Increasing Annual Subduction and Obduction Rates in the North Pacific Revealed by an Eddy‐Resolving Ocean Reanalysis

Abstract Subduction and obduction are water mass exchanges between the ocean mixed layer (ML) and the permanent pycnocline. Knowledge of how they change in a warming climate is of great importance to anticipating how warming will affect material circulation and biogeochemistry. We investigated interannual variations of annual subduction and obduction rates ( S ann and O ann ) over the North Pacific Ocean using an eddy‐resolving ocean reanalysis. Both S ann and O ann integrated over the basin increased during the last few decades. The volumes of subducted and obducted water parcels increased in most of the potential density range, and the regions where the volumes were large shifted northward after 2004. These increases of S ann and O ann were especially large north of the Kuroshio Extension (KE), despite a decrease of the potential density of the winter ML. The failure of the trends of either S ann or O ann to increase when mesoscale spatial variations were filtered out was consistent with calculations using a gridded product based on in situ observations, implying that eddy activity was responsible for the trends. The variations of S ann and O ann corresponded closely with that of eddy kinetic energy (EKE) north of the KE. This implies that northward migration of the KE increased available potential energy, thereby elevating EKE through baroclinic conversion. Warm eddies especially enhanced subduction and obduction. These results suggest that these processes activated vertical exchanges of water masses and materials, thereby surpassing enhanced stratification due to warming of the upper ocean.