Forced response and internal variability of summer climate over western North America

Abstract

23Over the past decade, anomalously hot summers and persistent droughts frequented over the western 24United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is 25 important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external 26 forcing and multidecadal modes of variability in sea surface temperature (SST), namely, the Pacific Decadal 27Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot 28 summers. In this study, 100-member ensemble simulations for 1951-2010 by an atmospheric general circulation 29 model (AGCM) were used to explore relative contributions of anthropogenic warming, atmospheric internal 30 variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing 31 historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing 32 agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean 33 temperature and frequency of hot summers (66% and 82%) over the wUS can be attributed to the anthropogenic 34 global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the 35 negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that 36 AMO-and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric 37 circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies 38 are not robust during the periods when PDO and AMO are in phase. The prolonged PDO-AMO antiphase period 39 since the late 20th century resulted in the substantial component of multidecadal anomalies in temperature and 40 precipitation over the wUS. 41 3

Removing Circulation Effects to Assess Central U.S. Land‐Atmosphere Interactions in the CESM Large Ensemble

“…Residual JJA SAT variability is highest along the arc of the Great Plains with almost no residual variability in the western U.S. (Figure b). The lack of residual σ west of the Rocky Mountains is consistent with the region being too dry overall for moisture variations to influence SAT (Kamae et al, ; Seneviratne et al, ). In the central U.S., the magnitude of residual variability suggests that the land surface feedbacks may be responsible for more than a quarter of total SAT σ .…”

Compound Drought and Heatwaves at a Global Scale: The Role of Natural Climate Variability‐Associated Synoptic Patterns and Land‐Surface Energy Budget Anomalies

“…The role of PDO in modulating the CDHW events over the eastern part of Asia has been previously reported (Yu et al, 2018). Previous studies also suggest that the anomalies over the northeastern and tropical Pacific have a significant impact on the occurrence of CDHW events in the conterminous U.S. (Dulière et al, 2013; Kamae et al, 2017; McCabe et al, 2004; Peterson et al, 2013).…”

Ocean warming pattern effects on future changes in East Asian atmospheric rivers

“…The ensemble simulations were developed with random initial and SST perturbations (Hirahara et al 2014). In this study, a 10-member ensemble mean was examined to reduce the effect of atmospheric internal variability (Kamae et al 2017a(Kamae et al , 2017b.…”

“…Hawkins andSutton 2009, Thompson et al 2015) by using large-member ensemble simulations (e.g. Deser et al 2012, Hagos et al 2016, Kamae et al 2017a, 2017b for quantitative evaluation of forced response to global warming.…”