WIND STRESS OVER INDIAN OCEAN Abhijit Sarkar, K Satheesan, Anant Parekh Ocean Sciences Division Space Applications Centre, INDIA ISRO-CNES Joint Programme on Atmosphere, Climate & Oceanography SAC, Ahmedabad, October 2005

IMPORTANCE The wind stress or the momentum flux drives the oceanic currents with the ocean acting as a sink for atmospheric momentum Wind stress fields are used to force global models Accurate fields of wind forcing are essential if realistic model simulations of ocean circulations are to be achieved

Sources of Wind Stress from Space ERS 2 AMI (1995) QuikSCAT (1999) Oceansat – 2 (2006) METOP (2006) T-P/JASON – I (1992/2003) ENVISAT (2002) TMI/SSMI (1997/1987) Megha – Tropiques (2008)

DS 2 OB 8 STUDY AREA Arabian Sea Bay of Bengal

Data Source ParameterSource Surface Pressure PBuoy Air TemperatureTaTa Buoy SSTTsTs Buoy HumidityRNCEP Daily Wind SpeedUQuikSCAT (available from ifremer) Wind Stress sbsb QuikSCAT (available from ifremer) Calculated from Buoy

Wind stress estimation by IFREMER To estimate surface wind stress, for each scatterometer wind vector, the bulk formulation is used |  | =  C D W 2 The 10 m neutral coefficient formulation over the ocean is (Smith 1988) C D = a + bW The values of a and b are determined for each wind speed range.

Effect of boundary layer stability on winds using LKB algorithm Liu et al., 1996

Pre-monsoon months in the NIO we have mostly high insolation and wind condition Tair-Tsst can be large This can also happens during monsoon season break conditions During active condition in monsoon its the other way round This leads to neutral stability winds to be very different from actual winds. Air – Surface Temperature in NIO

Bulk aero-dynamic formulations Where U= Wind speed at reference height U s = Wind speed at surface U * = Friction velocity Z= Height of the Sensor Z 0 = Roughness length  U = Stability function k = Karman constant

Annual cycle of Wind stress (N/m 2 ) over the north Indian Ocean

Where first term of right hand side is determines generation and second is dissipation of turbulent kinetic energy.  G and  D are non-dimensional constants (  G -  D = 1.25) * = Friction velocity = (  s /  w ) 1/2  s = Surface wind stress  w = Water Density  m = Depth independent background dissipation (2  m 2 /s 3 ) h = Depth Kim J., 1976, Shetye 1986

Inferences & Discussions  Wind stress computations with/without boundary layer parameters made – differences were small  Turbulent Kinetic Energy computations for different   Significant differences were found during monsoon seasons, particularly in the Bay of Bengal  This shows large overestimation when without boundary layer  Need for synergy of wind data with surface boundary layer