Natural Variability Has Slowed the Decline in Western U.S. Snowpack Since the 1980s

Siler, Nicholas; Proistosescu, Cristian; Po‐Chedley, Stephen

doi:10.1029/2018gl081080

Cited by 38 publications

(37 citation statements)

References 38 publications

(49 reference statements)

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“…Numerous modeling studies have also confirmed that the tropical‐SST‐induced upper‐level divergence anomaly can excite a Rossby wave train propagating poleward into the extratropics, causing significant remote atmospheric responses (Ding et al, ; Lin et al, ; O'Reilly, ). The SST forcing and associated response of upper‐level atmospheric circulation changes are also consistent with the recent findings that focus on the implications of atmospheric circulation/SST trends on snowpack changes in North America (Mudryk et al, ; Siler et al, ). In the present case, the AMIP models simulate a remote atmospheric response of AL weakening to the tropical SST forcing, and the northward propagating wave train acts as the important dynamical bridge linking the extratropical AL weakening to the changes in the tropical SST forcing.…”

Section: Resultssupporting

confidence: 87%

Spring Aleutian Low Weakening and Surface Cooling Trend in Northwest North America During Recent Decades

Sun

Kucharski

et al. 2019

JGR Atmospheres

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State‐of‐the‐art climate models predict an enhanced warming over global land areas in response to the greenhouse gas (GHG) forcing. In nature, however, the observed land surface warming is not spatially uniform in recent decades as GHG concentration has risen rapidly. There is no warming and even cooling trend in spring northwest North America (NWNA) surface temperature since 1980. Here, we link the observed multidecadal surface cooling trend in NWNA to the weakening of spring Aleutian low (AL), which is ecologically important during the growing season. The AL weakening leads to reductions of southwesterly wind and warm air transport from eastern North Pacific to NWNA. The spring AL weakening and associated NWNA surface cooling are reasonably reproduced by AMIP models, driven by the historical external forcing and imposed sea ice concentration (SIC) and sea surface temperature (SST) from observations. Further investigations into the roles of different forcing components suggest that the changes in the AL are most likely driven by the tropical SST changes in recent decades while the external forcing and SIC play minor roles. There is a zonal dipole pattern of trends in tropical SSTs since 1980, with central‐eastern Pacific cooling and warming in other tropical basins, and the atmospheric responses to this SST change show a meridionally propagating Rossby wave train from tropics into subarctic North Pacific, which mediates the SST forcing and the AL changes.

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Section: Resultssupporting

confidence: 87%

Spring Aleutian Low Weakening and Surface Cooling Trend in Northwest North America During Recent Decades

Sun

Kucharski

et al. 2019

JGR Atmospheres

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“…Contrary to the RKT results, few stations were detected to have significant trends ( α = 5%) in 1 April and peak SWE , timing of snowmelt, ROS frequency, and annual maximums of W . The lack of at‐site statistically significant trends was expected and consistent with previous studies because of the short SNOTEL data record and current mode of natural variability (Harpold et al, ; Siler et al, ).…”

Section: Discussionsupporting

confidence: 85%

“…In addition to the RKT, we performed trend analysis at individual SNOTEL stations using the nonparametric Mann-Kendall (M-K) test (Kendall, 1975;Mann, 1945) and simple linear regression ( Figures S8-S12). Contrary to the RKT results, few stations were detected to have significant trends (α = 5%) in 1 April and peak SWE, timing of snowmelt, ROS frequency, and annual maximums of W. The lack of at-site statistically significant trends was expected and consistent with previous studies because of the short SNOTEL data record and current mode of natural variability (Harpold et al, 2012;Siler et al, 2018).…”

Section: Discussionsupporting

confidence: 85%

Observed Spatiotemporal Changes in the Mechanisms of Extreme Water Available for Runoff in the Western United States

Yan

Sun

Wigmosta

et al. 2019

Geophysical Research Letters

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This paper presents the first study to identity, in historical records, regional changes in the mechanisms of extreme water available for runoff (W). We used a quality‐controlled Snowpack Telemetry data set (1979–2017) combined with the nonparametric regional Kendall test to examine changes in annual maximum W under four hydrometeorological conditions (melt only/rain‐on‐snow/all melt/all melt plus rainfall) over the mountainous regions of the western United States. Under a warming climate, our analyses indicated significant declining trends in annual maximum W at regional scale under all four conditions. The annual maximum of all melt plus rainfall decreased significantly by 15% in the southwestern United States, while the frequency of rain‐on‐snow events increased significantly by 32% in the northwestern United States. The annual maximum snowmelt only decreased significantly by 21% across the entire western United States. Our results confirmed that interaction between regional humidity and solar radiation with warming temperature helps drive these changes.

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“…Dynamical adjustment of precipitation is more challenging than for temperature (Saffioti et al 2016) because precipitation more spatially heterogeneous and thus less predictable from circulation. In addition, precipitation depends on location and orography (Houze 2012), and in some regions, forced precipitation changes are driven by atmospheric circulation (Yin 2005;He and Soden 2017;Siler et al 2019). Nonetheless, a large fraction of precipitation variability in Europe and North America is related to internal variability in atmospheric circulation (Saffioti et al 2016;Fereday et al 2018;Lehner et al 2018;Guo et al 2019).…”

Section: Fig 1 Conceptual Illustration Of Causal Relationships Assumedmentioning

confidence: 99%

Uncovering the Forced Climate Response from a Single Ensemble Member Using Statistical Learning

et al. 2019

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Internal atmospheric variability fundamentally limits predictability of climate and obscures evidence of anthropogenic climate change regionally and on time scales of up to a few decades. Dynamical adjustment techniques estimate and subsequently remove the influence of atmospheric circulation variability on temperature or precipitation. The residual component is expected to contain the thermodynamical signal of the externally forced response but with less circulation-induced noise. Existing techniques have led to important insights into recent trends in regional (hydro-) climate and their drivers, but the variance explained by circulation is often low. Here, we develop a novel dynamical adjustment technique by implementing principles from statistical learning. We demonstrate in an ensemble of Community Earth System Model (CESM) simulations that statistical learning methods, such as regularized linear models, establish a clearer relationship between circulation variability and atmospheric target variables, and need relatively short periods of record for training (around 30 years). The method accounts for, on average, 83% and 78% of European monthly winter temperature and precipitation variability at gridcell level, and around 80% of global mean temperature and hemispheric precipitation variability. We show that the residuals retain forced thermodynamical contributions to temperature and precipitation variability. Accurate estimates of the total forced response can thus be recovered assuming that forced circulation changes are gradual over time. Overall, forced climate response estimates can be extracted at regional or global scales from approximately 3–5 times fewer ensemble members, or even a single realization, using statistical learning techniques. We anticipate the technique will contribute to reducing uncertainties around internal variability and facilitating climate change detection and attribution.

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Natural Variability Has Slowed the Decline in Western U.S. Snowpack Since the 1980s

Cited by 38 publications

References 38 publications

Spring Aleutian Low Weakening and Surface Cooling Trend in Northwest North America During Recent Decades

Spring Aleutian Low Weakening and Surface Cooling Trend in Northwest North America During Recent Decades

Observed Spatiotemporal Changes in the Mechanisms of Extreme Water Available for Runoff in the Western United States

Uncovering the Forced Climate Response from a Single Ensemble Member Using Statistical Learning

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