long. It is reprocessed and interpolated into the same resolution of the bathymetry data.
Fig. 3: North-East monsoonal wind in January, Monsoon-transitional wind in May and South-West monsoonal wind in September.
3. Results and Discussions The past observation [Fig .4]; at the North-East monsoon end, the surface water of Andaman Sea seems
to be warmer, more saline and reaches the peak in May, in contrast, the end of South-West monsoon when the mixture of highly stratified and mix layers, cool and warm surface, clear and perturbation are observed. The river run-off gives the greatest drop of salinity in November and December and flows toward to the Bay of Bengal (Laviolette, 1967). In the transitional period from winter to summer in March to May, EICC is strongest while the regional wind is weakest during a year; equatorial Kelvin waves move northward and propagate into BOB at the North of Sumatra together with the divergence of Ekman transport. This cause Rossby waves contribute to the interior Bay of Bengal in driving the strong North-Eastward current of seasonal subtropical gyre (Shenoi et al., 2010). South-West monsoon from June to September, the EICC is much weaker and turbulent, the local alongshore winds contribute many turbulent currents on the west of BOB and they trend to move southward. These also draw the fresh water run-off from river at the top of Bay to the coast of India. In North-East monsoon from October to January, Ekman pumping and the local alongshore winds force the southward EICC (Hacker et al., 1998).
Fig. 4: The currents in the Bay of Bengal from the past observation on January, May and September.
42
The oceanic model by reprocessed data and ADCP measurement [Fig. 5, 6, 7]; they are in agreement for both directions and the speed magnitude. January currents have the magnitude of 0.5 - 0.7 m/s on the top and bottom of domain. They spin North-Eastward and South-Eastward on the top in opposite of the bottom where they flow South-Eastward. In August the current velocity of 0.7 m/s has been observed at the top and mid of domain. The current speed of 0.6 m/s happens at the top of domain in October. On the East of domain, the Andaman Sea covers Myanmar, Thailand and Malaysia coast; the 0.3 m/s North-Eastward current has been seen near Myanmar coast while the 0.4 m/s South-Westward current is at the North of Malaysia coast in January. In August, the 0.3 m/s North-Eastward current is observed near Myanmar shore whereas about 0.3 m/s South-Westward current is on Thailand and Malaysia border. In October, the 0.4 m/s South-East and North-Westward currents spin on Myanmar coast and 0.3 m/s North-Westward and North-Eastward spin near Phuket Island and Phung-nga province in Thailand. However the 0.3 - 0.4 m/s South-Eastward current through Malacca strait is observed throughout a year.
The shallow currents flow North-Eastward by equatorial Kelvin waves, Ekman pumping and Rossby waves from March to May. Local alongshore winds respond in many turbulent coastal currents with a tendency to move southward and draw at the fresh from the top of BOB southward from June to September. Both Ekman pumping and local alongshore winds drive the currents southward from October to January.
Fig. 5: The currents in the Bay of Bengal by oceanic model on January, May and September.
Fig. 6: ADCP ship data from R/V Knor 24th Jan - 5th Mar 1995 at 25-35 meter depth
Fig. 7: ADCP ship data from R/V Knor 30th Sep - 13th Oct 1995 at 25-35 meter depth
43
4. Conclusions The reprocessed data and created oceanic model suggest the local along-shored winds and Ekman
pumping are as important as the seasonal winds dominating the intra-seasonal currents. The result of simulation shows these intra-cycle seasonal currents influenced by local coastal winds, Rossby waves (both triggered by Ekman currents and radiated by equatorial Kelvin wave). They are similar with the ADCP measurements in term of direction and the magnitude of speed. In this region, the coastal currents flow in the opposite direction with offshore currents. The study reaches the conclusion that the oceanic model can implicitly explain these currents. However the field measurements and the small-scaled simulation based on local winds are needed for further investigations to capture the local coastal currents in detail. They give the resolution of the picture of these currents better than the large-scale wind-driven oceanic model.
5. Acknowledgements The team gratefully acknowledges Science and Technology Postgraduate Education and Research
Development Office (PERDO) of the Commission on Higher Education (CHE), Thailand for the financial support. The ADCP data in this study is obtained from the research vessel Knorr (R/V Knorr) which is owned by the U.S. Navy and operated by WHOI for the ocean research community. The appreciation is also given to Postdoctoral Researchers at Earth System Sciences research cluster (ESS) for the info and guidelines and Chortip Siwapornanan, a lecturer at Rajamangala University of Technology Rattanakosin, in formatting the data for visualization.
6. References [1] P. E. Laviolette. Temperature, Salinity, and Density of the World’s seas: Bay of Bengal and Andaman Sea. Naval
Oceanographic Office, 1967.
[2] F. Durand, D. Shankar, F. Birol, S.S.C Shenoi. Spatiotemporal structure of the East India Coastal Current from Satellite Altimetry, Journal of Geophysics. Res., 2009, 114.
[3] C. Amante, B.W. Eakins. 1 Arc–Minute Global Relief Model: Procedures, Data Sources and Analysis (ETOPO1), NOAA, 2008, National Geophysical Data Center, Colorado, U.S.A., 21.
[4] R.A. Locarnini, A.V. Mishonov, J. I. Antonov, T.P. Boye, H.E. Garcia. World Ocean Atlas 2005 (WOA2005), Volume 1: Temperature. S. Levitus, Ed, NOAA Atlas NESDIS 61, 2005, U.S. Gov. Printing Office, Washington, D.C., pp 182.
[5] P. Viterbo, A.K. Betts. Impact of the ECMWF Reanalysis Soil Water on Forecasts of the July 1993 Mississippi flood, Journal of Geophysical Research, 1999, 104, pp. 361-366.
[6] S.S.C Shenoi. Intra-seasonal variability of the coastal currents around India: A river of the evidences from new observations. Indian Journal of Geo-Marine sciences, 2010, 39(4), pp. 489-496.
[7] W. Wannawong , U. W. Humphries, P. Wongwises, S. Vongvisessomjai, W. Lueangaram. Numerical Analysis of Wave and Hydrodynamic Models for Energy Balance and Primitive Equations. International Journal of Mathematical and Statistical Sciences, 2010, 4, pp. 140–150.
[8] P. Hacker, E. Firing, J. Hummon, A. L. Gordon, J. C. Kindle. Bay of Bengal currents during the Northeast Monsoon. Geophysical Research Letters, 1998, 25(15), pp. 2769–2772.
