Presentation of the validation results: regional and monthly validation data using drifter data

 

In this section, we describe direct comparisons between the OSCAR velocity and the drifting buoy vectors, within sub-regions, throughout monthly periods, and over longer periods. The comparison results within each sub-area and for each monthly period are exhaustively provided to users. Here, for presentation, we only show vector maps relative to the PAC2 sub-area (central equatorial region) and to a monthly period (December 2003).

 

Method:

In these comparisons, as in the rest of the validation study, the drifting buoy data are minimally processed. The principle of this validation study is to compare OSCAR to the drifter data at the locations of the buoys.

 

For each individual buoy, the drifter data (locations and derived velocity vectors) are initially sampled every 6 hours, and are 20-day low-pass filtered along the drifter trajectory. Hence this validation study only concerns surface current variations on time-scales on the order of, or larger than, 20 days. This is consistent with the OSCAR surface current processing, which, by construction, is relative to periods larger than 10-20 days. The filtered drifter data are then sub-sampled onto the ~5 day OSCAR time base. For a set of drifter vectors defined by locations, times and velocity components, the collocated OSCAR vectors are obtained through linear interpolation at the same locations. The two collocated datasets can then be compared. This straightforward methodology only involves comparison between the absolute velocities of both datasets, and does not require any gridding procedure of the drifter data.

 

As an example of regional and monthly comparisons between OSCAR and drifter data, the Figure 2 shows the collocated drifter and OSCAR vectors during December 2003 in the PAC2 region. Such figures are available for all monthly periods between Oct. 1992 and present, and for all sub-regions in Pacific, Atlantic and Indian basins. We hereafter present the comparison analysis in this particular case (PAC2), and users on-line may apply the same reasoning to the region and time period of their choice. For monthly maps in general, there are at the most six vectors per drifting buoy. Hence, there were at least 51 drifters in this region at this time (N=311). As seen on the two maps on top of Figure 2, the OSCAR collocated vectors reproduce well the drifter velocities, which sketchily depict the large-scale pattern of the surface flow. The drifter data at this time cover several areas where surface current pathways can be seen at the frontier between the South Equatorial Current (SEC) and the North Equatorial Counter-Current (NECC). These pathways, located near 168W, 160W, 150W and 138W, are notably well reproduced by the OSCAR vectors. The match between OSCAR and drifters is also good on the equator and in the southern part. Hence, the meridional structure of the OSCAR velocity in the equatorial vicinity is quite realistic. This indicates that the OSCAR methodology performs well at reproducing fine mean and time-varying structures of the flow field.

 

The Figure 3 includes all data points for the entire period, and summarizes the results associated with this particular region. The two histograms at the bottom resemble Gaussian curves. For comparison, gaussian curves are also drawn (red curves) using the data-estimated mean and std parameters.