Western Pacific Subtropical High Research Articles

Influence of Typhoon Nangka Process on Ozone Pollution in Hainan Island

In order to explore the influence of the 2016 typhoon Nangka on ozone (O3) concentration in Hainan Island, this study employed correlation analysis and backward trajectory simulated methods, using the hourly air quality monitoring data, meteorological data, EAR5 reanalysis data, and bright temperature (TBB) infrared from the Himawari-8 satellite from October 10 to 14, 2020. The results showed that, during the typhoon process from October 11 to 13, the O3-8h (maximum of 8 hours sliding average) concentration exceeded the standard. The averaged ρ(O3-8h) of Hainan Island reached its peak value (130.5 μg·m-3) in October 12, 2020, with the O3-8h concentration exceeding the standard in four cities and counties, in which Lingao County reached the highest concentration of 198.44 μg·m-3. During the typhoon process, the hourly O3 concentration was negatively correlated with precipitation, relative humidity, and wind speed and positively correlated with air pressure and air temperature. Moreover, the correlation coefficients between O3 with precipitation, air pressure, and relative humidity exceeded the 99.9% confidence level. The southeastern coastal provinces of China were the main contribution source area for this ozone pollution process on Hainan Island. Affected by the downdraft inside the western Pacific subtropical high (WPSH) and the downdraft outside the circumference of typhoon Nangka, vertical transport in the atmosphere appeared over the contribution source area. The meteorological conditions were favorable for atmospheric photochemical reactions. Air pollutants were transported to Hainan Island along the northeast airflow around the typhoon, resulting in the occurrence of an O3 pollution event. In addition, this study built a weather conceptual model of O3 pollution in Hainan Island affected by typhoon Nangka, which can be used as a reference for the joint pollution prevention and control by air pollution prediction, early warning, and environmental management departments.

Regional Persistent Extreme Precipitation Events over Southwest China under Different Low-Latitude Intraseasonal Oscillations during the Rainy Season

Abstract This study investigates the mechanisms of low-latitude intraseasonal oscillations affecting regional persistent extreme precipitation events (RPEPEs) over Southwest China (SWC) during rainy seasons. Most of the RPEPEs over SWC are dominated by 7–20-day variability. The RPEPEs over SWC are preconditioned by two different types of 7–20-day Rossby waves with almost opposite phases over the western North Pacific (WNP). The two types of 7–20-day Rossby waves have direct and indirect effects on type-1 and -2 RPEPEs, respectively. For type 1, a coupled 7–20-day low-level anticyclone and suppressed convection originating from the tropical WNP propagate northwestward and cover the region from the South China Sea (SCS) to the Bay of Bengal before the RPEPEs. The anticyclone triggers ascending motion over SWC and transports more moisture to SWC, favoring the SWC RPEPEs. Before the type-2 RPEPEs, a coupled 7–20-day low-level cyclone and enhanced convection propagates from the tropical WNP to the SCS. The enhanced convection over the SCS leads to the westward extension of the western Pacific subtropical high (WPSH) and the eastward shift of the South Asian high (SAH). The variations in the WPSH and the SAH directly cause SWC RPEPEs by inducing ascending motion and transporting moisture. The mechanisms for type-2 RPEPEs tend to work under the background with a strong WPSH. Using a Lagrangian model, we found that both the 7–20-day oscillations and their background atmospheric circulations result in significant differences in moisture sources for the two types of RPEPEs. These findings benefit a better understanding of SWC extreme precipitation events.

Different Characteristics and Drivers of the Extraordinary Pakistan Rainfall in July and August 2022

