[PAOCQ] Leah Johnson (UW)

“The Fate of Subducted Waters: Examples of Transport and Transformation Below the Mixed Layer”

Waters subducted from the ocean mixed layer transport heat, carbon, and other tracers into the ocean interior, where their subsequent fate plays a critical role in physical and biogeochemical tracer budgets. This talk presents two observational examples of how subducted waters evolve below the mixed layer, highlighting the interplay between lateral stirring, vertical transport, and particle dynamics. The first example focuses on the Nordic Seas, where warm, saline North Atlantic waters mix with cold, fresh polar waters to form North Atlantic Deep Water that feeds the lower limb of the Atlantic Meridional Overturning Circulation. Using high-resolution in situ observations combined with SWOT velocity fields, we examine how subducted Atlantic waters are stirred and advected away from the Arctic Front. Observations reveal patchy intrusions of warm, saline water transported along isopycnals by small scale eddies. Synthetic particle tracking and in situ estimates yield along-isopycnal diffusivities consistent with previous drifter-based estimates, and underscoring the role of small-scale stirring in redistributing heat and tracers in this climatically important region. The second example examines the fate of organic matter during the North Atlantic spring bloom, using observations from the NASA EXPORTS Edge Experiment. Coordinated measurements across a mesoscale eddy edge reveal a deep intrusion of surface-derived, chlorophyll-rich water, indicating strong vertical advection. While dissolved and small particulate tracers follow isopycnals, larger marine snow particles are found deeper, reflecting the combined influence of advection and gravitational sinking. By combining estimated of vertical velocity and particle sinking rates, we demonstrate how these processes act together to regulate carbon export at the eddy edge. Together, these examples highlight the importance of three-dimensional transport processes on the fate of subducted tracers.

PAOC Colloquium —

Interdisciplinary seminar series that brings together the whole PAOC (Program in Atmospheres, Oceans, and Climate) community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, as well as talks about societal impacts of climatic processes.

Contact: paoc-colloquium-comm@mit.edu