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We used the monthly observations from Climatic Research Unit gridded Time Series Version 4.03 (CRU TS v.4.03) with a horizontal resolution of 0.5° latitude × 0.5° longitude as validation datasets (Harris et al., 2020). The three variables of monthly precipitation, temperature, and PET from 1960–2018, were employed in this study.
Monthly PET values in CMIP6 simulations were calculated following the Penman-Monteith approach (Penman, 1948; Monteith, 1965), which is derived based on surface moisture and energy balance and recommended by the Food and Agricultural Organization (FAO) of the United Nations (Allen et al., 1998). The detailed calculation of PET is as follows:
where PET (mm d–1) is the reference evapotranspiration rate, Rn (MJ m–2 h–1) and G (MJ m–2 h–1) are the net radiation and soil heat flux, respectively. Tas (°C) is 2 m air temperature, u2 (m s–1) is 2 m wind speed, Δ (kPa °C–1) represents the slope of the saturation vapor pressure–temperature relationship, and γ (kPa °C–1) is the psychrometric constant.
Specifically, es–ea (kPa) represents the difference between saturation vapor pressure (es) and actual vapor pressure (ea), i.e., vapor pressure deficit (VPD). In CMIP6 simulations, es and ea can be calculated following Eqs. (2–3):
where RH (%) is relative humidity.
According to Eqs. (1–3), the relevant input variables for calculating PET from CMIP6 outputs include the near-surface air temperature (tas), relative humidity (hurs), 10 m wind speed (uas, vas), longwave (rlds, rlus), and shortwave (rsds, rsus) radiation. Thus, we selected the historical simulations for the period 1980–2014 from 27 CMIP6 models (Eyring et al., 2016; Table 1) to assess their respective performance. This is the maximum number of models with available data that allow us to diagnose the PET simulation. Only the first realization of each model was employed in this study. To facilitate calculations and comparisons, all outputs were first re-gridded to 1.5° latitude × 1.5° longitude via a bilinear interpolation.
No. Model Name Institute, Country Resolution
(Lat×Lon×Level)1 ACCESS-CM2 Commonweaalth Scientific and Industrial Research Organization (CSIRO) and Bureau of Meteorology (BoM), Australia 144×192×85 2 ACCESS-ESM1-5 145×192×38 3 BCC-CSM2-MR Beijing Climate Center (BCC), China 160×320×46 4 CanESM5 Canadian Centre for Climate Modelling and Analysis (CCCma), Canada 64×128×49 5 CMCC-CM2-SR5 The Euro-Mediterranean Centre on Climate Change (CMCC) 192×288×47 6 CNRM-CM6-1 Centre National de Recherches Meteorologiques/Centre Europeen de Recherche et Formation Avancees en Calcul Scientifique (CNRM-CERFACS), France 128×256×91 7 EC-Earth3 EC-Earth-Consortium, Europe 256×512×91 8 EC-Earth3-CC EC-Earth-Consortium, Europe 256×512×91 9 EC-Earth3-Veg EC-Earth-Consortium, Europe 256×512×91 10 EC-Earth3-Veg-LR EC-Earth-Consortium, Europe 160×320×62 11 FGOALS-f3-L Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP-CAS), China 180×288×32 12 GFDL-CM4 NOAA’s Geophysical Fluid Dynamics Laboratory (NOAA-GFDL), USA 180×288×33 13 GFDL-ESM4 NOAA’s Geophysical Fluid Dynamics Laboratory (NOAA-GFDL), USA 180×288×49 14 GISS-E2-1-G NASA Goddard Institute for Space Studies (NASA-GISS), USA 90×144×40 15 HadGEM3-GC31-LL Met Office Hadley Centre (MOHC), UK 144×192×85 16 HadGEM3-GC31-MM Met Office Hadley Centre (MOHC), UK 324×432×85 17 INM-CM4-8 Institute for Numerical Mathematics (INM), Russia 120×180×21 18 INM-CM5-0 Institute for Numerical Mathematics (INM), Russia 120×180×73 19 IPSL-CM6A-LR Institute Pierre Simon Laplace (IPSL), France 143×144×79 20 KACE-1-0-G National Institute of Meteorological Sciences, Korea Meteorological Administration (NIMS-KMA), Korea 144×192×63 21 KIOST-ESM Korea Institute of Ocean