The unprecedented and long-lasting abnormal monsoon rainfall attacked Pakistan in the summer of 2022, causing severe flooding. This study investigated the sub-seasonal characteristics and mechanisms of this distinctively extreme precipitation event. The historical rainfall in July and August and extreme precipitation mainly occurred in northern Pakistan. Both the monthly rainfall in July and August 2022 and the extreme precipitation during the summer were far exceeding the historical record and involved unique spatial distribution. The rainfall in July 2022 is nationwide and mainly located in northern Pakistan, while the rainfall in August and extreme precipitation occurred in southern Pakistan. Different physical processes are responsible for the precipitation in July and August 2022. In July, the South Asian high (SAH) and Iranian high extended eastward. Meanwhile, the anticyclonic circulation anomalies occurred in northwestern Pakistan and the easterly winds enhanced in the south side of the Tibetan Plateau (TP), which strengthened water vapor transporting from the Bay of Bengal and cooperated with the cyclonic system over the Arabian Sea to enhance the precipitation over Pakistan. In August, the SAH further extended eastward and the Western Pacific Subtropical High extended westward to the TP. Meanwhile, the European blocking (EB) developed, and a deep trough appeared over northwestern Pakistan. This weakened the easterly flow along southern TP but enhanced the southerly flow accompanying the cyclone over the Bay of Bengal and the Arabian Sea, and thus guided the water vapor transporting to southern Pakistan and enhanced the precipitation. The extreme precipitation in July was mainly attributed to the unusually strong Indian monsoon, while the extreme precipitation in August was the result of a combination of the Indian monsoon and EB. The study provided important information about extreme precipitation in Pakistan, which will help policymakers take measures to deal with the effects of flooding.

Diagnosing Potential Impacts of Tibetan Plateau Spring Soil Moisture Anomalies on Summer Precipitation and Floods in the Yangtze River Basin

AbstractSoil moisture as a key variable of land processes greatly influences the weather and climate. This study investigates the observed linkage between Tibetan Plateau (TP) spring soil moisture and summer precipitation and floods in the Yangtze River basin during 1988–2008 using satellite and in‐situ observations. A significant (p = 1%) proportion of interannual variations of summer precipitation (about 25%) in the Yangtze River basin can be attributed to spring TP soil moisture anomalies which show a dipole pattern. When spring soil moisture anomalies are positive (negative) over eastern (western) TP, there is more summer precipitation and consequently river discharge in the Yangtze River basin, or vice versa. The possible mechanisms can be explained from the perspectives of surface energy balance and atmospheric thermodynamics. More (less) spring soil moisture over the eastern (western) plateau enhances summer diabatic heating, which may be related to soil moisture memory. The enhanced summer diabatic heating stimulates vigorous ascending motions over TP, which diverge in the upper troposphere (200 hPa) and descend over the western Pacific. This is conducive to the enhancement and mutual proximity of the South Asian High (SAH) and the Western Pacific Subtropical High (WPSH). As a result, the warm and humid air flows from the Bay of Bengal and the western Pacific and the cold and dry air flows from the boreal continent converge in the Yangtze River basin, causing excessive summer precipitation. Therefore, TP spring soil moisture can be considered a seasonal predictor of summer precipitation and possible subsequent floods in the Yangtze River basin.

A Method for Monthly Extreme Precipitation Forecasting with Physical Explanations

Monthly extreme precipitation (EP) forecasts are of vital importance in water resources management and storage behind dams. Machine learning (ML) is extensively used for forecasting monthly EP, and improvements in model performance have been a popular issue. The innovation of this study is summarized as follows. First, a distance correlation-Pearson correlation (DC-PC) method was proposed to identify the complex nonlinear relationship between global sea surface temperature (SST) and EP and select key input factors from SST. Second, a random forest (RF) model was used for forecasting monthly EP, and the physical mechanism of EP was obtained based on the feature importance (FI) of RF and DC–PC relationship. The middle and lower reaches of the Yangtze River (MLYR) were selected as a case study, and monthly EP in summer (June, July and August) was forecasted. Furthermore, the physical mechanism between key predictors with a large proportion of FI and EP was investigated. Results showed that the proposed model had high accuracy and robustness, in which R2 in the test period was above 0.81, and RMSE as well as MAE were below 10 mm. Meanwhile, the key predictors in the high SST years could cause eastward extension of the South Asian High, westward extension of the Western Pacific Subtropical High, water vapor rising motion and an increase in the duration of atmospheric rivers exceeding 66 h, which lead to increasing EP in the MLYR. The results indicated that the DC–PC method could replace Pearson correlation for investigating the nonlinear relationship between SST and EP, as well as for selecting the factors. Further, the key predictors that account for a large proportion of FI can be used for explaining the physical mechanism of EP and directing forecasts.

