Author: goni

Evaluation of the Ocean Observing System
for Ocean Surface Currents
Gustavo Goni(1), Rick Lumpkin(1) , and Pedro DiNezio(2)
(1) NOAA/AOML, Miami, Florida, USA.
(2) University of Miami/CIMAS, Florida, USA

The status of the observing system for surface currents obtained from quality-controlled, drogued Lagangrian drifter observations is derived. Sea height anomaly data are used to match with those from the drifters to evaluate the correlation between along-track sea height anomaly gradients and across-track drifter- derived geostrophic velocity anomalies. Global fields of correlations are presented and differences between estimates from both observations are evaluated. High correlations indicate where altimetry observations can be calibrated by the in-situ measurements to provide a good proxy for surface currents. On the other hand, low correlations may be indicate where errors in the wind field or in the Ekman model are problematic, where ageostrophic ocean dynamics are contributing significantly to the surface momentum budget, where the signal-to-noise ratio is low, or where there are depth-compensating effects in the upper layer causing the sea height to have low variability.

DATA
The altimetry data we use are the alongtrack delayed-time filtered AVISO data, with one data point every approximately 19 km.
The drifter data we use are low-passed filtered to 5 days. These drifter velocities then have the Ekman component removed using the Ralph and Niiler parameterization method (J. Phys. Oceanogr., 29, 2121-2129, 1999). The number of matches between altimeter alongtrack observations and surface drifters varies regionally and with time (left), depending on the number of active altimeters and drifters providing observations.

(far left) Number of matches between drifter and altimetry observations. (near left) Number of matches between altimetry observations and drifters per year.

ALTIMETRY ESTIMATES VS DRIFTER OBSERVATIONS
A match between altimetry and drifter observation is obtained when the altimeter groundtrack crosses the drifter trajectory within a radius of 30km allowing a maximum time lag of one day. The drifter velocities were projected in the direction perpendicular to the groundtrack to obtain the acrosstrack velocity. These matches were done between the sea height anomaly gradients and four drifter-derived variables: a) drifter acrosstrack geostrophic velocity anomaly, b) drifter acrosstrack geostrophic velocity, c) drifter acrosstrack velocity anomaly, and d) drifter acrosstrack velocity. Geostrophic velocity anomalies were computed from the sea height anomaly gradients, which are used to do the correlation analysis.
The matching drifter and altimetry observations were then grouped into squares of 1x1 degrees for the North Atlantic or in 2x2 degrees squares for the global ocean. Once the matching observations were grouped, a linear regression was done with the outliers removed. An outlier was defined using the Cook distance estimator, which quantifies the relative influence of each observation on the correlation coefficient. Correlations were only computed in squares where there were at least 8 matches. In the North Atlantic there are areas where few matches were found, such as the tropical Atlantic, the Gulf of Mexico and the south of the Gulf Stream axis. In general, the correlation coefficients are positive for most squares and for all the drifter velocity component (right).

(top) location of the matches between drifters (blue) and altimetry data (red) superimposed to the trajectories of drifters (gray lines).

(left) Correlation coefficient between across ascending track geostrophic velocity anomalies estimated from altimetry and drifters in 2x2 degree squares. Squares with less than ten coincident observations are shown in gray. No matches are indicated by white spaces.

(right) Example of regression plots for two 1x1 degree squares in the North Atlantic for the four altimetry-drifter velocity components.

(desc)

(asc)

CORRELATION GAIN
The correlation gain, the slope of the regression line, provides a means to estimate how the alongtrack altimetry observations compare with the drifter estimates. These gains have values mostly lower than one, indicating that for the data sets used here, the altimetry estimates overestimate the drifter estimates of geostrophic current anomalies. The ratio between the ascending and descending gains. Values close or equal to one indicate that the regression parameters are isotropic.

(top left) correlation gain for the correlations using the descending groundtracks in the North Atlantic Ocean. (top center) same for ascending groundtracks. (top right) ratio between the previous two fields.

COMPLEMENTING DRIFTER OBSERVATIONS
Areas with high correlation coefficient and with no drifter observations can be used to estimate the geostrophic velocities using altimetry data. There are regions, such as the tropical Atlantic where more drifter observations are still needed to evaluate the if altimetry could be used to produce estimates of geostrophic currents that match those estimated with drifters.

SURFACE CURRENTS PERFORMANCE REPORTS
For each quarter starting on AMJ 2005, NOAA Office of Climate Observation evaluates how well the Global Drifter Array is satisfying the GOOS/GCOS (1999) requirement for surface current measurements.
(www.aoml.noaa.gov/phod/soto/gsc/reports/).
Results obtained from this study will be used to complement the drifter observations, by using with altimetry estimates in regions where there are no drifter observations and correlation coefficients are high.

(top) Quarterly report on the state of the Global Ocean Observing System (GOOS) for near-surface currents.

(top) Drifter trajectories during a 10 day period (March 10-20, 2006) in the North Atlantic superimposed to boxes where the correlation coefficients between drifter and altimetry estimates of geostrophic velocity anomalies are larger than 0.5.

Acknowledgements: This work is supported by the NOAA Office of Climate Observation. Altimeter data is from AVISO.