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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.

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Presentation Transcript
slide1

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)

slide2
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.
slide7

78%

80%

slide8

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

slide9

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
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MEAN SEA LEVEL (cm)

HYCOM

NLOM

POP

ROMS

slide18

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

ROMS

FROM CORRECTED ALTIMETRY

slide19

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;

slide21

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

slide26

cm s-1

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

slide29

Suppose that:

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

Acceleration of a drifter:

slide32

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:

slide35

Acceleration of a drifter:

(horizontal velocity)