Coherent Pathways for Subduction From the Surface Mixed Layer at Ocean Fronts. Issue 5 (30th April 2021)

• Record Type: Journal Article
• Authors: Freilich, Mara; Mahadevan, Amala
• Is Part Of: Journal of geophysical research. Volume 126:Issue 5(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-04-30
• Subjects: frontal dynamics -- mixed layer -- process study -- submesoscale -- vertical velocity | Lagrangian; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC017042
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: In frontal zones, water masses that are tens of kilometers in extent with origins in the mixed layer can be identified in the pycnocline for days to months. Here, we explore the pathways and mechanisms of subduction, the process by which water from the surface mixed layer makes its way into the pycnocline, using a submesoscale‐resolving numerical model of a mesoscale front. By identifying Lagrangian trajectories of water parcels that exit the mixed layer, we study the evolution of dynamical properties from a statistical standpoint. Velocity‐ and buoyancy‐gradients increase as water parcels experience both mesoscale (geostrophic) and submesoscale (ageostrophic) frontogenesis and subduct beneath the mixed layer into the stratified pycnocline along isopycnals that outcrop in the mixed layer. Subduction is transient and occurs in coherent regions along the front, the spatial and temporal scales of which influence the scales of the subducted water masses in the pycnocline. An examination of specific subduction events reveals a range of submesoscale features that support subduction. Contrary to the forced submesoscale processes that sequester low potential vorticity (PV) anomalies in the interior, we find that PV can be elevated in subducting water masses. The rate of subduction is of similar magnitude to previous studies (∼100 m/year), but the Lagrangian evolution of properties on water parcels and pathways that are unraveled in this study emphasize the role of submesoscale dynamics coupled with mesoscale frontogenesis. Plain Language Summary: Exchange of properties such as heat, carbon, and nutrients between the ocean surface and the interior has implications for ocean circulation and ecosystems. But the sharp increase in density below the well‐mixed surface layer inhibits the transport of these waters into the region of increasing density that lies beneath, except along sloping surfaces of constant density, or fronts. Here, using a three‐dimensional computational model of an ocean front, we track and study the properties of water as it exits the surface mixed layer and travels deeper in a process we call subduction. The subduction occurs in specific regions, at scales ≲10 km, where the horizontal density variation is strongest. This study unravels important pathways for exchange between the ocean surface and interior that are not included in global models and result in 100 m of the mixed layer being subducted over the course of a year. Key Points: The dynamical pathways of subduction from the surface mixed layer to the pycnocline at a front are characterized using Lagrangian analysis The pathways of subduction are influenced by mixed layer mesoscale (geostrophic) and submesoscale (ageostrophic) frontogenetic processes Due to the influence of submesoscale along‐front variability, subduction is coherent on submesoscale, ≲10 km, length scales