Indian Journal of Geo-Marine Sciences Vol. 43 (3), March 2014, pp. 400-407 Seasonal variability of mixed layer depth (MLD) in the Bay of Bengal Mohammad Muslem Uddin, Md. Zahedur Rahman Chowdhury, Sabbir Ahammed & Shyamal Chandra Basak Institute of Marine Sciences and Fisheries, University of Chittagong, Chittagong-4331, Bangladesh [E-Mail: mmu_ims76@yahoo.com] Received 13 July 2012; revised 15 November 2012 Mixed Layer Depths (MLD) on the basis of temperature and density were calculated for the Bay of Bengal area. Long term archive data from World Ocean Database (WOD09) were used for calculating MLD in the study area. During the pre monsoon period the lowest thermal MLD was observed in the mid basin area with a value of 25 m. But within the monsoon period MLD was gradually increase in all area up to a layer of 50 m. In the post monsoon period MLD increase in the upper Northern region and during winter period highest MLD of 95 m was observed. In this time it creates a high depressed zone in this area. Density oriented Mixed Layer was found thinner for the whole year. It was observed as 5 m in the upper Northern part, but southern area MLD change with season to season and MLD show highest 65m in the winter monsoon and lowest 20 m pre monsoon period. Surface current flows of the ocean and sea surface temperature (SST) seem to control to change the MLD with seasonality, which was also discussed for an annual cycle. Introduction The mixed layer depth (MLD) variability is important to acoustic propagation 1, ocean-biology 2, long-term climate change, and understanding air-sea interactions associated with the exchange of heat, carbon dioxide, and fresh water and with other biological and physical processes. The Bay of Bengal, a northern extended arm of the Indian Ocean, is located north-south from 6 N to 22 N latitude and east-west from 80 E to 100 E longitude. The Bay of Bengal occupies about 2,172,000 km 2, 3 which is about 6% of the world ocean 4. The maximum length of the Bay is 2,090 km and the maximum width is 1,610 km, with an average depth of more than 2.6 km 5. Bangladesh has predominantly four major river systems- (1) the Brahmaputra-Jamuna, (2) the Ganges-Padma, (3) the Surma-Meghna, and (4) the Chittagong Region river system. However, Brahmaputra is the 22nd longest (2,850 km) and the Ganges is the 30th longest (2,510 km) river in the world. Among them the Ganges, Brahmaputra and Upper Meghna rivers are the main providers (92%) of the fresh surface water into the coastal zone 6. The influence of the ocean current, wind circulation and river discharge are help to make a quasi homogenous region in the ocean. Where, the parameter of temperature, density and salinity are homogeneous due to well mixing process. It will change with depth, where the temperature change suddenly and create a separation layer that s refers mixed layer depth (MLD) in an oceanic area Fig. 1. This layer will change with periodically. Materials and Methods Long term monthly averaged data sets of temperature, salinity, and density were used to produce Mixed Layer Depth (MLD) scenario of the Bay of Bengal. To study recent current condition current imagery of the year 2005 was used. Seasonality of the region was discussed as prevailing weather pattern and four seasons were noticed. Fig. 1 Mixed Layer Depth (MLD), source: Riebeek, (2008)
UDDIN et al.: SEASONAL VARIABILITY OF MIXED LAYER DEPTH IN BAY OF BENGAL 401 The study domain is the east arm of the northern Indian Ocean, from 5 N to 25 N latitude and 80 E to 100 E i.e., major part of the Bay of Bengal Fig. 2. The parameters viz. Potential temperature, Potential density, Surface current pattern, Sea surface temperature (SST) are examined in this study. These data sets were obtained from different satellite and field data achieve. Sea Surface Temperature (SST) data was obtained from National Oceanographic Data Center s (NODC) latest World Ocean Database WOD09. As there is no satellite for salinity measurement, salinity data in addition with temperature were also collected from the achieve of WOD09 (NODC). Readymade Sea Surface current imageries of the year 2005 were downloaded from Ocean Surface Current Analysis Real time (OSCAR). Ocean Data View and Saga GIS have been used to analysis these data. Mixed layer depth fields were calculated from the individual temperature and salinity profiles available from the WOD09, and then averaged within climatological months and 1 by 1 box. The MLD criteria 7 described in Section 3 were applied to the vertical profiles of temperature and density. The density profiles were computed from temperature and salinity profiles taken at the same location. No horizontal interpolation was applied. The mean mixed layer depth and standard deviation were then computed for each month and 1 box. MLD for 1 boxes with no observations were treated as missing. This