The temporal variability is examined using running decadal standard deviations for the combined Ellsworth Land snow accumulation record (Figure 2 b) revealing an abrupt increase after 1984, when values are more than double the average for the previous ~270 years. This jump coincides with the observed shift in interdecadal temperature variability observed a Byrd Station in West Antarctica, especially during the austral summer and winter [ Bromwich et al. , 2013 ].

(a) Normalized combined Ellsworth Land snow accumulation record and SLP (green) from 20CR in the ASL region (defined as 170–290°E, 60–75°S) as annual average (thin lines) and running decadal means (thick lines). (b) Running decadal standard deviation from the normalized Ferrigno (blue), Bryan Coast (red), and combined Ellsworth Land snow accumulation record. Horizontal solid lines represent baseline averages (1712–1899) and dashed lines +2 σ above the baseline average. Shading and dates highlight key transitions referred to in the text. (c) Running decadal correlations between the SOI and snow accumulation at Ferrigno (blue), Bryan Coast (red), and combined Ellsworth Land record (black) and correlation between SAM and combined Ellsworth Land record (pink). Horizontal dashed line indicates significance at p < 0.01.

Prior to 1900 the annual average snow accumulation at Ferrigno and Bryan Coast remained fairly constant at 33 cm yr −1 and 40 cm yr −1 , while after 1900 the snow accumulation increased at a rate of 0.13 cm yr −1 and 0.15 cm yr −1 , respectively. Snow accumulation during the most recent decade (2000–2009) is 27% higher at Ferrigno and 31% higher at Bryan coast than the baseline values determined from 1712 to 1899. This twentieth century increase is consistent with the Gomez ice core record from the southwestern Antarctic Peninsula (Figure 1 a (black)) which revealed a doubling of snow accumulation since 1854 with an increasing trend that began in the ~1930s and accelerated in the mid‐1970s [ Thomas et al. , 2008 ]. Determining the onset of the trend is heavily dependent on the statistical approach used; however, the Ellsworth Land ice cores appear to corroborate the onset of this snow accumulation increase. There is significant correlation between the two Ellsworth Land records and the Gomez record from the southern Antarctic Peninsula ( r 2 > 0.75, decadal), suggesting that these records are capturing local and regional (>350 km longitudinally) accumulation variability. Spatially averaging the records together reduces the amount of small‐scale noise, resulting from local wind redistribution and sublimation. Thus, a combined Ellsworth Land record was produced by averaging the normalized Ferrigno and Bryan Coast records (1712–2010), and a regional Ellsworth Land record was produced in the same way but includes the Gomez record (1854–2006). Using the combined Ellsworth Land record and selecting the period 1712–1899 as the baseline, we observe that after 1919 the running decadal mean exceeds the baseline average (Figure 2 a) and remains above it for the remainder of the twentieth century. The increase in snow accumulation accelerates in recent decades with the running decadal mean since 1995 consistently exceeding two standard deviations (2 σ ) above the baseline average. Looking at the individual records, the increase at Ferrigno is less pronounced with 8 of the last 30 years exceeding 2 σ above baseline average compared to 13 of the last 30 years at Bryan Coast. This is in contrast to the insignificant trend observed at WAIS divide (WDC05Q) ice core (Figure 1 a (green)) [ Banta et al. , 2008 ], and the negative trends observed from the Satellite Era Accumulation Traverse (SEAT 2010) ice cores from central West Antarctica [ Burgener et al. , 2013 ], suggesting that the observed twentieth century increase is confined to the Antarctic Peninsula and Ellsworth Land, with the magnitude decreasing from east (Gomez) to west (Ferrigno).

3.2 Climate Drivers

Spatial regression plots of annual average ice core snow accumulation with sea level pressure (SLP) and meridional wind (v10) from ERA‐Interim (Figure 3) reveal that the dominant feature governing precipitation variability at the Ellsworth Land and the Gomez sites is the climatological low pressure system that extends across the Amundsen and Ross Seas. This is a region of high synoptic activity and the largest contributor of the total Antarctic meridional moisture flux [Tsukernik and Lynch, 2013], with the highest interannual and seasonal variability. This persistent deep low‐pressure system, the result of the frequency and intensity of individual cyclones, is referred to as the Amundsen Sea Low [Baines and Fraedrich, 1989; Turner et al., 2013], the location and central pressure of which affects the climatic conditions on the Antarctic Peninsula and West Antarctica.

