For each quarter starting on Q2, 2005 (April-June), the Global Ocean Observing System (GOOS) is evaluated for near-surface current observations. The requirement for surface currents is 2 cm/s accuracy, one sample per month at a resolution of 600 km (GOOS/GCOS, 1999). We interpret the accuracy requirement as demanding observations from either a moored current meter (typically an acoustic-based point measurement within the upper 20m) or from a drogued Surface Velocity Program-type drifter. Drifter observations are quality controlled and distributed by the drifting buoy Data Assembly Center at NOAA/AOML; current meter observations are quality controlled and distributed by the TAO Project Office at NOAA/PMEL. The observing network of drifting and moored buoys providing at least one day of surface current observations is shown in the upper-right panel of the quarterly report.
We divide the ocean into 5.39° (600 km at the equator) boxes, and for each month we count the number of observations within each box. A single observation meets the GOOS/GCOS requirement. If a box was sampled at least once in all three months of the quarter, it is colored blue (100%) in the lower-left panel. If it was sampled for two of the three months, it is yellow (66%); one of the three months, red (33%); none of the three months, white (0%). Grey boxes are ice-covered, average less than 20m depth, or are mostly occupied by land. A time series of the overall success rate is shown in the lower-right panel: if 50% of the non-grey (or land) boxes were sampled at least once by a drifting or moored buoy in a month, the curve will have the value 50% on that month.
In the future, a plot will be added that quantifies the global bias in currents derived from satellite altimetry and winds, given the distribution of the observing system to correct for spatially-varying biases. These biases may be due to errors in the Ekman model or winds, other large ageostrophic terms such as centrifugal forces in strong rings, or surface/subsurface compensating effects that eliminate the sea level anomaly signature of near-surface currents. In the future, operational satellite-based surface current fields will be produced globally from satellite observations, calibrated using the in-situ observations from the Global Ocean Observing System as is done with SST today.
Special thanks are due to Paul Freitag, Peter Niiler, Meghan Cronin, and Todd Pearce for their contributions. Work to quantify the global bias in satellite-derived currents is underway between Rick Lumpkin and Gustavo Goni, both of NOAA/AOML. Funding is provided by NOAA's Office of Climate Observations.