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The above results indicate that HDEs over China exhibited significant decadal changes across the mid-1990s in observations. Numerical experiments are performed to estimate the contribution of anthropogenic forcing to these decadal changes, including changes in GHG concentrations and AA emissions, and to reveal the associated physical processes.
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A set of numerical experiments performed by the Global Ocean Mixed Layer 2.0 configuration of the Met Office Unified Model (MetUM-GOML2; Hirons et al., 2015; Peatman and Klingaman, 2018), which is an atmosphere–ocean–mixed-layer coupled model, is used to assess the contribution of the combined and individual GHG and AA forcings to the decadal changes in climate. The model is the same as that employed by Luo et al. (2019). The experimental setup [Table 1; same as Table 1 in Luo et al. (2019)] is the same as that in previous studies that investigated the decadal changes in the South Asian summer monsoon (Luo et al., 2019) and South China Sea summer monsoon (Lin et al., 2020). The decadal changes in summer precipitation over East Asia (Tian et al., 2018), temperature extremes over China (Chen and Dong, 2019), and heatwaves over China (Su and Dong, 2019) were investigated by performing the same experiments using an earlier configuration, MetUM-GOML1. The advantages of MetUM-GOML2 are its computational efficiency and its smaller biases in simulated sea surface temperature (SST) than coupled models with dynamic oceans. However, its lack of internal variability modes, such as El Nino–Southern Oscillation, may complicate the analysis of the response to prescribed forcing (Hirons et al., 2015; Dong et al., 2017; Luo et al., 2019).
Abbreviation Experiment Ocean Radiative forcing R0 Relaxation run Relaxation to PD (1994–2011) mean 3D ocean temperature and salinity to diagnose climatological temperature and salinity tendencies PD GHGs over 1994–2011 and AA emissions over 1994–2010 with AA after 2006 from RCP4.5 scenario (Lamarque et al., 2010, 2011) C-EP Early period (EP, 1964–81) Climatological temperature and salinity tendencies from relaxation run EP mean GHG and EP mean AA emissions C-PD Present day (PD, 1994–2011) GHG and AA forcings Climatological temperature and salinity tendencies from relaxation run PD mean GHG and PD mean AA emissions C-PD-GHG Present day (PD, 1994–2011) GHG forcing Climatological temperature and salinity tendencies from relaxation run PD mean GHG and EP mean AA emissions C-PD-AA Present day (PD 1994–2011) AA forcing Climatological temperature and salinity tendencies from relaxation run EP mean GHG and PD mean AA emissions Table 1. Summary of numerical experiments.
The HDE events in the model are defined according to the same criteria as in observations, except that the standard deviation of surface air temperature, the climatological mean ET, and SM percentile are computed based on both the C-EP and C-PD experiments.
The difference between any one of the PD experiments forced by a particular forcing and the C-EP experiment indicates the response to that forcing. The combined impact of changes in GHGs and AAs (hereafter ALL forcing), the individual impact of GHG changes (hereafter GHG forcing), and the individual impact of AA changes (hereafter AA forcing) are all examined. A two-tailed Student’s t-test is applied to assess the statistical significance of the mean changes.
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The HDE frequency and duration in the C-PD experiment are compared with those in observations for PD (Fig. 2). In observations, the HDE frequency and duration share similar spatial patterns, with relatively high values over SEC, NC and NEC (Figs. 2a and c), and relatively low values over western China. The spatial patterns of observed HDE frequency and duration are reproduced by MetUM-GOML2 (Figs. 2b and d), though the frequency and duration over NC are underestimated and the duration over SEC is overestimated. The area-averaged quantities of the HDE frequency and duration across China in the C-PD experiment (1.09 yr−1 and 0.61 pentads) are close to those observed (0.90 yr−1 and 0.51 pentads). The simulated area-averaged HDE frequency and duration over NEC (1.68 yr−1 and 0.87 pentads) and the HDE frequency over SEC (1.76 yr−1) are also similar to observations (1.56 yr−1, 0.81 pentads, and 1.53 yr−1). These agreements in the spatial pattern and area-averaged HDE frequency and duration between MetUM-GOML2 and observations indicate the model can capture the main observed features of HDEs over China.
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Figure 3 shows the spatial patterns of changes in HDE frequency and duration in response to different anthropogenic forcings in MetUM-GOML2 experiments. The HDE frequency and duration significantly increase over most of China in response to ALL forcing (Figs. 3a and b); the changes in frequency and duration share similar spatial patterns, with relatively large increases over SEC and NEC. The ALL-forced HDE changes are consistent with the observed changes (Figs. 3c and d), though the simulated response is weaker than the observed changes, especially over NC.