approach yields mean mixed layer depth values as well as standard deviations and Fig. 2 The study area domain number of observations. The standard deviations are computed for all months and 1 boxes that have two observations or more. The WOD09 standard level data provides vertical profiles of temperature and salinity at 4Q depths 8. MLD calculated from these profiles are not rounded to the standard levels, but rather linearly interpolated between them. Because the data coverage rapidly decreases with depth below about 500 m level, the MLD criteria were applied in the upper 500 m (standard levels I through 14). If an MLD criterion were not met within this depth range the MLD was set to missing. Such a cut off is relevant to limited polar areas during late winter / spring season. The following two criteria were used to produce the MLD maps. The fixed temperature difference criterion AT = 0.5 C (1) Where, AT=T(z=O)-T(z=MLD), T is the in-situ temperature. The temperature criterion based on the potential temperature would require use of the salinity profiles along with the in-situ temperature profiles hence would significantly reduce MLD coverage. - The fixed density difference criterion ACJ = 0.125 (sigma units) (2) Where, AO=o(z=O)lr(z=MLD), B=(p(g/cm3)-1)* 103=p(kg/m3)-1000, p is the potential density computed from the temperature and salinity based on the international equation of state of sea water 9. Taking into account the geographic variability of the sea surface temperature and salinity, the concept of a variable density criterion was introduced by and has been used for MLD computations by 10 and by 7. MLD criteria based on density account for the effects of salinity as well as temperature on MLD. Typically, a density criterion detects the minimum of the depths of the seasonal thermocline and the seasonal halocline. The value 0.125 in the fixed density criterion (2) approximately corresponds to a temperature difference of 0.5 C for water with salinity of S = 35.0% and temperature in the 17 C to 19 C range, which is characteristic for the mid-latitudes. Mixed layer depth results were visualized using the graphical software analysis package Ocean Data Viewer 4.3.2. I so surface variable for the equations (1 and 2) were created for the surface maps. Long term available data from CTD measurements, profiler readings and bathythermographs from WOD09 were used for the layer calculation. In order to realistically depict data
402 INDIAN J. MAR. SCI., VOL. 43, NO. 3, MARCH 2014 coverage, we have chosen to represent mixed layer depth by shading the results for each 1 box rather than use any contouring or extrapolation techniques. Results Both the Mixed Layer Depths (thermal and density driven) varied considerably in the Bay of Bengal with prevailing wind pattern in the sea. The results of the study show such variation as follows: In the month of March thermal mixed layer depth (MLD) extended up to 55 m in the Bay of Bangle (BOB). But at the Northern and eastern part of the oceanic region, it arises to 15 m and 25 m respectively. In the months of April, May and June MLD is observed at 15 m to 25 m in the whole area. But at the Northern region the layer went up to 30m-55 m depth (Fig. 3). In these months potential density oriented MLD decrease at the north portion Fig. 3 Average variability of MLD (m) of the Bay of Bangle (BOB) based on temperature in 12 months.
UDDIN et al.: SEASONAL VARIABILITY OF MIXED LAYER DEPTH IN BAY OF BENGAL 403 of the sea and it became 5 m to 10 m. But it is noticed 20 m to 30 m from mid basin to south portion of the sea (Fig. 4). During the period, the current in the Western and Southeast part of BOB increased in March. In this period current moves clockwise with a speed of 0.5 m/s. But in the mid basin current moves slowly and randomly with a seed of 0.1 m/s to 0.2 m/s. Surface current flow continue same direction during April month. But this direction was highly changed in the upper mid basin of Bay of Bangle (BOB) in the months of May and June. In the month of June high velocity current enter to the south west part of the BOB from the Indian Ocean. This current velocity is near 0.6 m/s (Fig. 5). Fig. 4 Average variability of MLD (m) of the Bay of Bangle (BOB) based on potential density in 12 months.
404 INDIAN J. MAR. SCI., VOL. 43, NO. 3, MARCH 2014 Fig. 5 Average current (m/s) pattern of the Bay of Bangle (BOB) for 12 months. During the monsoon months of July and August thermal MLD starts to increase. During this time a sharp fall of the layer was observed in the northern part of the Andaman Sea and MLD becomes up to 100 m (Maximum) during July. But in this time MLD is seen 5 m at the Western part and between 30 m-50 m in the mid basin. In the month of September, MLD areas are spread out at the western part deeper area of MLD becomes invisible and that reaches at 5 m (Fig. 3).
