Analysis of vertical profiles and an overview of the trends of circulation in the Arabian Sea
The shift of monsoon-winds hugely affects the circulation of the Arabian Sea. This circulation manipulates the biological productivity of the waters and therefore, understanding its trend is of great significance to deduce potential fishing grounds. In this study, we attempt to quantitatively represent the seasonal variability of the vertical profiles of temperature and density across the Arabian Sea using monthly long term mean sub-surface data from Levitus climatology (1900-1992) and obtain the implications for the changes in Barrier Layer Thickness. We divided the domain of study into six sub regions namely. Trivandrum, Mumbai, Arabia, Equatorial and Central to present their individual variations. Additionally, an attempt has been made to establish the duration of upwelling and downwelling motion along the coastal and central regions by analyzing the rate of vertical oscillations of isotherm(s). The findings are substantiated by zonal and meridonial wind data from coads climatology (1946-1989). After the study, we confirm that the process of upwelling commences in the early part of summer monsoon while sinking mostly occurs in winter along the coastal regions while the contrary is true for the open waters. The findings also suggest that the thermocline gradients for the western boundary regions of Arabian Sea are comparitively low (≈0.02Cm-1) while at the same time, a higher thermocline gradient exists for the equatorial (≈0.07Cm-1) and central regions (≈0.05Cm-1). It is also interesting to observe that the Barrier Layer Thickness for Trivandrum region almost hits 40m during the Post-monsoon season.
Keywords: Mixed layer depth, Main thermocline, Barrier layer thickness, upwelling, downwelling
|MLD||Mixed Layer Depth|
|BLT||Barrier Layer Thickness|
|MLDPT||Mixed Layer Depth Via Potential Temperature|
|MLDPD||Mixed Layer Depth Via Potential Density|
A generic vertical profile consists of a near surface well mixed layer, a layer of strong vertical gradient followed by a weak gradient layer. The area of large vertical gradients in temperature and density are termed as thermocline and pycnocline respectively. The depth and temperature of the mixed layer vary from day to day and from season to season in response to two processes: (1). Heat fluxes through the surface, and heat and cool the surface waters. Changes in temperature change the density contrast between the mixed layer and the deeper waters. The greater the contrast, the more work is needed to mix the layer downward and vice-versa. (2). Turbulence in the mixed layer mixes heat downwards. The turbulence depends on the wind speed and on the intensity of breaking waves. Turbulence mixes waters in the layers, and it mixes the water in the layer with the water in the thermocline. The mixed layer is characterized by near uniformity in properties such as temperature and salinity throughout a season or year(H. Stewart Robert,2006). The combined influence of temperature and salinity changes result in an abrupt density change or the pycnocline. One key factor to note is that density is more closely related to temperature than salinity i.e, cold saline water will sink and warm fresh water will float, but it is also possible that cold fresh water can have the same density as warm saline water. Therefore the thermocline also tends to be the layer where density gradient is the greatest.
In the tropics, between about 200m and 1000m, the temperature decreases with depth. Though the vertical thermal gradients are much weaker than the main thermocline, it is present in all seasons and is referred to as the permanent thermocline (Radhakrishnan K.G,1995). So, in the vertical profile presenting the seasonal variations, the permanent thermocline starts where the main thermocline of each season coincide.
Another significant parameter involved in the sub-surface profiles which is not discussed that often is the Barrier Layer Thickness (BLT). The BLT is a layer of water separating the mixed layer from the thermocline. A more precise definition would be the difference between MLD calculated from temperature and the MLD calculated using density. It can vary from 10m to almost 50m in the tropics. In the past, the only criterion for MLD was by determining the temperature variations and seeing which portion of the profile portrayed the isothermal region. More recently, a density criterion has also been used to define the MLD. The density derived MLD is defined as the depth where the density increases from the surface value by a prescribed temperature decrease while maintaining constant surface salinity value.
