1. Introduction
From late summer to midautumn in 2019, Southeast China (SEC; 25°–34°N, 110°–120°E as the key region) suffered an extreme drought, which devastated more than 3.3 million hectares of crops and caused serious economic losses (http://www.mem.gov.cn/xw/bndt/202001/t20200112_343410.shtml). The SEC area is one of the most developed and densely populated areas of China. The climate variability and extreme weather events in this area have been receiving significant academic and social attention; consequently, various mechanisms attributed to the phenomenon have been proposed.
Tropical sea surface temperature (SST) anomalies (SSTAs), such as El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD), are responsible for summer precipitation anomalies over East Asia (Ding and Chan 2005; Weng et al. 2011; Fan et al. 2013; Zhang et al. 2016a,b, 2017; Piao et al. 2020). In the developing summer of central Pacific El Niño, below-normal rainfall occurs over the Maritime Continent (MC) and the Yangtze River Valley, while above-normal rainfall occurs over South and Northeast China. Rainfall anomalies are accompanied by anomalous cyclone circulations over the subtropical western North Pacific (WNP; Weng et al. 2011; Yuan and Yang 2012; Chen et al. 2014). The IOD and El Niño Modoki played important roles in the extremely hot and dry summer of 1994 over East Asia (Guan and Yamagata 2003; Weng et al. 2011). These tropical SSTAs also influence the extratropical climate by the MC convection activities (Abdillah et al. 2017; Yang et al. 2020; Zhang and Duan 2021). During La Niña years, the MC-intensified heating is favorable for the formation of heavy droughts over Southwest China from January to March (Feng et al. 2014). Further, the MC convection activities can influence rainfall over the eastern edge of the Tibetan Plateau (TP), the southeastern TP, and Southwest China, and the East Asian summer monsoon through abnormal meridional–vertical circulation, transport of water vapor, and wave energy in summer (Jiang et al. 2015, 2016, 2017; Xu and Guan 2017; Zhuang and Duan 2019; Xia et al. 2020). Zhang and Duan (2021) indicated that the MC-suppressed convection serves as a crucial medium in which the tropical Pacific and Indian Ocean temperature anomaly mode influences the TP precipitation dipole mode during October.
The TP, which is an intense heat source in summer, significantly affects the interannual variability of precipitation over East China (Hu and Duan 2015; Wang et al. 2018; Y. Liu et al. 2020). The variation in latent heat of condensation associated with precipitation anomalies over the central and eastern TP dominates the interannual variation of the TP atmospheric heat source in summer, especially in July and August (Jiang et al. 2016; G. Wu et al. 2017; Sheng et al. 2022). The weak atmospheric heat source over the eastern TP favors below-normal precipitation along the Yangtze River valley during summer (Jian et al. 2004). Moreover, Wang et al. (2018) indicated that the positive thermal forcing of TP can intensify summer rainfall over central East China.
Additionally, mid- to high-latitude atmospheric Rossby waves strongly influence precipitation variability over East Asia by modulating atmospheric circulation (Kosaka et al. 2012; Lin 2014; J. P. Li et al. 2019). Enomoto et al. (2003) reported impacts of the Bonin high, which results from the propagation of stationary Rossby waves along the Asian jet in the upper troposphere, on the climate over East Asia in August. Zuo et al. (2013) demonstrated the role of a barotropic wave train occurring over the Atlantic–Eurasia region as a link between the East Asian summer monsoon and North Atlantic SST tripole in summer. Another study has found that the positive Atlantic–Eurasian teleconnection in summer promotes abundant precipitation over the mid- to-lower reaches of the Yangtze River (Li and Ruan 2018).
Several studies have emphasized on the role of tropical SSTAs in the 2019 SEC drought (Ma et al. 2020; Xu et al. 2020; Qi et al. 2021). Xu et al. (2020) and Ma et al. (2020) highlighted the effects of super positive IOD and El Niño Modoki from the August–October-averaged perspective. However, Qi et al. (2021) found that the IOD and El Niño Modoki were not the main contributors to the extreme precipitation deficits during autumn 2019. They demonstrated the role of the Madden–Julian oscillation. Hence, the role of IOD and El Niño Modoki with regard to the SEC drought in August 2019 and the role of the MC convection deserve further investigation. Moreover, substantial below-normal precipitation over the southern TP and a wave structure along the midlatitude westerly jet were observed in August 2019, which may have contributed to this drought; however, this has not yet been elucidated.
