vertical structure in satellite wind derived ocean currents
TRANSCRIPT
Kathleen Dohan Gary S. E. Lagerloef, and John T. Gunn
Earth & Space Research Seattle, Washington
IOVWST Barcelona May 2010 – p. 1
Introduction
Description of the basic OSCAR equations with the turbulence parameterization
Stommel model boundary conditions
Generalized Ekman boundary conditions
Compare results from OSCAR and Generalized Ekman to in situ data
Future directions for OSCAR
Introduction
Ocean Surface Currents Analyses-Realtime processing system (OSCAR) is a satellite-derived surface current database provided in near-real time based on a combination of quasi-steady geostrophic and locally wind-driven dynamics (Bonjean and Lagerloef, 2002).
The geostrophic term is computed from the gradient of surface topography fields (AVISO/CLS and NRL/MODAS).
Wind-driven velocity components are computed from an Ekman/Stommel formulation with variable viscosity using QuikSCAT winds (FSU/COAPS) and NCEP winds
with a thermal wind adjustment using Reynolds SST data.
Data available at http://podaac.jpl.nasa.gov and http://www.oscar.noaa.gov.
IOVWST Barcelona May 2010 – p. 3
OSCAR Gulf of Mexico
30’
28oN
30’
29oN
30’
30oN
30’
May.17,2010 km/day
22oN
24oN
26oN
28oN
30oN
May.17,2010 km/day
0
10
20
30
40
50
60
70
80
OSCAR product extended to daily output for the latest 20 days of data using real time products (AVISO RT and NCEP). Image from NASA’s Terra Satellite on May 17.
IOVWST Barcelona May 2010 – p. 4
OSCAR Real Time Gulf of Mexico
(Loading Gulf.mpg)
OSCAR product extended to daily output for the latest 20 days of data using real time products (AVISO RT and NCEP).
IOVWST Barcelona May 2010 – p. 5
Gulf.mpg
Media File (video/mpeg)
Calculating Surface Currents
Quasi-steady linear flow in a surface layer with turbulent mixing parameterized by a constant vertical eddy viscosity. Frontal model: buoyancy force θ is a function of SST horizontal gradients only. Surface layer velocity U by averaging over the top h=30m.
ifU = −g ζ + h
∂z .(2)
Use Stommel model boundary conditions in a second order differential equation for shear:
∂U
∂z (z = −H) = 0(4)
where: U = u + i × v, τ0 is surface wind stress, ζ is SSH, θ is buoyancy, based on SST (θ = gχT SST ), and ν is a vertical eddy viscosity, calculated as a function of wind
ν = a( |W|
Calculating Surface Currents: Generalized Ekman
ν = a( |W|
W0
)b(6)
Optimal choice for a in OSCAR blends from 8 × 10−5 m2s−1, b = 2.2 at the equator as in Santiago-Mandujano & Firing (JPO 1990), to 2.85 × 10−4 m2s−1, b = 2 for the global value.
Rather than solve for the shear, Cronin and Kessler (JPO 2009) solved for stress, using the Generalized Ekman boundary conditions and a vertically varying eddy viscosity which decays with depth so that the stress becomes zero at depth H, while the shear can remain nonzero.
Generalized Ekman boundary conditions
τ(z = −H) = ν(z) ∂U
∂z (z = −H) = 0.(8)
where: ν = A exp(z/D) − B, D = 125m and ν = 16e − 03 m2s−1 at 10m and zero at 250m.
IOVWST Barcelona May 2010 – p. 7
Comparison of OSCAR with TAO mooring
−6 −5 −4 −3 −2 −1 0 1 2
x 10 −5
−200
−150
−100
−50
0
−200
−150
−100
−50
velocity [m/s]
Vertical profiles of stress and velocity at a single day given a variable eddy viscosity ν(z) (Generalized Ekman) and a constant eddy viscosity (OSCAR).
Comparison of surface currents with 8N 125W TAO mooring ADCP.
