Response of the upper ocean to atmospheric forcing and mixed layer dynamics in the Bay of Bengal on seasonal and inter-annual scale

Physical Oceanography programme

Response of the upper ocean to atmospheric forcing in the Bay of Bengal on seasonal and inter-annual scale

Participants:

S. Prasanna Kumar Principal Investigator

P.M. Muraleedharan

Y.K. Somayajulu

G. Nampoothiri

National Institute of Oceanography, Dona Paula, Goa.

Objectives

The primary objectives of the proposed study is to address the role of physical processes in the overall biogeochemistry of the Bay of Bengal by determining (1) how far is the physical forcing coupled with biological processes and (2) to what extent physical forcing controls the carbon cycling in the Bay of Bengal.

This could be achieved by studying the response and spatio-temporal variability of the Bay of Bengal to (1) the monsoonal forcing, (2) fresh water forcing, and (3) role of cyclones in controlling the dynamics of the upper ocean and its coupling to the changes in water column chemistry and biology.

Background

It is well known that the biological production in warm tropical waters is controlled by the availability of nutrients and physical process sets the stage for making the nutrients available to the upper layers. Being a tropical basin, the biological production in the Bay of Bengal is expected to be nutrient limited and hence carbon cycling, to a large extent, must be modulated by the physical processes. The circulation and hydrography of the Bay of Bengal is complex due to the interplay of semi-annually reversing monsoonal winds and the associated heat and freshwater fluxes. Apart from this, the inflow of warm high saline waters of the Arabian Sea, the Persian Gulf and the Red Sea origin and a number of synoptic disturbances (cyclones) originating during both pre-monsoon (May) and post-monsoon (October) period also affects the upper ocean dynamics. It is generally known that cyclones are capable of inducing rapid changes in the heat and buoyancy fluxes over very short span of time (few days to weeks), but the magnitude of such processes is unknown. Neither do we fully understand the upper ocean variability in the seasonal and inter-annual time scales. Conventional hydrographic surveys for the past few decades after the International Indian Ocean Expedition (IIOE 1960-65) generated considerable amount of information regarding the hydrographic characteristics of the Bay of Bengal and to some extent the general circulation (Varkey et al., 1996 and the references thereof). Though there exists a broad understanding about the hydrography and general circulation of the Bay of Bengal, various aspects of circulation and mixed layer dynamics on a seasonal and inter-annual time scale is yet to be understood. Recent attempts to model the circulation in the Bay of Bengal (Potemra, 1991, Vinaychandran et al., 1996) and Indian Ocean (McCreary et al., 1993) suggested the role of planetary (Kelvin and Rossby) waves in influencing the circulation in the Bay of Bengal. These model studies showed that the low frequency waves on encountering a boundary, such as the eastern Bay of Bengal, propagate part of their energy as coastally trapped Kelvin along the eastern periphery, and part of the energy as westward propagating Rossby waves into the Bay of Bengal. This has been corroborated by Prasanna Kumar and Unnikrishnan (1995) and Unnikrishnan et al.(1997) based on the temperature climatology and altimeter data (Prasanna Kumar et al., 1996). However, the magnitude of the changes that these low frequency waves would induce in the upper oceans and the implications to water column chemistry and biology is presently not known. Another gray area is stratification associated with the fresh water influx. What is the role of stratification in controlling vertical and horizontal advection? How does this affect the de-gassing in the Bay of Bengal? How strong is the coastal upwelling in terms of nutrient supply to the upper ocean and biological production? Is the winter cooling process active in the Bay of Bengal ? If so, does it trigger winter convection and biological bloom? Does advective processes (from western and eastern boundaries) play a major role in fertilizing the Bay of Bengal. What are the magnitudes of the mixed layer changes associated with cyclones and how do they affect the biological production and Carbon flux? Do cyclones act as a switch which could reset the seasonal or inter-annual anomalies? In summary , how strongly is the biological processes coupled to physical forcing? We could answer this by addressing the above questions.

Method

The proposed study envisages the understanding of the surface and sub-surface expression of the upper ocean response to atmospheric forcing through in situ measurements (hydrographic data), satellite data on sea surface height (TOPEX/POSEIDON altimeter) and sea surface temperature (AVHRR) as well as ocean color. The upper ocean changes thus obtained will then be related to the atmospheric forcing via wind, heat and fresh water flux (evaporation-precipitation). The surface meteorological data required for this will be obtained during the field measurement programme as well as from climatologies such as FSU, ECMWF, COADS etc.

Measurements

I. Vertical profiles of (1) salinity and (2) temperature will be measured using CTD system at a spatial resolution of one degree. In addition to this temperature profiles at half-a-degree spatial resolution will be taken using ship-board XBT system when the ship is underway. This will enable to generate accurate temperature fields based on high density (spatially) data.

II. Surface meteorological parameters such as

(1) Air temperature

(2) Wet bulb temperature

(3) Wind speed & direction

(4) Pressure

will be measured using shipborne automatic weather station. In addition, the sea surface temperature (SST) will be measured using a bucket thermometer at every CTD and XBT stations. Sea surface salinity (SSS) as well as SST will be measured using ship-board digital thermo-salinograph along the ship-track.

III. Upper Ocean currents will be measure along the ship-track using shipborne ADCP.

References

McCreary, J.P., P.K. Kundu, and R.L. Molinari (1993) A numerical investigation of dynamics, thermodynamics, and mixed-layer processes in the Indian Ocean, Prog. Oceanog., 31, 181-224.

Potemra, J.T., M.E. Luther and O’Brien (1991) The seasonal circulation of the upper ocean in the Bay of Bengal, J. Geophys. Res., 96, 12667- 12683.

Prasanna Kumar, S. and A.S. Unnikrishnan (1995) Seasonal cycle of temperature and associated wave phenomena in the upper layers of the Bay of Bengal, J. Geophys. Res., 100, 13585-13593.

Prasanna Kumar, S., H.M. Snaith, P.G. Challenor and T.H. Guymer (1996) Large scale sea surface height variations of the Indian Ocean inferred from the TOPEX/POSEIDON altimeter, Internal Report, James Rennell Division for Ocean circulation, SOC, Southampton, U.K.

Unnikrishnan, A.S., S. Prasanna Kumar and G.S. Navelkar (1997) Large- scale process in the upper layers of the Indian Ocean inferred from temperature climatology, J. Mar. Res., 55, 93-115.

Varkey, M.J., V.S.N. Murty and A. Suryanarayana (1996) Physical oceanography of Bay of Bengal and Andaman Sea, Oceanography and Marine biology: an Annual Review, 34, 1-70.

Vinaychandran, P.V., S.R. Shetye, D. Sengupta and S. Gadgil (1996) Forcing mechanisms of the Bay of Bengal circualation, Curr. Sci., 71, 753-763.