Effect of the 2004 'Boxing Day' tsunami on water properties and currents in the Bay of Bengal.

Author:Reddy, Naveendra
 
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INTRODUCTION

An earthquake of magnitude 9.3 on the Richter scale occurred offshore in northwest Sumatra (epicenter 3.32[degrees] N, 95.85[degrees]E) on 26th December 2004. It generated a huge tsunami, which devastated the Andaman and Nicobar Islands, the east coast of India and south Kerala (1). The tsunami was tracked by the Jason altimetry satellite early morning around 0300 hours on the same day, 2 hours after the earthquake (2). This was the strongest occurrence since satellite altimetry started in the early 1970s.

The measurement shows an initial dominant wavelength of about 500 km, followed by significantly greater height variation in the Bay of Bengal compared with those observed in earlier cycles recorded 10-20 days before the event and afterwards shows a return to the undisturbed ocean. During the event, the position of the wave is consistent with shallow-water wave speed of about 200 m [sec.sup.-1] at approximately 4500 m depth. The wave would have travelled about 1500 km in the 2 h since the earthquake occurred off the coast of Sumatra (2).

The study area: The Bay of Bengal is a northern extended arm of the Indian Ocean, which is located between latitudes 5 and 22[degrees]N and longitudes 75 and 100[degrees]E as shown in Fig. 1. It is bounded in the west by the east coasts of Sir Lanka and India, on the north by the deltaic region of the Ganges, Brahmaputra and Meghna River systems and on the east by the Myanmar peninsula, extending up to the Andaman-Nicobar ridges. The southern boundary of the Bay is approximately along the line drawn from Dondra Head in the south of Sir Lanka to the north tip of Sumatra (3). The Bay occupies an area of about 2.2 million sq km and has an average depth of 2600 m with a maximum of 5258 m. Bangladesh is situated at the head of the Bay of Bengal.

[FIGURE 1 OMITTED]

Physical properties of ocean water in the Bay of Bengal: The important physical properties affecting seawater density and which controls the dynamic behavior of the ocean are temperature and salinity. Density is indirectly observed in the ocean, that is, it is computed from the measurements of the temperature and salinity fields as a function of depth. Therefore knowing the density, it is possible to deduce the movement of seawater.

Temperature: In the Bay of Bengal, the thermocline is usually at a depth of 50 m and occasionally goes down 120 m. Between February and March, the depth of the thermocline varied from 75-120 m in the western Bay and from 50-100 m in the eastern Bay. During the pre-summer season, a warm water cell is seen centered at 14[degrees]N, 85[degrees]E with a core temperature of 28[degrees]C at a depth of 100 m. Two cold water cells are noticed towards the west (18[degrees]C) and north (17[degrees]C) of the warm water cell (4). During the summer monsoon (monsoon is a wind pattern that reverses direction on a seasonal basis), a broad cold water (20[degrees]C) band oriented in a southwest-northeast direction in the central Bay characterizes the temperature distribution at this depth. During the post-summer season, a warm pocket (25[degrees]C) is located at 12[degrees]N, 83[degrees]E, while during the winter monsoon, penetration of warm waters (27[degrees]C) from the southeastern Bay towards the central Bay is seen[4]. Generally, examining the temperature, the thermocline is strong during the winter monsoon.

Salinity: Freshwater influx flows into the Bay of Bengal annually and about 50% of this comes during the summer monsoon months. During the summer monsoon, low saline water (~ 29.0 psu) spreads into the interior of the Bay in a southwesterly direction from the head of the Bay and the northern Andaman Sea (4), while during the winter monsoon, a large cell of low salinity water (34.9 psu) occupies the central Bay. At 500 m depth during the summer monsoon, the salinity distribution is characterized by zonal oriented isohalines with relatively high salinities in the central and southern Bay and low salinities north of 16[degrees]N. At 1000 m depth during the summer season, lower salinities in the northwestern Bay and higher salinities off the southern east coast of India has been observed. But during the winter monsoon, along the 91[degrees]E meridional section of the longitude, two pockets of low salinity centered at 8 and 15[degrees]N are seen in the upper 50 m. The halocline is located at about 75 m and a high salinity cell appears at depths of 90 m due to penetration of high salinity water (4).

Density: The low salinity surface waters of the Bay of Bengal causes them in all seasons to be isolated from the deep waters by a sharp pycnocline between depths of 50 and 100 m. The surface salinity variations within and between seasons are controlled by insulation, evaporative cooling and an influx of saline and freshwater. During the summer monsoon, the lowest density waters are seen in the north western Bay and the distribution pattern of density in this area resembles that of salinity. During the winter monsoon, surface water density is less than 1022 kg [m.sup.-3]. Two cells of very low density are found off the central east coast of India and northwest of the Andaman Islands, where freshwater discharges...

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