Episodic subduction patches in the western North Pacific identified from BGC-Argo float data

Abstract

Subduction associated with mesoscale eddies is an important but difficult-to-observe process that can efficiently export carbon and oxygen to the mesopelagic zone (100-1000g€¯dbar). Using a novel BGC-Argo dataset covering the western North Pacific (20-50g g€¯N, 120-180g g€¯E), we identified imprints of episodic subduction using anomalies in dissolved oxygen and spicity, a water mass marker. These subduction patches were present in 4.0g€¯% (288) of the total profiles (7120) between 2008 and 2019, situated mainly in the Kuroshio Extension region between March and August (70.6g€¯%). Roughly 31g€¯% and 42g€¯% of the subduction patches were identified below the annual permanent pycnocline depth (300g€¯m vs. 450g€¯m) in the subpolar and subtropical regions, respectively. Around half (52g€¯%) of these episodic events injected oxygen-enriched waters below the maximum annual permanent thermocline depth (450g€¯dbar), with >20g€¯% occurring deeper than 600g€¯dbar. Subduction patches were detected during winter and spring when mixed layers are deep. The oxygen inventory within these subductions is estimated to be on the order of 64 to 152g€¯gg€¯O2/m2. These mesoscale events would markedly increase oxygen ventilation as well as carbon removal in the region, both processes helping to support the nutritional and metabolic demands of mesopelagic organisms. Climate-driven patterns of increasing eddy kinetic energies in this region imply that the magnitude of these processes will grow in the future, meaning that these unexpectedly effective small-scale subduction processes need to be better constrained in global climate and biogeochemical models.