For plotting the pycnocline, the density values of sea water are rarely measured. Density is calculated from the measurements of temperature, conductivity or salinity, and pressure using the equation of state. The equation of state is an equation relating density to temperature, salinity and pressure. The relationship between these variables are non-linear.
Upwelling and Downwelling
In the coastal regions of oceans, the combination of persistent winds, Earth's rotation (the Coriolis effect), and restrictions on lateral movements of waters caused by shorelines and shallow bottoms, induces upward and downward water movements. The Coriolis effect plus the frictional coupling of wind and water (Ekman transport) cause net movement of surface water at about 90 degrees to the right of the wind direction in the Northern hemisphere and to the left in the Southern hemisphere. Coastal upwelling occurs where Ekman transport moves surface waters away from the coast; surface waters are replaced by water that wells up from below.
Upwelling and Downwelling also occur in the open ocean where winds cause surface waters to diverge (move away) from a region (causing upwelling) or to converge toward some region (causing downwelling).
The monsoon domain in the Indian Ocean is usually defined as northward of 10°S, where the circulation is characterized by seasonal reversal along with the monsoon annual cycle. These reversing currents include the Somali Current, the West Indian Coastal Current, the East India Coastal Current and the Java Current. Even though the basin-scale upper circulation is becoming clear through several decades of research (Schott and McCreary, 2001; Scott et al., 2009), the boundary currents and upwelling processes in the Indian ocean still remain far less well understood (Hood et al., 2015).
The upwelling and downwelling along the west coast of India are associated mainly with the west India coastal current (WICC). During the summer monsoon, the WICC flows towards the equator along the west coast of India and reverses its direction towards the pole during the winter monsoon (Shankar, Vinayachandran, and Unnikrishnan 2002). The west coast of India is characterised by upwelling during the Summer Monsoon (May to September) and downwelling during the winter monsoon (November to February) (Rao, Joshi, and Ravichandran 2008). Along the Western boundary of the Arabian Sea during the summer monsoon, we have the South Equatorial Current (SEC) and the East African Coastal Current (EACC) supplying the northward flowing Somali Current. A variety of factors like landmasses or topographic differences along with the wind variability can result in the formation of cells and gyres from the Somali current. After crossing the equator, the current results in a recirculation known as the Southern Gyre (SG) and in the north, a second gyre known as the Great Whirl (GW).The Socotra Eddy (SE) is also observed during the summer monsoons in the northeastern parts. The Southwest Monsoon Current (SMC) supplies to the West India Coastal Current (WICC) after circulating through the Laccadive Low (LL). During the winter monsoon, the EACC combines with the Somali current which results in the eastward flowing South Equatorial Counter Current (SECC) and at the eastern end of it, the Java current flows southeastward. The Northeast Monsoon Current (NMC) which cannot directly affect the circulation along the western coast of India due to topographic hindrance given by the Himalayas, supplies to the WICC after circulating through the Laccadive High (LH), similar to the summer monsoon, but this time, the WICC directs towards north.
This study attempts to analyze the variability of the mixed layer depths and the main thermocline in relation with the seasonally reversing monsoon winds. Previous studies on the region have given quantitative information of astounding implications about the variability and the multiple processes that have the potential to bring about the variations observed. The criteria called the barrier layer thickness has not been considered that often. Therefore, this research intends to show the variability of the BLT in response to the changing seasons and also confirm the findings of previous works on the duration of upwelling and downwelling in the central and coastal regions of the Arabian Sea by analyzing the vertical thermal structure(s) of the region(s) of interest. Decades of research has given us a deeper understanding on the circulation patterns of the West coast of India but the duration of upwelling and downwelling in the Arabia and Somalia Coast has not been studied in depth. Therefore an attempt is made here to establish the duration trends of these regions.