Impacts of Tibetan Plateau sensible heat and El Niño–Southern Oscillation on precipitation over South China under the background of the PDO

This study aims to investigate the impacts of the spring sensible heat (SH) over the Tibetan Plateau (TP) and the El Niño–Southern Oscillation (ENSO) in the preceding wintertime on midsummer (July–August) precipitation over South China under the different Pacific decadal oscillation (PDO) phases. More specifically, eight classifications are adopted at the demarcation point around 1996 when the spring SH over the TP and the midsummer precipitation in South China occurred as well as the PDO phase transition, including positive and negative SHs and ENSOs under a positive PDO phase (1979–1996) and a negative PDO phase (1997–2019), respectively, based on the Niño-3 index and the spring SH calculated from 48 stations over the central and eastern parts of the TP. The results show that both the spring SH and the ENSO in preceding wintertime have a significant impact on the midsummer precipitation over South China; that is, when the two factors are in their respective positive (negative) phase, the midsummer precipitation in South China is generally less (more). Importantly, the phase change of background field PDO can significantly enhance the effect of the SH and the ENSO on summer precipitation over South China. Moreover, compared with the preceding wintertime ENSO, the spring SH over the TP contributes more to the midsummer precipitation in South China based on analyses of their independent and synergistic effects. The main mechanism responsible for the anomalous midsummer precipitation over South China are the combined effects of the South Asian high (SAH) and the western Pacific subtropical high (WPSH), which are controlled by the spring SH anomaly over the TP and the ENSO, respectively. Deep understanding of the dominant factors of the midsummer precipitation over South China will help understand the local climate change and reduce the losses caused by drought and flood disasters.

Will extreme temperature events emerge earlier under global warming?

The increased frequency of climate extremes in recent years has profoundly affected terrestrial ecosystem functions and the welfare of human society. Previous studies have mainly focused on the impact of the frequency and intensity of climate extremes, but these studies have rarely documented the emergence time of climate extremes. Will extreme temperature events emerge earlier under global warming? To address this question, in this study, we focused on the emergence times of temperature extremes in China. Five indices were selected to quantify the trends in the emergence times of temperature extremes, including the annual maximum temperature and minimum temperature emergence dates, the ≤0 °C and ≥ 10 °C start dates, and the ≥10 °C end date. Additionally, the dynamic changes in extreme temperature emergence times and the influencing factors were analyzed. The results showed that from 1960 to 2018, the annual maximum and minimum temperature emergence dates showed no significant advancing trends in China. The ≥10 °C start date showed a significant advanced trend (P < 0.001), while the ≤0 °C start date and the ≥10 °C end date showed significant delayed trends (P < 0.01). The Qinghai-Tibet Plateau and the northwestern regions of China were the regions where the ≤0 °C start date and the ≥10 °C end date were most significantly delayed. Additionally, the changes in the emergence dates of temperature extremes were correlated with atmospheric circulation patterns. The change in the ≤0 °C start date showed a significant positive correlation with the Southern Oscillation index (P < 0.001) and the Western Pacific Subtropical High intensity index (P < 0.01). The ≥10 °C start date trend showed a significant negative correlation with the Northern Hemisphere Subtropical High area index (P < 0.01) and the Western Pacific Subtropical High intensity index (P < 0.05). The variation in extreme temperature emergence times showed a dependence on topographic factors. The maximum temperature emergence date and the ≤0 °C start date were delayed more significantly at higher elevations and latitudes in China. These results provide reference information for studying changes in extreme temperature emergence dates and taking measures to reduce the losses caused by temperature extremes.