Science and Technology (KIOST), Korea 96×192×32 22 MIROC6 Atmosphere and Ocean Research Institute (AORI, the University of Tokyo), National Institute for Environmental Studies, and Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan 128×256×80 23 MIROC-ES2L 64×128×62 24 MPI-ESM1-2-HR Max Planck Institute for Meteorology (MPI-M), Germany 192×384×95 25 MPI-ESM1-2-LR Max Planck Institute for Meteorology (MPI-M), Germany 96×192×47 26 MRI-ESM2-0 Meteorological Research Institute (MRI), Japan 160×320×80 27 UKESM1-0-LL Met Office Hadley Centre (MOHC), UK 144×192×85 Table 1. Introduction to the 27 CMIP6 models used in this study.
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Drylands are generally measured by the aridity index (AI; Middleton and Thomas, 1992; Hulme, 1996), which is defined as the ratio of annual precipitation to PET (i.e., AI = P/PET). In this study, we identified global drylands in both observations and simulations as regions with an annually averaged AI, based on 1960–2018 observations, of less than 0.65 following previous studies (Feng and Fu, 2013; Huang et al., 2016).
The drought index is measured by the Standardized Precipitation Evapotranspiration Index (SPEI), which is defined as a log-logistic probability distribution of the difference between precipitation and PET (P–PET) (Vicente-Serrano et al., 2010). The observed and simulated SPEI were calculated using monthly precipitation and PET from CRU and CMIP6 outputs, respectively. The SPEI can be calculated at different time scales (≥1 month). For the responses of arid biomes to droughts on short timescales (Vicente-Serrano et al., 2013), we employed the 6-month SPEI (SPEI-06) to identify drought events in this study. Droughts were then divided into four categories, i.e., above mild (SPEI≤–0.5), above moderate (SPEI≤–1.0), above severe (SPEI≤–1.5), and above extreme (SPEI≤–2.0) droughts, which correspond to mild, moderate, severe and extreme drought, respectively, in this study.
Four drought metrics, including the intensity, occurrence, duration, and fraction of affected area, for each drought category, were calculated to assess drought characteristics. Drought intensity is the mean SPEI of a drought event, occurrence (month yr–1) is the number of months under drought condition in a year, duration (months) is the months for an individual drought event, and fraction of affected area (%) is the percentage of area under drought condition relative to the global dryland area (Ukkola et al., 2018).
To represent drought-related hydrothermal conditions, four meteorological variables, including precipitation, temperature, PET, and P–PET, were examined. To exclude the seasonal cycle, the anomaly for each variable was first obtained as follows:
where i is the year ranging from 1980 to 2014, j is the month ranging from January to December, xi,j is the meteorological variable of month j for year i, and
${{\overline x_j}}$ is the 1980–2014 climatology for month j.Considering SPEI-06 represents the water balance for the preceding 6 months, we further calculated the previous 6-month average of the anomaly for each meteorological variable. Each meteorological anomaly during the four drought categories was then extracted and analyzed.
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Droughts and the corresponding primary meteorological elements were assessed in terms of their climatology, interannual variability, and long-term trends for the period 1980–2014. The interannual variability is measured by the temporal standard deviation (SD). The long-term trend is estimated by Theil-Sen non-parametric statistics (Sen, 1968; Theil, 1992) and is tested if the monotonic trend is robust by the Mann-Kendall non-parametric method (Mann, 1945; Kendall, 1955).
