Improvements to the Wind Driven Component of the OSCAR Surface Current Product

Transcription

1 OVWST Meeting May 18-2, 29 p. 1 Improvements to the Wind Driven Component of the OSCAR Surface Current Product Kathleen Dohan, Gary S. E. Lagerloef, and John T. Gunn Earth & Space Research Seattle, Washington

2 Introduction OVWST Meeting May 18-2, 29 p. 2 Ocean Surface Currents Analyses Realtime processing system (OSCAR) is a satellite-derived surface current database based on a combination of quasi-steady geostrophic and locally wind-driven dynamics (Bonjean and Lagerloef, 22). The geostrophic term is computed from the gradient of surface topography fields (AVISO/CLS). Wind-driven velocity components are computed from an Ekman/Stommel formulation with variable viscosity using QuikSCAT winds (FSU/COAPS) with a thermal wind adjustment using satellite SST data. Data available at

3 Introduction OVWST Meeting May 18-2, 29 p. 3 State of the improvements to the currently available OSCAR system. Examination of the wind-driven component Vertical eddy viscosity parameterization as a function of the wind Future directions.

4 OSCAR Latest Developments OVWST Meeting May 18-2, 29 p o N OSCAR One Degree Currents April 27, 29 m/s 1 45 o N OSCAR Third Degree Currents April 27, 29 m/s o N.8 42 o N o N.6 39 o N o N.4 36 o N o N.2 33 o N.2 1e 1 m/s.1 1e+ m/s.1 8 o W 75 o W 7 o W 65 o W 6 o W 55 o W 8 o W 75 o W 7 o W 65 o W 6 o W 55 o W Developments to OSCAR: increased grid spacing from one degree to 1/3 degree SSH gradient calculation for geostrophic component revised to suit 1/3 degree grid with extensive Cal/Val Larger coverage of data towards coasts Improved model in equatorial region Fewer spuriously large values (mostly along coasts). Available through ftp at ftp://ftp.esr.org/pub/datasets/sfccurrents/thirddegree/ and soon through

5 Comparison with Drifters Gulf Stream OVWST Meeting May 18-2, 29 p o N OSCAR Third Degree Currents 15 Jul 26 Drifters from 1 Jun to 1 Sep 26 m/s o N o N o N o N.2 1e+ m/s 8 o W 75 o W 7 o W 65 o W 6 o W 55 o W.1 Currents are interpolated onto the drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities.

6 Comparison with Drifters Gulf Stream OVWST Meeting May 18-2, 29 p. 6 OSCAR1deg & DRIFTER DATA: Jun.1,26 Sep.1,26 Background field: OSCAR1deg monthly mean OSCARthirddeg & DRIFTER DATA: Jun.1,26 Sep.1,26 Background field: OSCARthirddeg monthly mean Zonal OSCAR1deg vel (cm/s) N 1 cm/s 38N 34N 3N 8W 75W 7W 65W 6W 55W 42N 38N 34N 3N 8W 75W 7W 65W 6W 55W 1 cm/s N=696 rmsd=31cm/s rdstd=46% md= 5.4cm/s cor=.75 sk=.32 sco= Zonal Drifter vel (cm/s) Meridional OSCAR1deg vel (cm/s) 1 1 DRIFTERS OSCAR1deg N=696 rmsd=27cm/s rdstd=42% md= 4.2cm/s cor=.68 sk=.26 sco= Meridional Drifter vel (cm/s) Zonal OSCARthirddeg vel (cm/s) N 38N 34N 1 cm/s 3N 8W 75W 7W 65W 6W 55W 42N 38N 34N 3N 8W 75W 7W 65W 6W 55W 1 cm/s N=696 rmsd=26cm/s rdstd=22% md= 6.7cm/s cor=.82 sk=.42 sco= Zonal Drifter vel (cm/s) Meridional OSCARthirddeg vel (cm/s) 1 1 DRIFTERS OSCARthirddeg N=696 rmsd=24cm/s rdstd=17% md= 3cm/s cor=.75 sk=.33 sco= Meridional Drifter vel (cm/s)

7 Calculating Surface Currents OVWST Meeting May 18-2, 29 p. 7 Quasi-linear steady 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 Ū by averaging over the top 3m. (1) ifū = g ζ + h 2 θ + τ A U z h. h Stommel model boundary conditions (2) U (z = ) = τ/a U z (3) z (z = H) = where: U = u + i v, τ is surface wind stress, h = 3m, ζ is SSH, θ is SST and A is a vertical eddy viscosity, calculated as a function of wind (4) A = a( W W ) b.