The studies of Ma et al. (2020) and Xu et al. (2020) indicated the period from August to October as the extreme drought period. However, the circulation patterns in August, September, and October 2019 showed remarkable differences (see Fig. S1 in the online supplemental material), suggesting that the drought may have disparate attributions in different months. Based on this observation and the severity of the drought, the present study focused on the extreme drought event in August 2019. Figure 1 shows that the heaviest precipitation anomalies in August 2019 over the SEC reached −6.2 mm day−1 for the station dataset (Fig. 1a). The precipitation index (Pr_SEC) area-averaging the SEC precipitation anomalies for the station and Global Precipitation Climatology Project (GPCP) datasets suggest that this event was the third- and second-strongest drought for the periods 1951–2019 and 1979–2019, respectively (Fig. 1c).
(a) Precipitation anomalies (shading; unit: mm day−1) in August 2019 relative to 1951–2019 based on gridded data from station observations. Red dots denote stations where precipitation in August 2019 was below the 5th-percentile threshold during 1951–2019. (b) As in (a), but for the Global Precipitation Climatology Project (GPCP) dataset relative to 1979–2019. (c) Normalized time series of area-averaged precipitation anomalies over Southeast China [Pr_SEC; 25°–34°N, 110°–120°E; black box in (a) and (b)] in August for gridded data from station observations during 1951–2019 (red solid line) and the GPCP dataset during 1979–2019 (blue dashed line). Gray and black dashed lines represent ±0.8 and ±1.5 standard deviations, respectively; r = 0.99 indicates the correlation between the station and GPCP dataset during 1979–2019. The purple line in (a) and (b) indicates the Yangtze River Valley. Dark blue contours (3000-m topographic height) in (b) represent the location of the Tibetan Plateau (TP).
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
A general consensus is that the synergistic effects of tropical and mid- to high-latitude circulation anomalies may aggravate the occurrence of extreme events. This study aimed to investigate the synergistic influences of the mid- to high-latitude and tropical atmospheric circulation anomalies and analyze the role of the TP on the SEC extreme drought in August 2019. It highlights the role of the MC convection activity on the interannual variations of the SEC drought. Section 2 presents the data, methods, and models used. Section 3 shows the circulation and moisture anomalies in association with the SEC extreme drought event in August 2019 and their possible causes. The interannual linkage between the MC convective activity and the SEC precipitation is presented in section 4. Finally, a summary and discussion are provided in section 5.
2. Data, method, and model
a. Data
SST data with a 2° × 2° horizontal resolution were obtained from the improved Extended Reconstructed SST version 5 dataset (Huang et al. 2017). Winds and other circulation variables were obtained from the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) with a 0.75° × 0.75° horizontal resolution (Dee et al. 2011) and the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis datasets (Kalnay et al. 1996) for comparison. The above datasets were acquired as monthly means for 1979–2019.
Monthly precipitation products were acquired from the GPCP V2.3 (1979–2019; Adler et al. 2018), which combines observations and satellite precipitation data with a 2.5° × 2.5° horizontal resolution. Precipitation observations at 839 stations (1951–2019) provided by the National Meteorology Information Center of the China Meteorological Administration were interpolated to a 0.5° × 0.5° grid. A linear trend and monthly mean climatology were removed from each monthly dataset.
b. Method
The square brackets in Eq. (1) denote the area-averaged SSTAs over the tropical central (C; 10°S–10°N, 165°E–140°W), eastern (E; 15°S–5°N, 110°–70°W), and western (W; 10°S–20°N, 125°–145°E) Pacific, respectively.
The overbar and prime symbols in Eq. (2) represent the mean flows and anomalies, respectively; P indicates the pressure, V = (u, υ) represents the horizontal wind velocity, and φ denotes the geostrophic streamfunction. The subscripts x and y indicate the zonal and meridional derivatives, respectively.
Statistical methods, including the multivariate empirical orthogonal function (MV-EOF; Wang 1992), regression, correlation, and partial correlation (Saji and Yamagata 2003), were utilized to reveal the leading mode of precipitation and winds and the relationship between the two variables. A two-tailed Student’s t test was used to indicate the statistical significance of results.
c. Model
To explore the variation in the SEC moisture source in August 2019, atmospheric moisture backward trajectory simulations were conducted using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) 5.0 model (Stein et al. 2015). ERA-Interim data with 1.5° × 1.5° horizontal resolution, 6-h temporal resolution, and 37 pressure levels were used as HYSPLIT inputs. The SEC was divided into 56 points, and a total of approximately 20 832 trajectories were derived each year based on backward tracing of 240 h (10 days) for each grid at 500-, 1000-, and 1500-m altitudes above ground level. Following X. Liu et al. (2020), variation in the moisture source was determined by analyzing specific humidity along the backward trajectories.
