Under all four ice-thinning models, the Mt. Hunter snow accumulation rates in the 20th century are unprecedented in the last millennium (Figs 2, 3). The Campbell finite element ice flow model14 produces the highest degree of thinning since 1650 CE among any of the model simulations we used, and therefore is the most conservative model in that it produces the smallest precipitation increase on Mt. Hunter since 1840 CE. This is our preferred ice flow model because it is constrained by the detailed basin and glacier geometry, spatial accumulation patterns, and surface ice velocity of the Mt. Hunter ice core drill site. Prior to 1650, the Hooke model12 consistently produces higher accumulation values than the other thinning models (Fig. 3). This may be because parameters in the Hooke model are statistically optimized to match the timescale, as opposed to the other three models where thinning corrections are based on explicit calculations of vertical strain rate (see Supplemental Information). Even in the Hooke model, the accumulation rates before 1840 never approach 20th century values (Fig. 3).

Numerous accumulation reconstructions from alpine ice cores have been published during recent decades. Mt. Hunter is very similar to many of these sites with low velocities near the drill site (approximately 2 m a−1), surface-conformable stratigraphy, and very low surface slope. The average annual temperature on Mt. Hunter (−17 °C) is cooler than at most other alpine ice core sites but is similar to Ushkovsky volcano32 (−15.7 °C) and Dasuopu glacier33 (−16 °C). Many other alpine ice core accumulation records have been drilled at sites with saddle-like geometry including Aurora Peak16, Mt. Waddington21, Mt. Everest12, Belukha Saddle34 and Mt. Logan15. Thus, we would expect that the modeling approaches published in these studies would be applicable to the conditions on Mt. Hunter. Nevertheless, we use four different modeling approaches, one of which is designed for the specific Mt. Hunter site geometry, and all of which have very conservative and rigorously quantified ranges of uncertainty (Fig. 3 and Supplemental Information). To our knowledge, no other alpine ice core accumulation record has been developed with such a thorough characterization of the thinning regime or uncertainties. All of the thinning models produce a robust increase in accumulation since the mid-19th century above late-Holocene background values.

While all of our modeling simulations agree that during the early half of our record (810–1400 CE) precipitation remained well below present values, we cannot confidently conclude from our results whether precipitation was increasing or decreasing within this time period. One simulation (the Hooke model12) shows a slight decrease in accumulation rate between 810 and 1400 CE while the remaining three modeling approaches suggest a slight accumulation increase during this period with superimposed decadal to centennial scale variability. Relatively low snow accumulation on Mt. Hunter between 1000 and 1300 CE is consistent with low sodium concentrations in the Mt. Logan and Mt. Hunter records during this interval, indicative of a relatively weak Aleutian Low (Fig. S5)35,36.

We suggest that the recent doubling of Alaska Range snowfall recorded in the Denali ice core reflects an increase in absolute humidity with warming and an intensification of the Aleutian Low to its strongest levels (most positive PNA) of the past millennium. The precipitation increase we observe on Mt. Hunter is apparent in other Alaskan records. Between the mid-20th century and present day, several weather stations along coastal Alaska6 (e.g. Kodiak, Palmer, Juneau) have experienced similar (but smaller) increases in wintertime precipitation compared with Mt. Hunter (Fig. S6). Consistent with our results, a century-long ice core collected from Aurora Peak in the eastern Alaska Range16 (240 km from Mt. Hunter) also records a doubling of snow accumulation over the 20th century, with greater rates of precipitation increase than observed on Mt. Hunter (Figs S5, S6). The Mt. Logan ice core (Yukon, Canada) time series of accumulation, sodium, and water isotope ratios show trends over the past 200 years indicative of a strengthening Aleutian Low15,35,37 (Fig. S5). Specifically, Moore et al.15 found a positive and accelerating trend in Mt. Logan annual accumulation after the middle of the nineteenth century, and interpret it as indicative of a progressively more positive PNA. This was supported by Rupper et al.38 who confirm that high accumulation years on Mt. Logan are associated with a stronger wintertime Aleutian Low. Furthermore, the composite Mt. Logan and Mt. Hunter sea-salt Na+ calibrated Aleutian Low proxy similarly supports a progressive strengthening of the Aleutian Low over recent centuries to its strongest levels of the past 1500 years36 (Fig. S5).

