Dramatic hydroclimate shifts occurred in western North America during the last deglaciation, but the timing and mechanisms driving these changes are uncertain and debated, and previous modeling has largely relied on linear interpolations between equilibrium snapshot simulations. Using a published transient climate simulation and a range of proxy records, we analyze the region's climate evolution in order to identify the mechanisms governing hydroclimate shifts. A rapid loss of ice around 14,000 years ago causes an abrupt reorganization of the circulation, which precipitates drying and moistening of southwestern and northwestern North America, respectively. The atmospheric circulation transitions between two states on a timescale of decades to centuries, during which time the westerly jet shifts north by about 7°. In contrast to previous studies, we find that changes in the water budget of western North America prior to this event are not attributable to variations in the position of the jet, but rather to the intensity of moisture transport into the continent.

1 Introduction The climate evolution of western North America during the last glacial period (∼110–12 kyr B.P.; kyr, 103 years) indicates dramatic changes in regional water budgets and aridity [Whitlock and Bartlein, 1997; Asmerom et al., 2010; Wagner et al., 2010; Lyle et al., 2012; Munroe and Laabs, 2013; Ibarra et al., 2014; Reheis et al., 2014; Oster et al., 2015] linked to large‐scale Northern Hemisphere changes associated with global climatic events [Whitlock and Bartlein, 1997; Zic et al., 2002; Benson et al., 2003; North Greenland Ice Core Project members, 2004; Denniston et al., 2007; Asmerom et al., 2010; Wagner et al., 2010; Reheis et al., 2014; Marcott et al., 2014]. Western North America supported large pluvial lake systems during the last deglaciation in regions that today are arid and undergoing worsening drought with anthropogenic climate change [Seager et al., 2007]. These lakes experienced highstands before quickly falling to near‐modern levels of dryness around the onset of the Bølling‐Allerød interstadial (14.7 kyr B.P.) [Lyle et al., 2012; Munroe and Laabs, 2013; Kirby et al., 2013; Reheis et al., 2014]. A range of proxies from the wider region have been used to identify links between the local hydroclimate and Northern Hemisphere climatic events related to the North Atlantic [Zic et al., 2002; Benson et al., 2003; Denniston et al., 2007; Asmerom et al., 2010; Wagner et al., 2010], but considerable uncertainty remains regarding the atmospheric teleconnections and mechanisms that impacted the region's water budget during this interval. A common explanation for moisture fluctuations throughout the southwest involves variations in the position of the local jet stream and/or storm track [COHMAP Members, 1988; Hostetler and Benson, 1990; Bartlein et al., 1998; Benson et al., 1995; Munroe and Laabs, 2013; Wong et al., 2016]; however, these variations have only been qualitatively inferred from proxy data, ignoring possible changes in the relative positions of the jet, moisture delivery, and precipitation in the region. Moreover, the timing and magnitude of any such changes [Benson et al., 2003], as well as the importance of other contributions to moisture fluctuations like increased summer precipitation [Lyle et al., 2012], are debated. Meanwhile, the atmosphere and climate dynamics of the eastern North Pacific sector during this important interval have received relatively little attention in comparison to the North Atlantic [Pausata et al., 2011; Löfverström et al., 2014, 2016; Löfverström and Liakka, 2016]. The presence of massive North American ice sheets during the Last Glacial Maximum (LGM) affected the atmospheric circulation of the Northern Hemisphere by causing the westerly jet over the eastern North Pacific to split or be deflected south relative to its modern average position [Manabe and Broccoli, 1985; COHMAP Members, 1988; Bartlein et al., 1998; Bromwich et al., 2004; Löfverström et al., 2014], and climate changes in the region during the subsequent deglacial likely resulted in large part from changes in mechanical forcing from the ice sheets [Cook and Held, 1988; Kageyama and Valdes, 2000; Pausata et al., 2011; Löfverström et al., 2014, 2016]. Deglacial ice loss occurred roughly from southwest to northeast, with the Cordilleran ice sheet effectively disappearing by the end of the Pleistocene and the Laurentide becoming progressively confined to northeastern Canada. Retreat of these ice sheets peaked around the onset of the Bølling‐Allerød, with a massive loss of North American ice volume occurring sometime between 15 and 14 kyr B.P. (Figure S1 in the supporting information). After an initial separation of the Cordilleran and Laurentide ice sheets, ice sheet melting accelerated due to “saddle collapse” [Gregoire et al., 2012], and the resulting release of meltwater from North American ice sheets likely contributed a substantial fraction of Meltwater Pulse 1‐A (MWP1A) [Gregoire et al., 2012; Carlson and Clark, 2012; Tarasov et al., 2012]. This enormous meltwater event is constrained to have occurred between 14.6 and 14.3 kyr B.P., essentially coeval with Bølling warming [Deschamps et al., 2012]. Prior efforts to simulate the deglacial evolution of western North American climate, including in response to ice sheet retreat, have relied on interpolations between equilibrium climate experiments spaced in time [COHMAP Members, 1988; Hostetler and Benson, 1990; Bartlein et al., 1998], precluding a determination of the pace of simulated changes, transient responses, or threshold behaviors, as well as comparison with high‐resolution proxy records. Here we investigate the evolution of the climate of the North Pacific and western North American region in a transient simulation of the last deglaciation with a synchronously coupled atmosphere‐ocean general circulation model, asynchronously coupled to changing ice sheets (supporting information). We analyze published and publicly available output from the Transient Climate Evolution (TraCE) simulation of the last 22 kyr, run with the National Center for Atmospheric Research (NCAR) Community Climate System Model version 3 (CCSM3), which has been documented elsewhere [Liu et al., 2009; He, 2011; Shakun et al., 2012; He et al., 2013; Liu et al., 2014]. Briefly, the simulation is initialized from an equilibrium LGM state and forced by realistic transient forcing including orbitally driven insolation, atmospheric greenhouse gases, and prescribed meltwater forcing and land ice. TraCE compares favorably with key features of the global climate evolution as reconstructed from paleoclimate proxies [Shakun et al., 2012; He et al., 2013; Liu et al., 2014], despite known biases of the low‐resolution CCSM3, like excessive sea ice [Yeager et al., 2006]. TraCE attains Bølling warming in response to Atlantic meridional overturning circulation recovery following Heinrich Event 1, though forced by a sudden termination of freshwater input [Liu et al., 2009].

