Transformation of Deep Water Masses Along Lagrangian Upwelling Pathways in the Southern Ocean

Abstract

Upwelling of northern deep waters in the Southern Ocean is fundamentally important for the closure of the global meridional overturning circulation and delivers carbon and nutrient‐rich deep waters to the sea surface. We quantify water mass transformation along upwelling pathways originating in the Atlantic, Indian, and Pacific and ending at the surface of the Southern Ocean using Lagrangian trajectories in an eddy‐permitting ocean state estimate. Recent related work shows that upwelling in the interior below about 400 m depth is localized at hot spots associated with major topographic features in the path of the Antarctic Circumpolar Current, while upwelling through the surface layer is more broadly distributed. In the ocean interior upwelling is largely isopycnal; Atlantic and to a lesser extent Indian Deep Waters cool and freshen while Pacific deep waters are more stable, leading to a homogenization of water mass properties. As upwelling water approaches the mixed layer, there is net strong transformation toward lighter densities due to mixing of freshwater, but there is a divergence in the density distribution as Upper Circumpolar Deep Water tends become lighter and dense Lower Circumpolar Deep Water tends to become denser. The spatial distribution of transformation shows more rapid transformation at eddy hot spots associated with major topography where density gradients are enhanced; however, the majority of cumulative density change along trajectories is achieved by background mixing. We compare the Lagrangian analysis to diagnosed Eulerian water mass transformation to attribute the mechanisms leading to the observed transformation.

On the Superposition of Mean Advective and Eddy-Induced Transports in Global Ocean Heat and Salt Budgets

“…In general, the net air-sea heat flux (Fig. 4a) warms the equatorial ocean, low-latitude eastern boundary regions, and across the zonally nonuniform 408-558S band in the Southern Ocean (Tamsitt et al 2016), whereas it cools the ocean at midlatitudes in western boundary currents (WBCs) and at high latitudes (poleward of 508). Regions of maximum deep mixed layers (Fig.…”

“…In general, EIT acts against ADV, with a similar spatial distribution but opposite sign. In addition to frontal regions, ADV and EIT tend to be locally intensified above major topographic features such as the Drake Passage, the Kerguelen Plateau, and the Pacific-Antarctic Ridge, as well as in highly energetic regions such as the Brazil-Malvinas Confluence and the Agulhas Return Current (Tamsitt et al 2018).…”

A Deep Eastern Boundary Current Carrying Indian Deep Water South of Australia

“…Observational studies indicate narrow boundary pathways along the western and eastern boundaries of the Southern Hemisphere ocean basins and along major topographic ridges that are responsible for carrying the vast majority of upwelling deep waters from 30°S to the Antarctic Circumpolar Current (ACC; e.g., Faure & Speer, ; Hufford et al, ; McCartney & Donohue, ; Shaffer et al, ; Schulze Chretien & Speer, ; Well et al, ). The importance of these boundary pathways for Southern Ocean upwelling was more recently confirmed by Tamsitt et al () using Lagrangian experiments in three eddying ocean models. The western boundary pathways are, for the most part, well characterized deep western boundary currents (along the eastern coast of South America in the Atlantic, off the southern tip of Africa in the Indian, and east of New Zealand in the Pacific).…”

“…Lagrangian particle tracking experiments were conducted with SOSE iteration 100 daily averaged velocity output using Octopus (github.com/jinbow/Octopus) as described in Tamsitt et al () and Tamsitt et al (). Particles were released at 30°S across the Indian Ocean between 90°E and 115°E in each model grid cell between 1,000‐ and 3,500‐m depths (black lines in Figure a).…”

Climate Recorded in Seawater: A Workshop on Water-Mass Transformation Analysis for Ocean and Climate Studies

“…A powerful tool to reveal geographical hotspots of WMT are Lagrangian particle trajectories in combination with WMT diagnostics (Tamsitt et al 2018). However, attendees experimenting with this approach have highlighted issues with tracer budgets along Lagrangian trajectories: for example, the temperature difference between the initial and final locations of a Lagrangian particle can differ substantially from the thermal forcing and mixing integrated along the particle's trajectory.…”