Changes in the Subduction of Southern Ocean Water Masses at the End of the Twenty-First Century in Eight IPCC Models

Abstract

A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO 2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m 23 , a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by a mean of 14 Sv (23%; 1 Sv [ 10 6 m 3 s 21 ). Decreases in the projected subduction of the key Southern Ocean upper-water masses imply a slow down in the Southern Ocean circulation in the future, driven by surface warming and freshening. A reduction in the subduction of intermediate waters implies a likely future decrease in the capacity of the Southern Ocean to sequester CO 2 .

Multidecadal Warming and Shoaling of Antarctic Intermediate Water*

“…The winter SST ventilates the AAIW, and the SLP variations are related to wind variations, which are important regional drivers of ocean circulation and ventilation. The SAM and local SST variations have recently been suggested to influence AAIW formation (Naveira Garabato et al 2009;Downes et al 2010;Sallée et al 2010a,b). Here we examine basin-averaged SST (Smith et al 2008) from 608-558S (Fig.…”

“…This accumulation rate might change under global warming scenarios (Downes et al 2010), and the AAIW core warming, shoaling, and reduced density might be related to such a change.…”

Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

“…Several multimodel analyses from phase 3 of the Climate Model Intercomparison Project (CMIP3)/ Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) have assessed the influence of varying winds and buoyancy (heat and freshwater) fluxes on the Southern Ocean circulation. In these studies, the climate models suggest intensification and a poleward shift of the westerlies are associated with poleward shifts in the subtropical and subpolar gyres and ACC fronts, ocean warming, and increased upwelling and downwelling branches of the upper cell (Fyfe and Saenko 2006;Wang et al 2011;Downes et al 2010;Sen Gupta et al 2009). However, contrary to the expectations from ocean-only model simulations, changes in the transport of the ACC in CMIP3 class models are not associated with the wind stress changes (e.g., Sen Gupta et al 2009).…”

“…The models project a weakening of the lower cell, though associated changes in surface fluxes are uncertain. This may be associated with an inability of models to correctly simulate polar processes associated with Antarctic Bottom Water formation (e.g., Downes et al 2011). Using 13 phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, we will deduce whether changes in the ACC transport under global warming are due to eddies opposing winds or due to changes in surface buoyancy input.…”

The Ross Sea Dipole – temperature, snow accumulation and sea ice variability in the Ross Sea region, Antarctica, over the past 2700 years

“…With carbon dioxide (CO 2 ) and global temperatures predicted to continue to rise, model simulations of the Antarctic/Southern Ocean region show an increase in surface warming for the coming decades resulting in reduced sea ice extent, weakened Antarctic Bottom Water formation, intensified zonal winds that reduce CO 2 uptake by the Southern Ocean, a slowing of the southern limb of the meridional overturning circulation (MOC) and associated changes in global heat transport, and a rapid ice sheet grounding line retreat that contributes to global sea level rise (Kusahara and Hasumi, 2013;Spence et al, 2012;Marshall and Speer, 2012;Sen Gupta et al, 2009;Toggweiler and Russell, 2008;Russell et al, 2006;Downes et al, 2010;Anderson et al, 2009;DeConto and Pollard, 2016;Joughin and Alley, 2011;Golledge et al, 2015;DeVries et al, 2017). Observations confirm an ozone-depletion-induced strengthening and poleward contraction of zonal winds (Thompson and Solomon, 2002b;Arblaster et al, 2011), increased upwelling of warm, modified Circumpolar Deep Water (Jacobs et al, 2011), a warmer Southern Ocean (Böning et al, 2008;Gille, 2002;Abraham et al, 2013), meltwater-driven freshening of the Ross Sea (Jacobs et al, 2002), ice shelf and mass loss, grounding line retreat (Rignot et al, 2014;Pollard et al, 2015;Paolo et al, 2015;Joughin et al, 2014), reduced formation of Antarctic Bottom Water (Rintoul, 2007) and Antarctic Intermediate Water (Wong et al, 1999), changes in sea ice (regional decreases and increases in the Amundsen and Ross seas, respectively) (Holland and Kwok, 2012;Sinclair et al, 2014;Stammerjohn et al, 2012), and dynamic changes in Southern Ocean CO 2 uptake driven by atmospheric circulation (Landschützer et al, 2015).…”