8 Pacific Sample Region OVWST Meeting May 18-2, 29 p o N OSCAR Third Degree Currents 15 Jul 26 Drifters from 1 Jan to 1 Mar 27 m/s o N o N.2 45 o N o N 1e 1 m/s 144 o W 138 o W 132 o W 126 o W 12 o W.5 Currents are interpolated onto the drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities. Sensitivity to the parameter value a has been tested, with attention to different dynamical regimes. Coefficient a is varied from 1e-6 to 1e-3. Exponent b is varied from 2 to 2.2. Depth-scale H is varied from 3m to 15m.

9 Pacific Sample Region Comparison with Drifters OVWST Meeting May 18-2, 29 p o N 51 o N Pacific a=2.8e 4 b=2 DRIFTERS 1 Jan 27 to 1 Mar Pacific a=2.8e 4 b=2 Zonal velocity N=1444 Cor=.34 Sk=.6 RDS=.71 Slope= Pacific a=2.8e 4 b=2 Meridional velocity N=1444 Cor=.42 Sk=.9 RDS=.64 Slope= o N 45 o N 42 o N 54 o N 1e 1 m/s 144 o W 138 o W 132 o W 126 o W 12 o W Pacific a=2.8e 4 b=2 OSCAR 1 Jan 27 to 1 Mar Drifter velocity m/s OSCAR velocity m/s OSCAR m/s Drifter velocity m/s 51 o N 48 o N 45 o N 42 o N 1e 1 m/s 144 o W 138 o W 132 o W 126 o W 12 o W Well underestimating the magnitude of currents in the open basins: 1 degree winds, 1 day smoothing Optimal values for a varies between and with little variation. Correlations steadily increase from January to August (.3 to.6).

10 Equatorial Sample Region OVWST Meeting May 18-2, 29 p. 1 Currents are interpolated onto the drifter locations (which have been averaged over 1 day). Zonal and meridional currents vs drifter velocities. Sensitivity to the parameter value a has been tested, with attention to different dynamical regimes. Coefficient a is varied from 1e-6 to 1e-3. Exponent b is varied from 2 to 2.2. Depth-scale H is varied from 3m to 15m.

11 Equatorial Sample Region Comparison with Drifters OVWST Meeting May 18-2, 29 p o N Equatorial a=8.e 5 b=2.2 DRIFTERS 1 Jan 27 to 1 Mar o N Equatorial a=8.e 5 b=2.2 OSCAR 1 Jan 27 to 1 Mar 27 Equatorial a=8.e 5 b=2.2 Zonal velocity N=2756 Cor=.84 Sk=.45 RDS=.9 Slope= Equatorial a=8.e 5 b=2.2 Meridional velocity N=2756 Cor=.4 Sk=.4 RDS=.11 Slope= o N o 8 o S 16 o S 1e+ m/s 8 o N o 8 o S 16 o S 1e+ m/s OSCAR velocity m/s OSCAR velocity m/s o W 168 o W 16 o W 152 o W 144 o W 176 o W 168 o W 16 o W 152 o W 144 o W Drifter velocity m/s Drifter velocity m/s Optimal choice for a consistent with m 2 s 1 in Santiago-Mandujano & Firing (JPO 199). Little variation with a and b, except in amplitude of meridional: slope ranges from

12 Equatorial Region Climatology OVWST Meeting May 18-2, 29 p. 12 Climatological means are compared to drifter climatology using seasonal and regional climatologies along the equatorial region. Sample region and results shown here.

13 Equatorial Region Climatology OVWST Meeting May 18-2, 29 p U climatology Averaged over [218,266] o E 2 V climatology Averaged over [218,266] o E EQ Global Lat 4 Lat 7 Lat 12 DRIFT 1deg EQ Global Lat 4 Lat 7 Lat 12 DRIFT 1deg Parameter values are blended from a = m 2 s 1, b = 2.2 at the equator to m 2 s 1, b = 2 for a global value.

14 Summary OVWST Meeting May 18-2, 29 p. 14 OSCAR surface currents compare well with drifter velocities in regions of strong SSH gradients: boundary currents, zonal equatorial component, ACC. Amplitudes are underestimated outside the above regions, with lower velocity correlations. Wind-driven velocities are fairly insensitive to the value of the eddy viscosity, except for meridional amplitude around the equator. Mid-latitudes show a seasonal trend in drifter comparison. Wind-driven velocities are insensitive to the depth value H. Mooring analyses are inconclusive, with varying optimal viscosity values for equatorial region, season, and statistical quantity calculated.

15 Future Work OVWST Meeting May 18-2, 29 p. 15 Improve the wind-driven turbulent mixing scheme vertically varying eddy viscosity - e.g. law of the wall versus slab incorporate ARGO mixed layer depths vary models according to dynamical regions Incorporate faster timescales in wind driven OSCAR component Include time-dependent dynamics in OSCAR to include high-frequency wind-driven currents Separate by timescales the geostrophic from the wind-driven components 1 day winds Examine inertial motions in drifter datasets Extend OSCAR capability to nowcast and forecast Gulf Stream movie