Abstract
Abstract This study investigated the drivers and physical processes for the abrupt decadal summer surface warming and increases in hot temperature extremes that occurred over Northeast Asia in the mid-1990s. Observations indicate an abrupt increase in summer mean surface air temperature (SAT) over Northeast Asia since the mid-1990s. Accompanying this abrupt surface warming, significant changes in some temperature extremes, characterized by increases in summer mean daily maximum temperature (Tmax), daily minimum temperature (Tmin), annual hottest day temperature (TXx), and annual warmest night temperature (TNx) were observed. There were also increases in the frequency of summer days (SU) and tropical nights (TR). Atmospheric general circulation model experiments forced by changes in sea surface temperature (SST)/sea ice extent (SIE), anthropogenic greenhouse gas (GHG) concentrations, and anthropogenic aerosol (AA) forcing, relative to the period 1964–93, reproduced the general patterns of observed summer mean SAT changes and associated changes in temperature extremes, although the abrupt decrease in precipitation since the mid-1990s was not simulated. Additional model experiments with different forcings indicated that changes in SST/SIE explained 76% of the area-averaged summer mean surface warming signal over Northeast Asia, while the direct impact of changes in GHG and AA explained the remaining 24% of the surface warming signal. Analysis of physical processes indicated that the direct impact of the changes in AA (through aerosol–radiation and aerosol–cloud interactions), mainly related to the reduction of AA precursor emissions over Europe, played a dominant role in the increase in TXx and a similarly important role as SST/SIE changes in the increase in the frequency of SU over Northeast Asia via AA-induced coupled atmosphere–land surface and cloud feedbacks, rather than through a direct impact of AA changes on cloud condensation nuclei. The modelling results also imply that the abrupt summer surface warming and increases in hot temperature extremes over Northeast Asia since the mid-1990s will probably sustain in the next few decades as GHG concentrations continue to increase and AA precursor emissions over both North America and Europe continue to decrease.
Highlights
During the 20th century the global averaged surface air temperature (SAT) showed an unequivocal warming due to anthropogenic as well as natural causes (e.g., Kosaka and Xie, 2013; Trenberth et al, 2014; Steinman et al, 2015)
We sought to quantify the relative roles of changes in (1) sea surface temperature (SST)/sea ice extent (SIE), (2) anthropogenic greenhouse gas (GHG) forcing through its direct impact, and (3) anthropogenic aerosol (AA) forcing through aerosol–radiation and aerosol–cloud interactions, in shaping these regional changes in surface warming and temperature extremes, by performing a set of numerical experiments using the atmospheric component of HadGEM3
The changes in SAT due to aerosol forcing manifested as a band (40◦–55◦N) of positive anomalies extending from Europe to Northeast Asia, with a magnitude of 0.2◦C–0.8◦C (Fig. 6g); whereas, the direct impact of changes in GHG concentrations induced more localized warming over Europe, with weak warming over Northeast Asia (Fig. 6e). These results indicate that the direct impact of changes in anthropogenic GHG and AA changes plays an important role in midlatitude warming over the Eurasian continent, which is consistent with the recent studies of Kamae et al (2014a) and Andrews (2014)
Summary
During the 20th century the global averaged surface air temperature (SAT) showed an unequivocal warming due to anthropogenic as well as natural causes (e.g., Kosaka and Xie, 2013; Trenberth et al, 2014; Steinman et al, 2015). The SFND pattern and warming over Northeast Asia have emerged strongly since the mid-1990s (Kwon et al, 2007; Chen and Lu, 2014; Han et al, 2015; Ueda et al, 2015) This is roughly coincident with rapid increases in aerosol emissions from China and India and decreases in aerosol emissions from North America and Europe. Previous studies have demonstrated the effects of recent decadal SST variation on decadal change in summer precipitation and surface warming over East Asia (e.g., Feng and Hu, 2008; Han et al, 2015; Ueda et al, 2015), a comprehensive and quantitative understanding of the drivers and physical processes governing the SFND pattern, and the associated changes in temperature extremes over Northeast Asia, is still lacking.
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