A linear baroclinic model (LBM; Watanabe and Kimoto 2000; Watanabe et al. 2002) was used to investigate the atmospheric circulation response of a fixed diabatic heating force. The model input was derived from the NCEP1 reanalysis data for August 1979–2014. A 30-day integration was conducted for each experiment, which reached a steady state after approximately 10 days (Watanabe and Jin 2003). The results shown in the study were obtained from the average of 15–30 days.
To further understand the impact of the MC convection anomalies on the SEC precipitation, atmospheric general circulation model (AGCM) sensitivity experiments were conducted using version 2 of the finite-volume atmospheric model of the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences (CAS; J. X. Li et al. 2019), which is the atmospheric component of the climate system model CAS FGOALS-f2. The model evaluation indicated that the CAS FGOALS-f2 model can satisfactorily simulate circulation and precipitation over the TP and East Asia, characteristics of ENSO and IOD, and activities of tropical cyclones (Bao et al. 2018; J. X. Li et al. 2019; Yu et al. 2021).
3. Drought circulation and possible causes
a. Atmospheric circulation
The atmospheric circulation anomalies during the SEC extreme drought event of August 2019 showed that two prominent cyclones at 850 hPa appeared over Northeast China and the subtropical WNP (Fig. 2b). The cyclone over Northeast China was also conspicuous at 500 and 300 hPa, which signifies an equivalent-barotropic structure (Fig. 2a). Moreover, positive geopotential height anomalies at 500 and 300 hPa developed over the TP and accompanied the equivalent-barotropic anticyclonic wind anomalies surrounding the TP (Figs. 2a,b). Previous studies demonstrated that cyclone circulation anomalies over the subtropical WNP are conducive to the occurrence of a rainfall deficit over the Yangtze River Valley during summer (Weng et al. 2011; Yuan and Yang 2012; Chen et al. 2014). Recent studies related to the 2019 drought have also emphasized the role of cyclones over the subtropical WNP (Ma et al. 2020; Xu et al. 2020; Qi et al. 2021). However, we found that this cyclone is limited in accounting for such extreme events. The cyclone over Northeast China and anticyclone over the TP also played important roles in the August 2019 drought, which has been investigated in the subsequent section.
(a) 500-hPa geopotential height (shading; unit: 10 gpm) and 300-hPa wind (vectors; unit: m s−1) anomalies in August 2019 relative to 1979–2019. (b) As in (a), but for 850-hPa geopotential height and wind anomalies. (c) Meridional–vertical circulation anomalies averaged between 105° and 120°E. Shading and vectors represent vertical velocity (unit: 10−2 Pa s−1) and wind (unit: m s−1) anomalies, respectively. Black boxes in (a) and (b) indicate the SEC region. Dark blue contours (3000-m topographic height) in (a) and (b) represent the location of the TP.
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
The location of cyclone pairs over Northeast China and the subtropical WNP was crucial for the extreme drought. They were in phase in the meridional; hence, they were locked and sustained barotropic instability (Hoskins et al. 1985). The essence of the barotropic instability mechanism, which is characterized by a pair of Rossby waves (cyclones) propagating side by side, is that the induced velocity field overlaps significantly, keeps the other in step, and makes the other grow (Hoskins et al. 1985). Consequently, exceedingly strong low-level northerly wind anomalies developed over East China (Figs. 2b,c). The equivalent-barotropic cyclone over Northeast China and anticyclone over the TP were favorable for the development of northerly wind anomalies in the mid- to upper troposphere over East China (Figs. 2a–c). Therefore, strong northerly wind anomalies north of 24°N dominated from the lower troposphere to the stratosphere over East China (Fig. 2c). The 850-hPa northerly wind anomalies of August 2019 averaged over the 25°–50°N, 110°–120°E region were the strongest since 1979, with a normalized value of −2.12. Such strong northerly wind anomalies reduced the transportation of warm moisture from the south. This is elaborated in the next section. Concurrently, northerly wind anomalies also contribute to anomalous downdrafts through dry-cold moist enthalpy advection (B. Wu et al. 2017; Ma et al. 2020). Significant sinking motions observed between 24° and 34°N (Fig. 2c) may have inhibited the occurrence of local convection and precipitation.
b. Transport of water vapor
Moisture and dynamic conditions are the two precipitation controls. We examined the transport of water vapor using a backward trajectory from the HYSPLIT model to gain insight into the SEC climatological water vapor source in August and its change during the SEC extreme drought event of August 2019. The results indicated that the climatological water vapor of the SEC precipitation in August mainly originates from the south and east. Further, it is traced to the South China Sea, Indochina Peninsula, Bay of Bengal, tropical Indian Ocean, and WNP, while relatively less moisture originates from the north (e.g., from Eurasia). This is consistent with previous studies (Fig. 3a; Zhou and Yu 2005; Hu et al. 2021). Figure 3b shows anomalies in the number of particles weighted by specific humidity arriving at the SEC for a backward trajectory of 10 days during August 2019. Particles originating south of the SEC region were largely decreased, much more than those that increased from the east (Figs. 3b,c). This was consistent with the circulation presented in Fig. 2. Therefore, the loss of moisture from the south contributed to this SEC drought.
