Disentangling fast and slow responses of the East Asian summer monsoon to reflecting and absorbing aerosol forcings

Abstract

Abstract. We examine the roles of fast and slow responses in shaping the total equilibrium response of the East Asian summer monsoon (EASM) to reflecting (sulfate, SO 4 ) and absorbing (black carbon, BC) aerosol forcings over the industrial era using the Community Earth System Model version 1 (CESM1). Our results show that there is a clear distinction between fast and slow responses of the EASM to aerosol forcings and the slow climate response due to aerosol-induced change in sea surface temperature (SST) plays an important role in the impacts of aerosols on the EASM. The EASM is weakened by a decrease in land-sea surface thermal contrast in the fast response (FR) component to SO 4 forcing, whereas the weakening is more intensive due to the changes in tropospheric thermodynamic and dynamic structures in the slow response (SR) component to SO 4 . The total climate adjustment caused by SO 4 is a significant weakening of the EASM and a decrease in precipitation. The BC-induced fast adjustment strengthens the EASM both by increasing the local land-sea surface thermal contrast and shifting the East Asian subtropical jet (EASJ) northwards. The BC-induced slow climate adjustment, however, weakens the EASM through altering the atmospheric temperature and circulation. Consequently, the EASM is slightly enhanced, especially north of 30 • N, in the total response (TR) to BC. The spatial patterns of precipitation change over East Asia due to BC are similar in the total response and slow response.This study highlights the importance of ocean response to aerosol forcings in driving the changes of the EASM.

Changes in Anthropogenic PM and the Resulting Global Climate Effects Under the RCP4.5 and RCP8.5 Scenarios by 2050

“…Like other climatic variables, changes in the surface horizontal wind vector due to the reduction of PM 2.5 are very similar under RCP4.5 and RCP8.5 (Figures 9a and 9c). Enhancement of surface wind vectors from ocean to land is predicted over East Asia, as is the 10.1029/2019EF001285 cross-equatorial flow over the Indian Ocean; these results are consistent with those of previous studies (Wang, Lin, et al 2017; Figure 7. Figure 10 shows changes in low cloud cover (above 680 hPa), middle cloud cover (between 440 and 680 hPa), high cloud cover (below 440 hPa), and total cloud cover (TCC; sum of the low, middle, and high cloud fractions) due to changes in fine particles, and changes in TCC caused by coarse particles under RCP4.5 and RCP8.5 from 2014 to 2050.…”

“…The Monte Carlo Independent Column Approximation (McICA) scheme developed by Räisänen et al (2004) was applied to the model to resolve subgrid cloud variability. Previous studies have shown that this model simulates aerosol mass concentrations, optical properties, and climatology well Yang et al, 2016Yang et al, , 2017. The model has been used to study radiative forcing due to aerosols (Bond et al, 2013;Myhre et al, 2013;Wang, Zhang, Li, et al 2013;Zhang et al, 2012Zhang et al, , 2016 and its effects on climate Zhang et al, 2016;Zhao et al, 2017;Zhou et al, 2017).…”

The Role of Anthropogenic Aerosol Forcing in Interdecadal Variations of Summertime Upper‐Tropospheric Temperature Over East Asia

“…This study highlights the important role of anthropogenic aerosols in the interdecadal variations in UTT over East Asia. This may from a side imply that aerosol forcing affects the EASM by changing the thermodynamic structure in the upper troposphere rather than at the surface, as reported by Z. L. Wang, Lin, Yang, et al (2017). However, it should be noted that the aerosol-induced JA mean UTT trend over East Asia during the second half of the twentieth century is weaker than that of the reanalysis data ensemble mean (i.e., −0.14 vs. −0.2°C per decade).…”

“…There are significant positive stream function anomalies south of 40°N, with slight negative values between 10°N and 20°N, which lead to two anomalous clockwise meridional circulations, south of 20°N and between 20°N and 40°N, respectively. The aerosol-induced change in meridional circulation may be primarily attributed to the slow response of the ocean to aerosol forcing, that is, the asymmetric hemispheric change in SST (Ocko et al, 2014; Z. L. Wang, Lin, Yang, et al, 2017;Xu & Xie, 2015). However, there are negative stream function and counterclockwise circulation anomalies between 40°N and 70°N.…”

“…Scattering aerosols (e.g., sulfate, organic carbon, and nitrate), which cool the surface by reducing the solar radiation reaching the top of the atmosphere and surface, cause weak atmospheric forcing (Myhre et al, 2013). Recent studies have indicated that scattering aerosols can lead to significant tropospheric cooling via the ocean response to forcing (Ocko et al, 2014;Rotstayn et al, 2014;Z. L. Wang, Lin, Yang, et al, 2017;Xu & Xie, 2015).…”