Record-breaking summer rainfall in the Asia–Pacific region attributed to the strongest Asian westerly jet related to aerosol reduction during COVID-19

The monthly precipitation data during 1979–2020 are publicly available from the Global Precipitation Climatology Project (GPCP). The monthly aerosol optical thickness (AOT) is derived from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA2) during 1980–2020 (Gelaro et al 2017). The wind and air temperature data are from three Reanalysis datasets: National Centers for Environmental Prediction (NCEP) reanalysis (Kalnay et al 1996); the fifth major global reanalysis produced by European Centre for Medium-Range Weather Forecasts (ERA5) (Hersbach et al 2020); and the Japanese 55 year Reanalysis (JRA55) (Kobayashi et al 2015). In addition, the observation data of hourly mass concentrations of PM_2.5 were obtained from the Ministry of Ecology and Environment of the People's Republic of China. The meteorological data for Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) were obtained from the Meteorological Assimilation Data Ingest System (MADIS), a meteorological observation database with global coverage (Miller et al 2005). Among them, PM_2.5 data and MADIS meteorological data were used to estimate and drive the WRF-Chem.

2.2.1. Definition of the ASWJ and the westernmost point of the WPSH

2.2.2. Coupled Model Intercomparison Project (CMIP6) simulations

2.2.3. Statistical analysis

Additionally, Pearson correlation coefficient, regression analysis and t-test are applied to examine the connection between the ASWJ and air temperature in June–July during 1979–2020.

To discriminate specific heating processes in the atmosphere, we calculate apparent heat source (Q1) and apparent moisture sink (Q2) according to Yanai et al (1973):

$$\begin{equation}Q1 = { }{c_{\text{p}}}\frac{{\partial T}}{{\partial t}} - { }{c_{\text{p}}}\left( {\omega \sigma - {\boldsymbol{{V}}} \cdot \nabla T} \right),\end{equation} \tag{ 1 }$$

$$\begin{equation}Q2 = { } - L\frac{{\partial q}}{{\partial t}} - L{\boldsymbol{{V}}} \cdot \nabla q - L\omega \frac{{\partial q}}{{\partial p}},\end{equation} \tag{ 2 }$$

where c_p denotes the specific heat at constant pressure, T the temperature, t the time, ω the vertical p velocity, σ the static stability, p the pressure, V the horizontal velocity vector, L the latent heat of condensation, and q the specific humidity. Here, Q1 represents the total diabatic heating (including radiation, latent heating, and surface heat flux) and subgrid-scale heat flux convergences; Q2 represents the latent heating due to condensation or evaporation processes and subgrid-scale moisture flux convergences (i.e. strong rainfall process) (Yanai et al 1973).

2.2.4. WRF-Chem simulations

Figure 2(a) displays the climatological June–July precipitation during 1980–2020. Large values of precipitation appear over southern China extending northeastward to the south of Japan and southeastward to the Northwest Pacific. Figure 2(b) shows the rainfall anomalies in June–July 2020 relative to the climatology. As can be seen, an unprecedented quantity of rainfall struck the Asia–Pacific region (purple line in figure 2(b)) in June–July 2020 (figure 2(b)). The centers of the strong rainfall were located over the Yangtze River basin and the south of Japan (figure 2(b)), with a standardized regional-mean precipitation value of 4.07 mm d⁻¹, which is more than 35% higher than the climatological rainfall (i.e. 3.02 mm d⁻¹) and more than 10% higher than the second-largest value of precipitation in 2018 (i.e. 3.70 mm d⁻¹) (figure 2(c)).

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As noted in the introduction, the combination of a westward-extended WPSH and a strong ASWJ provide favorable conditions for rainfall in East Asia (Ding et al 2021, Li et al 2021). Many studies have reported that the WPSH was critical to the record-breaking summer rainfall in 2020 because of the moisture that it transported (Zhou et al 2021, Yang et al 2022). Here, we present a preliminary investigation into the question of whether the ASWJ led to the record-breaking rainfall by providing a favorable ascending condition.

