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Ruidan Chen
and
Riyu Lu

Abstract

Previous studies have suggested that, because of its particular location on the southeastern lee side of mountains, extreme heat (EH) over western north China (WNC) is affected by the foehn phenomenon. In this study, the EH days during summer over this region are categorized into foehn-favorable EH and no-foehn EH, according to whether there are anomalous northwesterlies over mountains, and composite analyses are performed on them. The analyzed results indicate that the no-foehn EH is characterized by an anticyclonic anomaly and a large-scale higher surface air temperature, while the foehn-favorable EH is featured by a cyclonic anomaly to the northeast and a localized higher temperature. Associated with the cyclonic anomaly, northwesterlies prevail over the mountain surface and provide a favorable environment for the occurrence of the foehn effect over WNC, which is located on the southeastern lee side of mountains. That is, both cyclonic and anticyclonic anomalies can induce EH over WNC (i.e., foehn-favorable EH and no-foehn EH, respectively). Further investigation indicates that large-scale cyclonic and anticyclonic anomalies tend to favor local descent and ascent anomalies over the lee side, respectively, through interaction with the particular terrain. Therefore, large-scale circulations and local terrain-induced winds play an offsetting role in affecting the surface air temperatures over WNC, and EH occurs when anomalous large-scale anticyclone or terrain-induced descent dominate. This study implies that attention should be paid to not only the upper-level/large-scale circulations but also to their impact on lower-level/local winds for temperature variability over the places with great topographic relief worldwide.

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Ruidan Chen
and
Riyu Lu

Abstract

The circulation associated with extreme heat (EH) typically shows an anomalous anticyclone that enhances temperature through adiabatic heating, but this study indicates obvious spatial variation in eastern China. The EH-related circulation pattern in eastern China can be classified into three categories: typical extratropical pattern, monsoonal pattern, and foehn pattern. EH over northeastern China and eastern north China is characterized by a typical pattern involving an anomalous anticyclone and subsidence, and the air temperature increases throughout almost the entire troposphere. In contrast, EH over the Yangtze River valley and south China is associated with the monsoonal pattern. Over these regions, the air temperature only increases in the lower troposphere as a result of anomalous subsidence and lower humidity that has resulted from a farther north transportation of water vapor by a stronger monsoonal southwesterly. Meanwhile, the air temperature decreases in the upper troposphere because of the decrease of latent heat caused by suppressed precipitation. On the other hand, western north China, with most of its stations located on the eastern leeside of mountains, is obviously influenced by the foehn effect on EH days. The foehn-related northwesterly anomalies bring drier and warmer air from the mountains to sink on the leeside and greatly increase the air temperature in the lower troposphere, particularly near the surface. Therefore, the impacts of monsoon and topography should be taken into consideration when EH-related circulations are discussed over the many regions of eastern China. As a result, the reliable projection of air temperature in these regions under global warming is a challenging problem.

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Ruidan Chen
and
Lorenzo Tomassini

Abstract

The southwesterly low-level jet (LLJ) located to the east of the Tibetan Plateau in southern China plays an important role in summertime convective initiation over north China. This study adopts a novel perspective and uses hindcast experiments in order to investigate the role of moisture in LLJ and associated heavy rainfall formation, employing a global atmospheric general circulation model (AGCM). In the sensitivity experiments, an increase of humidity in the inflow region leads to a weaker LLJ but stronger diurnal wind oscillations. The weaker LLJ is due to a decreased lower-tropospheric east–west pressure gradient resulting from a low pressure anomaly over southeastern China induced by deep convection and related condensational heating. On the other hand, the stronger diurnal variation of the LLJ originates from stronger day-and-night thermal differences over the sloping terrain, which is related to drier conditions over the mountain range. Moreover, the increased humidity and decreased LLJ counteract one another to impact precipitation in the outflow region. The change of precipitation is mainly determined by the altered moisture flux divergence. If the increase in humidity dominates, then the moisture flux convergence is enhanced and favors more precipitation over north China. Otherwise, if the decreased LLJ dominates, then the moisture flux convergence is reduced, which constrains precipitation. It is highlighted that the moist diabatic and dynamic processes are intimately coupled, and that a correct simulation of moisture flux convergence is vital for AGCMs to reproduce the LLJ-related precipitation, particularly the nocturnal precipitation peak, which is a deficiency in many current models.

