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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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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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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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Hong Ye and Riyu Lu

Abstract

The findings of the study reported in this paper show that, during ENSO decaying summers, rainfall and circulation anomalies exhibit clear subseasonal variation. Corresponding to a positive (negative) December–February (DJF) Niño-3.4 index, a positive (negative) subtropical rainfall anomaly, with a southwest–northeast tilt, appears in South China and the western North Pacific (WNP) in the subsequent early summer (from June to middle July) but advances northward into the Huai River Basin in China as well as Korea and central Japan in late summer (from late July to August). Concurrently, a lower-tropospheric anticyclonic anomaly over the WNP extends northward from early to late summer. The seasonal change in the basic flows, characterized by the northward shift of the upper-tropospheric westerly jet and the WNP subtropical high, is suggested to be responsible for the differences in the above rainfall and circulation anomalies between early and late summer by inducing distinct extratropical responses even under the almost identical tropical forcing of a precipitation anomaly in the Philippine Sea.

A particular focus of the study is to investigate, using station rainfall data, the subseasonal variations in ENSO-related rainfall anomalies in eastern China since the 1950s, to attempt to examine their role in weakening the relationship between the ENSO and summer mean rainfall in eastern China since the late 1970s. It is found that the ENSO-related rainfall anomalies tend to be similar between early and late summer before the late 1970s, that is, the period characterized by a stronger ENSO–summer mean rainfall relationship. After the late 1970s, however, the anomalous rainfall pattern in eastern China is almost reversed between early and late summer, resulting accordingly in a weakened relationship between the ENSO and total summer rainfall in eastern China.

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Riyu Lu and Yuanhai Fu

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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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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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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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Xinyu Li and Riyu Lu

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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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Ke Xu and Riyu Lu

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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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Xiaowei Hong and Riyu Lu

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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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