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Riyu Lu and Shu Lu

Abstract

The western North Pacific (WNP) monsoon variability plays an important role in East Asian climate, and it highlights the importance of understanding atmosphere–ocean interaction determining WNP variability. A key characteristic of atmosphere–ocean interaction is the local relationship between sea surface temperatures and precipitation (SST–P), which over the WNP exhibits a weak and negative correlation; this indicates that atmospheric variations lead to SST anomalies. This study investigates the underlying physical causes of this relationship, and it suggests that the inverse SST–P relationship over the WNP results from a local anomalous lower-tropospheric anticyclone or cyclone. A strong and negative SST–P correlation corresponds to a strong cyclonic/anticyclonic anomaly, while a weak SST–P relationship is related to a weak circulation anomaly. This study suggests that the remote effects play a crucial role in forming the inverse SST–P relationship over the WNP, while local SSTs tend to result in a positive SST–P correlation and partially offset the remote effects. Furthermore, the negative SST–P relationship over the WNP tends to be associated with rapid transitions of SST anomalies in the equatorial central and eastern Pacific, implying that atmosphere–ocean interaction over the WNP during summer may be affected by and in turn modify the evolution of ENSO.

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Riyu Lu and Shu Lu

Abstract

The summer precipitation anomalies over the tropical western North Pacific (WNP), which greatly affect East Asian climate, are closely related to Indian Ocean (IO) SST anomalies, and this WNP–IO relationship is widely assumed to be linear. This study indicates that the IO SST–WNP precipitation relationship is generally linear only when the IO SST anomalies are positive and not when the IO SST anomalies are negative, that is, a strongly cooler IO, in comparison with a moderately cooler IO, does not correspond to stronger precipitation enhancement over the WNP. Further analysis suggests that the phases of ENSO play a crucial role in modifying the impacts of IO SSTs on WNP anomalies. The reverse IO SST–WNP precipitation relationship, which exists without apparent ENSO development/decay, is intensified by El Niño decay through the enhancement of IO SST anomalies, but weakened by El Niño development and La Niña decay through the concurrence of SST anomalies in the tropical central and eastern Pacific. After removing El Niño developing and La Niña decaying cases, the IO SST and WNP precipitation anomalies show a clear linear relationship. Because of the effects of the phases of ENSO, the years of negative precipitation or anticyclonic anomalies over the WNP are highly concentrated over strongly warmer IO and El Niño decaying years, which is consistent with previous studies. However, the years of positive precipitation anomalies are scattered over cooler IO and moderately warmer IO years, implying a complexity of tropical SST forcing on positive WNP precipitation anomalies.

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Xinyu Li and Riyu Lu

Abstract

The meridional teleconnection over the western North Pacific and East Asia (WNP–EA) plays a predominant role in affecting the interannual variability of East Asian climate in summer. This study identified a breakdown of the meridional teleconnection since the early 2000s. Before the early 2000s, there are close tropical–extratropical relationships in light of both circulation and rainfall anomalies. For instance, the westward extension of the western North Pacific subtropical high (WNPSH) is closely associated with the southward shift of the East Asian westerly jet (EAJ), and more rainfall in the tropical WNP closely corresponds to less rainfall in the subtropical WNP–EA. However, after the early 2000s, the tropical–extratropical relationships are absent. Particularly, the tropical WNP precipitation anomalies can induce WNPSH anomalies, but the WNPSH anomalies cannot induce subtropical precipitation in the latter period, due to the absence of EAJ-related extratropical circulation anomalies. Further results indicate that in the latter period, the westward extension of the WNPSH is associated with the decay of central Pacific-like El Niño, and simultaneous summer sea surface temperature (SST) anomalies in the central eastern Pacific favor the northward shift of the EAJ, resulting in the disruption of the WNPSH–EAJ relationship. This evolution of tropical SSTs is sharply different from the decay of canonical El Niño and simultaneous summer tropical Indian Ocean warming, which favor the WNPSH–EAJ correspondence in the former period.

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Xingyan Zhou and Riyu Lu

Abstract

This study focused on the interannual variability of tropical cyclone (TC) activity over the western North Pacific in autumn. The results show that the frequencies of TC landfalls in the southern and northern coastal regions of East Asia are roughly independent, implying that they are affected by different factors and should be studied separately. Further analysis indicates that the frequency of TC landfall in the southern region is closely related to El Niño–Southern Oscillation, which affects both the upper- and lower-tropospheric circulation over the western North Pacific and East Asia and induces changes in the steering flow. By contrast, the frequency of TC landfall over the northern region has a close connection with a teleconnection pattern in the upper troposphere over the Eurasian continent, which seems to be triggered by an anomalous Rossby wave source over the North Atlantic. This teleconnection pattern leads to anomalous meridional winds over the western North Pacific and East Asia and induces significant changes in the steering flow.

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Ke Xu and Riyu Lu

Abstract

The modulation of tropical cyclone (TC) activity by the western North Pacific (WNP) monsoon break is investigated by analyzing the subseasonal evolution of TCs and corresponding circulations, based on 65 years of data from 1950 to 2014. The monsoon break has been identified as occurring over the WNP in early August. The present results show that TC occurrence decreases (increases) remarkably to the east of the Mariana Islands (southeast of Japan) during the monsoon break, which is closely related to local anomalous midtropospheric downward (upward) motion and lower-tropospheric anticyclonic (cyclonic) circulation, in comparison with the previous and subsequent convective periods in late July and mid-August. These changes of TC activity and the corresponding circulation during the monsoon break are more significant in typical monsoon break years when the monsoon break phenomenon is predominant. The reverse changes of TC activity to the east of the Mariana Islands and to the southeast of Japan during the monsoon break are closely associated with the out-of-phase subseasonal evolutions over these two regions from late July to mid-August, which are both contributed to greatly by 10–25-day oscillations. Finally, the roles of midlatitude and tropical disturbances on 10–25-day oscillations are also discussed.