UDDIN et al.: SEASONAL VARIABILITY OF MIXED LAYER DEPTH IN BAY OF BENGAL 405 During this period, density based MLD remain almost same at the Northern part with a thin layer from 5m to 10m. But in the month of July layer was thicken in the mid basin up to 45m in July and September (Fig. 4). During this period the high velocities current enter to the mid basin and reached to the upper Northern part of BOB. When this current reached in the upper Northern part current flow rate slowly decreased. During the month of August a small current flow occur in the upper Northern region due to the river discharge. In these months of August and September this condition remains same (Fig. 5). At the Northern part, thermal MLD again extended down to 80 m in the month of October and the depressed layer shifted to the east during November. During the post monsoon period other areas of the basin remains with a medium layer between 30 m to 35 m (Fig. 3). Density driven mixed layer remains almost unchanged during post monsoon with 5m to 10m in shallow regions and up to 25 m in deeper regions of the basin (Fig. 4). During these months oceanic current flows slowly from the Southern part to the Northern and moves in the upper, lower and mid basin counter clockwise and a little bit randomly (Fig. 5). At the Northern part, thermal MLD again extended down to 80 m in the month of October and the depressed layer shifted to the east during November. During the post monsoon period other areas of the basin remains with a medium layer between 30 m to 35 m (Fig. 3). Density driven mixed layer remains almost unchanged during post monsoon with 5 m to 10 m in shallow regions and up to 25 m in deeper regions of the basin (Fig. 4). During these months oceanic current flows slowly from the Southern part to the Northern and moves in the upper, lower and mid basin counter clockwise and a little bit randomly (Fig. 5). During the month of December, with cooler wind from north thermal MLD decreases at the Northern part to 10 m. Along with that the depth increase continuously in the mid basin with winter days and in the month of January and February MLD grows up to 95 m in the upper mid basin region. The extended MLD form a regular circular depression in February with a depth of 80 m in the centre. Southern part beyond the thickest zone, MLD is almost uniform at 50 m to 55 m for the whole winter season (Fig. 3). Density MLD, which is usually small layer compare to the earlier MLD, becomes deepest in the center during January with a depth of 60 m. In the month of February deeper MLD spreads around to form a regular shaped depressive zone in the mid basin region with a deeper value of 35 m (Fig. 4). During these months oceanic current flows occur slowly in the mid basin of Bay of Bangle (BOB). These months current moves anti clockwise with a speed of 0.1 m/s to 0.3m/s (Fig. 5). Discussion During the pre monsoon period the observed changes in the mixed-layer depth clearly demarcated a distinct north-south region with 15 N as the limiting latitude. North of this latitude thermal MLD remained shallow (20 m to 25 m) for most of the pre monsoon period (March-June), but the deeper Southern part appreciable seasonality, this area thermal MLD appreciable 30 m to 55 m. In this region the sea surface temperature (SST) is vary from 27 C to 29 C (Fig. 6) due to clear skies and solar energy. Highly river discharge occurs in this period except March. This period the sea surface current flows slowly in this region except June. As a result of this the upper Northern part of Bay of Bangle (BOB) shows same homogenous area. The pre monsoon period is characterized by light winds, clear and intense solar in solution and is a period of strong net heat gain, as can be seen by the steady rise in near surface temperatures. These strong surface heating and moderate winds are instrumental in heating up the ocean surface. During the course of April May, SST increases steadily, starting from a minimum of 27.4 C in April and reaching a maximum of 29.7 C in May. Temperature at depth of 20 m to 40 m started to rise. This can be attributed to penetrative radiation warming below shallow mixed layer 11. The freshwater discharge also influences the salinity gradient up to 300 m 12. Water salinity ranges from 30 ppt to 34.8 ppt. During the monsoon period thermal MLD show in the mid basin of the BOB was same 40 m to 50 m (Fig. 3). The potential density MLD was same in this period near the upper Eastern part 5 m to 10 m. Because, in this time high river discharge seen in the upper Eastern region due to heavy rainfall. As a result of this MLD in this area shows very thin, because low MLD indicates strong stratification coastal area. In this time all area shows almost same temperature
406 INDIAN J. MAR. SCI., VOL. 43, NO. 3, MARCH 2014 Fig. 6 Month to month variability of SST (ºC) in the Bay of Bengal, derived from WOD05 (1960-2004) 28.5 C to 29 C (Fig. 6). During this monsoon the current flow are help to maintain this situation, because these months surface current flows almost same direction from the southwest part to the mid ocean and it help to make a homogenous area. Where, the potential density MLD was almost same.
UDDIN et al.: SEASONAL VARIABILITY OF MIXED LAYER DEPTH IN BAY OF BENGAL 407 During the monsoon season, the Arabian Sea experiences some of the strongest and steadiest winds. In general, this period is characterized by strong winds, cloudy skies and moist air starting early June until mid September 11. Solar isolation decrease due to cloud cover and these strong wind play a major rule in determine MLD 13. Surface temperature first half of the summer monsoon showed a general cooling trend. Temperature within mixed layer depth remained fairly constant starting from July until the end of August. Despite the strong winds, evaporative heat loss from the ocean is limited by extreme humidity in the air 11. Over the entry period of June-September, the gained an abnormally low average of 18 Wm -2 and the SST cooled by 3.4 C. This abnormal net heat flux can be attributed to the break in summer monsoon in July 2002 14. During the Post monsoon (October- November), at the Northern part, thermal MLD again extended down to 80 m in the month of October and the depressed layer shifted to the east during November. But, the mid basin of the BOB thermal MLD formed a regular shaped, where MLD between from 30 m to 55 m. During this period the potential density MLD in upper region is 5 m to 10 m. These months the sea surface temperature (SST) is lowest (27 C) in the upper Northern part (Fig. 6). Because, this monsoon period sea surface current occur in the mid ocean randomly and slowly, but the upper Northern part this region is lower than other part. Fresh surface water is advected out of the Bay of Bengal by the westward currents associated to the winter monsoon. This is combined with a down welling Ross by wave and results in the formation of a thick barrier layer (BL) 15. These BLs reach their maximum thickness of 15 to 20 m (regional average) in January February. 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