Figures 3(a) and (b) depict the observed climatological distribution of zonal wind in June–July, showing a maximum speed at approximately 200 hPa and 40° N with an equivalent barotropic vertical structure. To quantity the change in the AWSJ, an ASWJ index is defined as the area-mean zonal wind speed over (35°–45° N, 45°–155° E; black frame in figure 3(a)), covering the regions around the Caspian Sea extending eastward to the Northwest Pacific. This index depicts changes in the strength of the ASWJ and shows an obvious weakening trend in the westerly jet, consistent with Dong et al (2022). However, the ASWJ is evidently strongest in 2020, with a standardized zonal wind speed of 2 m s⁻¹ (figure 3(c)). As shown in figure 3(c), the above features can be captured by all three reanalysis datasets (i.e. JRA-55, ERA5 and NCEP), indicating the robustness of the results (Dong et al 2022).

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A strong ASWJ usually companied by strong divergence in the upper troposphere along its southern flank due to anticyclonic shear of the zonal wind. As shown in figure 3(d), there is clear positive divergence on the south side of the ASWJ from 1 June to 31 July of 2020, which would have served as an Ekman pumping-like process providing an extremely favorable ascending motion condition and the low-level convergence (figure S1). Such favorable upward movement would have been accompanied by the abundant moisture condition generated by the westward-extended WPSH (Ding et al 2021, Li et al 2021, Zhao et al 2022). Hence, the unexpected westward extension of the WPSH and the unprecedented strengthening of the ASWJ collectively contributed to the occurrence of the record-breaking summer rainfall over the Asia–Pacific region in 2020 (figures 1, 3(e) and S2). The question now is why, given the background of a pronounced weakening trend over the past four decades, was the summer ASWJ so strong in 2020.

Based on the thermal wind balance (i.e. du/dp ∼ dT/dy, in which u is zonal mean zonal wind, p is pressure, and dT/dy is the meridional temperature gradient on constant pressure surfaces), an increased meridional temperature gradient in the troposphere tends to increase the vertical shear of the zonal wind, and therefore strengthen the upper-tropospheric jet. In addition, a strong temperature gradient can also increase the atmospheric baroclinicity and consequently accelerate the westerly wind via wave–flow interaction (An et al 2021). To investigate the causes of the observed strengthening of the summer ASWJ in 2020, we analyzed the distribution of the meridional gradients of air temperature (−dT/dy). As shown in figures 4(a)–(c), the increase in the temperature gradients over 30° N–45° N in Asia strengthens the ASWJ significantly, particularly at 500 hPa, which is caused by cooling over the mid-to-high latitudes of Asia and warming in the subtropics. Figures 4(d)–(f) show the distribution of the meridional gradients of temperature at 200 hPa, 500 hPa, and 700 hPa in June–July 2020, respectively. Consistent with the distribution of the temperature gradient over the midlatitudes in the strong westerly phase, the meridional gradients of temperature anomalies at these levels were significantly higher, near 40° N, in 2020. The meridional gradients of temperature at 500 hPa (black frame in figure 4(b)) reached their second largest level in history in 2020 (figure S3). As a result, the ASWJ in summer 2020 reached close to its strongest state of the past four decades (figure 3(c)). According to previous studies, a reduction in aerosols (figures 5(a) and (b)) may lead to these changes in the meridional gradients of temperature and associated zonal wind (Rotstayn et al 2013, 2014, Dong et al 2022).

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As we know, the COVID-19 pandemic led to reduced aerosols and their precursor emissions (Forster 2020, Wang and Zhang 2020, Hu et al 2022, Yang et al 2022), in particular PM_2.5 concentrations over southern China (Yang et al 2022). To identify the contribution of the reduction in aerosols during COVID-19 to the strengthening of the summer ASWJ, we use monthly AOT data from MERRA2. It has been shown that the AOT can be used as a proxy to represent the distribution and variability of atmospheric aerosols (Jia et al 2015, Yang et al 2022). As shown in figures 5(a) and (b), the AOT anomaly reaches its minimum over southern China (black rectangle in figure 5(a)) in June–July 2020. According to Yang et al (2022), the spatial distribution of changes in PM_2.5 concentrations due to COVID-19-related emission reductions show more substantial decreases (maximum decrease of up to 35%) in southern China, in particular for scattering aerosols, which might be due to either the reduced local emissions or less aerosol transport from South and Southeast Asia associated with emission reductions in these regions. It is worth noting that southern China was not a key region for this record-breaking rainfall, which in part avoids the consideration of rainfall leading to a reduction in aerosols through wet deposition. Therefore, a lower AOT related to COVID-19 over southern China favors higher temperatures in the lower and middle troposphere in this region by increasing downward shortwave radiation (He et al 2022, Yang et al 2022). As expected, significant negative correlations can be seen around 35°–45° N over East Asia between AOT and mean meridional gradients of 500 hPa air temperature (figure 5(c)). This suggests that a decrease in aerosol concentrations would lead to an increase in the meridional gradients of temperature in the mid troposphere over northern China (figure 5(c)), which would further contribute to an enhancement of the ASWJ (figure 5(d)). Note that the spatial pattern of the correlation between the meridional gradients of temperature and the AOT (figure 5(c)) around 35°–45° N over East Asia is similar to the observed meridional gradients of temperature in June–July 2020 (figure 4(e)).

