Both the Greenland and Antarctic ice sheets have been melting at an accelerating rate over recent decades. Meltwater from Greenland might be expected to initiate a climate response that is distinct, and perhaps different from, that associated with Antarctic meltwater. Which one might elicit a greater climate response, and what mechanisms are involved? To explore these questions, we apply climate response functions (CRFs) to guide a series of meltwater-perturbation experiments using a fully coupled climate model. In both hemispheres, meltwater drives atmospheric cooling, sea ice expansion, and strengthened Hadley and Ferrel cells. Greenland meltwater induces a slowdown of the Atlantic meridional overturning circulation (AMOC) and a cooling of the subsurface ocean in the northern high latitudes. Antarctic meltwater, instead, induces a slowdown of the Antarctic Bottom Water formation and a warming of the subsurface ocean around Antarctica. For melt rates up to 2000 Gt yr⁻¹, the climate response is rather linear. However, as melt rates increase to 5000 Gt yr⁻¹, the climate response becomes nonlinear. Due to a collapsed AMOC, the climate response is superlinear at high Greenland melt rates. Instead, the climate response is sublinear at high Antarctic melt rates, due to the halting of the northward expansion of Antarctic sea ice by warm surface waters. Finally, in the linear limit, we use CRFs and linear convolution theory to make projections of important climate parameters in response to meltwater scenarios, which suggest that Antarctic meltwater will become a major driver of climate change, dominating that of Greenland meltwater, as the current century proceeds. Significance Statement: Melting of the Greenland and Antarctic ice sheets is one of the most uncertain potential contributors to future climate change. In this study, we address the comparative role of Greenland and Antarctic meltwater in the climate system and explore the differing mechanisms at work in each hemisphere. We find that the climate response is linear for low melt rates but becomes nonlinear for high melt rates. As the century proceeds, we speculate that Antarctic meltwater will increasingly dominate that of Greenland meltwater, leading to atmospheric cooling, Antarctic sea ice expansion, and contraction and warming of the Antarctic Bottom Water. Greenland meltwater will, instead, affect smaller changes in the Northern Hemisphere.

The formation of a narrow band of the deep winter mixed layer (hereinafter “mixed layer wedge”) in the Indo–western Pacific Southern Ocean is examined using an eddy-resolving Parallel Ocean Program (POP) model simulation. The mixed layer wedge starts to deepen in June, centered at 47.5°S, with a meridional scale of only ~2° latitude. Its center is located ~1° north of the model’s Subantarctic Front (SAF). The Argo-based observed mixed layer is similarly narrow and occurs adjacent to the observed SAF. In the small sector of 130°–142°E, where the SAF is persistent and the mixed layer is deepest, the formation of the narrow mixed layer wedge coincides with destratification underneath the mixed layer. This destratification can be attributed primarily to the downwelling branch of a jet-scale overturning circulation (JSOC). The JSOC, which was reported in an earlier study by the authors, is driven by eddy momentum flux convergence and is therefore thermally indirect: its descending branch occurs on the warmer equatorward flank of the SAF, promoting destratification during the warm season. The model-generated net air–sea heat flux reveals a similar wedge-like feature, indicating that the flux contributes to the mixed layer depth wedge, but again this feature is preconditioned by the JSOC. Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of the region where the mixed layer initially deepens. These findings suggest that the eddy-driven JSOC associated with the SAF plays an important role in initiating the narrow, deep mixed layer wedge that forms north of the SAF.

The Southern Ocean plays a fundamental role in the global energy and carbon budgets via the uptake of large quantities of heat and carbon in subantarctic mode water (SAMW). Even though SAMW variability is closely tied to variations in mixed layer depth (MLD) and properties, little is known about what controls interannual to multidecadal MLD variations. Here we use atmospheric observations combined with a global 1/10° ocean‐sea ice model to examine the mechanisms controlling Southern Ocean mixed layer variability. We find that tropical teleconnections from the Indo‐Pacific to the Amundsen Sea Low via El Niño–Southern Oscillation and the Indian Ocean Dipole control mixed layer variability along the path of a Rossby wave train across the South Pacific during early winter. This suggests recent multidecadal variations in mixed layer properties are likely to have been significantly impacted by transitions in the Interdecadal Pacific Oscillation.

The relationship between Antarctic Circumpolar Current jets and eddy fluxes in the Indo–western Pacific Southern Ocean (90°–145°E) is investigated using an eddy-resolving model. In this region, transient eddy momentum flux convergence occurs at the latitude of the primary jet core, whereas eddy buoyancy flux is located over a broader region that encompasses the jet and the interjet minimum. In a small sector (120°–144°E) where jets are especially zonal, a spatial and temporal decomposition of the eddy fluxes further reveals that fast eddies act to accelerate the jet with the maximum eddy momentum flux convergence at the jet center, while slow eddies tend to decelerate the zonal current at the interjet minimum. Transformed Eulerian mean (TEM) diagnostics reveals that the eddy momentum contribution accelerates the jets at all model depths, whereas the buoyancy flux contribution decelerates the jets at depths below ~600 m. In ocean sectors where the jets are relatively well defined, there exist jet-scale overturning circulations with sinking motion on the equatorward flank and a rising motion on the poleward flank of the jets. These jet-scale TEM overturning circulations, which are also discernible in potential density coordinates, cannot be attributed to Ekman downwelling because the Ekman vertical velocities are much weaker and their meridional structure shares little resemblance to the rapidly varying jet-scale overturning pattern. Instead, the location and structure of these thermally indirect circulations suggest that they are driven by the eddy momentum flux convergence, much like the Ferrel cell in the atmosphere.