Divergence and Dispersion of Global Eddy Propagation from Satellite Altimetry

Abstract

It is well understood that isolated eddies are presumed to propagate westward intrinsically at the speed of the annual baroclinic Rossby wave. This classic description, however, is known to be frequently violated in both propagation speed and its direction in the real ocean. Here, we present a systematic analysis on the divergence of eddy propagation direction (i.e., global pattern of departure from due west) and dispersion of eddy propagation speed (i.e., zonal pattern of departure from Rossby wave phase speed). Our main findings include the following: 1) A global climatological phase map (the first of its kind to our knowledge) indicating localized direction of most likely eddy propagation has been derived from twenty-eight years (1993-2020) of satellite altimetry, leading to a leaf-like full-angle pattern in its overall divergence. 2) A meridional deflection map of eddy motion is created with prominent equatorward/poleward deflecting zones identified, revealing that it is more geographically correlated rather than polarity determined as previously thought (i.e., poleward for cyclonic eddies and equatorward for anticyclonic ones). 3) The eddy-Rossby wave relationship has a duality nature (waves riding by eddies) in five subtropical bands centered around 27°N and 26°S in the two hemispheres, outside which their relationship has a dispersive nature with dominant waves (eddies) propagating faster in the tropical (extratropical) oceans. Current, wind and topographic effects are major external forcings responsible for the observed divergence and dispersion of eddy propagations. These results are expected to make a significant contribution to eddy trajectory prediction using physically based and/or data-driven models.

How Mesoscale Eddies Masquerade as Rossby Waves in Merged Altimetric Products

“…According to previous conclusions, westward-propagating eddy velocities can be increased by westward mean flows or reversed in strong eastward mean currents (e.g., Chen et al, 2022;Klocker & Abernathey, 2014). In contrast, a surface-intensified eastward mean flow can increase the westward long Rossby wave speed, but a surface westward flow is ineffective (e.g., Colin de Verdière & Tailleux, 2005;Liu, 1999).…”

Major Migration Channels of Mesoscale Eddies in the Southern Ocean Derived From Satellite Altimetry

“…A more remarkable fact is that Chen et al. (2022) indicate that the global eddy propagation is distributed in all directions rather than a single westward or eastward. For the clustering of eddy trajectories with large variations in the spatial propagation direction, it seems necessary to consider their spatial direction.…”

“…Moreover, Peng et al (2022) found that the magnitude of change in current and wind field stress simultaneously affected the zonal propagation of eddies. Chen et al (2022) believed that the effect of wind fields on eddy motion is indirect. On the other hand, Fu (2009) pointed out that the mean propagation of the eddy generally follows the fronts (e.g., the Subtropical Front, the Sub-Antarctic Front, and the Polar Front) due to being steered by the topography feature in the Southern Ocean.…”

“…Nevertheless, in the ACC, nearly all eddies were advected eastward (e.g., Fu, 2009;Hughes et al, 1998), and the meridional propagation of AEs and CEs was deflected in opposite directions. Subsequently, by statistically analyzing 28 years of satellite altimetry data, Chen et al (2022) produced the first global climatological phase map, showing the localized direction of most likely eddy propagation. (b) The partition-and-group method, proposed by Lee et al (2007), analyzes data on moving objects, including vehicle position data, hurricane track data, and animal movement data.…”

“…Although the above methods provide valuable ideas for the study of eddy propagation, to obtain eddy channels that can exactly represent the propagation pathways of high-density eddies in a certain spatial distance and direction, the partition-and-group framework is chosen in this study. A more remarkable fact is that Chen et al (2022) indicate that the global eddy propagation is distributed in all directions rather than a single westward or eastward. For the clustering of eddy trajectories with large variations in the spatial propagation direction, it seems necessary to consider their spatial direction.…”