3 Discussion The large‐scale topography of the ice sheets excites stationary waves that affect the atmospheric circulation, and there is still considerable uncertainty in the thickness of ice sheets from various reconstructions [Ullman et al., 2014]. Nevertheless, models using a variety of LGM ice sheet reconstructions have shown that southward shifts of the midlatitude jet over the North Pacific and near the North American coast are robust for that interval [Manabe and Broccoli, 1985; Yanase and Abe‐Ouchi, 2007; Laîné et al., 2009; Kageyama et al., 2013; Ullman et al., 2014; Merz et al., 2015], and discrepancies decrease later in the deglaciation [Ullman et al., 2014]. Importantly, recent sensitivity studies to changing North American ice sheets also indicate a nonlinear transition in the circulation response to ice sheet extent and topography [Löfverström et al., 2014]. Löfverström et al. suggest that interactions of the mean flow with the stationary waves excited by the Laurentide ice sheet are muted by the preexisting stationary wave response to the Rocky Mountains, and therefore the circulation only responds discernibly when the ice sheet is large enough (close to LGM conditions) to overcome this effect at midlatitudes. In TraCE, the disposition of stationary waves prior to the transition at 13.9 kyr B.P. is similar to that at the LGM, with an enhanced trough and ridge in geopotential height over the North Pacific and North America, respectively, at 15.5 kyr B.P., and very little differences at 14.5 kyr B.P. (Figures 4g and 4h). But following the ice sheet retreat, both upstream and downstream troughs shallow, and the strong ridge over western North American diminishes considerably, affecting the midlatitudes. This change in the arrangement of the stationary waves coincides with the reorganization of the circulation, in agreement with this proposed mechanism [Löfverström et al., 2014]. We have shown that an abrupt transition of the simulated circulation occurs in direct response to a sudden drop in North American continental ice, which contrasts with more gradual long‐range responses to hemispheric climatic events of the deglaciation, and that the associated climate changes are consistent with various proxies from the region. However, neither the available proxies nor the TraCE simulation are able to conclusively identify the timing of this transition. The low‐resolution Lahontan lake level records suggest an earlier transition (Figure 1i), and the speleothem records are incomplete and noisy across this interval but show a decrease followed by an increase in δ18O around 14 kyr B.P. (Figure 1j). At the same time, continental ice sheet extent and topography in TraCE are prescribed and updated at irregular intervals [He, 2011] based on the ICE‐5G reconstruction [Peltier, 2004]. The transition between 15 kyr B.P. and 14 kyr B.P. ice sheets (Figure S1) is specified in the simulation at 13.87 kyr B.P. [He, 2011]. This corresponds to the conclusion of the prescribed Meltwater Pulse 1‐A, which in the simulation is specified between 14.35 and 13.85 kyr B.P. Subsequent work has constrained the timing of MWP1A to between 14.6 and 14.3 kyr B.P. [Deschamps et al., 2012], so the simulation's ice sheet retreat likely occurs too late by several hundred years, compared to other climate events of the deglaciation. Furthermore, the ice sheet change at 13.87 kyr B.P. corresponds to the largest single decrease of ice sheet volume for the last deglaciation but is prescribed to occur instantaneously. Though this step is obviously artificial, there are only marginal changes in the ICE‐5G ice sheets between 15 and 14.5 kyr B.P. Thus, the total ice sheet retreat corresponding to this step in the simulation likely occurred in under 500 years, in agreement with the estimate of timing of MWP1A [Deschamps et al., 2012], and still considerably shorter than the duration of the Bølling‐Allerød interstadial. The response of the climate system to this change is also extremely rapid (see, for example, Figure 3), so the reorganization of the circulation also plausibly occurred on a timescale of decades to centuries. Though previous idealized experiments with varying ice sheet height have identified nonlinear responses to ice sheet topography in some aspects of the climate system [Löfverström et al., 2014; Zhang et al., 2014; Lee et al., 2015], the sensitivity of the atmospheric circulation over the North Pacific to a more incremental separation of the Laurentide and Cordilleran ice sheets than incorporated in TraCE remains unresolved.

