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Ocean Surface Topography from Space
The Vertical Structure and Dynamics of Nonlinear Mesoscale Eddies


Dudley Chelton, Jr. - (Oregon State University)

  Roger Samelson
(Oregon State University, College of Oceanic & Atmospheric Sciences)


An approximate doubling of the resolution and the 18-year record length afforded by the SSH fields constructed by CLS/AVISO from pairs of simultaneously operating satellite altimeters has revolutionized observational studies of oceanic mesoscale variability. In lower-resolution SSH fields constructed previously from a single altimeter, the resolvable features were generally indistinguishable from linear Rossby waves. The SSH variability in the higher-resolution fields is so dominated by nonlinear eddies that there is almost no evidence of linear Rossby waves outside of the tropics.

Our efforts over the past four years were initially devoted to refinements of a global automated eddy identification and tracking procedure designed to isolate propagating compact structures in the SSH fields. The procedure was then implemented on the first 16 years of CLS/AVISO data and the resulting eddy dataset was posted online for the general public. A comprehensive global analysis of the kinematic properties and dynamical interpretation of the nonlinear eddies in this dataset has provided extensive new information about nonlinear eddies. Collocations with other satellite datasets (chlorophyll estimates derived from satellite measurements of ocean color, satellite-based SST fields, and scatterometer measurements of surface winds) have revealed the importance of nonlinear eddies in air-sea interaction over the eddies and in physical-biological interaction within the eddies.

Our proposed research includes carrying out periodic updates of our online eddy dataset. The eddy dataset has already been extended to 18 years and will soon be extended an additional 6 months. We also propose to apply the eddy tracking procedure to new experimental datasets that will be produced by CLS/AVISO with higher resolution in space and time than their existing online datasets. One of these new datasets consists of SSH fields at daily intervals, which we expect will improve the eddy tracking, especially at low latitudes. We believe that it may also be possible for CLS/AVISO to double the present spatial resolution of SSH for the 3-year period October 2002 through September 2003 during which four altimeters were operating nearly continuously. This will significantly expand the spectral range over which mesoscale variability can be investigated from altimetry.

We also propose to extend our recent analyses of nonlinear mesoscale eddies into two new areas of research. One of these is an investigation of the vertical structure of eddies, combining the altimeter results with statistical analysis of Argo float profiles of temperature and salinity and the output of numerical models, supplemented by idealized modeling of the depth-dependent eddy dynamics. The statistical analyses will be based on collocation of the variables of interest (e.g., temperature, salinity, density and velocity) to the interiors of the eddies identified from altimetric or model SSH fields by our automated tracking procedure. The other new area of research focuses on investigation of several aspects of the life-cycle dynamics of mesoscale eddies from a combination of altimetry, theory and numerical modeling. The topics of interest include:

  1. An assessment of whether the life cycles of mesoscale eddies (their initial growth and eventual decay) can be reliably determined from altimeter data;
  2. An investigation of the importance of self-induced Ekman pumping on eddy decay; and
  3. An investigation of eddy genesis and the internal dynamical processes that control eddy life cycles.

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