(a) Climatological number of particles weighted by specific humidity (unit: 103 g kg−1) arriving at the SEC for a backward trajectory of 10 days in August. (b) As in (a), but for anomalies in August 2019 relative to 1979–2019. (c) Pressure (unit: hPa) of particles arriving at the SEC for a backward trajectory of 10 days in August 2019. (d) 1000–300-hPa column-integrated water vapor flux (vectors; unit: kg m−1 s−1) anomalies and their divergence (shading; unit: 10−4 kg m−2 s−1) in August 2019 relative to 1979–2019. Black boxes indicate the SEC region. Dark blue contours (3000-m topographic height) represent the location of the TP.
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
The 1000–300-hPa column-integrated water vapor flux showed that anomalous southward transportation occurred over East China (Fig. 3d), which was consistent with the results of Ma et al. (2020). The vertically integrated water vapor flux across the four boundaries of the SEC region (Table 1) indicated that the net water vapor flux was 32 × 106 and −3 × 106 kg s−1 for August in climatology and 2019, respectively. This indicated a 109% loss of water vapor in August 2019 in terms of the climatology due to extremely strong northerly wind anomalies. Enhancement of the inflow of water vapor from the eastern boundary was offset by the enhancement of its outflow over the western boundary. These findings were consistent with the above diagnostic results obtained by the Lagrangian method; that is, the water vapor and dynamic conditions associated with the atmospheric circulation anomalies in August 2019 are beneficial to the development of the SEC extreme drought. In the next section, we have explored the cause of circulation anomalies.
The 1000–300-hPa column-integrated water vapor flux (unit: 106 kg s−1) across four boundaries of the SEC region. Inflow is indicated by positive values
c. Mechanism of atmospheric circulation anomalies
1) Tropical SSTAs: IOD and El Niño Modoki
Atmospheric circulation anomalies in the subtropics are closely related to the tropical SST forcing. As indicated by previous studies, the SST and 10-m wind anomalies in August 2019 presented a positive IOD event in the tropical Indian Ocean and El Niño Modoki in the tropical Pacific Ocean (Figs. S2a and S2b; Doi et al. 2020; Du et al. 2020; Lu and Ren 2020; Xu et al. 2020; Qi et al. 2021). This raises the following questions: Do the IOD and El Niño Modoki facilitate the SEC extreme drought in August 2019? If yes, what are the pathways?
An analysis of vertical circulation anomalies was performed to examine the connection between the IOD and El Niño Modoki and the northerly wind anomalies of the SEC extreme drought. The IOD and El Niño Modoki promoted the suppression of convection over the TSEIO and MC by weakening the Walker circulation. This caused a lack of precipitation and relevant diabatic heating (Fig. 1b; Ashok et al. 2007; Alsepan and Minobe 2020). An anomalous low-level anticyclone was stimulated in the northern Indian Ocean as a response to the baroclinic Rossby wave. This was verified by the LBM experiment (Fig. 2b; see also Figs. S2a and S3a,e). Consequently, more water vapor transported to the entire South Asian area strengthened the South Asian summer monsoon (Figs. 2b and 3d). Ratna et al. (2021) and Jiang et al. (2022) highlighted that the IOD is conducive to increasing summer rainfall over South Asia.
As the air column of the heating area is warmed up within the monsoon heating region and in the surroundings, the horizontal gradient of temperature is in phase with that of diabatic heating, and negative vorticity is produced to the north of the heating region (Liu et al. 2001). This impact was also observed by other studies (e.g., Kennett and Toumi 2005; Liu et al. 2013). The equivalent-barotropic anticyclonic circulation over the TP brought northerly and easterly winds and contributed to less moisture, further deep sinking, and reduced rainfall over the TP and SEC, suggesting positive feedback between circulation and precipitation anomalies (Figs. 1b, 2a–c, and 3d). Therefore, the precipitation presented a sandwich-like pattern from the equatorial Indian Ocean to the TP, which was similar to the indirect impacts of TP heating as given by He et al. (2019). These results were verified by the LBM experiments (Figs. 4a,b; see also Figs. S3c,d,g,h). The LBM result also indicated that the negative diabatic heating over the southern TP was favorable for the development of a low-level cyclone over the WNP following the Sverdrup vorticity balance [Eq. (4)] and an equivalent-barotropic cyclone over Northeast China through Rossby wave propagation (Fig. 4b; see also Fig. S3h). Hence, the TP is an important bridge to relay interaction between the midlatitude and tropical circulation.
