Global Teleconnections in Response to Freshening over the Antarctic Ocean

Abstract

In this paper, coupled ocean-atmosphere responses to freshening over the Antarctic Ocean are investigated in a fully coupled model with a series of sensitivity experiments. In the model, 1.0 Sv (1 Sv [ 10 6 m 3 s 21 ) of freshwater flux is uniformly imposed over the Antarctic Ocean for 400 yr, while the ocean and atmosphere remain fully coupled both locally and elsewhere. The model explicitly demonstrates that a freshening of the Antarctic Ocean can induce a significant local cooling coupled with an intensification of the westerly winds and expansion of sea ice. Furthermore, the cooling can extend to the entire southern extratropical and tropical oceans coupled with an intensification of southeasterly trades and the equatorial trade winds. Some modest warm anomalies also occur in the northern extratropical oceans, forming a sharp interhemispheric SST contrast.A series of sensitivity experiments are conducted to understand the mechanisms responsible for transmitting the southern high latitude cooling to the tropics and the Northern Hemisphere. Experimental results demonstrate the important role of the surface coupled wind-evaporation-SST feedback and in turn changes of the subtropical-tropical meridional overturning circulation in conveying the southern high-latitude temperature anomalies to the tropics. The interhemispheric seesaw originates from the tropical-northern extratropical atmospheric teleconnection and is sustained by the subductive process of Antarctic subsurface warming. The Atlantic meridional overturning circulation is intensified in the first few decades of the freshwater forcing over the Antarctic Ocean because of a shutdown of the Antarctic deep convection, but it subsequently decreases because of the spreading of the fresh anomalies from the Southern Ocean to the Northern Ocean.

Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

“…Many previous studies of the influence of buoyancy flux changes on AABW have been coupled to an atmospheric model (e.g., Aiken and England 2008;Ma and Wu 2011) such that the role of changing surface buoyancy fluxes are not independent from other atmospheric changes (i.e., subsequent induced wind shift/intensification). In this study, surface forcing perturbations are applied as a step change to the CORE forcing, removing any such atmospheric feedback and allowing for a mechanistic understanding of how each buoyancy flux and forcing term independently influences the AABW formation and properties.…”

“…This cooling is likely a result of increased surface freshwater fluxes strengthening the upper-ocean halocline and impeding the convection that precedes AABW formation (Aiken and England 2008;Trevena et al 2008;Swingedouw et al 2009;Ma and Wu 2011). Reduced convection increases the residence time of surface waters under atmospheric fluxes, facilitating a net cooling.…”

“…Previous modeling studies investigating the sensitivity of AABW to freshwater fluxes focus on a uniformly distributed surface freshwater flux over the Southern Ocean, simulating either changes in precipitation, icemelt, or ice-sheet runoff and/or collapse (e.g., Aiken and England 2008;Trevena et al 2008;Swingedouw et al 2009;Menviel et al 2010;Kirkman and Bitz 2011;Ma and Wu 2011;Bintanja et al 2013Bintanja et al , 2015Morrison et al 2015). However, changes to freshwater fluxes across Antarctica are not necessarily uniformly spread over the Southern Ocean.…”

“…Despite the sea surface cooling response simulated from anthropogenic forcing (as a result of a strengthened halocline hindering the upwelling of warm deep waters to the surface; refer to section 3a; Trevena et al 2008;Swingedouw et al 2009;Ma and Wu 2011), changes in surface heat fluxes remain an important component of any buoyancy adjustment. Increases in net radiative fluxes under climate change are dominated by an increase in downward longwave radiation (DLR; Stephens et al 2012;Zelinka and Hartmann 2012).…”

Freshwater fluxes in the Weddell Gyre: results from δ 18 O

“…Model studies indicate that dramatic effects on both regional and global climate can occur on relatively short time scales if the Southern Ocean freshwater balance is altered [5,6]. In an era of global warming, a modified hydrological cycle is expected (e.g.…”