WIND STRESS OVER INDIAN OCEAN

Abhijit Sarkar, K Satheesan, Anant Parekh

Ocean Sciences DivisionSpace 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

Parameter Source

Surface Pressure

P Buoy

Air Temperature Ta Buoy

SST Ts Buoy

Humidity R NCEP Daily

Wind Speed U QuikSCAT (available from ifremer)

Wind Stress 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

|| = 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.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-6 -4 -2 0 2 4 6

Tair-Tsst (C)

Un

/U1

0

WS 0.5

WS 2

WS 10

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

Us= Wind speed at surface

U*= Friction velocity

Z= Height of the Sensor Z0= Roughness length

U= Stability function

k = Karman constant 

kzzUUU Us //ln(/ 0*

Annual cycle of Wind stress (N/m2) over the north Indian Ocean

1 Jan 1 Feb1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec 1 Jan0.00

0.04

0.08

0.12

0.16

0.20W

ind

Str

ess

X Axis Title

Bay of bengal Arabian Sea

 

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

hATKE mwwDG 3*)(

Kim J. , 1976, Shetye 1986

1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec-10-505

101520253035

K E

Diff

eren

ce

Days

K E Difference

1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec0.660.690.720.750.780.810.840.870.90

Hum

idity

Humidity

1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.4

Air

- S

ea D

iffer

ence

Air - Sea Difference

1 Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec10021004100610081010101210141016

Pre

ssur

e Bay of Bengal Arabian Sea

Pressure

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

0 2 4 6 8 10 12 14 160

2

4

6

8

10

12

14

16

Qui

kSC

AT

Buoy

Bay of Bengal

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

14

16

18

20

Qui

kSC

AT

Buoy

Arabian Sea