1. Introduction
The large-scale circulation in the Southern Ocean is dependent upon the wind stress imparted by the atmosphere (Munk and Palmén 1951). The strong westerly winds of the Southern Hemisphere (SH) midlatitudes drive the Antarctic Circumpolar Current (ACC; Gnanadesikan and Hallberg 2000), a strong, unbounded flow that connects the major ocean basins and permits a global overturning circulation (Rintoul 2010). Farther south, easterly winds force the ocean around the Antarctic coast. Though the easterlies force a much narrower band of the ocean surface than the westerlies, they remain critical to the exchange of water masses across the Antarctic shelf break. In particular, they sustain the Antarctic Slope Front (ASF), across which newly formed dense water is exported from the continental shelf (Gill 1973; Jacobs 1991). Figures 1a and 1b show the multidecadal-mean surface wind stress over the Southern Ocean, and Fig. 1c contrasts the midlatitude westerlies with the band of easterlies around the Antarctic margins (see Table 1 for definitions of labeled reference points). Though for convenience we refer to the latter as “easterly winds,” it is more accurate to describe their direction as “anticyclonic” because they tend to be oriented parallel to the coastline rather than strictly easterly. The easterlies are supported by a combination of geostrophic flow associated with the pressure gradient between the high-altitude continental interior and the low pressure coastal trough and Coriolis deflection of offshore katabatic winds toward the west (Parish and Bromwich 1997). As a result, the shape of the continent obscures this band in a zonal average, so in Figs. 1d and 1e we plot the more dynamically relevant, alongshore component of the wind stress (see section 2 for details). Here the mean alongshore wind stress is easterly except at the Antarctic Peninsula, which protrudes into the ACC.
Multidecadal mean surface wind forcing around the Antarctic margins from 1979–2014 ERA-Interim data (Dee et al. 2011). (a) Mean zonal wind stress (N m−2), (b) mean meridional wind stress (N m−2), (c) zonal- and time-mean wind stress (N m−2), (d) mean along-slope wind stress (N m−2) as a function of along-slope distance, and (e) mean along-slope wind stress (N m−2) projected onto the circum-Antarctic 1000-m depth contour. The inset in (c) shows an alternative wind stress profile obtained by averaging in time and along the coast. Labels in (a) and (b) indicate reference points to aid interpretation of (d). See Table 1 for further details.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
Definitions of labeled reference points along the Antarctic coast
The most prominent change in the near-Antarctic climate has been the enhancement of the westerly winds over the past few decades (Thompson and Solomon 2002). This shift coincides with the trend toward the high-index polarity of the southern annular mode (SAM; Marshall 2003; Turner et al. 2005). When the SAM is in this positive phase, a high pressure signal exists over the SH midlatitudes with low pressure toward the higher latitudes. A positive SAM induces a stronger, poleward-shifted westerly wind belt (Thompson and Solomon 2002). The trend in the SAM has spurred more research and interest into the changes in the strength of the westerly winds and their impact on the ocean circulation (Meredith and Hogg 2006; Swart and Fyfe 2012; Hogg et al. 2015; Bracegirdle 2013). This trend is likely a response to both greenhouse gas (GHG) and ozone emissions (Wang et al. 2014) and is expected to result in long-term reductions in CO2 and heat uptake in the Southern Ocean (Sallée et al. 2010; Lenton and Matear 2007). The first EOF of the SAM projects onto the Antarctic coastline, so enhancement of the SAM may be expected to decrease the strength of the near-Antarctic easterlies (e.g., Langlais et al. 2015).
The easterly winds play a central role in modulating exchanges across the Antarctic shelf break. The oceanic transport of heat toward Antarctica’s marine-terminating glaciers and ice shelves is supplied by warm, salty Circumpolar Deep Water (CDW; Jenkins et al. 2016; Heywood et al. 2016). Spence et al. (2014) used a 1/4° ocean–sea ice model to show that a reduction of Ekman pumping occurs because of weakened easterly winds and strengthened, southward-shifting westerly winds. This leads to CDW intrusion and rapid heating of ice shelf waters. However, this mechanism may be less efficient where the Antarctic Slope Current (ASC) experiences strong tidal forcing (Flexas et al. 2015; Stewart et al. 2018). A weakening of the easterlies in the western Ross and Weddell Seas could also be expected to enhance shoreward CDW transport by raising the pycnocline at the shelf break (Thompson et al. 2014; Stewart and Thompson 2015a).
