Arun Chakraborty

Diagnosis of Tropospheric Moisture over Saudi Arabia and Influences of IOD and ENSO

Abstract A diagnostic study of atmospheric moisture data over Saudi Arabia derived from a 43-yr National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis revealed that moisture convergence in the lower troposphere and divergence in and above the middle troposphere occurs throughout the year. Although the amount of precipitable water content in the middle troposphere is high, precipitation is less than expected over this semiarid region during a boreal summer monsoon season because of strong moisture divergence. The net tropospheric moisture flux over the arid and semiarid regions of Saudi Arabia shows seasonal and interannual variability. The seasonal variability has a strong semiannual signal with its primary peak February–April and its secondary peak June–August. This pattern is consistent with a similar semiannual signal observed in rainfall climatology. The restricted moisture supply to southwestern Saudi Arabia during summer presumably explains the ...

Numerical simulation of Bay of Bengal circulation features from ocean general circulation model

The Bay of Bengal circulation features have been analyzed using a three-dimensional Ocean General Circulation Model (OGCM). The northward propagation of eastern boundary coastal Kelvin wave and its movement throughout the entire Bay is evident from the study. The trapping of Kelvin waves in the central Bay for many months causes warming of the central basin, which acts as a potential breeding ground of cyclones. The seasonal variations of circulation, temperature, and salinity have been reproduced by the model. These variations compare favorably with observations. The existence of a salt finger towards north and its eastward movement are two important results of the study. The cyclonic gyres in the Bay of Bengal during different months agree reasonably well with the SeaWiFS chlorophyll observations and indicate the upwelling zones over the basin. The presence of a subsurface Bay-of-Bengal dipole in summer monsoon months is another important result of the study.

Simulation of East India Coastal Features and Validation with Satellite Altimetry and Drifter Climatology

The circulation features of western coast of the Bay of Bengal (BOB) have been analyzed using Regional Ocean Modeling System (ROMS) with Comprehensive Ocean-Atmosphere Data Set (COADS) wind and thermal forcing. The model simulation shows that the coastal current is not continuous throughout the year similar to the structure seen from the drifter climatology. The western boundary current (WBC) is formed in February and persists till May. This boundary current is very strong during March and April due to formation of anticyclonic eddies. From July to September, the coastal current is disorganized because of sequential development of anticyclonic and cyclonic eddies. But in October the coastal current starts to flow southward as the East India Coastal Current (EICC) and it prevails till December with the formation of cyclonic eddies along the coast. The simulated sea surface height anomaly (SSHA) is competent to detect the upwelling and downwelling zones in the coastal region as supported by TOPEX/POSEIDON climatology.

The Mechanism of the 20°C Isotherm Depth Oscillations for the Bay of Bengal

The monthly variation of thermocline depth in terms of 20°C isotherm depth (Z20) in the Bay of Bengal has been studied using SODA and ARGO datasets. During a southwest monsoon, the Bay of Bengal is deepest in the western basin and shallowest in the eastern basin while it is the opposite during northeast monsoon. The Z20 oscillation is not only affected by semiannual reversing wind forcing but also influenced by coastal Kelvin waves from the eastern coast and remote effect from the Northern Indian Ocean and Malacca Strait. A linear relationship between SSHA and Z20 has been found from both SODA and observational studies.

Response of OceanSat II scatterometer winds in the Bay of Bengal circulation

Surface wind is one of the major forcing factors in any ocean circulation model. The response of satellite-derived OceanSat II scatterometer (OSCAT) winds during spring (February and March) 2010 in the Bay of Bengal (BOB) surface circulation is described in this study. Wind stress is calculated from wind speed derived from OSCAT by the bulk-aerodynamic formula. The Regional Ocean Modeling System is used in this study because it is a free-surface, terrain-following, primitive-equations ocean model widely used by the scientific community for a diverse range of applications. The model is used after a climatological simulation with Comprehensive Ocean–Atmosphere Data Set (COADS) forcing when the model simulation reached the annual cycle. The paper also carried out a comparative study of National Centers for Environmental Prediction (NCEP) forcing over the same time period. The comparison of model-simulated surface temperature to National Oceanic and Atmospheric Administration (NOAA) sea surface temperature (SST) indicates that meso-scale features in the BOB are resolved due to the finer resolution of this model. Comparisons of water mass characteristics to the available ARGO floats show good agreement in different locations within the BOB. This study confirms the usefulness of OSCAT winds in simulating the meso-scale feature in the BOB.

Seasonal and Monthly Variation of Vertical Structure of Temperature, Salinity and Heat Flux of the Bay of Bengal

Seasonal and monthly variations of heat flux have been investigated in this study using the Modular Ocean Model of version 3 (MOM 3) simulations and 52 years Simple Ocean Data Assimilation (SODA) products. These variations of the heat flux in different boxes of the Bay of Bengal (BOB) in different depths show the different behavior of the boxes. It is seen that the model and SODA results are comparable. The basin shows north-south variation in the surface from winter to spring whereas there is east-west variation in the mixed layer throughout the year except winter. The remote effect caused by warm water penetration from Pacific Ocean through the Strait of Malacca and coastal Kelvin waves keeps the basin warm most of the year. This article addresses the mechanisms of the seasonal variation of the vertical structure of the temperature and heat flux components.