The stress profile should depend on the wind and stratification, and has been observed to decrease to zero at the base of the transition layer (Dohan and Davis 2010).
IOVWST Barcelona May 2010 – p. 8
Comparison of OSCAR with TAO mooring
Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr −0.5
0
0.5
1
Generalized Ekman
TAO Mooring
Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr −0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
Meridional Surface Currents at 8N 125W
Comparison of surface current solutions with 8N 125W TAO mooring ADCP velocity at 30m depth, 10-day smoothed onto 5-day timebase.
Currents are calculated with Generalized Ekman ν(z) and OSCAR constant ν to 250m depth.
Moored data is distributed by NOAA/PMEL at http://www.pmel.noaa.gov/tao.
IOVWST Barcelona May 2010 – p. 9
Surface current differences
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May 0
0.05
0.1
0.15
0.2
0.25
m /s
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May −200
−100
0
100
de gr
ee s
Gen Ek
OSCAR 80m
OSCAR 250m
Difference in magnitude and angle of surface currents using Generalized Ekman ν(z), constant ν to 80m depth, and constant ν to 250m depth.
IOVWST Barcelona May 2010 – p. 10
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day).
Generalized Ekman solution.
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day).
OSCAR constant viscosity solution.
Summary
Global dataset dating back to 1992.
Validation of surface currents from the global drifting buoy array and subsurface currents from moored arrays.
The Generalized Ekman formulation, with a vertically varying eddy viscosity has more flexibility and is more physically realistic.
The profiles of velocity at depth do not agree with in situ data (not surprisingly).
Surface velocities are similar between viscosity choices, although with enough variation in magnitude and direction to distinguish between parameterization types.
IOVWST Barcelona May 2010 – p. 13
Future Work
vertically varying eddy viscosity
include a mixed layer and transition layer account for the shear-driven mixing in the transition layer (i.e. below mixed layer) incorporate ARGO mixed layer depths
vary models according to dynamical regions equatorial, ACC, high winds, near coasts, compensation layers/ salt fingers, deep convection, eddies ...
Develop time-dependent dynamics in OSCAR to include high-frequency wind-driven currents.
Extend OSCAR capability to nowcast and forecast.
IOVWST Barcelona May 2010 – p. 14
La Nina
ENSO cycle as indicated by 1st EOF of surface current and SST anomalies.
IOVWST Barcelona May 2010 – p. 15
Comparison with Drifters Gulf Stream
100 cm/s
34N
38N
42N
D R
IF T
E R
30N
34N
38N
42N
−100
0
100
200
300
)
N=696 rmsd=26cm/s rdstd=22% md=−6.7cm/s cor=0.82 sk=0.42 sco=5.3
−200 −100 0 100 200 −200
−100
0
100
200
)
N=696 rmsd=24cm/s rdstd=17% md=−3cm/s cor=0.75 sk=0.33 sco=5
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities are plotted on the scatter plot.
Drifter data distributed by NOAA/AOML www.aoml.noaa.gov/phod/dac/gdp.html
IOVWST Barcelona May 2010 – p. 16
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
−0.5 0 0.5
N=339 Cor=0.87 Sk=0.51 RDS=0.13 Slope= 0.7561
Drifter velocity m/s
N=339 Cor=0.60 Sk=0.11 RDS=−0.00 Slope= 0.60632
Drifter velocity m/s
/s
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities are plotted on the scatter plot.
Generalized Ekman solution.
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
−0.5 0 0.5
N=339 Cor=0.87 Sk=0.51 RDS=0.13 Slope= 0.75677
Drifter velocity m/s
N=339 Cor=0.60 Sk=0.12 RDS=0.01 Slope= 0.59494
Drifter velocity m/s
/s
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities are plotted on the scatter plot.
OSCAR constant viscosity solution.