To quantify the performances of the 27 CMIP6 models, we calculated the pattern correlation coefficients (PCC) and root-mean-square error (RMSE) between the simulation and observation, representing the spatial distribution similarity and model biases, respectively. Following Seo et al. (2013), we employed a skill score to evaluate both the PCC and normalized spatial SD, calculated as follows:
where PCC is the pattern correlation coefficient between simulation and observation,
$ \sigma $ is the simulated spatial SD divided by the observed SD (i.e., normalized spatial SD), and PCC0 is the maximum achievable correlation (set to 1 here).
No. | Model Name | Institute, Country | Resolution (Lat×Lon×Level) |
1 | ACCESS-CM2 | Commonweaalth Scientific and Industrial Research Organization (CSIRO) and Bureau of Meteorology (BoM), Australia | 144×192×85 |
2 | ACCESS-ESM1-5 | 145×192×38 | |
3 | BCC-CSM2-MR | Beijing Climate Center (BCC), China | 160×320×46 |
4 | CanESM5 | Canadian Centre for Climate Modelling and Analysis (CCCma), Canada | 64×128×49 |
5 | CMCC-CM2-SR5 | The Euro-Mediterranean Centre on Climate Change (CMCC) | 192×288×47 |
6 | CNRM-CM6-1 | Centre National de Recherches Meteorologiques/Centre Europeen de Recherche et Formation Avancees en Calcul Scientifique (CNRM-CERFACS), France | 128×256×91 |
7 | EC-Earth3 | EC-Earth-Consortium, Europe | 256×512×91 |
8 | EC-Earth3-CC | EC-Earth-Consortium, Europe | 256×512×91 |
9 | EC-Earth3-Veg | EC-Earth-Consortium, Europe | 256×512×91 |
10 | EC-Earth3-Veg-LR | EC-Earth-Consortium, Europe | 160×320×62 |
11 | FGOALS-f3-L | Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP-CAS), China | 180×288×32 |
12 | GFDL-CM4 | NOAA’s Geophysical Fluid Dynamics Laboratory (NOAA-GFDL), USA | 180×288×33 |
13 | GFDL-ESM4 | NOAA’s Geophysical Fluid Dynamics Laboratory (NOAA-GFDL), USA | 180×288×49 |
14 | GISS-E2-1-G | NASA Goddard Institute for Space Studies (NASA-GISS), USA | 90×144×40 |
15 | HadGEM3-GC31-LL | Met Office Hadley Centre (MOHC), UK | 144×192×85 |
16 | HadGEM3-GC31-MM | Met Office Hadley Centre (MOHC), UK | 324×432×85 |
17 | INM-CM4-8 | Institute for Numerical Mathematics (INM), Russia | 120×180×21 |
18 | INM-CM5-0 | Institute for Numerical Mathematics (INM), Russia | 120×180×73 |
19 | IPSL-CM6A-LR | Institute Pierre Simon Laplace (IPSL), France | 143×144×79 |
20 | KACE-1-0-G | National Institute of Meteorological Sciences, Korea Meteorological Administration (NIMS-KMA), Korea | 144×192×63 |
21 | KIOST-ESM | Korea Institute of Ocean Science and Technology (KIOST), Korea | 96×192×32 |
22 | MIROC6 | Atmosphere and Ocean Research Institute (AORI, the University of Tokyo), National Institute for Environmental Studies, and Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan | 128×256×80 |
23 | MIROC-ES2L | 64×128×62 | |
24 | MPI-ESM1-2-HR | Max Planck Institute for Meteorology (MPI-M), Germany | 192×384×95 |
25 | MPI-ESM1-2-LR | Max Planck Institute for Meteorology (MPI-M), Germany | 96×192×47 |
26 | MRI-ESM2-0 | Meteorological Research Institute (MRI), Japan | 160×320×80 |
27 | UKESM1-0-LL | Met Office Hadley Centre (MOHC), UK | 144×192×85 |