Vertical transport at curved ocean fronts

Ocean fronts are boundaries that separate distinct water masses and span a broad range of spatial scales, from basin-scale (103 km) western boundary currents such as the Gulf Stream and Kuroshio, to mesoscale (102 km) eddies and submesoscale (10 km) filaments. At mesoscale and submesoscale fronts, enhanced vertical velocities play a key role in exchanging heat, carbon, and other biogeochemical tracers between the ocean interior and the surface, which is in contact with the atmosphere. Because of Earth’s rotation, flow at ocean fronts tends to be directed parallel to the front and in geostrophic balance, with the Coriolis force balancing the cross-front pressure gradient. Since geostrophic flow is horizontally nondivergent, vertical motion relies on the ageostrophic component of the flow. Current theories of frontal vertical velocity are based on frontogenesis (front sharpening) in a straight front, where the ageostrophic velocity is solely in the cross-front direction. In reality, the ocean is far from the straight-front limit: meandering jets, eddies, and filaments are ubiquitous. We develop a front-following framework for ageostrophic flow that distinguishes cross-front divergence driven by frontogenesis from along-front divergence driven by front curvature. We first test this framework in an ocean model of curved fronts and then validate it using global surface drifter observations, showing that both along-front and cross-front ageostrophic velocities can be robustly estimated from sea surface height (SSH). We next quantify the relative importance of curvature and frontogenesis in driving ageostrophic velocities using SSH from conventional nadir altimetry and wide-swath observations from the Surface Water and Ocean Topography (SWOT) mission. We find that, at mesoscales and large submesoscales, the along-front ageostrophic velocity associated with front curvature is larger than the crossfront ageostrophic velocity driven by frontogenesis, highlighting the previously neglected role of front curvature in modulating vertical transport. Direct observational evidence for curvature-induced vertical transport is provided at the edge of a 10-km elliptical cyclonic eddy in the Balearic Sea, where the observed subduction of chlorophyll-rich surface waters is consistent with theoretical predictions based on curvature variations. In addition, we characterize the time evolution of elliptical eddies in terms of propagating vortex Rossby waves. In summary, our results indicate that curvature-induced vertical transport dominates over frontogenesis-induced vertical transport at curved fronts. This framework has potential to improve estimates of vertical velocity from SSH.

Grantor dc:publisher

Massachusetts Institute of Technology and Woods Hole Oceanographic Institution

Year dc:date.issued

2026

Author dc:creator

• Wu, Weiguang

Advisor dc:contributor.advisor

• Mahadevan, Amala

• Ocean fronts
• Vertical transport
• Flow curvature

DOI

10.1575/1912/73006

OAI identifier oai:identifier

oai:darchive.mblwhoilibrary.org:1912/73006