From surface to depth: SWOT Altimetry and Glider Observations of a Cuddy in the Northeast Pacific

Abstract

California Undercurrent Eddies (Cuddies) transport heat, salt, and biogeochemical properties from coastal regions to the open Northeast Pacific, but their three-dimensional structure, lateral exchange mechanisms and complete life-cycle are still not fully understood. This study presents the first detection of a Cuddy using the Surface Water and Ocean Topography (SWOT) satellite mission, demonstrating enhanced capabilities to resolve subsurface-intensified eddies. SWOT revealed a more intense eddy with sharper velocity gradients and submesoscale features, such as spiral filaments and jets, in the eddy periphery that are smoothed over in conventional altimetry. Surface chlorophyll filaments tracked over four days are consistent with SWOT-derived velocities and show differential rotation between the core and periphery. Glider observations and satellite data reveal a lens-shaped structure (100–500 m depth, core at 150 m) containing equatorial waters. The eddy exhibits a rotating core surrounded by partially detached spiral arms whose surface expression is consistent with vertically coherent peripheral structures that extend further than $500\ \mathrm{m}$. Multi-satellite tracking documented an eight-month evolution from coastal separation to possible merger, capturing both growth and decay phases. Throughout its lifetime, the eddy played a double role in the regional chlorophyll field by exporting high-chlorophyll coastal waters offshore and maintaining elevated chlorophyll concentrations within its interior.

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