Trends in the near-Antarctic easterly wind stress have impacts beyond their influence on the Antarctic coast. For example, (i) a reduction or a shift in the easterly wind stress around the continent may directly modify the rate of Antarctic Bottom Water (AABW) export across the continental slope (Stewart and Thompson 2012, 2013). Globally, AABW has been warming, freshening, and thinning over the past few decades (Purkey and Johnson 2010; Jullion et al. 2013; Meijers et al. 2016), though the cause remains unclear. The export of AABW from the Weddell Gyre is also mediated by the changes in the winds around the Antarctic margins (Jullion et al. 2010). (ii) Changes in the easterly winds have the potential to alter the strength of the southern branch of the ACC via the “free mode” mechanism, in which the depth-averaged velocity of the ACC is accelerated by the near-Antarctic wind stress along a circum-Antarctic 
In addition to changes associated with the enhancement of the westerly wind belt, the easterlies are influenced by changes in the Antarctic katabatic winds. The pole-to-coast downslope pressure gradient drives these winds down the sloping continent toward the coast, where they turn westward under the influence of the Coriolis force (Parish and Waight 1987). A large-scale low atmospheric pressure system over the Antarctic coast enhances katabatic winds (Parish et al. 1993; Parish and Bromwich 2007), which may in turn be expected to enhance alongshore winds.
In this article, we quantify the trends in the easterly surface forcing around Antarctica over the past several decades to examine their response to the enhancement of the SAM. In section 2, we describe our data sources and methods used to calculate the wind stress trends, the trends in the SAM, and the strength of the katabatic winds. In section 3, we show that the air–sea momentum exchange in the easterly wind band has remained approximately unchanged, but that there has been a substantial change in the seasonal cycle of the alongshore wind forcing. We further perform a comparison of these results against other observationally validated reanalysis products. In section 4, we summarize our findings and discuss the potential implications of the diagnosed trends, and in section 5, we provide concluding remarks and discuss the outlook for further research.
2. Data and methods
a. Data
We use the wind stress reported by reanalysis datasets for our calculations because insufficient direct observations are available to calculate the trends. Our primary dataset is ERA-Interim (Dee et al. 2011), which we use to calculate the trend in the easterly wind stress from the monthly mean surface fluxes. We compare ERA-Interim with the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al. 2015), the NCEP Climate Forecast System Reanalysis (CFSR; Saha et al. 2010), and the NASA Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA2; Gelaro et al. 2017). All of these products provide data over multiple decades and compare favorably with available observations (Bracegirdle and Marshall 2012). We focus on the surface easterly wind stress instead of wind velocity because wind stress more directly quantifies atmosphere–ocean momentum transfer and is thus more relevant to the wind-driven ocean circulation around the Antarctic margins. Note, however, that we are unable to distinguish between stress applied to the surface of the ocean and stress applied to the sea ice. While wind momentum imparted to sea ice may be transferred through to the ocean, densely packed sea ice that is immediately adjacent to land can also transfer momentum to the continent via internal stress (Leppäranta 2011).
The wind stress in ERA-Interim is calculated using the ECMWF bulk parameterization scheme (Renfrew et al. 2002). The ERA-Interim surface stress data have a spatial resolution of 0.75° × 0.75° and spans the years 1979–2014 (Dee et al. 2011). The sea ice, essential for proper flux measurements, is estimated by using the daily output from NCEP and the new global, operational, high-resolution, combined sea surface temperature (SST) and sea ice analysis system (OSTIA; Stark et al. 2007). In comparison with in situ measurements, ERA-Interim exhibits correlations in mean sea level pressure (MSLP) of greater than 0.98 and correlations between annual-mean surface temperatures of 0.88. ERA-Interim is the most accurate reanalysis dataset in its quantification of MSLP and geopotential height over Antarctica in the past three decades (1979–2008), with substantially smaller decadal-mean biases than other reanalysis products (Bracegirdle and Marshall 2012). ERA-Interim also most accurately reproduces the geostrophic winds around the Amundsen Sea when compared to other reanalysis products (Bracegirdle 2013).
For validation purposes, we calculate the easterly wind stress trend in MERRA2, CFSR, and JRA-55. All reanalysis products considered here use similar bulk parameterization schemes to assimilate wind speed observations. All products use a 3D variational assimilation method to compute air–sea fluxes from 6-hourly integrated fluxes that are based on roughness length parameterizations (Smith et al. 2011). The drag coefficients are derived from a modified Charnock formula and empirical stability functions. JRA-55 calculates the drag coefficient in a similar manner but does not include a stability term in the calculation of the Monin–Obukhov length scale. The 10-m wind speed is used to calculate the wind stress values, so there are strong qualitative similarities between the calculated wind stress trends reported in section 3 and the 10-m wind speed trends, as discussed in the appendix. In the appendix, we reproduce a subset of our wind stress trend calculations using an independent estimate of the stresses derived from the 10-m wind speeds via a simple quadratic bulk formula. These calculations demonstrate that the diagnosed trends are primarily due to changes in the winds, rather than, for example, changes in surface roughness because of changes in sea ice concentration.
