Observations of a splitting ocean cyclone resulting in subduction of surface waters.

Earth's oceans contain large numbers of cyclonic eddies, 10 to 25 kilometers in diameter and unresolved in climate simulations. However, we lack observations of these features due to their small size and fast evolution. Here, we present in situ observations of one such cyclonic eddy with intense chlorophyll at its center as it spontaneously splits into two smaller cyclonic eddies over a few days. This splitting rapidly transports surface waters to depth, with sustained vertical velocities of 60 meters per day, primarily from the center of the eddy where carbon concentrations are largest, facilitating efficient transfer of phytoplankton carbon to depth, below the well-mixed, sunlit surface layer. We reproduce the splitting process in an idealized ocean model and find that splitting is controlled by the initial elliptical eddy shape, size, and intensity. Our observations uncover a mechanism for subduction in the upper ocean and highlight the need for quantifying its global prevalence.