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Ruidan Chen
and
Riyu Lu

Abstract

Generally, tropical nights [TN; minimum temperature (Tmin) ≥25°C] occur under wet air conditions, while extreme heat [EH; maximum temperature (Tmax) ≥35°C] occurs under dry air conditions. This can be explained by higher humidity favoring TN through reducing longwave radiation cooling, and lower humidity favoring EH through enhancing solar radiation at the surface. The present study focuses on the atypical phenomena of dry TN (30% of all TN days) and wet EH (20% of all EH days) in Beijing during July and August, 1979–2008. It was found that meteorological conditions, including large-scale circulations and specific humidity, exhibit a resemblance between typical (wet TN and dry EH) and atypical (dry TN and wet EH) cases. That is, the meteorological anomalies for dry TN are similar to those for dry EH, and the anomalies for wet EH are similar to those for wet TN. For instance, descending anomalies, which lead to lower humidity and are thus associated with dry EH, appear for more than 70% of dry TN cases. In addition, the persistence of high temperature from day to night, and from night to day, also contribute significantly to dry TN and wet EH, respectively. About 50% of dry TN days and about 70% of wet EH days are preceded by EH and TN, respectively. It can be concluded from these results that both meteorological conditions and temperature persistence contribute greatly to dry TN and wet EH.

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Ruidan Chen
,
Zhiping Wen
, and
Riyu Lu

Abstract

South China experiences extreme heat (EH) most frequently in eastern China. This study specifically explores the large-scale circulation anomalies associated with long-lived EH events in south China. The results show that there is an anomalous cyclone (anticyclone) and active (inactive) convection over south China (the western Pacific) before the EH onset; then, an anticyclone develops and moves northwestward and dominates over south China on the onset day. The anomalous anticyclone maintains its strength over south China and then diminishes and is replaced by another cyclone migrating from the western Pacific after the final day of the EH event. Consequently, the temperature increases over south China around the onset day and is anomalously warm for approximately 10 days on average and then decreases shortly thereafter. The fluctuating anomalies over south China and the western Pacific are intimately related to two intraseasonal oscillation (ISO) modes, namely, the 5–25- and 30–90-day oscillations, which originate from the tropical western Pacific and propagate northwestward. The 5–25-day oscillation is vital to triggering and terminating EH, accounting for approximately half of the original temperature and circulation anomaly transitions. The 30–90-day oscillation favors the persistent warming during EH events, accounting for approximately one-third of the original prolonged warming and anticyclonic anomaly. This result suggests that different ISO modes play crucial roles at different stages of the events. Moreover, a higher annual frequency of long-lived EH days in south China is associated with the transition phase from El Niño to La Niña. It is suggested that both medium-range and interannual forecasting of long-lived EH in south China are possible.

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Ruidan Chen
,
Zhiping Wen
, and
Riyu Lu

Abstract

Southern China, located in the tropical–subtropical East Asian monsoonal region, presents a unique anticyclonic–cyclonic circulation pattern during extreme heat (EH), obviously different from the typical anticyclone responsible for EH in many other regions. Associated with the evolution of EH in southern China, the anticyclonic–cyclonic anomalies propagate northwestward over the Philippines and southern China. Before the EH onsets, the anticyclonic anomaly dominates southern China, resulting in stronger subsidence over southern China and stronger southerly (southwesterly) flow over the western (northern) margins of southern China. The southerly (southwesterly) flow transports more water vapor to the north of southern China, thus, together with the local stronger subsidence, resulting in drier air condition and accordingly favoring the occurrence of EH. Conversely, after the EH onsets, the cyclonic component approaches southern China and offsets the high temperature.

The oscillations of temperature and circulation anomalies over southern China exhibit a periodicity of about 10 days and indicate the influence of a quasi-biweekly oscillation, which originates from the tropical western Pacific and propagates northwestward. Therefore, the 5–25-day-filtered data are extracted to further analyze the quasi-biweekly oscillation. It turns out that the evolution of the filtered circulation remarkably resembles the original anomalies with comparable amplitudes, indicating that the quasi-biweekly oscillation is critical for the occurrence of EH in southern China. The quasi-biweekly oscillation could explain more than 50% of the intraseasonal variance of daily maximum temperature T max and vorticity over southern China and 80% of the warming amplitude of EH onsets. The close relationship between the circulation of the quasi-biweekly oscillation and the EH occurrence indicates the possibility of medium-range forecasting for high temperature in southern China.