4 Conclusions Though we are limited to a single model, TraCE performs well against a range of climate proxies [Liu et al., 2009; Shakun et al., 2012; He et al., 2013; Liu et al., 2014], including hydroclimate proxies from western North America. Our results suggest that the hydroclimate of the eastern North Pacific experienced an abrupt change in response to changing ice sheet forcing during the last deglaciation, in addition to more gradual changes associated with hemisphere‐wide climatic evolution. We find that estimates of local water budgets from the early deglaciation (20–15 kyr B.P.) are not diagnostic of the position of the jet stream or the storm track. But at around the beginning of the Bølling, a rapid change in moisture between the Pacific Northwest and the southwest and the associated regression of lakes are due to a sudden northward shift of westerlies, driven by rapid ice loss. While the exact timing (and pace) of the ice sheet changes is uncertain, based on dating of ice sheet retreat and MWP1A, it is likely that this transition occurred on the timescale of a few centuries. Similarly, it is plausible that changes in moisture transport and precipitation intensity at the onset of Bølling warming, and the abrupt reorganization of the circulation caused by ice sheet retreat, occurred nearly simultaneously, prompting the sudden regression of Great Basin lakes. Either way, it appears that two forcing mechanisms—namely, Bølling warming and ice sheet retreat, which themselves are probably causally connected—contributed to dramatic climate changes in the North Pacific and western North America, completely reorganizing the region's climate system in under 1000 years.

Acknowledgments We thank Z. Liu, B. Otto‐Bliesner, F. He, and their collaborators for designing, running, and making publicly available the results of the TraCE simulation. We further acknowledge the PMIP3 climate modeling groups (supporting information Table S1), as well as the NASA/Goddard Space Flight Center's Laboratory for Atmospheres and NOAA/OAR/ESRL PSD for computing and providing the GPCP combined precipitation data. We also thank R. Caballero and an anonymous reviewer for valuable comments that improved the manuscript. This work was supported by National Science Foundation fellowship AGS‐PRF‐1524866 to J.M.L. and CAREER award EAR‐1352212 to A.E.T., as well as “Laboratoire d'Excellence” LabexMER (ANR‐10‐LABX‐19), cofunded by a grant from the French government under the program “Investissements d'Avenir,” to A.E.T. Data used in this work are listed in the references and publicly available online at Earth System Grid Federation repositories.

Supporting Information Filename Description grl55194-sup-0001-supplementary.pdfPDF document, 8.4 MB Supporting Information S1 grl55194-sup-0002-supplementary.gifapplication/unknown, 19.3 MB Movie S1 Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.