Introduction
Introduction
Calculating Surface Currents
Comparison of OSCAR with TAO mooring
Comparison of OSCAR with TAO mooring
Surface current differences
Summary
Earth & Space Research Seattle, Washington
IOVWST Barcelona May 2010 – p. 1
Introduction
Description of the basic OSCAR equations with the turbulence parameterization
Stommel model boundary conditions
Generalized Ekman boundary conditions
Compare results from OSCAR and Generalized Ekman to in situ data
Future directions for OSCAR
Introduction
Ocean Surface Currents Analyses-Realtime processing system (OSCAR) is a satellite-derived surface current database provided in near-real time based on a combination of quasi-steady geostrophic and locally wind-driven dynamics (Bonjean and Lagerloef, 2002).
The geostrophic term is computed from the gradient of surface topography fields (AVISO/CLS and NRL/MODAS).
Wind-driven velocity components are computed from an Ekman/Stommel formulation with variable viscosity using QuikSCAT winds (FSU/COAPS) and NCEP winds
with a thermal wind adjustment using Reynolds SST data.
Data available at http://podaac.jpl.nasa.gov and http://www.oscar.noaa.gov.
IOVWST Barcelona May 2010 – p. 3
OSCAR Gulf of Mexico
30’
28oN
30’
29oN
30’
30oN
30’
May.17,2010 km/day
22oN
24oN
26oN
28oN
30oN
May.17,2010 km/day
0
10
20
30
40
50
60
70
80
OSCAR product extended to daily output for the latest 20 days of data using real time products (AVISO RT and NCEP). Image from NASA’s Terra Satellite on May 17.
IOVWST Barcelona May 2010 – p. 4
OSCAR Real Time Gulf of Mexico
(Loading Gulf.mpg)
OSCAR product extended to daily output for the latest 20 days of data using real time products (AVISO RT and NCEP).
IOVWST Barcelona May 2010 – p. 5
Gulf.mpg
Media File (video/mpeg)
Calculating Surface Currents
Quasi-steady linear flow in a surface layer with turbulent mixing parameterized by a constant vertical eddy viscosity. Frontal model: buoyancy force θ is a function of SST horizontal gradients only. Surface layer velocity U by averaging over the top h=30m.
ifU = −g ζ + h
∂z .(2)
Use Stommel model boundary conditions in a second order differential equation for shear:
∂U
∂z (z = −H) = 0(4)
where: U = u + i × v, τ0 is surface wind stress, ζ is SSH, θ is buoyancy, based on SST (θ = gχT SST ), and ν is a vertical eddy viscosity, calculated as a function of wind
ν = a( |W|
Calculating Surface Currents: Generalized Ekman
ν = a( |W|
W0
)b(6)
Optimal choice for a in OSCAR blends from 8 × 10−5 m2s−1, b = 2.2 at the equator as in Santiago-Mandujano & Firing (JPO 1990), to 2.85 × 10−4 m2s−1, b = 2 for the global value.
Rather than solve for the shear, Cronin and Kessler (JPO 2009) solved for stress, using the Generalized Ekman boundary conditions and a vertically varying eddy viscosity which decays with depth so that the stress becomes zero at depth H, while the shear can remain nonzero.
Generalized Ekman boundary conditions
τ(z = −H) = ν(z) ∂U
∂z (z = −H) = 0.(8)
where: ν = A exp(z/D) − B, D = 125m and ν = 16e − 03 m2s−1 at 10m and zero at 250m.
IOVWST Barcelona May 2010 – p. 7
Comparison of OSCAR with TAO mooring
−6 −5 −4 −3 −2 −1 0 1 2
x 10 −5
−200
−150
−100
−50
0
−200
−150
−100
−50
velocity [m/s]
Vertical profiles of stress and velocity at a single day given a variable eddy viscosity ν(z) (Generalized Ekman) and a constant eddy viscosity (OSCAR).
Comparison of surface currents with 8N 125W TAO mooring ADCP.
The stress profile should depend on the wind and stratification, and has been observed to decrease to zero at the base of the transition layer (Dohan and Davis 2010).
