Tropical Surface Current Velocities


Description:
These data are estimates of the horizontal near-surface currents of the
Tropical Pacific ocean, from October 1992 to current.
The near-surface velocity is directly derived from sea surface height (ssh),
wind velocity (W) and sea surface temperature (sst). It is the sum of  a
geostrophic term (Ug), a wind-diffusion term (UWh) and a buoyancy-gradient
term (UBh). The total velocity U=Ug+UWh+UBh is the vertical average over a
surface layer of thickness h=30m, and approximates the motion of a typical
15m drogue drifter (Bonjean and Lagerloef [2002]; 
http://www.esr.org/documents/bonjean/bl2002/bl2002.pdf).


The data used to calculate the velocity are:

- TOPEX/POSEIDON ssh anomalies (Lagerloef et al. [1999])

- wind velocity from SSM/I (Atlas et al. [1996]), and from QScat
(Pegion et al. [2000]; 
http://www.coaps.fsu.edu/scatterometry/Qscat/gcv_glob_L2B_1x1.html ).

SSM/I data were used from October 1992 to July 1999, QScat data from
August 1999 to November 2001

- sst (Reynolds and Smith [1994];
http://ingrid.ldeo.columbia.edu/SOURCES/.IGOSS/ )

- dynamic height (dh) derived from WOA (Levitus et al. [1994]). Dh was added
to the TOPEX/POSEIDON ssh anomalies in order to estimate the total
geostrophic currents.


Remarks and recommendations:
These near-surface currents are estimated through a simplified diagnostic
model of  the surface circulation. Notably, local acceleration and
non-linearities are not represented. The present velocity field is therefore
best used for description of  large scale and low frequency variations of
surface flow (T>=1 month, L>=5-10degree longitude). Until now, validation of
this velocity field has  largely been focused on these scales.
However we chose here to provide the scientific community  with an
unfiltered velocity field, on a 1deg X 1deg grid with a 5day resolution.
Therefore, smoothing may have been induced only by the processing of
the source data, that is ssh, W and sst (see corresponding references); also
some smoothing inherent to the method itself  was caused by calculation of
spatial gradient (for Ug and UBh only). We encourage researchers to
make comparisons between this velocity field and in-situ observations on
meso to short scales. As this surface current estimation is a work in
progress, we are interested in any result involving the present velocity
field, and we remain available for any help and discussion.


References:
Atlas R., R. Hoffman, S. Bloom, J. Jusem, J. Ardizzone, 1996, "A multi-year
global surface wind velocity data set using SSM/I wind observations", Bull.
Am. Meteorol. Soc., 77, 869-882

Bonjean F. and G.S.E. Lagerloef, 2002, "Diagnostic model and analysis of the
surface currents in the tropical Pacific ocean", J. Phys. Oceanogr., 
(in press).

Lagerloef G.S.E., G.T. Mitchum, R.B. Lukas and P.P. Niiler, 1999, "Tropical
Pacific near-surface currents estimated from altimeter, wind, and drifter
data", J. Geophys. Res., 104, C10, 23,313-23,326

Levitus S., T. Boyer, R. Burgett, 1994, "World Ocean Atlas 1994", vol 3-4,
Temperature, Salinity, NOAA Atlas NESDIS 3-4, NOAA, Silver Spring, Md.

P. J. Pegion, M. A. Bourassa, D. M. Legler, and J. J. O'Brien, 2000:
"Objectively-derived daily winds from satellite scatterometer data", Mon.
Wea. Rev., 128, 3150-3168

R. W. Reynolds and T. M. Smith, 1994, "Improved global sea surface
temperature analyses.," J. Climate, 7, 929--948.