The domain of study encompasses the region extending from equator to 25ºN and 45ºE to 80°E and is further divided into six 3°x3° sub-regions as: (A) Box 1→5°N to 8°N and 74°E to 77°E , (B) Box 2→18°N to 21°N and 70°E to 73°E, (C) Box 3→18°N to 21°N and 58°E to 61°E, (D) Box 4→2°N to 5°N and 48°E to 51°E, (E) Box 5→0° to 3°N and 64°E to 67°E and (F) Box 6→11°N to 14°N and 63°E to 66°E
The relationship of the density of sea water with temperature, salinity and pressure is non-linear and the pressure factor has to be considered at depths of 1000m or more for density calculations. In this study, the relation between density and the other mentioned variables are approximated to establish a linear relationship and without incorporating the pressure values for density calculations as the compressibility factor can almost be neglected.
The works done by Ravichandran, Rao and Joshi in 2008 using the POM numerical model analyzes the variability of vertical thermal structures across the West coast of India to reach conclusions regarding the temperature inversions during the monsoons. The criteria used to analyze is to consider the vertical structures for summer monsoon and winter monsoon for the regions of study. This is an effective method to confirm the duration of upwelling and downwelling. Therefore, the same methodology is adopted in this research as well. The paper published by Schott and McCreary in 2001 gives a detailed overview of all the currents present in the Indian Ocean and a detailed description of the formation of the Somali current and the cells and gyres which result from it that can alter the vertical thermal structures of the coastal regions. It also elucidates the major currents and gyres like the Laccadive Low (LL), Southwest Monsoon Current (SMC) and West Indian Coastal Current (WICC) along the eastern boumdary of the Arabian Sea and the Socotra eddy(SE), Great Whirl (GW), Southern Gyre (SG), East African Coastal Current (EACC) and the South Equitorial Counter Current (SECC) along the western boundary and equatorial regions. The work done by KV Ramesh & R Krishnan in 2005 and P Amol et.al in 2014 analyzed the intraseasonal and seasonal variability of WICC and their findings suggest that during the summer monsoon, the surface currents are toward the equator and the thermocline shoals near the coast and during the winter monsoon, the surface currents are northward and and the thermocline deepens near the coast. The general conclusion is that the upslope of isotherms indicate periods of upwelling and vice-versa and therefore the same methodology is used in this research as well. The study done by Radhakrishnan K.G (2005) on the thermocline variability in the Arabian Sea analyzed the trends of shoaling and deepening of the main thermocline in seven major locations within the Arabian Sea. It established the trend of the variability of the top of thermocline and the contribution of upwelling and downwelling towards the observed variability. His work also showed the quantitative variability of thermocline gradient from each of the locations that he chose to study. The same method is used in this research to obtain the thermocline gradient. The methodology that he used to obtain the duration of upwelling and downwelling is by observing the oscillations of the 23ºC and 18ºC isotherms in the vertical thermal structures. The same method is utilized for this research as well by considering the 24ºC isotherm as it is prominent throughout all regions and all seasons in the thermal structures.
To produce the results for the above mentioned objective, monthly long term mean sub-surface data of ocean temperature and salinity was obtained from the Levitus (1900-1992) climatological dataset. It was then converted using the climatological axes of Ferret NOAA tool to aquire seasonal datasets. The supplementary dataset of Levitus which inlcudes the MLDPT and MLDPD values for each month was also used to obtain values of BLT for the specified locations in the study region. The thermoclines were drawn for the six locations marked in the study region by specifying the coordinates of the center of the 3ºx3º grids in Ferret tool. The pycnoclines were drawn by using the temperature and salinity values from Levitus dataset and plugging into equation (2) given below:
The density of seawater can be represented as a function of temperature, pressure and salinity.
ρ = ρ ( p, T , S )
In the differential form, dρ/ρ=γTdp − αTdT + βdS
αT=1/ρ(∂ρ/∂T)→Coefficient of thermal expansion, γT=1/ρ(∂ρ/∂p)→Isothermal compressibility coefficient, β=1/ρ(∂ρ/∂T)→Coefficient of haline contraction
This equation can be used to calculate the increase/decrease in density with the changes in any of the variables in the function if a standard condition for density, temperature, salinity and pressure is predefined. Equation (1) reduces to the form as given below if the pressure variations are neglected.