Global reanalysis products remain at resolutions too coarse to resolve coastal features, and orography height biases contribute to errors in model temperature (Bracegirdle and Marshall 2012; Bromwich and Fogt 2004). Therefore, we additionally compare ERA-Interim with the Antarctic Mesoscale Prediction System (AMPS; Powers et al. 2003, 2012), which spans 2006–15. AMPS does not provide surface stresses, so instead we compare the 10-m velocity fields (see Fig. A1). AMPS employs higher spatial resolution and is able to resolve a consistently swifter band of easterly winds around the Antarctic coast. Our comparison (see the appendix) shows that the enhancement of the alongshore winds in AMPS, relative to ERA-Interim, is typically higher than the diagnosed trends in the 30-yr ERA-Interim data. This is a caveat to the present study: It is likely that the magnitudes of the ERA-Interim trends reported in this study would change with a higher spatial resolution.
b. Methods
1) Wind stress trends
We calculate the annual and seasonal linear trends in the zonal and meridional components of the wind stress over the 36-yr 1979–2014 ERA-Interim time span as well as the 1973–2012 JRA-55 (40 years), 1979–2015 MERRA2 (36 years), and 1980–2010 CFSR (31 years) datasets. Because of the variable latitudinal extent of the Antarctic coastline, neither the zonal nor the meridional components of the wind stress products provide a robust measure of the alongshore wind stress, which is arguably the most relevant for near-Antarctic circulation because it sets the cross-slope Ekman transport (Nøst et al. 2011; Stewart and Thompson 2015b; Zhou et al. 2014). We therefore additionally quantify the mean and trend of the wind stress directed along the Antarctic continental shelf break, which we define as the circum-Antarctic 1000-m depth contour, taken from ETOPO-1 (Amante and Eakins 2009). We emphasize the component of the wind stress parallel to the Antarctic coast, as this component directly influences water mass exchanges across the continental slope (e.g., Thompson et al. 2014; Stewart and Thompson 2016). This also motivates our use of the 1000-m depth contour to define the Antarctic “coastline,” as this consistently tracks the location of the continental shelf break.
To facilitate calculation of the along-slope winds, we derive a high-resolution 1000-m depth contour to represent the coastline from the ETOPO-1 bathymetric dataset (Amante and Eakins 2009). For compatibility with the spatial resolution of the ERA-Interim wind stress data, we approximate this contour as a series of piecewise-linear (in latitude–longitude space) sections of 1° in length. We then interpolate and rotate the wind stress data in order to calculate the wind stress directed along each section. To compensate for our somewhat arbitrary selection of the 1000-m depth contour to define the coastline, we calculate the wind stress along each section as an average over a box that spans the length of the section and a prescribed width perpendicular to the section. More precisely, along each section we define a coordinate system 
2) Interannual variability in the SAM and the pole-to-coast pressure gradient
Motivated by the impact of the SAM on the southward-shifting westerly winds and the hypothesized influence on the nearshore easterly wind stress, we calculate the correlations between the calculated interannual easterly wind stress trends and the calculated interannual 
Meteorological station locations used to calculate 
Anticipating that the easterlies should also vary in response to the pole-to-coast pressure gradient that drives the katabatic winds, we define 
3. Circum-Antarctic wind stress trends
a. Annual-mean wind stress trend
Figures 2a and 2b show the overall zonal and meridional wind stress trends calculated from ERA-Interim data. The most prominent feature of this trend is an enhanced eastward stress due to the strengthening of westerly winds, primarily between 45° and 60°S. This eastward strengthening approaches 0.05 N m−2 in the Pacific sector and bears a pattern that qualitatively resembles the first principal component of the SAM (e.g., Langlais et al. 2015). However, this eastward stress trend does not extend to the Antarctic coastline. With the exceptions of the western Ross Sea and the tip of the Antarctic Peninsula, the trend shows an enhanced westward wind stress along the coast. There are also substantial regional variations in the amplitude of this strengthening trend, with the area around the Amery Ice Shelf exhibiting a particularly pronounced intensification of the westward stress over this time period.