The doubling of accumulation rates at Mt. Hunter, Aurora Peak and other sites near the Gulf of Alaska stand seemingly in contrast to the more modest 2–3% increases in precipitation per degree of warming predicted by climate models (e.g. Held and Soden3). The rapid precipitation increase in Alaska represents a regional signal amplified primarily by enhanced advection of moisture and storms into the Gulf of Alaska associated with a strengthening Aleutian Low. With a deeper Aleutian Low, the largest precipitation increases would be expected to occur where southerly flow on the eastern side of the low encounters the Alaskan coastline and mountain ranges. This is exactly the pattern we observe, with increasing precipitation at coastal Alaskan weather stations including Kodiak, Juneau and Valdez, and the strongest precipitation increases at orographic barriers such as Mt. Hunter and Aurora Peak (Fig. 4d; Fig. S6). Previous work has shown that this configuration leads to drier conditions within the rain shadow of these orographic barriers39, which is apparent as negative correlations between winter Mt. Hunter accumulation and precipitation at Tanana, McKinley Park, and Fairbanks located north of the Alaska Range (Table 1). Additionally, Tanana winter precipitation has experienced a significant decrease since 19506. We do not, therefore, infer that the Mt. Hunter precipitation increase is representative of the expected broader mid-high latitude precipitation increase predicted by global scale climate projections40. Rather, the Mt. Hunter record captures a specific regional phenomenon (a deepening Aleutian Low) that has occurred in conjunction with warming, resulting in a dramatic increase in regional precipitation. Finally, ice core17 and other paleoproxy records41 from outside the North Pacific region do not document long-term precipitation increases, consistent with our finding that the North Pacific hydroclimate intensification is a regional feature associated with a strengthening Aleutian Low.

Dynamical studies suggest that the most plausible way to intensify the Aleutian Low is through warmer tropical SSTs, particularly in the western tropical Pacific and Indian Oceans42. Enhanced convective precipitation and latent heat release associated with higher tropical Pacific and Indian Ocean SSTs induce a Rossby wave extratropical response manifesting as the PNA positive pattern42,43 (Fig. 4b). Consistent with this dynamical theory, the annual Mt. Hunter accumulation record from 1910–2010 is significantly correlated (r = 0.59, p < 0.01) with reanalysis SSTs in the western tropical Pacific and Indian Oceans (Fig. 5), supporting a strong link between Alaskan precipitation and tropical SSTs. This high correlation is due in part to the highly trended nature of both records, with warming temperatures over the tropical Pacific and Indian Oceans and rising accumulation in the Alaska Range. However, a significant correlation remains after detrending both time series (r = 0.39, p > 0.01), demonstrating the robust nature of the link between the tropical Pacific and Indian Ocean and North Pacific climate identified by previous studies42,43.

Figure 5 Relationship between tropical Pacific and Indian Ocean temperatures and accumulation on Mt. Hunter (triangle). (A) Spatial correlation between annual Mt. Hunter accumulation and annual surface temperature. (B) Time series of wintertime western tropical Pacific and Indian Ocean temperatures compared to Mt. Hunter wintertime accumulation. 20th Century Reanalysis V2 data27 from 1910–2010 is used in each panel. The black box delineates the Indian and western tropical Pacific Ocean region averaged in panel B. Colors only filled in areas exceeding 90% significance. Images obtained using Climate Reanalyzer (http://cci-reanalyzer.org), Climate Change Institute, University of Maine, USA. Full size image

In Fig. 6, we compare the Denali accumulation record to multiproxy records of SSTs in the Western Pacific and Indian Oceans44,45. Sustained and significant changes since ~1840 CE are evident in each dataset (Denali = 1840 CE, Western Pacific = 1834 CE, Indian = 1827 CE), which we confirm using the change detection methods described in Abram et al.44 (see Methods). This is consistent with the doubling of Denali snow accumulation since ~1840 being forced, at least in part, by warmer western Pacific and Indian Ocean SSTs through the atmospheric bridge43. The secular increase in Denali and Mt. Logan ice core sea salt Na+ concentrations over recent centuries has also been interpreted as an extratropical response to warming Pacific SSTs35,36, as has a dramatic increase in the Mt. Logan δ18O record at 184037 (Fig. S5). Similarly, a proxy record of Hawaii precipitation includes a secular change towards more PNA positive conditions, reflected as a long-term precipitation decrease (Fig. 6) associated with anomalously high pressure in the subtropical North Pacific46 (Fig. 4b). Thus, through the atmospheric teleconnection, warmer western tropical Pacific and Indian Ocean waters have supported an increase in Alaskan precipitation and a concomitant decrease in Hawaiian precipitation.