Effects of Typhoon In‐Fa (2021) and the Western Pacific Subtropical High on an Extreme Heavy Rainfall Event in Central China

AbstractAn extreme heavy rainfall event in Henan Province (HnHRE) of central China in July 2021, which is jointly affected by the western Pacific subtropical high (WPSH) and Typhoon In‐Fa (2021), is comprehensively studied through five experiments conducted using the Weather Research and Forecasting Model (WRF). The patterns of the WPSH, In‐Fa, and precipitation are well reproduced in the control experiment. The magnitude of potential vorticity (PV) anomaly associated with the WPSH (In‐Fa) is increased and decreased by 50% in the initial condition based on the piecewise PV inversion method, namely, SH150 and SH050 (TC150 and TC050), respectively. The analysis shows that the 54‐hr rainfall accumulations in TC150 and TC050 are reduced by 39.5% and 31.8%, respectively, more than 28.8% in SH150 and 20.1% in SH050, indicating that Typhoon In‐Fa plays a more critical role in the rainfall amplification than the WPSH. This result also confirms that the HnHRE occurs within the quite favorable configuration of WPSH and In‐Fa circulations. The WPSH and In‐Fa majorly control the meridional and zonal moisture transports, respectively. The diagnosis of moist ageostrophic turns out that diabatic heating is closely related to the vertical motion during the HnHRE. Besides, the heavy rainfall region is also closely attributed to the distributions of equivalent potential temperature and divergence.

Increasing Compound Hazards of Tropical Cyclones and Heatwaves over Southeastern Coast of China under Climate Warming

Abstract Compound hazards are more destructive than the individual ones. Using observational and reanalysis datasets during 1960–2019, this study shows a remarkable concurrent relationship between extreme heatwaves (HWs) over southeastern coast of China (SECC) and tropical cyclone (TC) activities over western North Pacific (WNP). Overall, 70% of HWs co-occurred with TC activities (TC–HWs) in the past 60 years. Although the total frequency of TCs over WNP exhibited a decreasing trend, the occurrences of TC–HWs over SECC have been increasing, primarily due to the increasing HWs in the warming climate. In addition, TC–HWs are stronger and longer lasting than HWs that occur alone (AHWs). And in the long-term perspective, both AHWs and TC–HWs exhibit increasing trends, especially since the mid-1980s. The enhancement on HWs caused by TC activities is sustained until TCs make their landfalls and then collapse. Based on composite analysis, TC activities enhance HWs by modulating atmospheric circulations and triggering anomalous descending motion over southern China mainland which intensifies the western Pacific subtropical high (WPSH) and favors increased temperatures therein. Given the severe adverse impacts of TC–HWs on coastal populations, more research is needed to assess the future projections of TC–HWs, as HWs are expected to be more frequent and stronger as the climate warms, whereas TCs over WNP may occur less often.

Spatial-Temporal Variations of Extreme Precipitation Indices in the Xinjiang Cold Area over the Past 60 Years

The damage of extreme precipitation to the social economy and ecological environment has attracted extensive attention. Daily precipitation data of 59 meteorological stations located in Xinjiang Uygur Autonomous Region were selected, where the temporal and spatial distribution and variation characteristics of extreme precipitation from 1961 to 2017 were comprehensively analyzed by percentile threshold method, precipitation tendency rate method, the trend-free pre-whitening Mann Kendall trend test, Mann Kendall mutation test, and other methods. Further, three methods, the continuous wavelet transform, cross wavelet transform, and wavelet coherence analysis, were selected to analyze the correlation between extreme precipitation index and six atmospheric circulation indexes. Results show that from 1961 to 2017, the precipitation in Xinjiang increased, and the extreme precipitation (R99P), extreme precipitation days (R99D), extreme precipitation contribution rate (R99C), and extreme precipitation intensity (R99I) showed a significantly increasing trend. Due to the influence of topography, the spatial distribution of extreme precipitation is bounded by the Tianshan Mountains, decreasing from northwest to southeast, meeting the characteristics of “Northern Xinjiang is greater than Southern Xinjiang,” indicating obvious spatial heterogeneity. Among the selected atmospheric circulation indexes, Western Pacific Subtropical High Intensity Index (WPSHII) and Western Pacific Subtropical High Intensity Index (SCSSHII) contribute significantly to extreme precipitation events in Xinjiang. There is an obvious positive correlation between SCSSHII and extreme precipitation index. In other words, the greater the SCSSHII, the greater the frequency and frequency of extreme precipitation events in the study area, and the more prone it is to natural disasters such as rainstorms and floods.