Multidecadal trends in the mean surface wind stress (N m−2) around the Antarctic margins from 1979–2014 ERA-Interim data (Dee et al. 2011). Black contours envelop regions that are statistically significant at the 5% level. (a) Mean zonal wind stress trend (N m−2), (b) mean meridional wind stress trend (N m−2), and (c),(d) along-slope wind stress trend as a function of along-slope distance and mapped onto the 1000-m depth contour.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
Figures 2c and 2d show the trend in the alongshore wind stress (defined positive for a “westerly” or cyclonic trend) as a function of alongshore distance and mapped onto the 1000-m depth contour, respectively. This trend further suggests that the annual-mean easterly wind stress around the Antarctic coast is strengthening, albeit weakly, which is contrary to the expected weakening associated with the SAM. Averaged around the coastline, the easterly wind stress increased in magnitude by 0.0044 N m−2 from 1979 to 2014. Note, however, that this strengthening is not statistically significant at the 5% level (
b. Trend in the seasonal cycle
The pronounced seasonal cycle around Antarctica (Meehl 1991; Simmonds and Jones 1998; Mathiot et al. 2011; Wang et al. 2012; Gordon 1981) warrants a calculation of the trends in the seasonally averaged wind stress. Figures 3 and 4 show the seasonal breakdown of the zonal/meridional and alongshore wind stress trends. There are pronounced local variations in the signs and amplitudes of the alongshore trends in Fig. 3. For example, in Fig. 2, we find a weakening of the easterlies in the western Ross Sea and the western Weddell Sea. Figures 3a and 3c show that the weakening is statistically significant in DJF in the western Weddell Sea and in DJF and MAM in the Ross Sea. Austral summer and autumn also have significant northerly strengthening trends in these regions, which will augment the trend of weakening circumpolar-averaged anticyclonic wind stress. In contrast, parts of East Antarctica show a strengthening of the prevailing easterly wind stress trend in all seasons, with this trend being significant in all seasons at the 5% level with the exception of DJF.
Multidecadal trends in the seasonal-mean zonal and meridional wind stress (in N m−2) calculated from 1979–2014 ERA-Interim data (Dee et al. 2011). Black contours envelop trends that are statistically significant at the 10% level. (a) DJF zonal wind stress trend, (b) DJF meridional wind stress trend, (c) MAM zonal wind stress trend, (d) MAM meridional wind stress trend, (e) JJA zonal wind stress trend, (f) JJA meridional wind stress trend, (g) SON zonal wind stress trend, and (h) SON meridional wind stress trend.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
Multidecadal mean and trend of the seasonal-mean along-slope wind stress (N m−2), from 1979–2014 ERA-Interim data (Dee et al. 2011). (a) DJF mean, (b) DJF trend, (c) MAM mean, (d) MAM trend, (e) JJA mean, (f) JJA trend, (g) SON mean, and (h) SON trend. (i),(j) Compilation of the mean and trend of along-slope wind stress, respectively, as functions of along-slope distance.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
Figures 3 and 4 both visually suggest an overall pattern of austral summer weakening and austral winter strengthening of the prevailing easterly winds. More quantitatively, from 1979 to 2014, DJF and MAM exhibit weakening trends of magnitudes of 0.0055 N m−2 (
In summary, the annual-mean circumpolar alongshore wind stress trends are small but indicate a slight strengthening of the easterlies, in contrast to the expected weakening due to enhancement of the SAM. However, the trends are regionally and seasonally much larger, notably in DJF and SON, in the Amundsen and East Antarctic sectors. At the continental scale, this implies a trend toward a larger seasonal cycle (see Fig. 4j), with a weakening of the alongshore easterlies in summer and a strengthening in winter. We discuss implications of this trend in section 4.
c. Attribution of the circum-Antarctic wind stress trends
The trends in the circumpolar wind stress in ERA-Interim show a weakening during austral summer and autumn and a strengthening during austral winter and spring. As stated previously, the high-index polarity of the SAM is characterized by lower pressure at midlatitudes with higher pressures and cold temperatures over the Antarctic continent (Thompson and Solomon 2002; Shindell and Schmidt 2004). Periods of anomalously cold temperatures are also associated with slope-inversion pressure gradients in interior East Antarctica (Kodama and Wendler 1986). We now examine both multidecadal variations in 
Figure 5 shows the seasonal trend in the ERA-Interim 
The seasonal breakdown (a) DJF, (b) MAM, (c) JJA, and (d) SON of the multidecadal trend in the 
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
To attribute the long-term seasonal trends in the alongshore wind stress, we now consider seasonal trends in 
ERA-Interim seasonal trends in the circumpolar-averaged alongshore wind stress and meridional pressure gradients with the corresponding p values as superscripts; p values in bold are significant at 10%.
