Vol. 85, No. 2, 13 January 2004

New Global Drifter Data Set Available

Stephen E. Pazan, Ocean Prospects, Encinitas, Calif.; and Peter Niiler, Scripps Institution of Oceanography, La Jolla, Calif.

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Copyright 2004 American Geophysical Union

Since 1978, oceanographers, meteorologists, and the U.S. Navy have deployed a large number of Argos tracked drifters into all of the major ocean basins (Table 1). Since 1987, the Data Buoy Cooperation Panel (DBCP) of the World Meteorological Organization/Intergovernmental Oceanographic Commission (WMO/IOC) has coordinated the deployment of drifters via international cooperative projects in various ocean basins. In any given month since 1993, there has been an array of more than 600 drifters in the global ocean (http://www.aoml.noaa.gov/phod/dac/dacdata.html).

Most of the raw observations and processed data from ARGOS fixed drifters have been accumulated at the Meteorological and Environmental Data Service (MEDS), Canada, which is the world center for drifter data. We have processed the raw data on file at MEDS, and from other sources, and merged these with the processed data from the Drifter Data Center (DDC) at the Atlantic Oceanographic and Meteorological Laboratory (AOML) to form a single file. This complete file is now available from the Scripps Institution of Oceanography. Most of the ARGOS location data from 1978 to 2003 is available from Global Drifter Program drifters and Surface Velocity Program (SVP) drifters (6570 SVPs) (http://www.dbcp.noaa.gov/dbcp/index.html), which have a mean time before failure exceeding 1 year; the U.S. Navy AN/WSQ-6 meteorological (WSQ) drifters (1782 WSQs) [Pazan and Niiler, 2001], which have a mean time to failure of about 40 days; and First Global GARP Experiment (FGGE)/Tropical Ocean and Global Atmosphere (TOGA) drifters (840 FGGEs) for data collected from 1978 to 1994 [Large and Van Loon, 1989]. Quality control of the data sets has been reviewed in [Pazan and Niiler, 2001].

Argos location data is used to compute a velocity following the path of the drifters by time differencing the processed 6-hourly locations. The SVP is drogued to 15 m depth, and it is designed to follow the water to within ± .013 ms^-1 in 10 m/s winds [Niiler et al., 1995]. The WSQ or FGGE generally had no drogue and these and the SVPs that lose their drogues (called SVPL hereinafter) are subject to a leeway drift relative to the SVP drifters that retain their drogues. The movements of WSQs, FGGEs, and SVPLs have been corrected for leeway drift using previously developed algorithms [Pazan and Niiler, 2001]. This corrected data set adds nearly 50% to the global near-surface velocity observations. Forming ensemble averages of these observations within various size spatial-temporal scales yields maps of the near-surface circulation and its variability over 72% of the global ocean [Niiler, 2001].

Since the decorrelation time scale of Lagrangian velocity following a drifter is about 2 days [Niiler, 2001], the number of 2-day mean velocity vectors per month is displayed in the inset to Figure 1 (top). The International Ice Patrol has deployed 121 drifters of the SVP type in the western part of the Labrador Sea and North Atlantic; some of these drifters were drogued at 50 m (before 1996). NOAA's Fisheries Oceanography Coordinated Investigations (FOCI) deployed more than 200 SVP drifters with drogues centered at 40 m in the north Pacific and the Bering Sea. These data are included for calculations of eddy kinetic energy.

The FGGE/TOGA drifters were of various designs, which were built during the First Global GARP Experiment [Large and Van Loon, 1989]. The drifters in this data set were deployed in the southern hemisphere between 1978 and 1994 for FGGE, and later, for the TOGA program (Figure 1, top).

NAVY-AN/WSQ-6 were deployed in several configurations. The drifters included in our archive all had the same float assembly but a drogue of unknown survival characteristic; antenna and sensor arrangements differed [Pazan and Niiler, 2001]. NAVY-AN/WSQ-6 drifters were deployed in the northern hemisphere and their distribution is displayed in Figure 1 (top). The global spatial distribution of 2-day mean SVP drogued and undrogued drifter data is displayed at 1/2° longitude by 1/2° latitude resolution in Figure 1 (middle).

Fig. 1. (Top) The number of 2-day periods with First Global GARP Experiment (FGGE) and Navy drifter observations per 1/2° latitude by 1/2° bin is shown. The Navy data set is comprised of AN/WSQ-6 drifters. The FGGE data set is comprised of drifters deployed by a variety of scientific programs starting with the First GARP Global Experiment on 1 December 1978. The inset figure in the upper left is a global summary of the number of 2-day periods with observations per month for (black) fully operational WOCE/TOGA Lagrangian drifters and (red) WOCE/TOGA Lagrangian drifters from buoys that have lost their drogues. Undrogued buoy velocities have been corrected for leeway drift; (green) FGGE Lagrangian drifters deployed by a variety of scientific programs starting with the First GARP Global Experiment on 1 December 1978; (blue) Navy AN/WSQ-6 drifters; (cyan) NOAA FOCI program drifters in the North Pacific and Bering Sea. (Middle) The number of 2-day periods with WOCE/TOGA Lagrangian drifter observations per 1/2° latitude by 1/2° bin. This is the Surface Velocity Program (SVP) data set; it is comprised of all Lagrangian drifters deployed by the WOCE/TOGA scientific programs. (Bottom) A global map of root mean square velocity (m/s) on a 1/2° longitude by 1/2° latitude grid derived from SVP, SVPL, and FGGE drifting buoy observations is shown. A "zoomed" view of the map near the Hawaiian archipelago is displayed in the inset.