(a) Location of below-normal precipitation-related diabatic heating source over the southern TP at a sigma level of 0.67 in the LBM. (b) Response of 200-hPa streamfunction (shading; unit: 106 m2 s−1) and wind (vectors; unit: m s−1) for 15–30-day averaged results in LBM. (c) 200-hPa geopotential height (shading; unit: 10 gpm) and wave activity flux (vectors; unit: m2 s−2) anomalies in August 2019 relative to 1979–2019. Gray contours present 20 m s−1 zonal wind at 200 hPa in August 2019 and indicate the location of westerly jet. (d) Regression of 200-hPa geopotential height (shading; unit: 10 gpm) and wave activity flux (vectors; unit: m2 s−2) against the North Atlantic tripole (NAT) SSTA index in August. Stippled area denotes values exceeding the 90% confidence level. Black boxes in (b)–(d) indicate the SEC region. Dark blue contours (3000-m topographic height) represent the location of the TP.
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
2) Midlatitude dynamics
In addition to the contribution from tropical circulation anomalies, the cyclone over Northeast China also contributed to maintaining the SEC extreme drought. This poses the following question: Are there other triggering mechanisms of the equivalent-barotropic cyclone apart from the Rossby wave response to the TP precipitation deficiency (Fig. 4b; see also Fig. S3h)?
Geopotential height and wave activity flux anomalies at 200 hPa in August 2019 demonstrated that an evident wave train extended from the northern North Atlantic, Europe, and north of the TP to the Northeast China along the midlatitude westerly jet, which was similar to the active centers of the Atlantic–Eurasian teleconnection (Fig. 4c; Li and Ruan 2018). This wave train induced the equivalent-barotropic cyclone over Northeast China (Figs. 2a,b and 4c). To understand the external forcing that led to this stationary Rossby wave, the SST and atmospheric circulation over North Atlantic were examined. A tripole SSTA pattern was found over the North Atlantic (Fig. 5a). The distribution of the 2-m air temperature anomaly over the North Atlantic was similar to the SSTA tripole pattern with weaker magnitude, demonstrating the thermal impact of the warm SST on the atmosphere (Figs. 5a,d; Yu et al. 2021). The northerly winds were located above the two warm poles in association with surface sensible heating due to local warm sea surface [Eq. (4)] and with the southward meridional SSTA gradient. Consequently, abnormal anticyclonic circulations developed to the west of the warm poles while abnormal cyclonic circulations developed to the east of the warm poles. The pair of anomalous anticyclonic and cyclonic circulation developing over the southern warm pole was located near 30°N where the subtropical easterly flow prevails (Figs. 5a,c), and it is more difficult for the energy of the disturbance to be dispersed eastward. On the other hand, the abnormally strong cyclone circulation developed to the east of the northern warm pole near 60°N where the climatological westerly flow dominates (Fig. 5c). Following the geostrophic advection limit in the mid- to high latitudes (Smagorinsky 1953), this circulation disturbance in the troposphere presented an equivalent-barotropic structure (Figs. 2a,b). The strong cyclonic anomaly in the upper troposphere over the northeastern North Atlantic was found near the midlatitude westerly jet; therefore, the energy of the disturbance can be easily dispersed eastward by the propagation of quasi-stationary Rossby-wave activity along the jet waveguide (Fig. 4c).
(a) SSTAs (shading; unit: °C) and 10-m wind (vectors; unit: m s−1) anomalies in August 2019 relative to 1979–2019. (b) Normalized time series of the August NAT SSTA index during 1979–2019. (c) Climatological 10-m winds (vectors; unit: m s−1) in August. (d) 2-m air temperature anomalies (shading; unit: K) in August 2019 relative to 1979–2019. Green boxes indicate the northern (N; 55°–62°N, 38°–25°W) and southern (S; 23°–32°N, 50°–38°W) poles of the NAT SSTA pattern and the red box indicates the central (C; 42°–48°N, 52°–40°W) pole in (a), (c), and (d).