In summary, a combined consideration of interannual correlations between the large-scale pressure gradients 
d. Consistency among reanalysis products
As outlined in section 2a, ERA-Interim is used to quantify the trend in the multidecadal easterly wind stress and provides the closest comparison to Antarctic meteorological stations (Bracegirdle and Marshall 2012). However, as stated previously, reanalysis data are less constrained by observations in high latitudes than in other parts of the world (Dee et al. 2011). Therefore, we check the consistency in the diagnosed trends of 
Figure 6 compares the multidecadal seasonal wind stress trends from the four reanalysis products during DJF and SON, the seasons that exhibit the strongest and most significant trends in ERA-Interim. As expected, all trends differ quantitatively from one another. Aside from CFSR, all products also demonstrate a strengthening of the easterly wind stress during SON. Moreover, all four products capture the divergence between the DJF and SON trends, implying that all products exhibit a widening of the gap between the weaker DJF easterly wind stress and the stronger SON easterly wind stress. This enhancement appears to be concentrated in approximately the same sectors of the Antarctic coastline. For instance, the Amundsen region shows a stronger change in the difference of the wind stress trend between DJF and SON, consistent with the deepening of the Amundsen Sea low (ASL; Turner et al. 2013).
Multidecadal DJF and SON along-slope circumpolar wind stress (N m−2 century−1) trends from (a) 1980–2010 CFSR (Saha et al. 2010), (b) 1972–2013 JRA-55 (Kobayashi et al. 2015), (c) 1980–2015 MERRA2 (Gelaro et al. 2017) data, and (d) 1979–2014 ERA (Dee et al. 2011) products. Red labels indicate reference points to aid geographical interpretation. See Table 1 for further details.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
To visualize the regional variations of the changes in seasonality and their amplitude compared to the interannual variability, Fig. 7 shows the difference in the SON and DJF wind stress trends as a function of distance along the Antarctic slope. Here, it is clear that the magnitude of the change in the seasonality of the wind stress depends highly on location. Areas that visually have large summer-to-winter diverging seasonal cycles, especially the Amundsen and East Antarctic region, exhibit the largest trends in the SON–DJF wind stress changes. One caveat specific to the Amundsen region is the impact of ENSO on the depth of the ASL in winter and spring (Clem et al. 2016), which may be a driver of our calculated trends during these seasons.
Multidecadal trends in the SON–DJF wind stress difference (N m−2 century−1) as a function of along-slope distance, error bars are standard deviations from the multidecadal mean SON–DJF wind stress for (a) 1980–2010 CFSR (Saha et al. 2010), (b) 1972–2013 JRA-55 (Kobayashi et al. 2015), (c) 1980–2015 MERRA2 (Gelaro et al. 2017), and (d) 1979–2014 ERA-Interim (Dee et al. 2011) products. Red labels indicate reference points to aid geographical interpretation. See Table 1 for further details.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
Table 4 summarizes the annual-mean wind stress trends, the DJF and SON wind stress trends, and the 
Trends in the circumpolar-averaged alongshore wind stress and meridional pressure gradients with the corresponding statistical p values in the superscript of each trend; p values in bold are significant at 10%.
Multidecadal trends in DJF for (a) 1980–2010 CFSR (Saha et al. 2010), (b) 1973–2012 JRA-55 (Kobayashi et al. 2015), (c) 1979–2015 MERRA2 (Gelaro et al. 2017), and (d) 1979–2014 ERA (Dee et al. 2011) products. The DJF 
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
Table 4 reveals that during SON, all products except for CFSR show an increase of strength in the easterly wind stress and an increase in 
Similarly to Fig. 8, Fig. 9 compares the SON easterly wind stress trends and the calculated 
Multidecadal trends in SON for (a) 1980–2010 CFSR (Saha et al. 2010), (b) 1973–2012 JRA-55 (Kobayashi et al. 2015), (c) 1979–2015 MERRA2 (Gelaro et al. 2017), and (d) 1979–2014 ERA (Dee et al. 2011) products. The SON 
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
4. Implications of increased alongshore wind seasonality for near-Antarctic circulation
In section 1, we provided an overview of the impacts of the easterly winds on near-Antarctic circulation. In our results in section 3, we found that the annual-mean wind stress has not changed significantly, but that the seasonality of the wind stress has changed substantially. For example, Fig. 6d shows that the ERA-Interim DJF and SON wind stresses have diverged from one another, with strengthening easterly wind stress in SON and weakening easterly wind stress in DJF. Figure 8 demonstrates the consistency among reanalysis products, and each shows an enhancement of the wind stress difference between DJF and SON.
