Mesoscale Eddies in Altimeter Observations of SSH

Dudley B. Chelton and Michael G. Schlax

IMPORTANT NOTE: Some issues have been identified in the Version 4 eddy dataset described below. Previous versions of our eddy datasets were based on 7-day sampling of the AVISO SSH fields. Changing to daily sampling for Version 4 resulted in occasional very large jumps in eddy locations. This problem did not affect our previous versions of the eddy datasets.

The reason for the problem has been identified and an improved version of our eddy identification and tracking procedure has been developed in collaboration with CLS/DUACS.

A replacement and update of the Version 4 eddy dataset described below has been produced by the CLS/DUACS team and is now available from AVISO. For details, see the AVISO announcement. The eddy dataset will be maintained and updated hereafter by CLS/DUACS and AVISO.

The previous version of our eddy dataset from the AVISO weekly SSH fields can be accessed HERE.

This 4th release of the trajectories of mesoscale eddies differs substantially from previous releases. Most importantly, this version is based on the DT-2014 daily "two-sat merged" sea level anomaly (MSLA) fields (formerly referred to as the REF dataset) posted online by AVISO for the 22-year period January 1993–April 2015. As described in detail by Pujol et al. (2016) , the AVISO data processing included new sensor-specific instrumental and atmospheric corrections, new inter-calibration of the various altimeters, a new tide model and a longer reference period of 20 years for mean sea level, rather than the 7-year average used previously. The objective mapping procedure use to generate the DT-2014 MSLA fields interpolated directly to a ¼° grid at daily intervals using somewhat shorter length scales of the covariance function that modestly improve the spatial resolution compared with previous MSLA fields produced by AVISO.

In addition to the improved quality of the MSLA fields, our eddy identification and tracking procedure was modified substantially. Rather than defining eddies by an outermost closed contour of SSH as in our previous three eddy datasets, each eddy was defined on the basis of connected pixels that satisfy specified criteria. The procedure is a modified version of the method presented by Williams et al. (2011). A description of our implementation of the eddy identification procedure can be found HERE.

As in our three previously released eddy datasets, this new 22-year dataset retains only those eddies with lifetimes of 4 weeks or longer. Commensurate with the daily sampling of the DT-2014 SSH fields, the trajectories in this new version of the eddy dataset are available at 1-day time steps. There is a total of 245,389 such eddies. Compared with previous datasets, the eddies in this new dataset have somewhat larger amplitudes, smaller radii and higher rotational speeds. They are thus somewhat more nonlinear and have somewhat higher eddy kinetic energy than those in our previous eddy datasets. Statistically, the eddies have longer lifetimes and propogate longer distances. These changes from the eddy characteristics in our previous datasets are attributable mostely to the changes in the AVISO processing to produce the DT-2014 MSLA dataset.

A detailed analysis of the first version of our dataset can be found in Chelton et al. (2011)., which summarizes the characteristics of the eddies with lifetimes of 16 weeks and longer. The figures in that paper made from this new 22-year version of the eddy dataset can be downloaded HERE as a PDF file.

Chelton, D. B., M. G. Schlax, and R. M Samelson, 2011: Global observations of nonlinear mesoscale eddies. Prog. Oceanogr., 91, 167—216.

Pujol, M.-I., Y. Faugère, G. Taburet, S. Dupuy, C. Pelloquin, M. Ablain, and N. Picot, 2016: DUACS DT2014: The new multi-mission altimeter dataset reprocessed over 20 years. Ocean Sci., 12, 1067—1090, doi:10.5194/os-12-1067-2016, 2016.

Williams, S., M. Petersen, P.-T. Bremer, M. Hecht, V. Pascucci, J. Ahrens, M. Hlawitschka and B. Hamann, 2011: Adaptive extraction and quantification of geophysical vortices. IEEE Transactions Visualization and Computer Graphics, 17, 2088—2095.