Figure 6 Paleoclimate context for Mt. Hunter precipitation increase. The Mt. Hunter annual accumulation time series (light grey) with a 21-year running mean (solid black) compared with reconstructed Hawaii rainfall46 (green), Western Pacific (red) and Indian Ocean (blue) SST anomalies44,45, and 21-year running mean GCM reconstructions of Western Pacific temperature anomalies (orange), and the NPI index (purple) from the CMIP5 Last Millennium experiments. With the exception of the GCM reconstructed NPI index, each dataset shows a sustained directional trend from the early 1800s to the present. Full size image

Inspection of Fig. 5b reveals that both the western tropical Pacific-Indian Ocean SSTs and Mt. Hunter wintertime accumulation have continued to progressively increase since the 1976/77 PDO transition. This post-1977 increase contrasts with steady wintertime precipitation at south coastal Alaska weather stations following the precipitation jump in 1976/77 (Fig. 4E). This may indicate a heightened sensitivity to tropical SST teleconnections at higher elevation in Alaska. Other studies from the nearby St. Elias range have inferred a similar sensitivity of high elevation hydroclimate to conditions in the tropical Pacific Ocean35,37. The Rossby waves responsible for these tropical teleconnections are best defined in the free troposphere43, which may explain the preferential expression of tropical connections at high altitudes in Alaska. Additionally, mountain summit areas are more representative of free-air conditions compared to stations located in valleys or along the coast where local effects may mask the tropical climate signal47.

We examined output from seven CMIP5 GCM simulations (Table S1) to assess whether Last Millennium Experiment reconstructions capture the Aleutian Low intensification we infer from the Mt. Hunter record. All of the GCM simulations show warming in the western Pacific (15°S–15°N, 110°–160°E) with an ensemble average warming of 0.57 °C during the last two centuries comparing well with the 0.5° warming estimate from paleoclimate records45. In contrast, only 3 of the 7 models show significant (p < 0.05) deepening of the Aleutian Low (defined by the NPI index) (Fig. 6). None of the models match the observed increase in precipitation over the central Alaska Range or southern Alaska (58°–62°N, 135°–155°W), nor does the ensemble show a consistent precipitation decrease in Hawaii (Fig. S7). Those models that do suggest a precipitation increase in southern Alaska (MPI-ESM48 and IPSL-CM5A-LR49) produce a 5–10% rise in precipitation during the last two centuries, considerably less than shown in the Denali accumulation record. Furthermore, these two models are both among the simulations that generate an Aleutian Low deepening since 1840, and have the greatest increase in Western Pacific SSTs, although the SST increase in both models is significantly larger than in proxy reconstructions45. We suggest that current GCMs underestimate the sensitivity of North Pacific atmospheric teleconnections to tropical SSTs, and thus further model development focused on improving the extra-tropical responses to tropical SST warming may improve North Pacific hydroclimate projections in a warming world.

Although the future response of central-eastern tropical Pacific SSTs to increased radiative forcing remains unclear50, GCM simulations show enhanced ocean warming in the western tropical Pacific and Indian Ocean through the 21st century in response to higher GHG concentrations51,52. The Mt. Hunter accumulation record corroborates and significantly extends instrumental (Fig. 4) and paleoclimate15,16 records showing precipitation increases in southern Alaska during the 20th century. While hemispheric-scale reconstructions have yet to reveal coeval intensification of 20th century hydroclimate anomalies outside of the North Pacific41, our data imply that regions such as Mt. Hunter, which are sensitive to tropical teleconnections, may continue to experience hydroclimate variability well outside the natural range of the past millennium.