An increase in the seasonality of the wind stress may have consequences for several aspects of the near-Antarctic circulation. For instance, ice motion trends during autumn, especially in the Pacific and Atlantic sectors of the Antarctic coast, are strongly correlated to the changes in the autumn 10-m wind forcing (Holland and Kwok 2012). Other potential impacts of the near-Antarctic winds on the circulation include changes in AABW export, changes in the properties of AABW, and variability in heat transport toward ice shelves.
Easterly winds modify the rate of AABW formation and export (Jullion et al. 2010; Wang et al. 2012; Su et al. 2014), a process that has global-scale impacts for ocean circulation (Marshall and Speer 2012). Stronger easterly winds in the southern Weddell and Ross Seas (see Fig. 2) enhance the export of dense shelf water simply by virtue of a stronger shoreward Ekman transport and deep offshore return flow (Stewart and Thompson 2012, 2013) and/or by depressing the pycnocline at the shelf break and enhancing the overflow through the Filchner Trough (Kida 2011; McKee et al. 2011). The wind stress trends shown in Fig. 2 may also be expected to produce a stronger baroclinicity of the Weddell and Ross Gyres (McKee et al. 2011). Export of dense water would be enhanced as an effect of stronger thermal wind forcing. A stronger gyre would promote a doming of isopycnals in the center of the gyre and a deepening of isopycnals at the gyre rim (Jullion et al. 2010).
The seasonal trends (Fig. 4), as well as the enhancement of the seasonal cycle in the strength of the easterly winds (Fig. 6), project onto the mechanisms that form and export dense water from Antarctica. Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) are precursors to AABW in the Weddell Sea. The export of these dense water masses from the Filchner Trough and the transport toward the Antarctic Peninsula is mediated by seasonal variations in the along-slope winds (Wang et al. 2012). Additionally, a change in wind stress seasonality may influence export of WSDW northward from the deep Weddell Gyre via the Orkney Passage by modulating the seasonal cycle in the gyre’s baroclinicity (Meredith et al. 2008; Su et al. 2014). AABW export reaches a peak outflow from the Weddell Gyre in austral autumn and a minimum in austral spring (Gordon et al. 2010). Therefore, an increase (decrease) in the easterlies in austral autumn (spring) may lead to an increase (decrease) in AABW export.
Properties of AABW are closely tied to the seasonal circum-Antarctic wind forcing. In the Ross Sea, winter intensification of the winds shifts the Antarctic Slope Front poleward and suppresses high-salinity shelf water (HSSW) export (Gordon et al. 2015), a process that may have intensified as a result of the SON enhancement of the easterlies. Warm and fresh bottom water is produced with an increase in the strength of the easterlies, which is in contrast to the cold, salty bottom water produced during autumn and winter when there are generally weaker winds. The amplification of the seasonal trends in the wind stress may therefore act to enhance the seasonal variations in AABW properties.
Seasonal changes in wind forcing may also project onto oceanic heat transport toward the continent. The trend toward weaker austral summer and spring easterly wind stress adjacent to the Amundsen and Bellingshausen Seas (see Figs. 4f,j), where the ACC lies closest to the continent, may be enhancing the shoreward flux of CDW (Thoma et al. 2008), and contributing to the ongoing warming of these west Antarctic continental shelves (Schmidtko et al. 2014). Figure 4j shows this substantial (~0.05 N m−2) weakening of the easterlies in austral spring. The ice shelf thinning seen in West Antarctica (Rignot et al. 2013) may be tied to the trends apparent in the wind stress.
Offshore of the Fimbul Ice Shelf, two wind-influenced mechanisms of shoreward heat transport have been identified that are tied to the easterly wind stress: Ekman transport of relatively warm summer surface water (Zhou et al. 2014) and deep intrusion of CDW (Nøst et al. 2011; Hattermann et al. 2014). Our calculated wind stress trends that show a reduction of the strength of the easterlies in austral summer will limit the available warm water at the ice shelf front during the summer, thus reducing melt. However, the same trend in the easterlies will enhance the intrusion of CDW, with more CDW intrusion expected in summer and autumn (Spence et al. 2014). Antarctica’s floating ice shelves may therefore be expected, on average, to exhibit reduced melt adjacent to the ice shelf face and enhanced melting close to their grounding lines, though with substantial variations associated with the spatially inhomogeneous wind stress trends.