The standard values for the variables are ρo=1000kg m-3, To=283K,So=35psu and α=155x10-6 K-1, β=765x10-6 psu-1 and these values are assumed to be constant as the relationship is linear.
In the following tables, the seasonal variations in the thermoclines and pycnoclines are presented to a depth of 1000m. The tables are divided for presenting the Eastern boundary, Western Boundary and the Open water regions to delve inside each domain with specificity. The thermocline is on the right and the pycocline is on the left of each figure of the table. Blue color is for the winter (DJF), red for pre-monsoon(MAM), grey for monsoon(JJA) and green for post-monsoon(SON) seasons. After the following figures, two tables are presented showing quantitative variations in BLT and the thermocline gradient.
Region B [Mumbai (69.5°E, 17.5°N)]: The thermocline variations in terms of initial and final temperatures are not that prominent in this region but the shoaling and deepening of the thermoclines are significant. During the winter season, the thermocline extends till 110m and the temperature range is from 26.5ºC to 21.0ºC. During the pre-monsoon season, the thermocline extends till 100m and the temperature range is from 28.8ºC to 23.2ºC. During the monsoon season, the thermocline shoals to 75m depth and the temperature range is from 27.8ºC to24.1ºC. During the post-monsoon season, the thermocline shoals by another 25m to extend till 50m depth, with a temperature range from 27ºC to 26.4ºC. There are no abrupt changes in the pycnoclines like the southeastern region of Arabia and the variations are prominent only for the post-monsoon season.
Region D [Somalia(47.5°E, 2.5°N)]: For the winter season, thermocline extends till 80m and the temperature ranges from 26.4°C to 25.3°C. During the pre-monsoon period, the thermocline remains at 75m depth and the temperature range is between 26.5°C and 23.1°C. During the monsoon period, the thermocline deepens to a depth of 120m with the temperature range being 25.8°C to 21.2°C. During the post-monsoon period, the thermocline shoals to a depth of 75m with the temperature ranging from 26.7°C to 23.5°C. There is an abrupt change in density for the winter and monsoon seasons from the observation of the pycnoclines. The gradient for the other two seasons are gradual and no pycnocline has multiple inflection points. The changes are not distinct due to the low influx of freshwater in these regions and therefore, the primary contributor to the observed changes should be the currents present in the region.
Region E [Equatorial(64.5°E, 0.5°N)]: During the winter season, the thermocline extends till 60m and the temperature range is from 28.2°C to 27.3°C. In the pre-monsoon season, the thermocline deepens to 75m from the surface and the temperature range is from 28.8°C to 25.7°C. One key thing to notice is that the surface temperature is hitting 29°C and almost remains constant during these two seasons. But during the monsoon season, the surface temperature drops to 28.7°C and the main thermocline remains at 75m. The temperature range of the thermocline is 28.1°C to 25.8°C. During the post-monsoon time period, the thermocline shoals to back to 50m with the temperature ranging from 27.9°C to 26.1°C. The pycnoclines for the winter and monsoon seasons almost coincide with each other, while the other two pycnoclines differ greatly which is distinctly visible in the chosen scale.
Region F [Central(64.5°E, 12.5°N)]: During the winter season, the thermocline extends till 75m and the thermocline range is from 26.9°C to 25.6°C. In the pre-monsoon season, the thermocline deepens to 100m and the temperature range is from 27.8°C to 25.1°C. During the monsoon season, the main thermocline again deepens to 125m with the temperature ranging from 26.6°C to 23°C. In the post-monsoon season, the main thermocline shoals to 75m depth and the temperature ranges from 26.7°C to 24.4°C. The pycnoclines for the winter and post-monsoon seasons are similar in shape and the abrupt increase in density after the mixed layer is very prominent for the pre-monsoon pycnocline.
|Regions||MLD via potential temperature(m)||MLD via potential density(m)|
Barrier Layer Thickness(m)
RESULTS AND DISCUSSION
The prime contributor to the observed seasonal variations in the parameters of vertical profiles of temperature and density is wind-driven circulation, although fresh-water flux is also found to contribute to some of the observed variabilities in previous studies. Therefore, correlating the circulation trends to the observed variabilites can help us understand the mechanism by which it occurs since the findings will be of astounding implication to the fisheries and naval sectors.
