WIND STRESS OVER INDIAN OCEAN

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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
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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

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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)

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STUDY AREA
Bay of Bengal
Arabian Sea
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Data Source
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Wind stress estimation by IFREMER
To estimate surface wind stress, for each
scatterometer wind vector, the bulk formulation
is used
? ?CDW2
The 10 m neutral coefficient formulation over the
ocean is (Smith 1988)
CD a bW
The values of a and b are determined for each
wind speed range.
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Effect of boundary layer stability on winds using
LKB algorithm
Liu et al., 1996
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Air Surface Temperature in NIO
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.
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Bulk aero-dynamic formulations
  Where U Wind speed at reference height
Us Wind speed at surface U Friction
velocity Z Height of the Sensor
Z0 Roughness length ?U
Stability function k Karman
constant  
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Annual cycle of Wind stress (N/m2) over the
north Indian Ocean
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  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 ? 10-8
m2/s3) h Depth
Kim J. , 1976, Shetye 1986
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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

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Thank You
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