Using Eulerian and Lagrangian Approaches to Investigate Wind-Driven Changes in the Southern Ocean Abyssal Circulation

Abstract

This study uses a global ocean eddy-permitting climate model to explore the export of abyssal water from the Southern Ocean and its sensitivity to projected twenty-first-century poleward-intensifying Southern Ocean wind stress. The abyssal flow pathways and transport are investigated using a combination of Lagrangian and Eulerian techniques. In an Eulerian format, the equator-and poleward flows within similar abyssal density classes are increased by the wind stress changes, making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework. Lagrangian particle analyses are used to identify the major export pathways of Southern Ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around Antarctica in response to the wind forcing. Both the Lagrangian particle and Eulerian analyses identify transients as playing a key role in the abyssal export of water from the Southern Ocean. Wind-driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the Southern Ocean barotropic circulation.

Role of Mesoscale Eddies in Cross-Frontal Transport of Heat and Biogeochemical Tracers in the Southern Ocean

“…increase in the meridional pressure gradient, as reported in previous modeling studies (Hirabara et al 2012;Shakespeare and Hogg 2012;Cheon et al 2014;Spence et al 2014).…”

Lagrangian Timescales of Southern Ocean Upwelling in a Hierarchy of Model Resolutions

“…The increase in timescale at coarser resolutions or with temporal smoothing partially reflects the reduced LUTs, which are also likely related to the decrease in eddy variability (as also found in Spence et al, 2014). Tamsitt et al (2017) show that the upward motion of particles upwelling along isopycnals is enhanced in regions where the ACC interacts with bottom topography and eddy kinetic energy is enhanced.…”

“…While isopycnal diffusion does not affect RUT, it is possible that its omission in CM2-1deg-5day is partially responsible for the low LUT. In CM2.6-monthly, the temporal smoothing of the velocity field may inhibit the ability for particles to cross fronts of the ACC, as found in Spence et al (2014), potentially affecting the LUT.…”

Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds

“…First, increased Antarctic Peninsula surface air temperatures since the 1950s [ Chapman and Walsh , ; Turner et al ., ] (Figure S1a) are associated with warmer near‐surface waters in summer on the western side of the peninsula but with little observed impact below 100 m [ Meredith and King , ]. Second, a positive trend in the Southern Annular Mode, associated with strengthened and poleward‐shifted southern hemisphere midlatitude westerly winds since the 1950s [ Thompson and Solomon , ] (Figure S1b), may aid the intrusion of water onto the Antarctic continental shelf [ Chavanne et al ., ; Wåhlin et al ., ; Hellmer et al ., ], alter sea ice extent and thickness [ Bintanja et al ., ], and impact dense water formation and transport [ Spence et al ., ]. Last, increased Southern Ocean precipitation and glacial runoff can inhibit the vertical mixing of cold surface waters with the underlying warmer waters [ Bintanja et al ., ] and are linked with freshening Southern Ocean water masses [ Rintoul , ; Durack and Wijffels , ].…”