New facets of the near-surface ocean circulation are being discovered in these data, two of which are discussed below. A large number of publications have resulted from the use of these data, and most of these have been compiled in a regularly updated list at the Web site: http://www.aoml.noaa.gov/phod/dac/drifter_bibliography.html.

Lagrangian drifters are capable of resolving ocean circulation and the meso-scale, or eddy energy, on global scales. These data are also useful for comparison with data derived from a satellite altimeter, and for revealing features in the equatorial zones where satellite data cannot be used directly for the computation of time dependent motion [Niiler, 2001]. Figure 1 (bottom) is a global map of ensemble root mean square time variable velocities, derived as a deviation from the Eulerian mean of SVP, SVPL, FGGE/TOGA, and NAVY drifter observations. The means are taken for November 1978 through February 2003. These extend from 60°S to 60°N and through all oceans. Western boundary currents such as the Kuroshio, the Gulf Stream, the Agulhas, and even the Brazilian and east Australian currents are clearly defined regions of high variability of circulation. The Antarctic circumpolar current is visible from 60°W to 240°E. The regions of greatest current variability are in the equatorial zones.

The spatial resolution of the drifter data is shown as an inset in an expanded variability map near Hawaii. Evidence of the variability of currents associated with the intermittent Hawaiian Ridge Current (NHRC) can be seen in the high levels behind the island of Hawaii and extending westward from its southern tip, where the NHRC has been observed [Qiu et al., 1997; Bingham, 1998]. Ocean circulation models demonstrate that the island of Hawaii creates meso-scale eddies in its lee and from its southern tip [Qiu and Durland, 2002].

The circulation patterns of the major western boundary currents of the northern hemisphere have been studied intensively [e.g., Church et al., 2001]. More remote regions can be studied with the global drifter data. The map of the time-mean near-surface velocity in the Agulhas Current Extension reveals seven semi-permanent meanders that have not been described in any ocean data set before (Figure 2). These meanders are present whether data from 1978-1994 or 1994-2003 are graphed separately. The Agulhas Current can be seen flowing south along the east coast of South Africa, separating from the coast and retroflecting into the Agulhas Current Extension. As the Aghulas flows east and eventually merges with the Antarctic Circumpolar Current, the northward and southward meanders alternate seven times. The meanders are probably due to the tendency of a strong eastward current to conserve vorticity in the general planetary vorticity gradient [Niiler and Robinson, 1967].

Fig. 2. Binned mean surface currents on a 1/2° longitude by 1/2° latitude grid are shown; each value is the mean of SVP, SVPL, and FGGE drifting buoy observations taken together. The arrow vectors are red (blue) when the meridional component of the mean velocity is northward (southward). The vectors overlie a color map of the topographic slope in the local direction of the mean velocity. Yellow (green) shades indicate that the bottom topography is rising (falling) in the direction of the mean surface. A white shade is used for slopes less than 0.00125.

We have compiled the global drifter velocity data set into an easily accessible and usable file and wish to make these data available to the scientific community. To request free, processed, corrected Surface Velocity Program, Navy, Coast Guard, and FGGE/TOGA data, write to Peter Niiler, 9500 Gilman Dr., La Jolla, California, 92093-0213; or e-mail: pniiler@ucsd.edu. These near-surface velocity data are updated every 6 months as more observations accumulate. They differ principally from the files maintained by the Canadian MEDS or NOAA AOML by inclusion of the FGGE/TOGA, NAVY, and International Ice Patrol data sets, and they are corrected for both wind slip of the SVP data and leeway relative to the drogued data. Requests can be made in various geographical regions, time-space means, or subsets that facilitate comparison with circulation models. The real-time deployment pattern of drifters and various other data files maintained by the DDC at AOML can be obtained at http://www.aoml.noaa.gov/phod/dac/dacdata.html.

Acknowledgments

This work was supported by NASA Grant NAG 5-8351. We thank the Canadian Marine Environmental Data Service (MEDS), FNMOC, Andy Sybrandy, Sharon Lukas, and Mayra Pazos for their help.

References

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Church, J, G. Siedler, and J. Gould, (eds.), Ocean Circulation and Climate, 715 pp., Academic Press, San Diego, 2001.

Large, W. G., and H. Van Loon, Large scale, low frequency variability of the 1979 FGGE surface buoy drifts and winds over the Southern Hemisphere, J. Phys. Oceanogr., 19, 216-232, 1989.

Niiler, P., Ocean Circulation and Climate, pp. 193-204, Academic Press, San Diego, Calif., 2001.

Niiler, P. P., and A. R. Robinson, The theory of free inertial jets. II. A numerical experiment for the path of the Gulf Stream, Tellus, 19, 601-619, 1967.

Niiler, P. P., A. L. Sybrandy, K. Bi, P. Poulain, and D. Bitterman, Measurements of the water-following capability of holey sock and TRISTAR drifters, Deep Sea Res., 42, 1837-1858, 1995.

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Qiu, B., D. A. Koh, C. Lumpkin, and P. Flament, Existence and formation mechanism of the North Hawaiian Ridge Current, J. Phys. Oceanogr., 27, 431-444, 1997.

Qiu, B., and T. Durland, Interaction between an island and the ventilated thermocline: Implications for the Hawaiian Lee Countercurrent, J. Phys. Oceanogr., 32, 3408-3426, 2002.