5. Conclusions
In this article, we quantified the multidecadal change in the easterly surface wind forcing around Antarctica using ERA-Interim data and compared our findings with three other reanalysis products that compare favorably with Antarctic meteorological measurements (Bracegirdle 2013; Bromwich et al. 2011). The emphasis in this study is on the alongshore component of the surface forcing, as this component is most relevant for cross-slope exchange of water masses (e.g., Thompson et al. 2014). We find that, despite the increase in SAM in recent decades, the easterlies have on average strengthened by 0.0044 N m−2, or by around 7%, though this trend is not statistically significant. This corresponds to an anomalous shoreward Ekman transport of around 0.6 Sv (1 Sv ≡ 106 m3 s−1) around the ~18 000 km length of the Antarctic shelf break. The annual-mean wind stress trend differs substantially between reanalysis products, but an aggregation of these products similarly indicates little overall change in the annual-mean wind stress (see Table 4).
In contrast, ERA-Interim wind stress exhibits substantial seasonal trends, in particular a widening of the gap between the weaker summer easterlies and the stronger winter easterlies (see Table 3). In DJF and MAM, this amounts to the easterly wind stress trend weakening by 0.0055 (12%) and 0.0022 N m−2 (3%), equivalent to offshore Ekman transport anomalies of approximately 0.7 and 0.3 Sv, respectively. In JJA and SON, the easterlies have strengthened by 0.112 (15%) and 0.0143 N m−2 (22%), equivalent to shoreward Ekman transport anomalies of 1.5 and 1.9 Sv, respectively. There are substantial regional departures from these circumpolar-average trends, as shown in Figs. 3 and 4, with seasonal trends reaching 0.05 N m−2 along various stretches of the shelf break. Despite widely varying annual-mean trends, all reanalysis products examined here exhibit a qualitatively similar widening of the gap between the SON and DJF easterly wind stresses (see Figs. 6, 7).
In section 3c, we investigated the extent to which the wind stress trends can be attributed to the trends in the SAM, quantified via 
Throughout this study, we have examined the surface wind stress, rather than the wind speed. We made this choice because it is the momentum transfer from the atmosphere to the ocean that is most relevant to the near-Antarctic circulation, as outlined in sections 1 and 4. However, we are unable to distinguish directly if changes in the sea ice cover, which changes the surface roughness (Lüpkes and Birnbaum 2005), might be contributing to the trends. In the appendix, we show that the trends in the ERA-Interim 10-m wind speed are qualitatively similar to the trends in the wind stress. The alongshore wind speed exhibits increases in seasonality that are qualitatively similar to the wind stress trends in all reanalysis products. Furthermore, in the appendix, we construct an independent approximation of the alongshore wind stress trends, derived from the wind speeds via a simple quadratic bulk formula, and find quantitatively similar trends as reported by the model. We therefore infer that the trends in the wind stress are primarily a result of changes in the winds, rather than changes in sea ice conditions. However, sea ice can efficiently redistribute momentum laterally, and further work will be required to determine how changes in sea ice properties are contributing to the trends in the momentum input to the ocean (Leppäranta 2011).
Another caveat to this analysis is that reanalysis products are relatively poorly constrained close to Antarctica because of sparse measurements (see Saha et al. 2010; Dee et al. 2011; Kobayashi et al. 2015; Gelaro et al. 2017). This motivated a comparison of our results across four different products that have been evaluated using Antarctic meteorological measurements, with ERA-Interim exhibiting substantially smaller decadal-mean biases in MSLP and geopotential height than the other three (Bracegirdle and Marshall 2012). In turn, ERA-Interim’s low MSLP biases motivated our attribution of the diagnosed wind stress trends to annular meridional pressure gradients, specifically 
In section 4, we discussed qualitatively how the calculated trends may impact specific aspects of the regional Antarctic circulation. Given the wide range of Antarctic ocean processes that are tied to the easterly winds, there may be a specific connection between these wind stress trends, the observed multidecadal changes in water mass properties around the Antarctic margins (Schmidtko et al. 2014), and the ongoing warming and freshening of AABW (Purkey and Johnson 2010). Further work is necessary to quantify the ocean’s response to multidecadal changes in the easterly wind belt.
Acknowledgments
This material is based in part upon work supported by the National Science Foundation under Grants OCE-1538702 and PLR-1543388. The authors thank ECMWF, NCAR, and NASA GES DISC for access to the available ERA-Interim, CFSR, MERRA2, and JRA-55 datasets. The authors thank Andrew Thompson for his constructive comments that improved this paper.
