goos/gcos measurements of near-surface currents
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GOOS/GCOS measurements of near-surface currents
GOOS/GCOS measurements of near-surface currents
Rick LumpkinRick Lumpkin(Rick.Lumpkin@noaa.gov)(Rick.Lumpkin@noaa.gov)
National Oceanic and Atmospheric Administration (NOAA)Atlantic Oceanographic and Meteorological Laboratory (AOML)
Miami, Florida USA
Silvia Garzoli and Gustavo GoniSilvia Garzoli and Gustavo GoniNOAA/AOML
Peter NiilerPeter NiilerNOAA/JIMO
Office of Climate Observations 6th Annual System Review, 3 September 2008
Drifters + altimetry in the South Atlantic
Shading: SSTA trend, 1993—2002 (C) from NCEP/NCAR.v2 reanalysis.
Brazil-Malvinas Confluence
Lumpkin & Garzoli (2008)
Lumpkin & Garzoli (2008)
Latitude of the Brazil-Malvinas Confluence
Trend: 0.860.06 degrees per decade
Lumpkin & Garzoli (2008)
-1.060.56 degrees per decade
Lat
itu
de
of
max
imu
m
win
d s
tres
s cu
rlB
asin
-ave
rag
ed S
ST
an
om
aly
(°C
)
Assessment of the global observing array
Analogy with SST analysis:Potential Satellite Bias Error
Satellite measurements Model Field at surface
Biases in model: biases in resulting field.
In-situ observations: reduce bias.
Resulting bias error is a function of observing system configuration and biases in various platforms.
Near-surface currentsWhat model converts satellite measurements to near-surface currents?
OSCAR: the most mature satellite-based surface current product.
Web page offers comparisons with moored and drifting buoys in various regions.
However, the OSCAR currents aren’t accompanied by formal error bars needed to asses bias.
Drifter motion:
.residslipgeoEk uuuuu
Five-day lowpass: filter tides, inertial oscillations, submesoscale.
GOAL: forecast surface velocities (and drifter trajectories) with wind and altimetry products, including error bars.
Assess Potential Satellite Bias Error.
.residslipgeoEk uuuuu
Ekman: Ralph and Niiler (1999), Niiler (2001).
Geostrophic mean from hydrographic climatology, variations addressed by averaging in 2° 5° bins.
./ fAuEk Mean angle 54° off the wind.
For NCEP winds, best fit A=0.081 s-1/2.
.residslipgeoEk uuuuu
Geostrophic: For many studies (e.g., Rio and Hernandez, 2003),
,'SLAgeo uuu .
''
f
guSLA
Left: Altimeter EKE minus drifter EKE (Fratantoni, 2001).
Several reasons that these can differ in general, even if Ekman and slip are perfectly removed:
• centrifugal force, submesoscale motion, etc.
• mismatch between spatial smoothing of altimetry, temporal smoothing of drifters, and energy spectra of motion.
.residslipgeoEk uuuuu
Niiler et al., (2003): .)( 'SLACgeo uxGuu
Drifter measurements
Drifter mean (biased)
Unbiased mean
SLA geo.vel.anomaly
G(x)
Absolute sea level height (cm) (from Niiler, Maximenko & McWilliams, 2003)
.residslipgeoEk uuuuu
Niiler et al., (1995), Niiler and Paduan (1995):
Pazan and Niiler (2001): uundrogued=udrogued+(7.910-3)W.
.WR
Auslip
Drag area ratio
Best fit: A=0.07
Wind speed
Holey-sock drifters: R=40.
Slip is 1.8 cm/s in 10 m/s wind.
Slip in high wind/wave stateNiiler et al. (1995) measurements of slip were in W8 m/s.
Slip may exceeds linear relationship at high wind/wave state.
Niiler, Maximenko and McWilliams (2003): absolute sea height change, 40—60°S: 2.34m, all drifters; 1.98m, only drifters in W8 m/s; 1.55 m, hydrography referenced to floats (Gille, 2003).
Discrepancies with models: problems with models or with data?
Left: mean zonal drifter speed (AOML climatology) minus mean zonal speed of ECCO-GODAE 1° state estimation, 15yr mean (figure courtesy M. Mazloff, WHOI).
Consistent offset in Southern Ocean.
How we can improve our understanding of drifter motion?
• Use high resolution scatterometer-based wind product and include ocean currents when calculating wind stress.
• Simultaneously project motion into geostrophic, wind-driven, and residual components.
slipMDT uuu
./)ˆˆ(ˆˆ)1( 5432'
1 nτu jxixjxixx av
Solve in bins using Gauss-Markov estimation.
(Lumpkin and Elipot, in preparation)
Wind and wind stress
Winds: 6 h, 25 km resolution Variational Analysis Method (VAM) product (Atlas et al., 1996; Atlas et al., 2008) derived from SSM/I, AMSR-E, TMI, QuickSCAT, SeaWinds, and in-situ observations and ECMWF analysis.
Stress: Smith (1988) algorithm as implemented in COARE 3.0 (Fairall et al., 2003) applied to VAM wind and drifter downwind speed (if drifter speed=wind speed, stress=0).
A priori errors 2/Rf
gu
Results
Globally-averaged gain: 1.13±0.06
Gain coefficient (shading) and time-mean currents (arrows)
Wind-driven motion
Comparison with Ralph & Niiler (1999)./ fAuEk .//* AfH
Where do these results differ from RN99?
Where do these results differ from RN99?
.1 32ag
U s a: using significant wave height.
: using peak wave period.
Difference vs. wind, waves
Residuals
Potential Satellite Bias Error (globally averaged)
SummaryWithin assumed error, 5d lowpassed drifter velocities can be estimated as sum of geostrophic and wind-driven. Residual has structure related to EKE maxima.
AVISO altimetry generally underestimates observed EKE, presumably due to smoothing in OI. But some regions are overestimated with a gain of 1. Ageostrophic terms in surface momentum budget.
Wind-driven component is consistent with Ralph and Niiler (1999) in much of the tropics, subtropics. However, high wind areas have larger downwind motion. The spatial variations in the wind-driven part may be due to Stokes drift in wave field. This will be included explicitly in the next version of the model.
Stokes drift: 10—20 cm/s increase in time-mean, in some regions of the Southern Ocean.
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