The active monsoon winds and the precipitation received in the regions of study play a significant role in accounting for the changes observed. As mentioned before, we kept track of the deepening and shoaling of the thermoclines and pycnoclines for the various seasons and in order to confirm that it is consistent with the upwelling and downwelling time periods, we considered the vertical oscillation of the 24º C isotherm to ultimately establish the causation. Using the oscillations of the isotherm, a general trend for the time-period of upwelling and downwelling can be obtained and any slight deviation that might be observed may be due to remote forcing or the influence of prominent inertial waves.
The magnitude of upwelling and downwelling for a specific isotherm were plotted for each month by assigning positive values for all upward movements and negative values for all downward movements. The magnitude for each month was obtained by keeping its previous month as the reference.
When we analyzed the oscillations of the isotherm, we observed that the periods of upwelling along the eastern boundary were as follows: from April to August for region A with the maximum peak touching 15m and from June to October for region B with the maximim peak reaching 13m. Along the western boundary, the months of upwelling were as follows: from June to August with the peak upwelling being almost 35m for region C and from April to July for region D with maximum upwelling hitting 35m. For the equatorial and central open waters, the upwelling time periods were as follows: from September to December for region E with peak upwelling being almost 11m and from September to November for region F with magnitude of peak nearly 30m. The periods of downwelling for the equatorial and central waters were as follows: from January to April for region E with peak downwelling of almost 10m and January to August for region F with peak downwelling touching 11m. The downwelling duration for the western boundary were as follows: from September to April with a peak magnitude of nearly 10m for region C and from October to March with a peak downwelling of nearly 20m for region D. The duration of downwelling for the eastern boundary were as follows: October to January with a peak downwelling of nearly 23m for region A and from November to March with a peak downwelling magnitude of 15m.
From the monthly SST variations, we were able to observe that the periods of upwelling for each region of study is characterised by subsequent decrease in the SST which reaffirms the fact that when cold denser nutrient rich water is brought to the top, there will be an evident decrease in temperature at the surface. The time periods of upwelling and downwelling is also completely compatible with the duration of the deepening and shoaling of the thermoclines and the pycnoclines as observed in the data. The reversal of West India Coastal current, the formation of the South Equatorial current and its countercurrent and the subsequent changes in the Somali current is again consistent with the conclusions regarding the circulation of coastal and central regions that are made from the Ekman Transport concept in the Northern Hemisphere. Significant changes in the Barrier Layer Thickness is observed mainly along the Eastern boundary of Arabian Sea i.e, A and B, with region A demonstrating an abrupt drop from nearly 36m to 14m during the pre-monsoon season and a sharp increase to nearly 40m in the post-monsoon season and region B showing sharp increment during the post-monsoon season. The major contributor to the fluctuations in BLT are inertial waves like rossby and kelvin waves. Comparitively lower gradients of temperature are observed along the western boundary of the Arabian sea with region C having a gradient of ≈0.027Cm-1and region D having a gradient of ≈0.023Cm-1 without demonstrating abrupt deviations from these values.
Utmost gratitude to the Director of National Institute Of Oceanography, Goa for giving the oppurtunity to produce fruitful work through the research. We also wish to thank the developers of Ferret tool and datasets ( Levitus,COADS) without which the analysis wouldn't have been possible. I would also like to thank AuthorCafe for providing a top notch platform to present our ideas.
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