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Permanent Meanders in the California Current System and Comparison of Near-Surface Observations with OGCM Solutions. Luca Centurioni (SIO-PORD). Collaborators: Peter Niiler, Carter Ohlmann.

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Permanent Meanders in the California Current System and Comparison of Near-Surface Observations with OGCM Solutions

Luca Centurioni

(SIO-PORD)

Collaborators: Peter Niiler, Carter Ohlmann

Acknowledgments (PI):Harley Hurlburt (NLOM), Julie McClean (POP), Jim McWilliams (ROMS), Ruth Preller (HYCOM)


Outline Comparison of Near-Surface Observations with OGCM Solutions

  • Summary of observations from 15 depth drifters data;

  • the bias problem: best estimate of 15 m depth geostrophic velocity field;

  • comparison of some observation-derived quantities with OGCM solutions;

  • Conclusions.






78% ignored):

80%


Vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO


  • UNBIASED FIELD (V velocity residuals from drifters and AVISOC):

  • (Niiler et al. 2003)

  • A running average (30 hrs) filter is applied to Lagrangian time series

  • Ekman currents (Ralph & Niiler 1999) are removed to compute geostrophic velocities from drifters;

  • Drifter geostrophic velocities (VDG) are binned in time (7 days) within each cell (0.5ºX0.5º) and anomalies are computed;

  • Geostrophic velocities anomalies from AVISO (VS) gridded maps are computed and interpolated at drifter locations and (binned) times;

  • Assume the following model: VG(ti;x)=A(x)VS(ti;x)+VC(x)

  • Estimate A and VC by minimizing {{(VG-VDG)2}} where {{}} denotes time average over concurrent drifter and satellite velocity data, i.e


Slope of the linear model V velocity residuals from drifters and AVISOG(t,x,y)=A(x,y)VS(t,x,y)+VC(x,y)

A


UNBIASED GEOSTROPHIC VELOCITY VECTOR FIELD AT 15 m DEPTH velocity residuals from drifters and AVISO


Unbiased geostrophic velocity field: zonal component (cm s velocity residuals from drifters and AVISO-1)


MEAN GEOSTROPHIC EKE velocity residuals from drifters and AVISO0.5 FROM CORRECTED ALTIMETRY

cm s-1


MEAN SEA LEVEL (cm) velocity residuals from drifters and AVISO

HYCOM

NLOM

POP

ROMS


EKE velocity residuals from drifters and AVISO0.5 FROM NUMERICAL MODELS (0-20 cm s-1)

HYCOM

POP

NLOM

ROMS


EKE velocity residuals from drifters and AVISO0.5 COMPARISON (0-20 cm s-1)

ROMS

FROM CORRECTED ALTIMETRY


Conclusions velocity residuals from drifters and AVISO1)Data confirm that the CCS (during the last 10 years and in the area examined) had 4 permanent meanders which are co-located with jets of zonal flow that extend nearly to Hawaii;2)Time biases from the drifter data can be removed with the aid of satellite altimetry; Comparison of observed quantities with OGCM outputs can be addressed;3)Preliminary comparisons show that ROMS is likely to be the model with the highest degree of realism;



Ageostrophic, non-linear velocity in ROMS and simple GFD model of cold eddy interacting with wind (Lee et al 1998)


s model of cold eddy interacting with wind (Lee et al 1998)-1


s model of cold eddy interacting with wind (Lee et al 1998)-1


FOLLOWING THE DRIFTERS model of cold eddy interacting with wind (Lee et al 1998)


Ekman force ( ) is determined from model of cold eddy interacting with wind (Lee et al 1998)

(Ralph&Niiler 1999)


cm s model of cold eddy interacting with wind (Lee et al 1998)-1

MEAN EKE0.5 at 15 m DEPTH (from drifters)


EKMAN CURRENT AT 15m DEPTH model of cold eddy interacting with wind (Lee et al 1998)


Can we explain the jets of zonal flow? model of cold eddy interacting with wind (Lee et al 1998)


Suppose that: model of cold eddy interacting with wind (Lee et al 1998)

And use the following barotropic model to compute the stream function of volume transport per unit depth:

Acceleration of a drifter:


From AVISO and Unb. Vel. Field. model of cold eddy interacting with wind (Lee et al 1998)

From drifters



Consider a one layer ocean of depth D=D model.0+D’ with a wind stress t acting over it:

The stream function of the mass transport can be computed as:



Acceleration of a drifter: 21 days long

(horizontal velocity)


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