The 1948–2007 atmospheric state includes several observed decadal trends with links to Antarctic coastal ocean variability. First, increased Antarctic Peninsula surface air temperatures since the 1950s [ Chapman and Walsh , 2007 ; Turner et al ., 2005 ] (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 , 2005 ]. 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 , 2002 ] (Figure S1b), may aid the intrusion of water onto the Antarctic continental shelf [ Chavanne et al ., 2010 ; Wåhlin et al ., 2010 ; Hellmer et al ., 2012 ], alter sea ice extent and thickness [ Bintanja et al ., 2013 ], and impact dense water formation and transport [ Spence et al ., 2014 ]. 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 ., 2013 ] and are linked with freshening Southern Ocean water masses [ Rintoul , 2007 ; Durack and Wijffels , 2010 ]. However, the Southern Ocean precipitation trends are weak in CORE‐IAF (Figure S1c), and observations of Antarctic coastal runoff insufficient to include it in the 1948–2007 model forcing, so runoff is held constant.

In the first experiment, GFDL‐MOM025 was integrated through six cycles of 1948–2007 CORE Inter‐Annual Forcing (CORE‐IAF) provided at 6 h intervals. We analyze decadal averages from 1958 to 1967 and 1998 to 2007 in the last forcing cycle.

2.2.2 Idealized Wind Forcing

The observed Southern Annular Mode trend is consistently projected to persist through the 21st century due to continued anthropogenic forcing [Fyfe et al., 2007; Zheng et al., 2013]. We isolate the impact of the projected 21st century Southern Annular Mode trend on the Antarctic coastal ocean in a series of idealized perturbed Southern Ocean wind simulations initiated from a control GFDL‐MOM025 simulation (CNTRL). CNTRL is equilibrated for 100 years under CORE Normal Year Forcing (CORE‐NYF). CORE‐NYF provides a 1 year climatological mean atmospheric state at 6 h intervals, along with representative synoptic variability [Large and Yeager, 2009].

In simulations denoted as W 4°S , W +15% , and W 4°S+15% the CORE‐NYF 10 m winds between 25°S and70°S are shifted to 4°S, increased in magnitude by 15%, or both (Figure S2). Simulations denoted as W 4°S (62°S–70°S) , W 4°S+15% (62°S–70°S) , and W 4°S+15% (62°S–80°S) have reduced latitudinal ranges (i.e., 62°S–70°S and 62°S–80°S) of perturbed wind forcing. The northern tip of the Antarctic Peninsula is near 62°S. Both meridional and zonal wind components are modified to prevent unrealistic decomposition of the CORE synoptic variability. Smoothing is applied within the 5° latitude of the perturbation boundaries. Anomalies are determined by differencing perturbed simulations from the concomitantly extended CNTRL simulation, with this approach acting to remove effects from model drift.

The idealized wind perturbation scenarios were guided by an assessment of the late 21st century change in Southern Ocean zonal winds induced by “business as usual” anthropogenic forcing (RCP8.5) in 32 climate models from the Fifth Coupled Model Intercomparison Project (CMIP5) (Figure S3; see also Fyfe et al. [2007] and Zheng et al. [2013]). The barycenter of westerly winds south of 20°S is shifted 3.6°S in the idealized GFDL‐MOM025 wind shift scenarios. Twenty‐five percent of the CMIP5 models analyzed have a mean barycenter shift of 2.9°S, and three models have a more southward barycenter shift than in our idealized approach. We avoid using CMIP5 model mean wind anomalies as forcing due to their equatorward Southern Ocean westerly wind position bias relative to reanalysis data (Figure S3; see also Swart and Fyfe [2012]).

As the westerly winds shift poleward under anthropogenic forcing they reduce the meridional extent and strength of the polar easterlies (Figure S3). The transition from zonal average easterly winds to zonal average westerly winds, which occurs at roughly 65°S in CNTRL, is shifted 4°S in the idealized wind forcing simulations. In the CMIP5 models analyzed the transition to zonal average easterly winds is shifted 2.5°S ± 0.5°S for the 32 multi‐model ensemble mean, and 4.5°S ± 0.5°S for 25% of the models (Figure S3). The zonal average wind speed anomaly averaged between 65°S and 70°S for the W 4°S (62°S–70°S) simulation is 1.44 m/s (positive westward), while it is 0.81 m/s for the 32 CMIP5 model ensemble mean and 1.38 m/s for top 25% of the models (Figure S3).

The idealized wind forcings are applied as a consistent anomaly to the CORE‐NYF forcing atmospheric state, and as such they do not properly account for temporal or spatial variations of the projected SAM trend. In particular, the observed spatial pattern of the SAM during summer is largely zonally symmetric, and in winter it exhibits increased zonal wave number 2–3 variability [Codron, 2005, 2007]. While observational data identify that the largest SAM trend has occurred during spring/summer in recent decades [Marshall, 2003], in the future the SAM is projected to trend across all seasons as greenhouse gases take over from ozone depletion as the primary driver of a midlatitude jet shift in the Southern Hemisphere [Thompson et al., 2011].