Decadal variation of the relationship between the previous autumn IOD and the South China Sea summer monsoon

Using reanalysis data from the ECMWF and Hadley Centre of the UK Meteorological Office, the present study analyzes the relationship between Indian Ocean Dipole (IOD) in the previous autumn (September–November) and South China Sea Summer (June–August) Monsoon (SCSSM) from 1951 to 2021. It is found that the relationship between the two indices was significantly enhanced from the late 1970s to the beginning of the 21st century, with a significant resonance cycle of 2–4a. The previous autumn IOD forced the Gill pattern in the upper troposphere by changing the Sea Surface Temperature Anomaly (SSTA) of the following summer in the Tropical Indian Ocean (TIO), resulting in the anticyclonic circulation anomaly in the Philippine Sea. It eventually alters the southwest monsoon near the South China Sea and the Philippine Sea. However, for a decade or more, as the Pacific Decadal Oscillation (PDO) phase turns from warm to cold and the relationship between El Niño–Southern Oscillation (ENSO) and IOD weakens, the impact of the previous autumn IOD on Indian Ocean SSTA in the summer of the following year weakens, resulting in a decrease in the correlation between the previous autumn IOD and SCSSM. The impact of the previous autumn’s Indian Ocean SSTA on the following year’s SCSSM is more considered in terms of the eastern Bay of Bengal and the Somali Sea basin. In addition, during the warm phase of PDO, the IOD of the previous autumn can cause abnormal summer precipitation in South China by strengthening or weakening the Western Pacific Subtropical High (WPSH) of the following summer, but there is no significant effect in the cold phase of PDO.

Changes of extreme precipitation in the middle and lower reaches of the Yangtze River and their correlation with atmospheric circulation

Under the backdrop of climate change, the rising frequency and intensity of extreme precipitation events in the middle and lower Yangtze River (MLYR) region are causing serious consequences and economic losses. To assess regional risk and understand the impact of atmospheric circulation on extreme precipitation, it's crucial to analyze the spatial and temporal fluctuations of these events, focusing on the time-frequency phase relationship. Using the observed precipitation data from 1979 to 2015, nine extreme precipitation indices were calculated and a trend analysis and wavelet coherence analysis were performed to evaluate their variation. The results show that: 1) an overall increase in the frequency, intensity, and duration of precipitation in the MLYR, with significant rises in the wet daily precipitation and highest precipitation amount in 1-day period; 2) a higher likelihood of extreme precipitation events in the southeast of the MLYR, and an uneven spatial distribution; 3) the Western Pacific Subtropical High (WPSH), the South China Sea high (SCSH), and the East Asian westerly jet (EAJ) all play a role in precipitation changes in the MLYR, with the WPSH having the most significant impact, followed by the SCSH, and the EAJ being weaker. The results deepen our understanding of the various characteristics and causes of extreme precipitation in the MLYR.

Significant Northward Jump of the Western Pacific Subtropical High: The Interannual Variability and Mechanisms

AbstractThe intensity and position of the western Pacific subtropical high (WPSH) have crucial effects on climate and disaster events in East Asia during summer. The WPSH significant northward jump (SNJ) events are the main manifestation of the seasonal evolution of WPSH, which are important for the precipitation over East Asia. Using the daily reanalysis data sets from year 1979 to 2020, this study further defines the early and late SNJ events of WPSH on the interannual timescale, which are connected separately with the tropical, midlatitude subseasonal signals and the local air–sea interaction. However, the mechanisms of the WPSH‐SNJ events are different in the anomalous early and late years. In the early SNJ years, the subseasonal signals from the midlevel East Asia–Pacific teleconnection pattern or the low‐level boreal summer intraseasonal oscillation cause the positive 500 hPa geopotential height anomalies, which contributes to the significant WPSH northward jump in the first pentad of July. However, the above factors are unable to cause the WPSH‐SNJ in the late years. Until the second pentad of August, the collaborative effects between midhigh latitudes wave trains over high levels and cold sea surface temperature (SST) anomalies in the core region lead to the barotropic geopotential height anomalies and the lagged northward jump of WPSH. It seems that the meridional position of WPSH has a complex interannual variability, which is modulated by the solar radiation, the atmospheric disturbances at different levels, and the SST anomalies at the same time.

