Global observations of westward energy propagation:  Rossby waves or
nonlinear eddies?

Dudley B. Chelton, Oregon State University


Numerous analyses of the TOPEX/POSEIDON (T/P) altimeter data over the past decade have documented nearly ubiquitous westward propagation
with speeds close to the phase speed of nondispersive, linear Rossby  waves.  This has led to widespread interpretation of the observed
variability as baroclinic linear Rossby waves.  Other investigators  have advocated a different point of view in which the sea surface
height (SSH) variability is dominated by nonlinear eddies.  This alternative view is difficult to support observationally from T/P data
alone because of the coarse resolution of SSH fields that can be constructed from a single altimeter dataset.  These two seemingly
contradictory points of view are investigated in this presentation from SSH fields derived from the merged T/P and ERS satellite
altimeter datasets.  The higher-resolution afforded by the merged data reveals that SSH variability over much of the World Ocean consists
largely of long-lived, eddy-like structures that propagate nondispersively westward. By three different measures, the observed
variability is found to be nonlinear in the eddy-rich regions, although not strongly so. The kinematic properties of these eddies are
determined from application of an automated tracking procedure that identifies nearly 151,000 eddies with lifetimes of a month or longer
over the 10-year data record.  The observed characteristics of the eddy-like structures favor the nonlinear eddy interpretation of the
observed SSH variability.