A drifter-derived climatology of global near-surface currents

Last revised: 19 October 2011. Most recent data included: 1 January 2011.

On this page, you will find a climatology of near-surface currents and SST for the world, at one degree resolution, derived from satellite-tracked surface drifting buoy observations. Please cite as reference:

Lumpkin, R. and Z. Garraffo, 2005: Evaluating the Decomposition of Tropical Atlantic Drifter Observations. J. Atmos. Oceanic Techn. I 22, 1403-1415.

For an analysis of the monthly climatology in the Tropical Atlantic Ocean, see:

Lumpkin, R. and S. L. Garzoli, 2005: Near-surface Circulation in the Tropical Atlantic Ocean. Deep-Sea Res. I 52(3),495-518, 10.1016/j.dsr.2004.09.001.

For more information about the drifters, go to the Global Drifter Program information page.

The climatology is available in NetCDF, Matlab and ASCII formats. E-mail me if you would like to get this data in a different format.

Two versions of the climatology are available. The first version ( NetCDF/ Matlab binary format/ ASCII) contains annual mean values of the near-surface currents and subskin sea surface temperature. The Matlab file contains the following variables:

Lon (1x361): longitude (degrees), negative=West.
Lat (1x159): latitude (degrees), 73S to 85N.
V (141x361): northward speed (m/s) vs. Lat/Lon.
U (141x361): eastward speed (m/s).
SST (141x361): sea surface temperature (degrees C).
V_g (141x361): Ekman-removed (see below) northward speed.
U_g (141x361): Ekman-removed eastward speed.
eV, eU, etc: standard error bars for V, U, etc.
N (141x361): Number of drifter-days per square degree.

The ASCII version contains a subset of these data. The colums of the ASCII dataset are: Lat, Lon, U, V, SST, eU, eV, eSST.

The second version ( NetCDF/ Matlab binary format/ ASCII) contains monthly mean values of these fields. The variable names of the Matlab file are the same as above. The third index of matrix U, V, etc. corresponds to the month: 1=January, 2=February, etc. In the ASCII dataset, the first column is the month; the rest of the columns are the same as in the annual climatology file described above.

Description: satellite-tracked SVP drifting buoys (Sybrandy and Niiler, 1991; Niiler, 2001) provide observations of near-surface circulation at unprecedented resolution. In September 2005, the Global Drifter Array became the first fully realized component of the Global Ocean Observing System when it reached an array size of 1250 drifters. A drifter is composed of a surface float which includes a transmittor to relay data, a thermometer which reads temperature a few centimeters below the air/sea interface, and a submergence sensor used to detect when/if the drogue is lost. The surface float is tethered to a subsurface float which minimizes rectification of surface wave motion (Niiler et al., 1987; Niiler et al., 1995). This in turn is tethered to a holey sock drogue, centered at 15 m depth. The drifter follows the flow integrated over the drogue depth, although some slip with respect to this motion is associated with direct wind forcing (Niiler and Paduan, 1995). This slip is greatly enhanced in drifters which have lost their drogues (Pazan and Niiler, 2000). Drifter velocities are derived from finite differencing their raw position fixes. These velocities, and the concurrent SST measurements, are archived at AOML's Drifting Buoy Data Assembly Center where the data are quality controlled and interpolated to 1/4-day intervals (Hansen and Herman, 1989; Hansen and Poulain, 1996).

In this study, daily winds from the NCEP/NCAR reanalysis were interpolated onto the drifter positions and used to estimate and remove the slip (Niiler and Paduan, 1995; Pazan and Niiler, 2000). Wind stress and the local Coriolis parameter were used to estimate the Ekman component (Ralph and Niiler, 1999), which was subtracted to generate a separate set of Ekman-removed velocities. All velocities and SSTs were lowpassed at five days to remove high frequency variability (diurnal, tidal, inertial).

Drifters sample regions of the ocean inhomogeneously, which can cause aliased time-mean values if the presence of strong seasonal variations are neglected. To address this, Lumpkin (2003) developed a methodology to simultaneously decompose the drifter observations into time-mean, seasonal and eddy components. Lumpkin showed that this methodology produces significantly different results than standard bin averaging.