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
The square brackets in Eq. (5) denote the area-averaged SSTAs over the central (C), northern (N), and southern (S) North Atlantic, respectively. The 200-hPa geopotential height and wave activity flux regressed onto the NAT index in August illustrate that there is typical Rossby wave at 200-hPa extending from the northern North Atlantic to Northeast China, similar to the one observed in August 2019 (Figs. 4c,d).
In summary, the IOD, El Niño Modoki, and North Atlantic tripole SSTA collectively induced the local circulation anomalies, namely the low-level cyclone over the WNP, equivalent-barotropic anticyclone over the TP, and equivalent-barotropic cyclone over Northeast China. These circulation anomalies were responsible for the development of the SEC extreme drought in August 2019 by reducing the transport of warm moisture from oceans and promoting sinking motions.
4. Relationship between the MC convection and SEC drought in August
The statistical relationship between the IOD or El Niño Modoki and the Pr_SEC index is unstable on an interannual time scale (Xu et al. 2020), which is similar to that of the steady midlatitude Rossby wave. However, the MC convection anomaly, which fundamentally contributed to the SEC extreme drought in August 2019, is associated well with the interannual variability of the SEC precipitation. This section assesses impacts of the variability of long-term MC convection to explore whether the above mechanisms of the SEC extreme drought in August 2019 are generally applicable.
a. Observation
To further understand the role of the MC convection, an MC precipitation index (Pr_MC) was defined as the area-averaged precipitation anomaly over the 3°S–5°N, 100°–120°E region in August. The correlation coefficient between the Pr_SEC and Pr_MC index is 0.46 (0.43 after excluding the year 2019), which significantly exceeds the confidence level of 99%. Further, it can partially represent the relation between IOD and El Niño Modoki, with correlation coefficient of −0.7 between the Pr_MC index and SSTA gradient defined as differences of the SSTA between the eastern Indian Ocean (10°S–10°N, 90°–130°E) and central Pacific (10°S–10°N, 165°E–140°W).
The correlation between the meridional–vertical circulation and the negative Pr_MC index (multiplied by −1.0) indicates the development of local meridional circulation, as indicated in the case of August 2019 (Figs. S2c,d and S4a,b). Consequently, anomalous northeasterly winds and substantial below-normal precipitation appear over the southern TP and SEC (Fig. 6a). These anomalous wind and rainfall patterns are analogous to those observed by Jiang et al. (2016). However, their SEC precipitation signal is insignificant, suggesting that the variability of the SEC precipitation, unlike TP rainfall, is somewhat sensitive to the domain of the MC convection. Generally speaking, precipitation and circulation associated with the negative Pr_MC index correspond to those of August 2019 in the tropical and subtropical regions. Interestingly, the first dominant modes of precipitation and 850-hPa winds in August are extracted through MV-EOF analysis, which explains the 17.6% of the total variance and shows high consistency with the spatial correlation against the Pr_MC index (Figs. 6a,b). According to North’s test, this mode can be well distinguished from other modes (North et al. 1982). The correlation coefficient between the Pr_MC index and the corresponding time series of MV-EOF1 is 0.94, thus confirming the crucial role of the MC convection anomalies in triggering the precipitation and circulation modes. These results are insensitive to the selected area for the MV-EOF analysis.
(a) Correlation between August precipitation (shading)/850-hPa winds (vectors) and negative Pr_MC index (multiplied by −1.0) during 1979–2019. (b) Spatial pattern of the first mode of MV-EOF analysis (MV_EOF1) of precipitation (shading; unit: mm day−1) and winds (vectors; unit: m s−1) over 10°S–40°N, 70°E–180° in August during 1979–2019. (c) As in (a), but for partial correlation patterns with the impacts of precipitation over southern TP (Pr_STP) excluded. (d) As in (c), but with the impacts of precipitation over the Philippine Sea (Pr_PS) excluded. Stippled area and purple vectors denote values exceeding the 95% confidence level. Black and red boxes in (a)–(d) indicate the SEC region and the key region over the MC, respectively. Green and orange boxes in (c) and (d) indicate the key region over southern TP and the PS, respectively. Dark blue contours (3000-m topographic height) represent the location of the TP.
