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

Mark L Wells,Shuangling Chen,Fei Chai,Jingyuan Xi,Huijie Xue,Rui Xin Huang

doi:10.5194/bg-18-5539-2021

Abstract

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 (100–1000 dbar). Using a novel BGC-Argo dataset covering the western North Pacific (20–50∘ N, 120–180∘ E), we identified imprints of episodic subduction using anomalies in dissolved oxygen and spicity, a water mass marker. These subduction patches were present in 4.0 % (288) of the total profiles (7120) between 2008 and 2019, situated mainly in the Kuroshio Extension region between March and August (70.6 %). Roughly 31 % and 42 % of the subduction patches were identified below the annual permanent pycnocline depth (300 m vs. 450 m) in the subpolar and subtropical regions, respectively. Around half (52 %) of these episodic events injected oxygen-enriched waters below the maximum annual permanent thermocline depth (450 dbar), with >20 % occurring deeper than 600 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 152 g 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.

Highlights

Ocean subduction is the process of transporting water from the wind-mixed surface layer into or below the permanent thermocline, resulting in the efficient injection of heat, carbon, and oxygen to the ocean interior (Fig. 1)
Subduction associated with eddy pumping is a recognized important contributor to the transfer of carbon and other materials from the surface euphotic layer to the ocean interior (McGillicuddy, 2016; Bord et al, 2019), but investigating the spatial distributions, physical dynamics, and biogeochemical consequences of these episodic small-scale processes is difficult
Subduction patches below the seasonal and permanent pycnoclines can be identified in vertical profiles by anomaly matrices of temperature, salinity, and dissolved oxygen (DO)

Summary

Introduction

Ocean subduction is the process of transporting water from the wind-mixed surface layer into or below the permanent thermocline, resulting in the efficient injection of heat, carbon, and oxygen to the ocean interior (Fig. 1). Many studies focus on the subduction of mode waters driven by large-scale circulation and the seasonal cycle of the mixed-layer dynamics (Williams, 2001; Qu et al, 2002; Qiu et al, 2007; Koch-Larrouy et al, 2010; Kawakami et al, 2015; Nie et al, 2016). Recent advances have highlighted the importance of small-scale (1–100 km) dynamical processes for vertical transport and biogeochemistry in the upper ocean, driven by mesoscale eddies and sub-mesoscale processes (Lévy et al, 2001; Xu et al, 2014; Omand et al, 2015; McGillicuddy, 2016; Llort et al, 2018; Resplandy et al, 2019). Chen et al.: Episodic subduction from BGC-Argo term subduction processes because of their episodic characteristics (Xu et al, 2014; Llort et al, 2018)

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Conclusion