Pacific North American circulation pattern links external forcing and North American hydroclimatic change over the past millennium

Abstract

Land and sea surface temperatures, precipitation, and storm tracks in North America and the North Pacific are controlled to a large degree by atmospheric variability associated with the Pacific North American (PNA) pattern. The modern instrumental record indicates a trend toward a positive PNA phase in recent decades, which has led to accelerated warming and snowpack decline in northwestern North America. The brevity of the instrumental record, however, limits our understanding of long-term PNA variability and its directional or cyclic patterns. Here we develop a 937-y-long reconstruction of the winter PNA based on a network of annually resolved tree-ring proxy records across North America. The reconstruction is consistent with previous regional records in suggesting that the recent persistent positive PNA pattern is unprecedented over the past millennium, but documents patterns of decadal-scale variability that contrast with previous reconstructions. Our reconstruction shows that PNA has been strongly and consistently correlated with sea surface temperature variation, solar irradiance, and volcanic forcing over the period of record, and played a significant role in translating these forcings into decadal-to-multidecadal hydroclimate variability over North America. Climate model ensembles show limited power to predict multidecadal variation in PNA over the period of our record, raising questions about their potential to project future hydroclimatic change modulated by this circulation pattern.paleoclimate | atmospheric circulation | Holocene | climate change |

Timing and Potential Causes of 19th-Century Glacier Advances in Coastal Alaska Based on Tree-Ring Dating and Historical Accounts

“…Together, these regional data suggest that increased winter snow accumulation may have contributed to positive mass balances on glaciers throughout southern coastal Alaska. This conclusion is supported by proxy records in downwind North America, where reconstructions of the wintertime PNA exhibit shifts toward more positive PNA patterns in the early CE 1800s, a teleconnection pattern that also implies a strengthening AL and hence increased winter precipitation in the GOA (Figure 8C; Liu et al, 2017). Additionally, oxygen-isotope proxies of paleoclimate from carbonates in lake sediment and from the Mount Logan ice core suggest that an abrupt shift to a stronger AL occurred between the mid-18th and early-19th centuries ( Figure 8D; Fisher et al, 2004;Anderson et al, FIGURE 8 | Shifts or trends toward a stronger Aleutian Low pressure system preceded the 19th century glacier advance in Coastal Alaska (blue box).…”

“…(B) Accumulation rate of snow on Mount Hunter in the Alaska Range (Winski et al, 2017). (C) Reconstructed Pacific North American atmospheric circulation pattern based on tree rings and varved lake sediments from downwind North America, where more positive values coincide with a stronger Aleutian Low (Liu et al, 2017). (D) Oxygen isotopes of carbonate deposited in Jellybean Lake in the Yukon Territory (Anderson et al, 2005).…”

Navajo Nation, USA, Precipitation Variability from 2002 to 2015

“…PNA is known to exert strong control over winter storm tracks across North America (Wallace and Gutzler ). Previous work has suggested a weak association between PNA and 20th‐century winter precipitation anomalies (Leathers et al ) in the southwestern USA, although long‐term paleoclimate data have suggested that variation in PNA is correlated with drought in the region over the past millennium (Liu et al ). This correlation suggests that long‐term trends in the PNA pattern, such as those suggested by paleoclimate records (Liu et al ), could impact future winter precipitation and water resources in the Navajo Nation.…”

Robust Increases in Extreme Pacific North American Events Under Greenhouse Warming

“…Several modeling studies have investigated how greenhouse warming affects changes in the PNA mean state and found a trend toward a more PNA+ pattern in response to increased greenhouse gas emissions (Allan et al, 2014;Chen et al, 2018;Gan et al, 2017;Ning & Bradley, 2016;Zhou et al, 2014). There is also some geological evidence that the twentieth century PNA patterns tend to be more positive than those of the past centuries or millennia (Hubeny et al, 2011;Liu et al, 2017;Moore et al, 2002;Trouet & Taylor, 2010). However, few studies have explored how extreme PNA events will change in response to greenhouse warming.…”