“ Combining Ocean Velocity Observations and Altimeter Data for OGCM Verification ” Peter Niiler Scripps Institution of Oceanography with original material from N. Maximenko, M.-H.Rio, L. Centurioni, C. Ohlmann, B. Cornuelle, V. Zlotnicki,, D.-K. Lee

Method of Calculating Ocean Surface Circulation Combines Drifter and Satellite Observations Between 1/1/88 and 12/1/ ,561 drifters drogued to 15m depth were released in the global ocean, with array of 1250 since 9/18/05

Satellite Observations sea level = altimeter height - geoid height Altimeters: GEOS, T/P, JASIN, ERS I&II Data from Present (rms noise: +/- 4cm relative to geoid) GRACE’04: Estimated accuracy of geoid: +/-3 cm at 400 km horizontal scale Sea level gradient, or geostrophic velocity, depends upon method and scale of averaging, or mapping, of sea level data

Drifter velocity observations are accurate (+/ m/sec daily averages), but spatial distribution of data can result in biased averages in space and time

Altimeter data is used to calculate geostrophic velocity with “smoothing” scales (and amplitude correction) consistent with drifter data: e.g. AVISO N/S ; *E/W AVISO Correlation Scales * * B. Cornuelle

Drifter observed rms velocity variance [ + ] 1/2 N.Maximenko

Log10 (Eddy Energy / Mean Energy)1/2 N. Maximenko

The” simple method” of obtaining a velocity map

Vector Correlation between drifter and altimeter derived AVIO geostrophic velocity anomalies N.Maximenko

“East Sea”: 3 day average velocity from “simple method” vs drifter obs. 8/01-11/03 D.-K. Lee

Comparison drifter and ECCO 15m zonal velocity components in tropical Pacific B. Cornuelle

Vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO in California Current L.Centurioni

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 C. Ohlmann

L.Centurioni and C. Ohlmann Unbiased drifter and satellite derived geostrophic 15m velocity (on left) and ROMS 5km resolution sea level (right)

Geostrophic zonal velocity from drifter and altimeter data L. Centurioni

Decadal MEAN SEA LEVEL (cm) in models of the California Current C. Ohlmann

OGCM (Eddy Energy) 1/2 : California Current NLM HYCO POP ROMS

Geostrophic EKE 0.5 ROMS (left) corrected AVISO (right) (0-20 cm s-1) L. Centurioni and C. Ohlmann

Ageostrohic 15m velocity and MSL in 5km resolution ROMS of California Current C. Ohlmann

THE GLOBAL SOLUTIONS 1. Time mean surface momentum balance for surface sea level gradient: Observed drifter = “D” Computed Ekman = “E”

2. Compute sea level that minimized the global cost function in least square The solution is also minimized relative to parameters of Ekman force and GRACE altimeter referenced sea level, G o, is averaged on 1000km scales. Maximenko-Niiler

3. Perform an objective mapping of sea level, with mesoscale based, geostrophic, correlation functions, as a linear combination of: Levitus 1500m relative steric level, GRACE referenced altimeter derived sea level Drifter geostrophic velocity. RIO(05), Knudsen-Andersen

Mean Sea Level: Maximenko (05)

Zonal, unbiased geostrophic velocity (-10,+10 cm/sec)

Mean Sea Level: RIO (05) M.-H. Rio

Difference between Maximenko(‘05)-Rio(‘05) MSL with both data adjusted to period M.-H. Rio : RMS difference of 5cm

Comparison of 15m velocity from SURCOLF and MERCATOR near real time maps of Gulf Stream region with drifter data M.-H. Rio

Mean Sea Level: Knudsen-Anderson V. Zlotnicki

ECCO-2 CUBE49 (18km horizontal, global assimilation with flux and diff.par.optim.) V. Zlotnicki

ECCO-2 cube 37 and 49 east velocity difference from Maximenko (05) and Knudsen-Anderson V. Zlotnicki

CONCLUSIONS Combined drifter and altimeter derived velocity anomalies can be used to make regional, realistic, near real time maps of 15m ocean circulation. Global, absolute sea level on 50km scale from combined data displays new circulation features. OGCM solutions are most stringently tested with velocity fields derived from combined drifter and altimeter observations.

“Why does our view of ocean circulation always have such a dreamlike quality…”...Henry Stommel THE DREAM HAS COME TRUE… we are observing the circulation peter niiler

East-west average vorticity balance at 15m depth (black line is from Ekman’s 1906 model, shaded is drifter data; 100 km coastal and western boundary currents excluded)

The eddy transport of vorticity The eddy transport vector of vorticity is computed around the Gulf Stream eddy energy maximum.

North Atlantic: 0.25º resolution sea level (upper) and “simple” geostrophic velocity (lower).

The time average quasi- geostrophic eddy vorticity flux vector in the Gulf Stream region.

The mean kinetic energy at 15m depth from drifters. This quantity graphed is ( /2g) and represents the sea level change caused by Bernoulli effect of ocean time variable eddies.

SST convergence (x10 -7 Cºsec -1 ) at 15m depth:

Average drifter velocity with QSCT/NCEP blended wind-stress divergence

Conservation of vorticity in the Agulhas Extension Current

Drifter geostrophic velocity compared with ocean circulation model sea-level in California Current in POP (left) and UCLA/ROMS (right)

Global streamlines of average 15m depth velocity

Zonal, unbiased geostrophic velocity (-40,+40cm/sec)