IOVWST Barcelona May 2010 – p. 8
Comparison of OSCAR with TAO mooring
Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr −0.5
0
0.5
1
Generalized Ekman
TAO Mooring
Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr −0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
Meridional Surface Currents at 8N 125W
Comparison of surface current solutions with 8N 125W TAO mooring ADCP velocity at 30m depth, 10-day smoothed onto 5-day timebase.
Currents are calculated with Generalized Ekman ν(z) and OSCAR constant ν to 250m depth.
Moored data is distributed by NOAA/PMEL at http://www.pmel.noaa.gov/tao.
IOVWST Barcelona May 2010 – p. 9
Surface current differences
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May 0
0.05
0.1
0.15
0.2
0.25
m /s
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May −200
−100
0
100
de gr
ee s
Gen Ek
OSCAR 80m
OSCAR 250m
Difference in magnitude and angle of surface currents using Generalized Ekman ν(z), constant ν to 80m depth, and constant ν to 250m depth.
IOVWST Barcelona May 2010 – p. 10
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day).
Generalized Ekman solution.
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day).
OSCAR constant viscosity solution.
Summary
Global dataset dating back to 1992.
Validation of surface currents from the global drifting buoy array and subsurface currents from moored arrays.
The Generalized Ekman formulation, with a vertically varying eddy viscosity has more flexibility and is more physically realistic.
The profiles of velocity at depth do not agree with in situ data (not surprisingly).
Surface velocities are similar between viscosity choices, although with enough variation in magnitude and direction to distinguish between parameterization types.
IOVWST Barcelona May 2010 – p. 13
Future Work
vertically varying eddy viscosity
include a mixed layer and transition layer account for the shear-driven mixing in the transition layer (i.e. below mixed layer) incorporate ARGO mixed layer depths
vary models according to dynamical regions equatorial, ACC, high winds, near coasts, compensation layers/ salt fingers, deep convection, eddies ...
Develop time-dependent dynamics in OSCAR to include high-frequency wind-driven currents.
Extend OSCAR capability to nowcast and forecast.
IOVWST Barcelona May 2010 – p. 14
La Nina
ENSO cycle as indicated by 1st EOF of surface current and SST anomalies.
IOVWST Barcelona May 2010 – p. 15
Comparison with Drifters Gulf Stream
100 cm/s
34N
38N
42N
D R
IF T
E R
30N
34N
38N
42N
−100
0
100
200
300
)
N=696 rmsd=26cm/s rdstd=22% md=−6.7cm/s cor=0.82 sk=0.42 sco=5.3
−200 −100 0 100 200 −200
−100
0
100
200
)
N=696 rmsd=24cm/s rdstd=17% md=−3cm/s cor=0.75 sk=0.33 sco=5
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities are plotted on the scatter plot.
Drifter data distributed by NOAA/AOML www.aoml.noaa.gov/phod/dac/gdp.html
IOVWST Barcelona May 2010 – p. 16
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
−0.5 0 0.5
N=339 Cor=0.87 Sk=0.51 RDS=0.13 Slope= 0.7561
Drifter velocity m/s
N=339 Cor=0.60 Sk=0.11 RDS=−0.00 Slope= 0.60632
Drifter velocity m/s
/s
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities are plotted on the scatter plot.
Generalized Ekman solution.
Comparison of OSCAR with Drifters
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned DRIFTERS 05−Apr−1997 to 05−Apr−1998
136oW 132oW 128oW 124oW 120oW 5oN
6oN
7oN
8oN
9oN
10oN
1−day base Binned OSCAR 05−Apr−1997 to 05−Apr−1998
−0.5 0 0.5
N=339 Cor=0.87 Sk=0.51 RDS=0.13 Slope= 0.75677
Drifter velocity m/s
N=339 Cor=0.60 Sk=0.12 RDS=0.01 Slope= 0.59494
Drifter velocity m/s
/s
OSCAR surface velocities are interpolated onto drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities are plotted on the scatter plot.
OSCAR constant viscosity solution.
Introduction
Introduction
Calculating Surface Currents
Comparison of OSCAR with TAO mooring
Comparison of OSCAR with TAO mooring
Surface current differences
Summary