Indian Journal of Geo-Marine Sciences Vol. 43 (3), March 2014, pp. 400-407

Seasonal variability of mixed layer depth (MLD) in the

Mohammad Muslem Uddin, Md. Zahedur Rahman Chowdhury, Sabbir Ahammed & Shyamal Basak Institute of Marine Sciences and Fisheries, University of Chittagong, Chittagong-4331, Bangladesh [E-Mail: [email protected]]

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 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 surface temperature (SST) seem to control to change the MLD with seasonality, which was also discussed for an annual cycle.

Introduction change with depth, where the temperature change The mixed layer depth (MLD) variability is suddenly and create a separation layer that’s refers important to acoustic propagation1, ocean-biology2, mixed layer depth (MLD) in an oceanic area Fig. 1. long-term climate change, and understanding air-sea This layer will change with periodically. interactions associated with the exchange of heat, carbon dioxide, and fresh water and with other Materials and Methods biological and physical processes. Long term monthly averaged data sets of The Bay of Bengal, a northern extended arm of the temperature, salinity, and density were used to , is located north-south from 6° N to produce Mixed Layer Depth (MLD) scenario of 22° N latitude and east-west from 80° E to 100° E the Bay of Bengal. To study recent current longitude. The Bay of Bengal occupies about condition current imagery of the year 2005 was used. 2,172,000 km2, 3which is about 6% of the world Seasonality of the region was discussed as prevailing ocean4. The maximum length of the Bay is 2,090 km weather pattern and four seasons were noticed. and the maximum width is 1,610 km, with an average depth of more than 2.6 km5. Bangladesh has predominantly four major river systems- (1) the Brahmaputra-Jamuna, (2) the -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 zone6. 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 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 number of observations. The standard deviations Indian Ocean, from 5° N to 25° N latitude and 80° E are computed for all months and 1° boxes that have to 100° E i.e., major part of the Bay of Bengal two observations or more. Fig. 2. The parameters viz. Potential temperature, The WOD09 standard level data provides vertical Potential density, Surface current pattern, Sea surface profiles of temperature and salinity at 4Q depths8. temperature (SST) are examined in this study. MLD calculated from these profiles are not rounded These data sets were obtained from different to the standard levels, but rather linearly interpolated satellite and field data achieve. Sea Surface between them. Because the data coverage rapidly Temperature (SST) data was obtained from National decreases with depth below about 500 m level, the Oceanographic Data Center’s (NODC) latest World MLD criteria were applied in the upper 500 m Ocean Database WOD09. As there is no satellite for (standard levels I through 14). If an MLD criterion salinity measurement, salinity data in addition with were not met within this depth range the MLD was temperature were also collected from the achieve of set to missing. Such a cut off is relevant to limited WOD09 (NODC). Readymade Sea Surface current polar areas during late winter / spring season. imageries of the year 2005 were downloaded from The following two criteria were used to produce Ocean Surface Current Analysis Real time (OSCAR). the MLD maps. The fixed temperature difference Ocean Data View and Saga GIS have been used to criterion analysis these data. AT = 0.5°C (1) Mixed layer depth fields were calculated from the individual temperature and salinity profiles available Where, AT=T(z=O)-T(z=MLD), T is the in-situ from the WOD09, and then averaged within temperature. The temperature criterion based on the climatological months and 1° by 1 box. The MLD potential temperature would require use of the salinity criteria7 described in Section 3 were applied to profiles along with the in-situ temperature profiles the vertical profiles of temperature and density. hence would significantly reduce MLD coverage.

The density profiles were computed from temperature - The fixed density difference criterion and salinity profiles taken at the same location. ACJ = 0.125 (sigma units) (2) No horizontal interpolation was applied. The mean mixed layer depth and standard deviation were then Where, AO=o(z=O)lr(z=MLD), B=(p(g/cm3)-1)* computed for each month and 1° box. MLD for 103=p(kg/m3)-1000, p is the potential density 1° boxes with no observations were treated as computed from the temperature and salinity based 9 missing. This approach yields mean mixed layer on the international equation of state of sea water . depth values as well as standard deviations and 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 by10 and by7. 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

