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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

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:

Acceleration of a drifter:

(horizontal velocity)