APPENDIX
a. Analysis of the seasonal wind trends
We focus on the seasonal easterly wind stress trends, rather than the wind speed trends, because the wind stress more directly impacts the ocean circulation. For comparison, we now repeat our seasonal trend analysis for the wind speed. We first provide in Fig. A1 seasonal plots of the zonal and meridional 10-m winds in ERA-Interim. For comparison, we also calculate the 10-m wind trends from 1980–2010 CFSR (Saha et al. 2010), 1979–2015 MERRA2 (Gelaro et al. 2017), 1973–2012 JRA-55 (Kobayashi et al. 2015), and 1979–2014 ERA-Interim (Dee et al. 2011). Figure A2 shows the SON and DJF alongshore wind trends diagnosed from all four reanalysis products. These figures indicate that the wind stress trends are primarily due to changes in the winds, rather than, for example, changes in surface roughness associated with sea ice.
Multidecadal trends in the seasonal-mean zonal and meridional 10-m winds (in m s−1), calculated from 1979–2015 ERA-Interim data (Dee et al. 2011). Black contours envelop trends that are statistically significant at the 10% level. (a) DJF zonal wind trend, (b) DJF meridional wind trend, (c) MAM zonal wind trend, (d) MAM meridional wind trend, (e) JJA zonal wind trend, (f) JJA meridional wind trend, (g) SON zonal wind trend, and (h) SON meridional wind trend.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
The multidecadal trend in the DJF and SON along-slope 10-m winds (in m s−1), calculated from 1979–2015 MERRA2 (Gelaro et al. 2017) data, 1980–2010 CFSR Saha et al. (2010) data, 1973–2012 JRA-55 (Kobayashi et al. 2015) data, and 1979–2014 ERA-Interim (Dee et al. 2011) data.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
The multidecadal trend in the DJF and SON along-slope wind stress (in N m−2 century−1) calculated from the respective 10-m wind speeds for 1979–2015 MERRA2 (Gelaro et al. 2017) data, 1980–2010 CFSR (Saha et al. 2010) data, 1973–2012 JRA-55 (Kobayashi et al. 2015) data, and 1979–2014 ERA-Interim (Dee et al. 2011) data.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
b. Comparison against the AMPS
To evaluate the impact of reanalysis resolution on representation of the near-coastal winds, we compare the ERA-Interim winds against the output of the AMPS forecast (Hines et al. 2011; Powers et al. 2012) over the period that AMPS data are available (2006–15). Figure A4 shows the difference between the mean 10-m winds in ERA-Interim and AMPS. For this comparison, the AMPS wind velocity data were rotated and interpolated onto the same regular latitude–longitude grid as ERA-Interim. A key difference is that the coastal winds in ERA-Interim are consistently slower than in AMPS, likely because of the higher resolution used in the latter. The AMPS midlatitude westerly winds are also stronger than in ERA-Interim. The differences in the mean wind are of comparable magnitude to the diagnosed multidecadal trends in ERA-Interim.
The difference in the 2006–14 averaged 10-m zonal wind between ERA-Interim (Dee et al. 2011) and AMPS (Powers et al. 2003, 2012). AMPS data has been provided by the Byrd Polar Research Center’s Polar Meteorology Group at The Ohio State University. Land points have been removed to highlight the ocean regions relevant to this study.
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
c. Calculation of katabatic index from surface pressure
As in Fig. 8, but calculating the DJF 
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
As in Fig. 9, but calculating the SON 
Citation: Journal of Climate 32, 6; 10.1175/JCLI-D-18-0402.1
d. Calculation of the interannual trends using a consistent 1980–2010 time period
In section 3, we reported trends calculated over the full time span of each reanalysis product, which differ slightly from one another. We show the p values of the interannual correlations between the wind stress, PKat, and PSam in Table A1. In Table A2, we present a recalculation of the trends and p values reported in Table 4 but use a uniform time period of 1980–2010 across all reanalysis products. There is little change in the JRA-55 and CFSR trends because their analysis periods differ little between Tables 4 and A2. In ERA-Interim and MERRA2, there are some more substantial changes. For example, while these products still report a strengthening of the easterly wind stress in SON over 1980–2010, these trends are no longer statistically significant.
The P values of the interannual correlations between 
Trends in the circumpolar-averaged alongshore wind stress and meridional pressure gradients with the corresponding statistical p values in the superscript of each trend for years 1980–2010; p values in bold are significant at 10%.
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