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Ruidan Chen
,
Zhiping Wen
,
Riyu Lu
, and
Wenjun Liu

Abstract

This study reveals the interdecadal changes in the interannual variability of the summer temperature over Northeast Asia (NEA), which presents an enhancement around the early 1990s and a reduction after the mid-2000s. The stronger NEA temperature variability after the early 1990s is favored by the enhanced influence of the Pacific–Japan (PJ) teleconnection, which is remotely modulated by the southeastern tropical Indian Ocean (SETIO). After the early 1990s, the mean state over the SETIO presents relatively warmer SST and ascending motion, favoring a good relationship between the local SST and convection. Therefore, the SETIO SST could prominently influence the local convection and subsequently modulate the convection over the western North Pacific (WNP) via a cross-equatorial overturning circulation. The abnormal convection over the WNP further triggers the PJ teleconnection to influence NEA. However, these ocean–atmosphere processes disappear before the early 1990s. In this period, the mean state over the SETIO features relatively colder SST and subsiding motion, accompanied by a poor relationship between the local SST and convection. Therefore, the variability of convection over the SETIO is weak, and thus the atmospheric variability over the WNP is also weakened and the PJ teleconnection presents a different distribution that could not influence NEA. The reduced variability of NEA temperature after the mid-2000s is related to the feeble influence of the PJ teleconnection and the reduced variability of the SETIO SST, which is modulated by the SST over the tropical central–eastern Pacific during the preceding winter to spring.

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Yuan Zhao
,
Zhiping Wen
,
Xiuzhen Li
,
Ruidan Chen
, and
Guixing Chen

Abstract

This study reveals a close relation between the Mascarene high (MH), atmospheric transient eddies (hereafter transient eddies or eddies for short), and the sea surface temperature (SST) front over the southern Indian Ocean in austral winter. Climatologically, the subpolar westerly jet couples well with transient eddies via eddy–mean flow interaction and the anticyclonic vorticity to its north helps with anchoring the MH. On the interannual time scale, the MH exhibits a dominant meridional variation accompanied by intensity variability. When the MH moves poleward and intensifies, positive quasi-barotropic geopotential height anomalies associated with a warm temperature feature the southern flank of MH. As a result of the modified mean temperature gradient, the subpolar jet and transient eddies’ activity are enhanced near the jet exit; in contrast, the subtropical westerly jet and eddies are weakened over the jet entrance, mainly via the baroclinic energy conversion. As feedback, the anomalous transient eddies can trigger the poleward shift of MH by diverging the extended Eliassen–Palm (E-P) flux from subpolar to subtropical region and thus the intensification of subpolar jet and weakness of subtropical jet. Such positive feedback between the meridional variation of MH and transient eddies could be attributed to the underlying SST anomalies. Early SST warming appears over the southwestern Indian Ocean 3 months prior and shifts the Agulhas SST front poleward. The poleward Agulhas SST front could further induce a southward displacement of the activity of transient eddies by changing the low-level atmospheric baroclinicity. Hence, the SST anomalies over the southern Indian Ocean may trigger the meridional variation of MH via the positive eddies–mean flow feedback.

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Ke Xu
,
Riyu Lu
,
Baek-Jo Kim
,
Jong-Kil Park
,
Jiangyu Mao
,
Jae-Young Byon
,
Ruidan Chen
, and
Eun-Byul Kim

Abstract

The large-scale circulation anomalies associated with extreme heat (EH) in South Korea and southern–central Japan are examined using data during the time period 1979–2016. Statistical analysis indicates that EH days in these two regions are concentrated in July and August and tend to occur simultaneously. These EH days are therefore combined to explore the physical mechanisms leading to their occurrence. The composite results indicate that the anomalous atmospheric warming during EH days is dominantly caused by a significant subsidence anomaly, which is associated with a deep anomalous anticyclone over East Asia. Further investigation of the evolution of circulation anomalies suggests that the anomalous anticyclone over East Asia related to EH is primarily initiated by wave trains originating from upstream regions, which propagate eastward along the Asian westerly jet in the upper troposphere. These wave trains can be categorized into two types that are characterized by the precursor anticyclonic and cyclonic anomalies, respectively, over central Asia. The distinction between these two types of wave train can be explained by the wavenumbers of the Rossby waves, which are modulated by both the intensity and the shape of the Asian westerly jet as the background basic flow.

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Chunhui Lu
,
Jie Jiang
,
Ruidan Chen
,
Safi Ullah
,
Rong Yu
,
Fraser C. Lott
,
Simon F. B. Tett
, and
Buwen Dong
Open access