Evolution and prediction of the extreme rainstorm event in July 2021 in Henan province, China

AbstractThe Once‐in‐a‐Century extreme rainstorm event caused severe floods over Henan province during July 18–21, 2021, which resulted in large casualty and property losses. Although the rainstorm event occurred in Henan after July 18, the excessive rainfall had occurred to the east of Henan before July 18, with the 4‐day accumulated rainfall exceeding +130 mm during July 14–17, 2021. How the rainfall evolving westward and intensifying after July 18 remained a puzzle, which is the focus of this study. The prerainstorm stage (July 14–17) was related to the South Asian High (SAH) extending eastward and the western Pacific subtropical high (WPSH) extending northwestward, and a low vortex between the SAH and WPSH caused above‐normal rainfall to the east of Henan. The rainstorm stage (July 18–21) was associated with an inverted trough and excessive southerly and southeasterly water vapor transportation above Henan, which resulted from the combined effects of a deep trough in the upper troposphere and typhoon activities. Additionally, three subseasonal forecasting systems predicted this rainstorm event 3 days in advance, with the European Center for Medium Range Weather Forecasts (ECMWF) performing the best, which was related to a better prediction of the inverted trough and the water vapor transportation in the middle‐lower troposphere. These results advance our understanding of the extreme rainstorm event in July 2021 in Henan.

Oceanic and atmospheric anomalies associated with extreme precipitation events in China 1983-2020.

Observed synoptic anomalies in connection with China's extreme precipitation events/floods in the summers of 1982/83, 1997/98, 2010, 2014, 2015/16, and 2020 are studied. These events mainly occur within the middle and lower Yangtze basins. The dominant moisture source is the Northern Indian Ocean and the Southwestern Pacific Ocean of the Indo-Pacific warm pool (IPWP). Both of these bodies of water have warmed since 1979. In East Asia, the strong land-sea thermal contrast driven by global warming drives the increased East Asian summer monsoon (EASM) circulation, which develops deep convective precipitation. The total precipitable water in the Indo-Pacific region has also been increasing since 1979. The intense southwest Indian monsoon transports moist air to the Yangtze basin in mid-June and forms the Meiyu (plum rain) front. Strengthened Okhotsk/Ural blocking highs in East and West Asia, as well as the Western Pacific subtropical high (WPSH) and the South Asian high (SAH) over south Eurasia, remain stationary for long periods and interact to exacerbate the precipitation. The western edge of the WPSH expands westward towards East Asia to transport moisture. To the north, the WPSH combines with the two blocking highs to trigger more rain. The intensified SAH expands eastward and merges with the extended WPSH to add rain. On the other hand, rainfall is modulated by the El Niño-Southern Oscillation (ENSO), notably in relation to the super El Niño events in 1982-1983, 1997-1998, 2015-2016, and 2020. The research described in this paper highlights changes in the weather systems with warming and, in particular, the enormous and dominating impact of the warming and expanding IPWP on rainfall extremes. Improved seasonal forecasts and planning ahead will protect lives and livelihoods.

Enhanced ozone pollution in the summer of 2022 in China: The roles of meteorology and emission variations

