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 (V C ): (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; 1.Drifter geostrophic velocities (V DG ) are binned in time (7 days) within each cell (0.5ºX0.5º) and anomalies are computed; 2.Geostrophic velocities anomalies from AVISO (V S ) gridded maps are computed and interpolated at drifter locations and (binned) times; 3.Assume the following model: V G (t i ;x)=A(x)V S (t i ;x)+V C (x) 4.Estimate A and V C by minimizing {{(V G -V DG ) 2 }} where {{}} denotes time average over concurrent drifter and satellite velocity data, i.e

Slope of the linear model V G (t,x,y)=A(x,y)V S (t,x,y)+V C (x,y) A

UNBIASED GEOSTROPHIC VELOCITY VECTOR FIELD AT 15 m DEPTH

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

MEAN GEOSTROPHIC EKE 0.5 FROM CORRECTED ALTIMETRY cm s -1

HYCOMNLOMPOPROMS spatial domainglobal ~1000 x 2000 km (USWC) vertical coordinateshybridlayerslevelssigma (ETOPO5) horizontal resolution1/12° (~7 km)1/32° (~3.5 km) 1/10° (~10 km)~5 km vertical layers/levels266 + ML4020 time step6 hour 15 minute mixed layerKPPKraus-TurnerKPP wind forcingECMWFNOGAPS/HRNOGAPSCOADS (seasonal) heat forcingECMWFNOGAPSECMWFCOADS (seasonal) buoyancy forcingCOADS (restored to Levitus) Levitus (restoring) Levitus (restoring) COADS (seasonal); parameterization for Columbia River outflow integration time years assimilationnoneSST, SSHnone otherLow computational cost open boundaries

POP HYCOM NLOM ROMS MEAN SEA LEVEL (cm)

EKE 0.5 FROM NUMERICAL MODELS (0-20 cm s -1 ) POP HYCOM NLOM ROMS

EKE 0.5 COMPARISON (0-20 cm s-1) ROMS FROM CORRECTED ALTIMETRY

Conclusions 1)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

FOLLOWING THE DRIFTERS

Ekman force ( ) is determined from (Ralph&Niiler 1999)

MEAN EKE 0.5 at 15 m DEPTH (from drifters) cm s -1

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

The stream function of the mass transport can be computed as: Consider a one layer ocean of depth D=D 0 +D’ with a wind stress  acting over it:

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

Acceleration of a drifter: (horizontal velocity)