To further illustrate the close connections between the enhanced ASWJ and the reduction in aerosols during COVID-19 and the associated record-breaking rainfall in 2020, simulations from four CMIP6 models are analyzed (table S1). The effects of emissions reduction due to the COVID-19 pandemic are represented by the difference between the SSP2-4.5-covid experiments (with emissions reduction during COVID-19) and the SSP2-4.5 experiments (without emissions reduction). Visually, there is good agreement between the observed rainfall pattern and that from the COVID-19 experiments in figures 2(b) and 5(e), meaning that CMIP6 models in this study can capture rainfall in summer 2020 as a whole. The COVID-19 experiments indicate that the reduction in aerosols during COVID-19 led to an increase in the downward solar radiation over South Asia, which in turn led to an increase in meridional gradients of temperature in the lower and mid troposphere at midlatitudes, in particular over the North China Plain and Japan Sea, and thus accelerated the ASWJ (figures 5(e)–(h)). The stronger ASWJ provided a favorable ascending condition for the occurrence of record-breaking rainfall in the Asia–Pacific region (figures 5(e) and (h)). To exclude the impact of internal variability, we checked the ensemble mean rainfall for different models and found that the ensemble-mean of four models captured the rainfall over the Asia–Pacific region as a whole, giving us confidence that our results are robust, that is, aerosols reduction as an external factor induces record-breaking rainfall by accelerating the ASWJ (figure S4). To further verify these observed and CMIP6 model simulated results, we conduct two groups of experiments (CTL and EXP) based on WRF-Chem using different emissions. The CTL and EXP were based on emissions in 2019 and 2020 respectively. The difference between EXP and CTL was used to study the role of the reduction in aerosol on summer rainfall in the Asia–Pacific region during COVID-19. The results show that the regional model can also capture the summer rainfall in 2020 caused by aerosols reduction during COVID-19 (figure S5). Specifically, daily average surface downwelling solar radiation is obviously increased in southern China (figure S5(d)), which supports the acceleration of the westerly jet, especially in northern China and the Sea of Japan by increasing the meridional gradient of temperature (figure S5(e)). As a result, rainfall occurs in the Asia–Pacific region (figure S5(f)). These results all support the hypothesis that the reduction in aerosols during the COVID-19 pandemic contributed to record-breaking rainfall in the Asia–Pacific region by strengthening the ASWJ. Of course, the diabatic heat released by strong rainfall may also strengthen the ASWJ. Therefore, we checked another strong ASWJ event in 2009 (figures 1 and 3(c)) that did not feature strong rainfall in the Asia–Pacific region owing to a WPSH that was not situated abnormally to the west (figure 1). This suggests that the contribution of the diabatic heat released by strong rainfall to the strong ASWJ may have been negligible. In addition, compared with the meridional gradients of the latent heating due to condensation processes related to the record rainfall, we find that the meridional gradients of the total diabatic heating that may leads to the strong ASWJ, is obviously stronger, meaning that heating (i.e. due to aerosols reduction; 53%) other than latent heat released by rainfall (47%) are also important (figure S6) for the strongest ASWJ in June–July of 2020.

In brief, observations, CMIP6 models, and WRF-Chem model results both suggest that the strong summer ASWJ in 2020 was closely related to the reduction in aerosols during COVID-19 in southern China and Southeast Asia, which was conducive to the record rainfall in the Asia–Pacific region that year.