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
To further understand the bridge role of precipitation anomalies over the southern TP and PS, impacts of August precipitation area-averaged over the southern TP (24°–34°N, 82°–100°E) and PS (10°–25°N, 120°–150°E) are removed from the subsequent partial correlation analyses. The low-level cyclone over the WNP weakens when the impact of southern TP precipitation anomalies is removed, and the northerly anomalies east of the TP are reduced (Fig. 6c). This indicates the influence of TP latent heat release on the local circulation around the TP and the WNP, which is consistent with the LBM results (Figs. S3d,h) and with the results of Xie and Duan (2017) and Wang et al. (2018). After excluding the impact of precipitation anomalies over the PS, the other bridge, precipitation and 850-hPa winds associated with the Pr_MC index, becomes insignificant over the SEC. However, it still responds with northerly anomalies and below-normal precipitation (Fig. 6d). The below-normal precipitation also becomes insignificant over most parts of the southern TP, suggesting a significant interaction between them. Thus, precipitation anomalies over the southern TP and PS indeed play important bridge roles in which the MC convection anomalies affect the SEC precipitation.
b. Numerical experiments
AGCM sensitivity experiments were performed using the CAS FGOALS-f2 to further understand the influence of the MC convection on the SEC precipitation in August and the role of the TP. The MC-suppressed convection is accompanied by the observed local cold SSTAs, with a correlation coefficient of 0.62 between the Pr_MC index and area-averaged SSTAs over the region 20°S–10°N, 90°–140°E in August. Thus, following Jiang et al. (2016), the negative and positive (multiplied by −1.0) SSTAs in July, August, and September 2019 over 20°S–10°N, 90°–140°E superimposed onto the corresponding monthly mean climatological SST during 1979–2019 were used as the forcing field in sensitivity experiments, and were referred to as MC_Neg and MC_Pos, respectively. The other external forcing fields (e.g., aerosols, greenhouse gases, ozone, and volcanic and solar activity) were fixed at climatological values. The two experiments were continuously integrated for 25 years, and only data of the last 20 years were used for analysis.
The differences in August precipitation, 850-hPa wind, and meridional–vertical circulation between the MC_Neg and MC_Pos are presented in Fig. 7. The MC negative SST forcing stimulates substantial local negative precipitation anomalies (Fig. 7a). Thus, differences between the MC_Neg and MC_Pos can be considered as a response to the MC-suppressed convection. The results show that the intensified South Asian summer monsoon and the meridional circulation, which descends over the TSEIO and MC and ascends over the Bay of Bengal and PS, are well simulated and transport moisture toward the South Asia and the WNP (Figs. 7a,c,d). Further, below-normal precipitation, which is weaker than that in the observation, is observed over the southern TP in association with the cyclone circulation at 850 hPa over northwest of Bay of Bengal (Figs. 1b, 6a, and 7a). Additionally, along with the development of convection and above-normal precipitation over the WNP, a significant cyclone circulation at 850 hPa is established over the WNP, with a larger range relative to the observations (Figs. 2b, 6a, and 7a,d). Consequently, the MC-suppressed convection causes strong northerly wind anomalies and significant below-normal precipitation over the SEC, as indicated by the observations (Figs. 1a,b, 2a–c, 6a, and 7a).
(a) Precipitation (shading; unit: mm day−1) and 850-hPa winds (vectors; unit: m s−1) differences between the MC_Neg and MC_Pos experiments. (b) As in (a), but for differences between the MC_Neg_noTP and MC_Pos_noTP experiments. Dark blue solid and dashed contours (3000-m topographic height) in (a) and (b) represent the location of the TP, respectively. Black box in (a) and (b) indicates the SEC region. (c) Differences in the meridional–vertical circulation averaged over 85°–110°E between the MC_Neg and MC_Pos experiments. Shading and vectors represent the vertical velocity (unit: 10−2 Pa s−1) and wind (unit: m s−1), respectively. (d) As in (c), but for 110°–135°E.
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
We conducted two additional experiments without the TP topography. The topography height over the region 20°–45°N, 60°–110°E was set as 500 m, but other sets were the same as those of the MC_Neg and MC_Pos experiments, and were named MC_Neg_noTP and MC_Pos_noTP, respectively. The differences between the MC_Neg_noTP and MC_Pos_noTP experiments demonstrate that the negative SST forcing over the MC stimulates substantial local below-normal precipitation after eliminating the topography. However, the MC-suppressed convection fails to efficiently result in the SEC drought because of the lack of strong anomalous northerly winds associated with the TP anticyclone and the WNP cyclone. This experiment also verifies the influence of the TP on the development of the WNP cyclone, which is elucidated by the LBM experiments (Fig. 7b and Fig. S3h). Thus, it is proved that the TP plays a crucial role in the influence of tropical convection on the SEC precipitation.