Fig. 2—The study area domain 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 In the month of March thermal mixed layer depth depth by shading the results for each 1° box rather than (MLD) extended up to 55 m in the Bay of Bangle use any contouring or extrapolation techniques. (BOB). But at the Northern and eastern part of the oceanic region, it arises to 15 m and 25 m Results respectively. In the months of April, May and June Both the Mixed Layer Depths (thermal and density MLD is observed at 15 m to 25 m in the whole driven) varied considerably in the Bay of Bengal with area. But at the Northern region the layer went up to prevailing wind pattern in the sea. The results of the 30m-55 m depth (Fig. 3). In these months potential study show such variation as follows: 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 with a seed of 0.1 m/s to 0.2 m/s. Surface current flow 20 m to 30 m from mid basin to south portion of the continue same direction during April month. But this sea (Fig. 4). direction was highly changed in the upper mid basin of During the period, the current in the Western and Bay of Bangle (BOB) in the months of May and June. Southeast part of BOB increased in March. In this period In the month of June high velocity current enter to current moves clockwise with a speed of 0.5 m/s. the south west part of the BOB from the Indian Ocean. But in the mid basin current moves slowly and randomly 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 MLD is seen 5 m at the Western part and between thermal MLD starts to increase. During this time a 30 m-50 m in the mid basin. In the month of sharp fall of the layer was observed in the northern September, MLD areas are spread out at the western part of the Andaman Sea and MLD becomes up to part deeper area of MLD becomes invisible and 100 m (Maximum) during July. But in this time 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 Density MLD, which is usually small layer almost same at the Northern part with a thin layer compare to the earlier MLD, becomes deepest in the from 5m to 10m. But in the month of July layer center during January with a depth of 60 m. In the was thicken in the mid basin up to 45m in July and month of February deeper MLD spreads around to September (Fig. 4). form a regular shaped depressive zone in the mid During this period the high velocities current enter basin region with a deeper value of 35 m (Fig. 4). to the mid basin and reached to the upper Northern During these months oceanic current flows occur part of BOB. When this current reached in the upper slowly in the mid basin of Bay of Bangle (BOB). Northern part current flow rate slowly decreased. These months current moves anti clockwise with a During the month of August a small current flow speed of 0.1 m/s to 0.3m/s (Fig. 5). occur in the upper Northern region due to the river discharge. In these months of August and September Discussion this condition remains same (Fig. 5). During the pre monsoon period the observed At the Northern part, thermal MLD again extended changes in the mixed-layer depth clearly demarcated down to 80 m in the month of October and the a distinct north-south region with 15°N as the limiting depressed layer shifted to the east during November. latitude. North of this latitude thermal MLD remained During the post monsoon period other areas of the shallow (20 m to 25 m) for most of the pre monsoon basin remains with a medium layer between 30 m to period (March-June), but the deeper Southern 35 m (Fig. 3). part appreciable seasonality, this area thermal Density driven mixed layer remains almost MLD appreciable 30 m to 55 m. In this region the sea unchanged during post monsoon with 5m to 10m in surface temperature (SST) is vary from 27°C to 29°C shallow regions and up to 25 m in deeper regions (Fig. 6) due to clear skies and solar energy. of the basin (Fig. 4). Highly river discharge occurs in this period except During these months oceanic current flows slowly March. This period the sea surface current flows from the Southern part to the Northern and moves slowly in this region except June. As a result of this in the upper, lower and mid basin counter clockwise the upper Northern part of Bay of Bangle (BOB) and a little bit randomly (Fig. 5). shows same homogenous area. At the Northern part, thermal MLD again extended The pre monsoon period is characterized by light down to 80 m in the month of October and the winds, clear and intense solar in solution and is a depressed layer shifted to the east during November. period of strong net heat gain, as can be seen by the During the post monsoon period other areas of steady rise in near surface temperatures. These strong the basin remains with a medium layer between surface heating and moderate winds are instrumental 30 m to 35 m (Fig. 3). in heating up the ocean surface. During the course of Density driven mixed layer remains almost April – May, SST increases steadily, starting from a unchanged during post monsoon with 5 m to 10 m in minimum of 27.4°C in April and reaching a maximum shallow regions and up to 25 m in deeper regions of the of 29.7°C in May. Temperature at depth of 20 m basin (Fig. 4). During these months oceanic current to 40 m started to rise. This can be attributed to flows slowly from the Southern part to the Northern penetrative radiation warming below shallow mixed and moves in the upper, lower and mid basin counter layer11. The freshwater discharge also influences the clockwise and a little bit randomly (Fig. 5). salinity gradient up to 300 m12. Water salinity ranges During the month of December, with cooler wind from 30 ppt to 34.8 ppt. from north thermal MLD decreases at the Northern During the monsoon period thermal MLD show in part to 10 m. Along with that the depth increase the mid basin of the BOB was same 40 m to 50 m continuously in the mid basin with winter days and in (Fig. 3). The potential density MLD was same in this the month of January and February MLD grows up to period near the upper Eastern part 5 m to 10 m. 95 m in the upper mid basin region. The extended Because, in this time high river discharge seen in the MLD form a regular circular depression in February upper Eastern region due to heavy rainfall. As a result with a depth of 80 m in the centre. Southern part of this MLD in this area shows very thin, because beyond the thickest zone, MLD is almost uniform at low MLD indicates strong stratification coastal area. 50 m to 55 m for the whole winter season (Fig. 3). 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 direction from the southwest part to the mid ocean and the current flow are help to maintain this situation, it help to make a homogenous area. Where, the potential because these months surface current flows almost same density MLD was almost same. UDDIN et al.: SEASONAL VARIABILITY OF MIXED LAYER DEPTH IN BAY OF BENGAL 407

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