Figure 3. Spatial patterns of changes in HDE (a, c, e) frequency (units: yr−1) and (b, d, f) duration (units: pentads) in the growing season in response to changes in (a, b) ALL forcing, (c, d) GHG forcing, and (e, f) AA forcing, from EP to PD, masked by China’s boundary. The slashes highlight the regions where the differences are statistically significant at the 90% confidence level, based on a two-tailed Student’s t-test.
Comparing the changes forced by GHG and AA forcings shows that the GHG changes cause the increase in HDE frequency and duration. The spatial patterns of these changes in response to GHG forcing are similar to those in response to ALL forcing (Figs. 3c and d), but the magnitude of the GHG-forced changes is slightly larger than the ALL-forced ones. The AA-forced changes are weak, but local decreases in HDE frequency and duration are significant over SEC and NC (Figs. 3e and f).
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Figure 4 illustrates the area-averaged changes in HDE frequency and duration throughout China and the three sub-regions of NEC, NC and SEC in observations and model experiments. The exact quantities are summarized in Table 2. The averaged responses of HDEs to ALL forcing basically agree with observations throughout China and the three sub-regions. The increases in China-averaged HDE frequency and duration in the ALL forcing experiment are 0.53 yr−1 (77% of the observed 0.69 yr−1) and 0.17 pentads (61% of the observed 0.28 pentads), respectively. In the sub-regions, the ALL-forced changes in HDE frequency and duration over SEC are similar to observed. These results suggest that the observed decadal changes in HDE frequency and duration across the mid-1990s throughout China and over SEC can be primarily attributed to anthropogenic forcing. However, the simulated changes in frequency and duration over NC and NEC are much weaker than the observed changes. On the one hand, these regional discrepancies between model-simulated changes and observed changes might reflect model deficiencies in responses to anthropogenic forcing or a missing external forcing, such as volcanic forcing or solar forcing (e.g., Song et al., 2014; Shim et al., 2019), which are not included in our model experiments. On the other hand, decadal SST variability, such as the Pacific Decadal Oscillation (e.g., Qian and Zhou, 2014; Zhang et al., 2020) and the Atlantic Multidecadal Oscillation (e.g., Qian et al., 2014; Zhang et al., 2020), might also be an important factor for the observed decadal changes. These modes of variability are not simulated in MetUM-GOML2, because the model lacks ocean dynamics.
Figure 4. Area-averaged changes in HDE (a) frequency (units: yr−1) and (b) duration (units: pentads) in the growing season averaged over China, SEC, NC, and NEC, in observations and simulations forced by ALL forcing, GHG forcing, and AA forcing. The error bars indicate the 90% confidence intervals based on a two-tailed Student’s t-test.
HDE property Area OBS Forcings ALL GHG AA Frequency (yr−1) China 0.69* 0.53* 0.74* −0.16* SEC 1.11* 0.67* 0.92* −0.20* NEC 1.48* 0.53* 0.74* −0.12 NC 0.90* 0.28* 0.26* −0.1* Duration (pentads) China 0.28* 0.17* 0.24* −0.07* SEC 0.37* 0.25* 0.32* −0.01* NEC 0.67* 0.15* 0.22* −0.04 NC 0.41* 0.10* 0.11* −0.07* * HDE changes significant at the 90% confidence level based on a two-tailed Student’s t-test. Table 2. Area-averaged HDE changes in observations and model experiments.
The GHG and AA changes have opposite effects on HDEs (Figs. 3 and 4). GHG forcing increases the HDE frequency and duration across China and the three sub-regions, but AA forcing significantly decreases them, except over NC. The magnitude of GHG-forced changes is much larger than the AA-forced changes, indicating the dominant role of GHG forcing in anthropogenically forced increases in HDE frequency and duration over China.
Abbreviation | Experiment | Ocean | Radiative forcing |
R0 | Relaxation run | Relaxation to PD (1994–2011) mean 3D ocean temperature and salinity to diagnose climatological temperature and salinity tendencies | PD GHGs over 1994–2011 and AA emissions over 1994–2010 with AA after 2006 from RCP4.5 scenario (Lamarque et al., 2010, 2011) |
C-EP | Early period (EP, 1964–81) | Climatological temperature and salinity tendencies from relaxation run | EP mean GHG and EP mean AA emissions |
C-PD | Present day (PD, 1994–2011) GHG and AA forcings | Climatological temperature and salinity tendencies from relaxation run | PD mean GHG and PD mean AA emissions |
C-PD-GHG | Present day (PD, 1994–2011) GHG forcing | Climatological temperature and salinity tendencies from relaxation run | PD mean GHG and EP mean AA emissions |
C-PD-AA | Present day (PD 1994–2011) AA forcing | Climatological temperature and salinity tendencies from relaxation run | EP mean GHG and PD mean AA emissions |