The variation of surface ozone (O3) is linked to changes in meteorology and emission. A record-breaking high temperature struck China in the summer of 2022, resulting in positive anomalies of daily maximum temperature of 0.86 ± 1.02 °C. Consequently, the mean daily maximum 8 h average (MDA8) ozone (O3) increased by 6.46 ± 13.0 μg m−3 compared to the mean summertime MDA8 O3 from 2014 to 2021 in China. This research aimed to understand the impact of meteorology and emission changes on O3 variations in different regions in the summer of 2022 by an explainable machine learning method. Results showed that the changes in emissions dominated the O3 enhancement in the Pearl River Delta (PRD), Beijing-Tianjin-Hebei (BTH), and Twain-Hu Basin (THB) with contributions of 89%, 86%, and 53% respectively. Satellite observations suggested that the increased O3 due to emission changes in BTH and PRD were related to the decreases in NO2 emissions and increases in volatile organic compound (VOC) emissions. On the contrary, variations in meteorology contributed 63%, 63%, and 51% respectively to the increases of O3 in the Sichuan Basin (SCB), Yangtze River Delta (YRD), and Fen-Wei Plain (FWP). The increase in O3 levels due to meteorological changes was associated with an anomalous westward extension of the Western Pacific Subtropical High during the summer of 2022. Specifically, the decrease in relative humidity, increases in temperature and boundary layer height were the dominant factors contributing to the increase of meteorology related O3 in SCB, THB, and YRD, with contributions over 99%. This study recommends a continuous reduction in O3 precursor emissions to mitigate O3 pollution under the global warming background.

Comparison of Atmospheric Circulation Anomalies between Daytime and Nighttime Extreme High Temperature in North China

Many previous studies have shown that atmospheric circulation anomalies are usually the direct cause of extreme high temperatures (EHT). However, the atmospheric circulation anomalies associated with daytime and nighttime EHTs in North China and their differences are less discussed. The present study divides the summer EHTs in North China into independent daytime EHT (ID-EHT) and independent nighttime EHT (IN-EHT) according to the 90th percentile thresholds of the daily maximum and minimum temperature from CN05.1 and compares their atmospheric circulation anomalies. Composite results show that the sinking motion anomaly over North China and the southward displacement of the Western Pacific Subtropical High (WPSH) cause less low cloud cover and water vapor, which is conducive to absorbing more solar radiation at the surface, and leads to the daytime high temperature of ID-EHT. With the disappearance of solar radiation at night, the heat is rapidly dissipated, and the high temperature cannot be maintained. A wave train from high latitudes can affect ID-EHT weather. On the contrary, the upward motion anomaly over North China cooperates with the northward displacement of the WPSH, leading to more clouds and water vapor over North China. As a result, the absorption of solar radiation in North China during the daytime is reduced, and EHT has difficulty in forming during the day. The higher humidity causes slower heat loss from daytime to nighttime, resulting in an IN-EHT. IN-EHT is more likely to be affected by a wave train such as the Silk Road pattern from the midlatitudes.

Thermal Impact of the Southern Tibetan Plateau on the Southeast Asian Summer Monsoon and Modulation by the Tropical Atlantic SST

Abstract While it is commonly accepted that the thermal effect of the Tibetan Plateau (TP) strengthens the Asian summer monsoon, a recent analysis based mainly on idealized model experiments revealed that the TP effect weakened the Southeast Asian summer monsoon (SEASM). Based on both observational analyses and model experiments, the current study further deciphers the physical mechanism for the TP’s thermal impact on the SEASM and the modulation of this impact by the sea surface temperature (SST) in the tropical Atlantic. When diabatic heating is enhanced over the southern TP, the South Asian high (SAH) intensifies and extends eastward, leading to convergence over the southeastern flank of the anomalous upper-level anticyclone and sinking motion that cause downward advection of negative vorticity. Accompanied by this anomalous anticyclonic pattern, the western Pacific subtropical high (WPSH) extends westward and the monsoon over Southeast Asia is weakened. The TP–SEASM relationship is enhanced when SST and convection increase over the tropical Atlantic, which cause an anomalous barotropic wave train propagating southeastward from eastern North America to East Asia, leading to an eastward extension of the SAH and a westward extension of the WPSH. The anomalous heating over the tropical Atlantic also modulates the Walker circulation through two anomalous vertical cells, with ascending motions over the Maritime Continent and the eastern tropical Indian Ocean, inducing a lower-level anticyclone over Southeast Asia as a Gill-type response. Thus, a warming tropical Atlantic can intensify the TP’s thermal forcing, weaken the SEASM, and then modulate the TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation. Significance Statement The Southeast Asian summer monsoon (SEASM) exhibits significant interannual variability. This study is aimed at better understanding the thermal impact of the Tibetan Plateau (TP) on the interannual variability of SEASM intensity and the possible modulating effect of the tropical Atlantic on the TP–SEASM relationship. We find that enhanced heating over the southern TP weakens the SEASM circulation and its associated precipitation by extending the South Asian high eastward and expanding the western Pacific subtropical high westward. Tropical Atlantic warming enhances this TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation. Thus, seasonal prediction of the SEASM can be improved by further considering the synergistic impact of the TP’s thermal forcing and the tropical Atlantic surface temperature.