5. Summary and discussion
An extreme drought, the third- and second-strongest drought during the periods 1951–2019 and 1979–2019, respectively, occurred over the SEC in August 2019, and caused immense economic losses. The low-level cyclone over the WNP, equivalent-barotropic anticyclone around the TP, and equivalent-barotropic cyclone over Northeast China collectively induced the Southeast China (SEC) drought by contributing to the development of strong northerly wind anomalies in the entire troposphere over East China. These northerly wind anomalies largely reduced the warm water vapor from the south of SEC and induced significant local sinking motions. The factors that triggered the local circulation anomalies are schematically summarized in Fig. 8.
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The SST gradient, generated by simultaneous extreme IOD and moderate El Niño Modoki events, dominated the significant suppressed convection over the TSEIO and MC by altering the zonal Walker circulation (gray dashed arrows).
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The suppressed convection promoted local circulation anomalies associated with the SEC drought via two pathways:
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The suppressed convection over the TSEIO and MC contributed to the below-normal precipitation over the southern TP and associated equivalent-barotropic anticyclonic circulation around the TP through the local meridional circulation (blue hollow arrows on the left), which rose near 10°N and sank around the equator and north of 22°N, as well as thermal effects of the horizontal gradient of enhanced convection in South Asia.
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The local meridional circulation (blue hollow arrows on the right), which descended over the MC and ascended over the PS, and the intensified South Asian summer monsoon (green curved arrow) contributed to the development of convection over the PS. Subsequently, the prominent low-level cyclone was established over the WNP following the Sverdrup vorticity balance.
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The dispersion of Rossby waves upstream along the subtropical waveguide in association with the North Atlantic tripole SSTA pattern and the Rossby wave response to the TP precipitation deficiency generated an equivalent-barotropic cyclone over Northeast China (light gray arrows). The synergistic effects of the midlatitude and tropical circulation systems dominated the occurrence of SEC extreme drought in August 2019.
Schematic diagram of the formation mechanism of the SEC extreme drought in August 2019. (a) Shading denotes 200-hPa geopotential height anomalies (identical to Fig. 4c). (b) Shading of tropical Indian Ocean, tropical Pacific Ocean, and North Atlantic denotes SSTAs (identical to Fig. 5a and Fig. S2a). Shading north of 10°N between 60°E–180° represents precipitation anomalies (identical to Fig. 1b), and purple vectors represent 850-hPa wind anomalies (identical to Fig. 2b).
Citation: Journal of Climate 35, 22; 10.1175/JCLI-D-22-0138.1
The observation and simulation results indicate a significant relationship between the MC convection and the SEC precipitation on an interannual time scale. Precipitation anomalies over the southern TP and around the PS serve as pivotal media by which the MC convection affects the SEC precipitation through the above two pathways. This is similar to the case of August 2019, and is verified by the partial correlation analysis.
The variation in the MC convection is closely related to local SST anomalies and those in the Indo-Pacific (Hendon 2003; Xu et al. 2019; Alsepan and Minobe 2020). Therefore, a convection anomaly may be predicted if tropical SST anomalies are predicted well. Other factors, including atmospheric circulation anomalies in the Southern Hemisphere such as the Mascarene high and Australian high (Xue et al. 2003), the spring Arctic Oscillation (Gong et al. 2011), the summer North Atlantic Oscillation (Wang et al. 2018), the frequency of persistent blocking and ridge events (Zhang et al. 2019), and global warming (Ma et al. 2020) might influence the variability of the SEC precipitation during summer.
This study makes a novel contribution to the literature because the SEC drought in August 2019 was extremely severe and resulted in extensive social and economic losses. Our results will advance the prediction of similar SEC extreme drought events in the future, and suggest the synergistic effects of the three oceans. The quantitative contributions of midlatitude and tropical circulation to the SEC extreme event require further investigation and serve as suitable focus areas for future research.
Acknowledgments.
The study is supported by National Natural Science Foundation of China (41730963), Guangdong Major Project of Basic and Applied Basic Research (2020B0301030004), and Strategic Priority Research Program of Chinese Academy of Sciences (XDB40030204).
Data availability statement.
All the data in this paper are publicly available from the following websites: SST from https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html; the ERA-Interim reanalysis dataset from https://apps.ecmwf.int/datasets/data/interim-full-moda/levtype=pl/; the GPCP dataset from https://psl.noaa.gov/data/gridded/data.gpcp.html; daily station precipitation from the National Meteorology Information Center at the China Meteorological Administration from http://data.cma.cn; the Hybrid Single‐Particle Lagrangian Integrated Trajectory (HYSPLIT) 5.0 model from https://www.arl.noaa.gov/hysplit/; and the Linear Baroclinic Model (LBM) from https://ccsr.aori.u-tokyo.ac.jp/∼lbm/sub/lbm.html.
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