Revisiting Interior Water Mass Responses to Surface Forcing Changes and the Subsequent Effects on Overturning in the Southern Ocean

Abstract

Two coupled climate models, differing primarily in horizontal resolution and treatment of mesoscale eddies, were used to assess the impact of perturbations in wind stress and Antarctic ice sheet (AIS) melting on the Southern Ocean meridional overturning circulation (SO MOC), which plays an important role in global climate regulation. The largest impact is found in the SO MOC lower limb, associated with the formation of Antarctic Bottom Water (AABW), which in both models is enhanced by wind and weakened by AIS meltwater perturbations. Even though both models under the AIS melting perturbation show similar AABW transport reductions of 4–5 Sv (50%–60%), the volume deflation of AABW south of 30°S is four times greater in the higher resolution simulation (−20 vs. −5 Sv). Water mass transformation (WMT) analysis reveals that surface‐forced dense water formation on the Antarctic shelf is absent in the higher resolution and reduced by half in the lower resolution model in response to the increased AIS melting. However, the decline of the AABW volume (and its inter‐model difference) far exceeds the surface‐forced WMT changes alone, which indicates that the divergent model responses arise from interactions between changes in surface forcing and interior mixing processes. This model divergence demonstrates an important source of uncertainty in climate modeling, and indicates that accurate shelf processes together with scenarios accounting for AIS melting are necessary for robust projections of the deep ocean's response to anthropogenic forcing.

The Impact of Orbital Precession on Air‐Sea CO2 Exchange in the Southern Ocean

“…Indeed, results from our model indicate that the eddy-induced meridional overturning circulation strengthens in response to SWW changes under high precession (counterclockwise anomalies in Figure S7b in Supporting Information S1), suggesting that our coarse resolution ocean model component is capable of capturing changes in unresolved eddy advection. However, there are studies that suggest that this response is model dependent and would likely alter the circulation response seen here (Bishop et al, 2016;Böning et al, 2008;Meredith et al, 2012;Tesdal et al, 2023). Future work should explore the responses identified here using a higher resolution configuration of capable of resolving these processes.…”

Global Consequences of Regional Connectivity Along the Antarctic Margin

“…Understanding evolving dynamics at the Antarctic margin is complicated by the fact that the ice shelves mutually affect the ocean beneath them, with observations indicating meltwater itself modifies the CDW transported across and along the shelf (Wählin et al., 2021). Recent modeling studies suggest AIS meltwater alters vertical stratification and ocean‐atmosphere‐ice interactions (Bronselaer et al., 2018), weakens DSW production (Moorman et al., 2020; Tesdal et al., 2023), and may accelerate along‐slope and along‐shelf currents (Beadling et al., 2022; Moorman et al., 2020). This growing body of literature has presented several plausible positive (Bronselaer et al., 2018) and negative feedback (Beadling et al., 2022; Moorman et al., 2020) mechanisms that may be acting or will act to control water properties and AIS mass loss into the future.…”

“…that these processes are sensitive to freshening from the AIS (Beadling et al, 2022;Moorman et al, 2020;Tesdal et al, 2023).…”

“…Shelf residence times of CDW have recently been estimated and mapped using Lagrangian particle trajectories (Tamsitt et al, 2021), highlighting the importance in considering residence time in conjunction with onshore heat transport when evaluating potential for heat delivery to ice shelves. Several recent studies have highlighted strong connections between the ASC, DSW formation, and subsequent Antarctic Bottom Water production (Moorman et al, 2020;Tesdal et al, 2023) and (Thompson et al, 2018). The Antarctic Coastal Current is found on the shallow continental shelf with its westward pathway generally following the base of the ice shelves or the Antarctic coastline (Heywood et al, 2004;Schubert et al, 2021).…”

Reduced Deep Convection and Bottom Water Formation Due To Antarctic Meltwater in a Multi‐Model Ensemble

“…Additional surface freshening enhances the stratification by making surface waters less dense and thus the water column less prone to deep convection. Idealized model experiments also show that Antarctic meltwater forcing reduces convective depth (Fogwill et al., 2015) and suppresses the production of AABW (Lago & England, 2019; Li et al., 2023; Tesdal et al., 2023).…”