Combining Ocean Velocity Observations and Altimeter Data for OGCM VerificationPeter NiilerScripps Institution of Oceanography
with original material fromN. 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 12/1/06 1988 10,561 drifters drogued to 15m depth were released in the global ocean, with array of 1250 since 9/18/05
Satellite Observationssea level = altimeter height - geoid heightAltimeters: GEOS, T/P, JASIN, ERS I&IIData from 1992 - Present (rms noise: +/- 4cm relative to geoid)GRACE04: Estimated accuracy of geoid: +/-3 cm at 400 km horizontal scaleSea level gradient, or geostrophic velocity, depends upon method and scale of averaging, or mapping, of sea level data
Drifter velocity observations are accurate (+/- 0.015 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 [+]1/2N.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. 8/01-11/03D.-K. Lee
Comparison drifter and ECCO 15m zonal velocity components in tropical PacificB. Cornuelle
Vector correlation and scatter plots of geostrophic velocity residuals from drifters and AVISO in California CurrentL.Centurioni
HYCOMNLOMPOPROMSspatial domainglobalglobalglobal ~1000 x 2000 km (USWC)vertical coordinateshybridlayerslevelssigma (ETOPO5)horizontal resolution1/12 (~7 km)1/32 (~3.5 km)1/10 (~10 km)~5 kmvertical layers/levels266 + ML4020time step6 hour6 hour6 hour15 minutemixed layerKPPKraus-TurnerKPPKPPwind forcingECMWFNOGAPS/HRNOGAPSCOADS (seasonal)heat forcingECMWFNOGAPSECMWFCOADS (seasonal)buoyancy forcingCOADS(restored to Levitus)Levitus(restoring)Levitus (restoring)COADS (seasonal); parameterization for Columbia River outflowintegration time1990-20011991-20001990-20009 yearsassimilationnoneSST, SSHnonenoneotherLow computational costopen boundaries
Unbiased drifter and satellite derived geostrophic 15m velocity (on left) and ROMS 5km resolution sea level (right) L.Centurioni and C. Ohlmann
Geostrophic zonal velocity from drifter and altimeter dataL. Centurioni
Decadal MEAN SEA LEVEL (cm) in models of the California CurrentC. Ohlmann
OGCM (Eddy Energy)1/2: California CurrentROMS
Geostrophic EKE0.5 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 CurrentC. 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, Go, 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.
1992-2002 Mean Sea Level: Maximenko (05)
Zonal, unbiased geostrophic velocity (-10,+10 cm/sec)
1993-1999 Mean Sea Level: RIO (05)M.-H. Rio
Difference between Maximenko(05)-Rio(05) MSL with both data adjusted to 1993-1999 periodM.-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-AndersonV. Zlotnicki
CONCLUSIONSCombined 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 StommelTHE DREAM HAS COME TRUE we are observing the circulationpeter niiler
East-west average vorticity balance at 15m depth (black line is from Ekmans 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: 0.25 resolution sea level (upper) and simple geostrophic velocity (lower).
The 1992-2000 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 (x10-7Csec-1) at 15m depth:
1978-2003 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 1992-2002 average 15m depth velocity
Zonal, unbiased geostrophic velocity (-40,+40cm/sec)
In a 50 m upper ocean column one unit of 10(-7)deg.C/sec corresponds to 10 watts/m(2) of surface heat flux convergence.