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Xiaowei Hong and Riyu Lu

Abstract

The Silk Road pattern (SRP), which depicts the teleconnection pattern along the Asian jet, has been extensively investigated and commonly described as the leading mode of upper-tropospheric meridional wind anomalies in summer. In this study, the SRP is identified as having a significant relationship with the meridional displacement of the Asian jet (JMD), which manifests as the leading mode of upper-tropospheric zonal wind anomalies. This significant relationship is confirmed by the correlation coefficient between the indices for JMD and SRP, which is 0.39 and reaches statistical significance at the 0.01 level. When the Asian jet is in a northward (southward) displacement, the phase of SRP tends to be shown as anticyclonic (cyclonic) anomalies over western Asia and East Asia and cyclonic (anticyclonic) anomalies over Europe and central Asia. The authors propose an internal atmospheric mechanism for this relationship. In addition, it is found that the JMD is significantly affected by the tropical surface temperature anomalies. In particular, the negative (positive) SST anomalies in the tropical central and eastern Pacific of the preceding spring lead to significant cooler (warmer) tropical tropospheric temperatures in summer and may induce the northward (southward) displacement of the Asian jet through modifying the meridional gradient of tropospheric temperatures. The tropical tropospheric temperature anomalies may also affect the SRP through the JMD.

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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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Ke Xu and Riyu Lu

Abstract

A significant decadal change is detected in the break of the western North Pacific summer monsoon (WNPSM) around 2002/03. For the period 1979–2002, the monsoon break occurs in early August, accompanied by noticeable convection suppression over the ocean to the east of the Mariana Islands (10°–20°N, 140°–160°E). However, for the period 2003–11, the monsoon break there is delayed until mid-August. This decadal change is attributable to the differences in the evolution of the WNPSM. Over this break region, convection becomes weaker after its peak in late July for the former period, and the monsoon break appears in early August. In contrast, for the latter period, convection continues strengthening in late July and reaches its peak in early August, and the monsoon break is delayed until mid-August. The differences in the evolution of sea surface temperature (SST) in the western Pacific warm pool region are responsible for the decadal change in the evolution of the WNPSM. In contrast to the former period, for the latter period the southern extent of the warm pool is remarkably warmed, and tends to be higher than the northern extent in mid- and late July, which enhances atmospheric convection nearby but inhibits the development of convection over the northern extent through a local meridional circulation. As the SST in the northern extent continues warming and becomes higher than that in the southern extent, the convection over the northern extent reaches its maximum intensification in early August. The presented results highlight that the spatial pattern of SST changes can modulate the subseasonal evolution of the WNPSM.

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Xinyu Li and Riyu Lu

Abstract

The Yangtze River basin (YRB), a typical East Asian monsoon region, experiences a large year-to-year variability in summer precipitation and is subject to both floods and droughts. There is a well-known seesaw relationship in precipitation between the tropical western North Pacific and the YRB, but more than half of the variance in precipitation in the YRB cannot be explained by this seesaw pattern. The authors therefore investigated other physical factors that might affect precipitation in the YRB. The results indicate that the northeasterly anomaly in the lower troposphere to the north of the YRB plays an important role in the variability in precipitation. This northeasterly anomaly is paired with the southwesterly anomaly to the south of the YRB. They both play an important role in water vapor accumulation over the YRB and intensify the meridional gradient of the equivalent potential temperature θ e over the YRB by bringing dry and cool air from the north and wet air from the south. This intensified θ e gradient favors convective instability and heavier rainfall in the YRB, as previous studies on mei-yu weather have indicated. Furthermore, it is found that the zonally oriented teleconnection along the Asian westerly jet and the meridional displacement of the jet can affect circulation in the lower troposphere and precipitation in the YRB. These results highlight the role of extratropical circulation anomalies and thus contribute to a more comprehensive understanding of the variability of precipitation in the YRB.

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Riyu Lu and Yuanhai Fu

Abstract

The authors examine the projected change in interannual variability of East Asian summer precipitation and of dominant monsoonal circulation components in the twenty-first century under scenarios A1B and A2 by analyzing the simulated results of 12 Coupled Model Intercomparison Project phase 3 (CMIP3) coupled models. Interannual standard deviation is used to depict the intensity of interannual variability. An evaluation indicates that these models can reasonably reproduce the essential features of the present-day interannual variability in both East Asian rainfall and the rainfall-related circulations.

The models project an enhanced interannual variability of summer rainfall over East Asia in the twenty-first century, under both scenarios A1B and A2. Over the East Asian summer rain belt, 10 of the 12 models under scenario A1B and 9 of the 10 models under scenario A2 show enhanced variability in the twenty-first century relative to the twentieth century. The multimodel ensemble (MME) results in increased ratios of interannual standard deviation of precipitation averaged over this region of about 12% and 19% under scenarios A1B and A2, respectively. Furthermore, it is found that the interannual variability is intensified much more remarkably in comparison with mean precipitation.

Two circulation factors, the western North Pacific subtropical high (WNPSH) and East Asian upper-tropospheric jet (EAJ), which are closely related to the interannual variability of East Asian summer rainfall, are also projected by the models to exhibit enhanced interannual variability in the twenty-first century. This provides more evidence for the enhancement of interannual variability in East Asian summer rainfall and implies intensified interannual variability of the whole East Asian summer monsoon system. On the other hand, the relationships of East Asian rainfall with the WNPSH and EAJ do not exhibit clear changes in the twenty-first century under scenarios A1B and A2, and there are great discrepancies in the changes of the relationships among the individual models.

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