Figure 3 Open in figure viewer PowerPoint Thomas et al., 2008 Dee et al., 2011 p < 0.10 and **p < 0.05) with SLP in ASL region (highlighted box (Figures p < 0.05, and black stars indicate ice core locations (note the different axis in Figure Burgener et al., 2013 Regression plots of annual average snow accumulation from Gomez [], Bryan Coast, and Ferrigno with annual average (a–c ) SLP and meridional (d–f) V winds from ERA‐Interim 1979–2009 [] with correlation coefficients (*< 0.10 and **< 0.05) with SLP in ASL region (highlighted box (Figures 3 a– 3 c)) shown for each record. Brighter coloring indicates< 0.05, and black stars indicate ice core locations (note the different axis in Figure 3 c). Green star indicates location of SEAT 2010 record with arrows (Figure 3 f) representing the simplified clockwise rotation of the ASL. (g) Annual snow accumulation (cm weq) at Bryan Coast (red), Ferrigno (blue), and stacked SEAT 2010 record [] (green) with trends shown (1968–2010).

ERA‐Interim SLP (1979–2010) from the ASL region (defined 170–290°E, 60–75°S) exhibits a deepening trend [Hosking et al., 2013] and is negatively correlated with snow accumulation at all sites (Figure 3). The strongest relationship is observed at Ferrigno and the weakest at Gomez, the most eastern site. At all sites high snow accumulation years are associated with a reduction in regional pressure in the vicinity of the ASL, leading to strengthened circumpolar westerlies and enhanced northerly flow (Figures 3d–3f) with back trajectory analysis, revealing that over half of all air masses reaching Ellsworth Land originate in the south Pacific sector of the Southern Ocean, but anomalous high snow accumulation years occur when transport is confined to this region [Thomas and Bracegirdle, 2015]. Winds in this region directly impact global sea level rise through ocean‐driven basal melt, resulting in widespread ice shelf thinning on the coast of West Antarctica [Pritchard et al., 2012]. The onshore winds to the east of the ASL are potentially poorly represented within the CMIP5 climate models due to the smoothed low‐lying representation of the Antarctic Peninsula which in the real‐world acts as a barrier to direct flow toward Ellsworth Land. The mechanism of lower SLP in the Amundsen Sea sector creates a dipole pattern of enhanced precipitation in Ellsworth Land and reduced precipitation over West Antarctica, reflecting the clockwise rotation of air masses and moisture advection paths around the ASL (Figure 3f). Comparison with the SEAT 2010 [Burgener et al., 2013] records reveal a negative trend in West Antarctic snow accumulation, equivalent to the positive trends (1968–2010) observed in the Ellsworth Land cores (Figure 3g). Assuming 20CR has some skill in reconstructing SLP, then the relationship between snow accumulation and SLP in this region appears to be stable beyond the instrumental period (Figure 2a), [Compo et al., 2011] (1871–2010). However, it is difficult to assess the reliability of the 20CR for the southern ocean and Antarctica, which reportedly contains nonclimatic inhomogeneities [Ferguson and Villarini, 2014].

The transient eddy coastal flux in the Amundsen Sea region, the main driver for atmospheric moisture flux, exhibits a decreasing trend since 1979 [Tsukernik and Lynch, 2013] in contrast to the increased accumulation observed in the ice core records. Another possible driver for increased snow accumulation at these sites could be the observed reduction in sea ice in the Amundsen Sea resulting in enhanced availability of surface level moisture and increased poleward atmospheric moisture transport [Tsukernik and Lynch, 2013]. This could explain the longitudinal differences between the ice core sites, with the least significant changes in accumulation in the west (Ferrigno) and the greatest changes observed in the eastern sites (Gomez), where adjacent sea ice exhibits the largest decreasing trend [Turner et al., 2009]. However, the role of surface level moisture in the total moisture flux and the mechanisms behind the decreasing trends in transient eddy flux remain unclear.