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“ New Ocean Circulation Patterns from Combined Drifter and Satellite Data ” 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, Y.-Y. Kim, R. Lumpkin, M. Pazos

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Method of Calculating Ocean Surface Circulation Combines Drifter and Satellite Observations A. Between 1/1/88 and 12/1/07 1988 14,320 drifters drogued to 15m depth were released in the global ocean, with array of 1250 since 9/18/05 (drifter slip relative to wind corrected to (+/-1cm/sec)

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B. Satellite Observations sea level = altimeter height - geoid height Altimeters: GEOS, T/P, JASIN I&II, ERS I&II Data from 1992 - Present (rms : +/- 4cm relative to geoid) GRACE’04: Accuracy of geoid: +/-3 cm at 400 km scale Sea level gradient, or geostrophic velocity, depends upon method and scale of averaging, or mapping, of sea level data

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Drifter velocity observations are accurate (+/- 0.01.5 m/sec daily averages), but spatial distribution of data can result in biased averages in space and time

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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

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Streaklines and speed (log scale) of mean drifter derived circulation at 15m depth (N. Maximenko)

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1982-2007- Drifter observed rms velocity variance [ + ] 1/2 (Y.Y. Kim)

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(Eddy Energy / Mean Energy)1/2 (Y.Y. Kim)

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The” simple method” of obtaining a joint drifter and altimeter velocity map

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

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

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

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

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

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Geostrophic zonal velocity from drifter and altimeter data L. Centurioni

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

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Drifter tracks in the South China Sea during NE Monsoon with speed in color (left) and Ekman transport divergence (right) (L.Centurioni)

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

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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

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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 et al., (2005),

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1992-2002 Mean Sea Level: Maximenko (05)

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1993-1999 Mean Sea Level: RIO (05) M.-H. Rio

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Difference between Maximenko(‘05)-Rio(‘05) MSL with both data adjusted to 1993-1999 period M.-H. Rio : RMS difference of 5cm

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Zonal, unbiased geostrophic velocity

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Short spatial scale (<400km) zonal geostrophic current distribution. Black squares are used to locate regions of subsurface temperature analyses (N. Maximenko)

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The subsurface temperature and surface dynamic height anomalies tha are congruent with the surface expression of near zonal striations in sea level (black) in the North Pacific (upper) and South Pacific (lower) (N. Maximenko)

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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.

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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.

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

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