Response of an ocean general circulation model to wind and thermodynamic forcings

The stretched-coordinate ocean general circulation model has been designed to study the observed variability due to wind and thermodynamic forcings. The model domain extends from 60‡N to 60‡S and cyclically continuous in the longitudinal direction. The horizontal resolution is 5‡ x 5‡ and 9 discrete vertical levels. First a spin-up experiment has been done with ECMWF-AMIP 1979 January mean fields. The wind stress, ambient atmospheric temperature, evaporation and precipitation have been used in order to derive mechanical and thermodynamical surface forcings. Next, the experiment has been extended for another 30 years (3 cycles each of 10 year period) with varying surface boundary conditions (from January 1979 to December 1988 of ECMWF-AMIP monthly fields for each cycle) along with 120 years extended spin-up control runs results as initial conditions. The results presented here are for the last 10 years simulations. The preliminary results of this experiment show that the model is capable of simulating some of the general features and the pattern of interannual variability of the ocean.

Variability of sensible heat flux over the Bay of Bengal and its connection to Indian Ocean Dipole events

The Bay of Bengal (BOB) is known to possess complex thermodynamics which show distinct seasonal patterns. Surface heat fluxes in the BOB are very much dependant on upper ocean heat exchanges and wind. Sensible heat flux (SHF) is also one among those fluxes that depends on air-sea temperature difference and wind. However, this study further proves that a strong relationship exists between barrier layer thickness (BLT) and SHF variability that has not been focussed on in earlier literatures. This study also investigates the seasonal as well as inter-annual variability of SHF and its relationship with BLT and sea surface temperature (SST) patterns in more detail with statistical analyses. It is found that both SST and BLT are responsible for the evolution of SHF signal in the BOB although their effects are spatially distributed. During the post monsoon period, freshwater induced enhanced BLT is more related to SHF than the summer time when effect of SST is found to be dominant. During Indian Ocean Dipole (IOD) years, the correlation between SHF and BLT in the eastern BOB is more pronounced compared to SHF and SST. The western BOB however is dominated by SST variations for the respective IOD phase which also contribute to SHF signals there. Northernmost BOB shows high standard deviation due to river discharge effects.

An Inter-Comparison of Daily and Monthly In Situ, Satellite Derived and Reanalyzed Sea Surface Temperature Climatology Fields Over the Bay of Bengal

Sea surface temperature (SST) is an important and one of the most studied variables in oceanography. In the current study in situ, satellite derived and reanalyzed SST climatology fields are compared for the Bay of Bengal (BOB) region at daily and monthly scales. The analysis of these SST climatologies shows not only spatial difference but also monthly average patterns with slight variations in the form of maximum temperature peak shifts. The climatology fields reveal the spatial extent of intra-seasonal variations of BOB. These variations are more distinct in TMI satellite and in situ SST data. Root mean square error (RMSE) and correlation analysis reveal that TMI and AMSR satellite daily data are more indicative of true state of SSTs and are suitable for surface initial conditions in ocean models, while reanalyzed SSTs are suitable for monthly scale forecast.

Simulating the Effects of Tidal Dynamics on the Biogeochemistry of the Hooghly Estuary

A three-dimensional coupled hydrodynamic biogeochemical sediment transport model is proposed to simulate the response of plankton dynamics in the Hooghly estuary to freshwater discharge under the presence and absence of tide, and in-depth understanding the significant processes involved in estuarine biogeochemistry based on Regional Ocean Modeling System (ROMS). Riverine-estuarine circulations are used to evaluate the tidal consequence on wavering of biogeochemical properties in the Hooghly estuary. Therefore, three numerical experiments are performed: 1) model run with tidal forcing, river discharge, and applied biogeochemical and sediment properties along with coastal current; 2) model run with tidal forcing along with coastal current plus applied biogeochemical and sediment properties; and 3) model run with river discharge along with coastal current plus applied biogeochemical and sediment properties to determine the chief role of this physical parameters on the transport of bloom in a well-mixed estuary using this coupled model for the first time in Hooghly estuary. The model divulges its capability in imitating observed temporal variability in tidal oscillation representing skill coefficients more than 0.80. The satellite remotely sensed Ocean Color Monitor Data is assimilated using an SOR algorithm to incorporate in the model as the initial field for chlorophyll-a concentration (chla) to retrieve the productivity of the estuary. Productivity is found maximum in the area of soaring suspended sediment implying maximum bottom stress contributed by tidal currents along with coastal currents evident from the realistic case. The RMSE calculated is less than 0.6, which implies that the model can perform reasonably well.