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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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Luca Centurioni (SIO-PORD)

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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

  • 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.


Number of 6-hrs interval observations in a 0.5º x 0.5º bin.


MEAN VELOCITY FIELD At 15 m DEPTH


FROM mean field at 15 m


Momentum balance (mean) at 15 m depth (dissipation is ignored):


78%

80%


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


  • UNBIASED FIELD (VC):

  • (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 VG(t,x,y)=A(x,y)VS(t,x,y)+VC(x,y)

A


UNBIASED GEOSTROPHIC VELOCITY VECTOR FIELD AT 15 m DEPTH


Unbiased geostrophic velocity field: zonal component (cm s-1)


MEAN GEOSTROPHIC EKE0.5 FROM CORRECTED ALTIMETRY

cm s-1


MEAN SEA LEVEL (cm)

HYCOM

NLOM

POP

ROMS


EKE0.5 FROM NUMERICAL MODELS (0-20 cm s-1)

HYCOM

POP

NLOM

ROMS


EKE0.5 COMPARISON (0-20 cm s-1)

ROMS

FROM CORRECTED ALTIMETRY


Conclusions1)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;


Number of 6-hrs interval observations in a 0.5º x 0.5º bin.


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


s-1


s-1


FOLLOWING THE DRIFTERS


Ekman force ( ) is determined from

(Ralph&Niiler 1999)


cm s-1

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


EKMAN CURRENT AT 15m DEPTH


Can we explain the jets of zonal flow?


Suppose that:

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.

From drifters


of zonal volume transport per unit depth from barotropic model.

cm s-1


Consider a one layer ocean of depth D=D0+D’ with a wind stress t acting over it:

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


Absolute sea level 27 Oct, 1993 with drifter tracks that are 21 days long


Acceleration of a drifter:

(horizontal velocity)


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