Changing patterns of the East Asian monsoon drive shifts in migration and abundance of a globally important rice pest.

Numerous insects including pests and beneficial species undertake windborne migrations over hundreds of kilometers. In East Asia, climate-induced changes in large-scale atmospheric circulation systems are affecting wind-fields and precipitation zones and these, in turn, are changing migration patterns. We examined the consequences in a serious rice pest, the brown planthopper (BPH, Nilaparvata lugens) in East China. BPH cannot overwinter in temperate East Asia, and infestations there are initiated by several waves of windborne spring or summer migrants originating from tropical areas in Indochina. The East Asian summer monsoon, characterized by abundant rainfall and southerly winds, is of critical importance for these northward movements. We analyzed a 42-year dataset of meteorological parameters and catches of BPH from a standardized network of 341 light-traps in South and East China. We show that south of the Yangtze River during summer, southwesterly winds have weakened and rainfall increased, while the summer precipitation has decreased further north on the Jianghuai Plain. Together, these changes have resulted in shorter migratory journeys for BPH leaving South China. As a result, pest outbreaks of BPH in the key rice-growing area of the Lower Yangtze River Valley (LYRV) have declined since 2001. We show that these changes to the East Asian summer monsoon weather parameters are driven by shifts in the position and intensity of the Western Pacific subtropical high (WPSH) system that have occurred during the last 20 years. As a result, the relationship between WPSH intensity and BPH immigration that was previously used to predict the size of the immigration to the LYRV has now broken down. Our results demonstrate that migration patterns of a serious rice pest have shifted in response to the climate-induced changes in precipitation and wind pattern, with significant consequences for the population management of migratory pests.

Late Holocene temperature and precipitation variations in an alpine region of the northeastern Tibetan Plateau and their response to global climate change

Knowledge of temperature and precipitation variations on the northeastern Tibetan Plateau (NETP) during the recent past can improve our understanding of late Holocene regional climate change and its response to global climate change, in the past and potentially the future. Based on records of multiple geochemical indicators and branched glycerol dialkyl glycerol tetraethers from the sediments of alpine Lake Bihu on the NETP, we reconstructed high-resolution precipitation sequences and quantified the variation of the mean air temperature of months above freezing (MAF, between May and October) over the past ∼3500 years. The MAF reconstruction shows obvious fluctuations on decadal to centennial timescales, and the absolute values of the reconstructed temperature range from 0.70 to 3.98 °C, with an average of 2.19 °C. The precipitation record reflects the high-frequency variability of the East Asian summer monsoon (EASM). The maximum precipitation and temperature on the NETP occurred during the Medieval Warm Period (MWP, ∼800–1400 CE), rather than during the Current Warm Period (CWP, the last 150 years). We also observed broadly similar patterns of warm–wet and cold–dry climatic variations over the NETP and the broader EASM region; moreover, the decadal to centennial temperature fluctuations were consistent on a large spatial scale. Our results indicate that temperature variability during the late Holocene was primarily controlled by solar radiation, interrupted by multiple decades of successive volcanic eruptions. In addition, anomalous and rapid warming during the CWP may also be related to the unprecedented increase in atmospheric greenhouse gases. Precipitation variability in this region, driven by changes in the EASM, may have been primarily a response to changes in total solar irradiance and associated changes in atmospheric–oceanic modes (e.g., the El Niño–Southern Oscillation, Intertropical Convergence Zone, and Western Pacific Subtropical High). Overall, our results provide robust evidence for global climate teleconnections between different regions, especially in high mountains, in the past, present, and potentially in the future.