This methodology was further developed and evaluated using SST observations and products, and simulated drifters in the MICOM model (Lumpkin and Garraffo, 2005). The method produces significantly improved estimates of the mean currents and SST, and simultaneously provides the annual and semiannual amplitudes and phases at a nominal resolution of one degree squared.

When this methodology is applied to the modern data set of tropical Atlantic drifter observations, many features of the near-surface circulation become apparent which were not resolved by older ship-drift-based climatologies or by SEQUAL/FOCAL drifter trajectories (Lumpkin and Garzoli, 2005).

To derive the fields contained in the climatology provided here, the time-mean, annual and semiannual components of total velocity and of SST were combined and integrated to yield monthly averages. Results were smoothed via Optimum Interpolation (c.f., Cuny et al., 2002), assuming a Gaussian autocorrelation function with an isotropic e-folding scale of 150 km. Some regions do not contain values (placeholder "NaN"). With future deployments focusing upon these regions, and the ever-growing array soon able to achieve comparable resolution of interannual variations, future analyses efforts promise exciting new results in this region.

Credits: This climatology was developed by Rick Lumpkin (NOAA/AOML) in collaboration with Silvia Garzoli and Mayra Pazos (NOAA/AOML), Jessica Redman (CIMAS), and Zulema Garraffo (RSMAS, Univ. Miami). Code to convert Matlab files into NetCDF format was provided by Derrick Snowden (NOAA/AOML).

References

Cuny, J., P. B. Rhines, P. P. Niiler and S. Bacon, 2002: Labrador Sea boundary currents and the fate of the Irminger Sea Water. J. Phys. Oceanogr. 32, 627-647.

Hansen, D. and A. Herman, 1989: Temporal sampling requirements for surface drifting buoys in the tropical Pacific. J. Atmos. Oceanic Technol. 6, 599-607.

Hansen, D. and P.-M. Poulain, 1996: Quality control and interpolations of WOCE-TOGA drifter data. J. Atmos. Oceanic Technol. 13, 900-909.

Lumpkin, R., 2003: Decomposition of surface drifter observations in the Atlantic Ocean. Geophys. Res. Letters 30(14), 1753, 10.1029/2003GL017519.

Lumpkin, R. and S. L. Garzoli, 2005: Near-surface Circulation in the Tropical Atlantic Ocean. Deep-Sea Res.I 52 (3), 495-518, 10.1016/j.dsr.2004.09.001.

Lumpkin, R. and Z. Garraffo, 2005: Evaluating the Decomposition of Tropical Atlantic Drifter Observations. J. Atm. Oceanic Techn. 22, 1403-1415.

Niiler, P. P., 2001: The world ocean surface circulation. In Ocean Circulation and Climate, G. Siedler, J. Church and J. Gould, eds., Academic Press, Volume 77 of International Geophysics Series, 193-204.

Niiler, P. P., R. Davis and H. White, 1987: Water-following characteristics of a mixed-layer drifter. Deep-Sea Res. 34, 1867-1882.

Niiler, P. P. and J. D. Paduan, 1995: Wind-driven motions in the northeast Pacific as measured by Lagrangian drifters. J. Phys. Oceanogr. 25, 2819-2830.

Niiler, P. P., A. Sybrandy, K. Bi, P. Poulain and D. Bitterman, 1995: Measurements of the water-following capability of holey-sock and TRISTAR drifters. Deep-Sea Res. 42, 1951-1964.

Pazan, S. E. and P. P. Niiler, 2000: Recovery of near-surface velocity from undrogued drifters. J. Atmos. Oceanic Technol. 18, 476-489.

Ralph, E. A. and P. P. Niiler, 1999: Wind-driven currents in the Tropical Pacific. J. Phys. Oceanogr. 29, 2121-2129.

Sybrandy, A. L. and P. P. Niiler, 1991: WOCE/TOGA Lagrangian drifter construction manual. WOCE Rep. 63, SOI Ref. 91/6, 58pp, Scripps Inst. of Oceanogr., La Jolla, Calif.