“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

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)

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

B. Satellite Observationssea 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

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

Altimeter data is used to calculate geostrophic velocity with “smoothing” scales (and amplitude

correction) consistent with drifter data: e.g. AVISO

0 10 20 30 40 50

40

60

80

100

120

140

160

Bin Center Latitude

Geostrophic Velocity Smoothing Scale vs. Latitude

N. Hemi

S. Hem i

N. Hemi dmn

S. Hemi dmn

N. Hemi nolev

S. Hemi nolev

• • •

••

••

••

• N/S ; *E/W AVISO Correlation Scales

*

*

B. Cornuelle

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

1982-2007- Drifter observed rms velocity variance [<u’2>+<v’2>]1/2 (Y.Y. Kim)

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

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

€

rV G =

r V o + A <

r V A >

r V GD =

v V D −

r V EK (W , f ,...)

Minimize E relative to A,r V o :

E = <r V GD −

drifter obs <>

∑r V o − A

r V A >2

Solution depends on vector correlation : C =<

r V DG

r V A >

[<r

V A ⋅r

V DG ><r V A ⋅

r V DG >]1/ 2

MAP :r V MAP =

r V o + A

r V A +

r V EK

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

18

Mean U (cm/s) at 15m depth from Drifters

B. Cornuelle

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

L.Centurioni

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

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

C. Ohlmann

Drifter tracks in the South China Sea during NE Monsoon with speed in color (left) and Ekman

transport divergence (right) (L.Centurioni)

THE GLOBAL SOLUTIONS

1. Time mean surface momentum balance for surface sea level gradient:

• Observed drifter = “D”

• Computed Ekman = “E”

€

−< ∇η >= ˆ f x<r v D −

r v E > + < d

r v D / dt >≡ −

r F

€

uE + ivE = A(lat)(Wx + iWy )( NCCEP wind)

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

€

∂∂ηo

{4π 2

L2| Go

globe

∫∫ − ηo |2 dS + | ∇η o −r F |2

globe

∫∫ dS + 4δ 2 | ∇2η o |2

globe

∫∫ dS} = 0

with (ηoglobe

∫∫ )dS = 0; L = 1000km; δ =1

2

o

(km)€

ηo

€

ηo

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),

1992-2002 Mean Sea Level: Maximenko (05)

1993-1999 Mean Sea Level: RIO (05)

M.-H. Rio

Difference between Maximenko(‘05)-Rio(‘05) MSL with both data adjusted to 1993-1999 period

M.-H. Rio : RMS difference of 5cm

Zonal, unbiased geostrophic velocity

Short spatial scale (<400km) zonal geostrophic current distribution. Black squares are used to locate

regions of subsurface temperature analyses (N. Maximenko)

QuickTime™ and a decompressor

are needed to see this picture.

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)

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.

The mean kinetic energy at 15m depth from drifters. This quantity graphed is (<v’•v’>/2g) and represents the sea level change caused by Bernoulli effect

of ocean time variable eddies.

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

€

< r

v > ⋅∇(< SST >)

€