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Lei Song and Renguang Wu

Abstract

The present study reveals that the Madden–Julian oscillation (MJO)-related temperature anomalies over East Asia have notable differences among positive, neutral, and negative Arctic Oscillation (AO) phases. In MJO phases 2–3, cold anomalies over eastern China occur mainly during positive AO. In MJO phase 7, warm anomalies over eastern China are observed mostly during neutral AO, and in MJO phase 8 warm anomalies appear in positive and neutral AO. Regional mean temperature anomalies over northeastern East Asia tend to be negative during negative AO but positive during positive AO in six of eight MJO phases. In MJO phases 2–3, the AO-related mid- to high-latitude wave train over Eurasia and the MJO convection-triggered poleward wave train work together in contributing to negative height anomalies over eastern China and leading to cold anomalies there. The mid- to high-latitude wave train is stronger when the AO is negative than positive, which is associated with stronger zonal winds. In MJO phases 7–8, the positive AO-related mid- to high-latitude wave train over Eurasia and the MJO-induced poleward wave train cooperate in inducing positive height anomalies and leading to warm anomalies over eastern China. The mid- to high-latitude wave train is the main contributor to negative height anomalies over eastern China when the AO is negative during MJO phases 7–8. Meanwhile, the intensity of the South Asian wave source associated with the MJO convection is subjected to the modulation of southeastward dispersion of wave energy from western Europe during negative AO.

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Lei Song and Renguang Wu

Abstract

The present study shows that winter cold events over eastern China can be induced by Madden–Julian oscillation (MJO)-associated anomalous convection over the Maritime Continent. We conduct composite analysis separately for identified intraseasonal cold events over eastern China that occur following anomalous convection over the Maritime Continent and the tropical Indian Ocean. For cold events related to anomalous convection over the Maritime Continent, the southward intrusion of cold air into eastern China takes an eastward path in association with an eastward location of an anomalous Siberian high compared to cold events related to anomalous convection over the tropical Indian Ocean. The Maritime Continent convection-related cold events tend to occur with a negative Arctic Oscillation (AO), whereas the relationship between the tropical Indian Ocean convection-related cold events and the AO is weak. Anomalous convective heating over the Maritime Continent triggers a poleward Rossby wave train, which, together with an AO-related southward wave train from northern Eurasia, contributes to the deepening of the East Asian trough. The poleward wave energy dispersion is similarly triggered by anomalous convective heating over the tropical Indian Ocean. In both types of cold events, anomalous tropical heating induces a meridional vertical circulation, with large-scale airmass convergence in the upper midtroposphere and descending of air on the northern branch of the vertical cell over Siberia. The upper-level mass convergence and the radiative cooling over Siberia work together for the enhancement and southeastward expansion of the Siberian high and the southward intrusion of cold anomalies to eastern China.

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Lei Song and Renguang Wu

Abstract

A strong cold event hit eastern China around 24 January 2016 with surface air temperature reaching more than 10°C below the climatological mean in most regions of eastern China south of 40°N. A total of 37 strong cold events similar to the January 2016 event with temperature anomalies over eastern China exceeding −5°C have been identified during the winters from 1979/80 to 2015/16. A comparative analysis of events with surface temperature anomalies of the same intensity but limited to north of 40°N indicates that the southward invasion of cold air to eastern China south of 40°N is related to two factors. One is the latitudinal location of the upper-level wave train, the surface Siberian high, and the midtropospheric East Asian trough over the mid- to high-latitude Eurasian continent. The other is a subtropical upper-level wave train emanating from the midlatitude North Atlantic. The emergence of the subtropical wave train is related to the positive phase of the North Atlantic Oscillation (NAO). When the mid- to high-latitude wave train is located too far northward and the subtropical wave train induces an anomalous midtropospheric high over southern China, the East Asian trough does not extend southwestward and the Siberian high does not expand southeastward. In such a case, the cold air mainly affects northeastern China and northern Japan.

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Lei Song, Lin Wang, Wen Chen, and Yang Zhang

Abstract

The East Asian trough (EAT) is a distinct component of the boreal winter circulation whose strength corresponds to the amplitude of the Northern Hemispheric stationary waves. In this study, the mechanism and climatic impacts of the intraseasonal variations of the EAT’s strength are investigated through composite analysis and dynamical diagnostics. The significant roles played by the low-frequency Rossby wave (RW) and synoptic transient eddy (TE) are revealed. Before the peaks of strong EAT events, an upper-tropospheric RW train propagates across northern Eurasia and interacts with preexisting surface cold anomalies over central Siberia. This pattern intensifies the Siberian high and causes RW convergence toward the EAT, leading to 30% of the EAT’s amplification directly via the RW-induced feedback forcing. Meanwhile, RW weakens the background baroclinicity and reduces TE activities near the entrance region of the North Pacific storm track. The TE-induced feedback forcing leads to another 30% of the EAT’s amplification. The evolution and dynamical processes of the weak EAT events generally resemble those of the strong events with opposite signs. These results are consistent with the knowledge on the mechanism of the strong and weak EAT events regarding the role of RWs with additional quantitative description and provide new insights regarding the role of TEs. Variations of the EAT’s strength exert significant climatic impacts on East Asia and its downstream region. Near-surface air temperature is below (above) normal over East Asia during the growth and peak stages of the strong (weak) EAT events and above (below) normal over North America afterward.

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Tianjiao Ma, Wen Chen, Hans-F. Graf, Shuoyi Ding, Peiqiang Xu, Lei Song, and Xiaoqing Lan

Abstract

The present study investigates different impacts of the East Asian winter monsoon (EAWM) on surface air temperature (Ts) in North America (NA) during ENSO and neutral ENSO episodes. In neutral ENSO years, the EAWM shows a direct impact on the Ts anomalies in NA on an interannual time scale. Two Rossby wave packets appear over the Eurasian–western Pacific (upstream) and North Pacific–NA (downstream) regions associated with a strong EAWM. Further analysis suggests that the downstream wave packet is caused by reflection of the upstream wave packet over the subtropical western Pacific and amplified over the North Pacific. Also, the East Asian subtropical westerly jet stream (EAJS) is intensified in the central and downstream region over the central North Pacific. Hence, increased barotropic kinetic energy conversion and the interaction between transient eddies and the EAJS tend to maintain the circulation anomaly over the North Pacific. Therefore, a strong EAWM tends to result in warm Ts anomalies in northwestern NA via the downstream wave packet emanating from the central North Pacific toward NA. A weak EAWM tends to induce cold Ts anomalies in western-central NA with a smaller magnitude. However, in ENSO years, an anomalous EAJS is mainly confined over East Asia and does not extend into the central North Pacific. The results confirm that the EAWM has an indirect impact on the Ts anomalies in NA via a modulation of the tropical convection anomalies associated with ENSO. Our results indicate that, for seasonal prediction of Ts anomalies in NA, the influence of the EAWM should be taken into account. It produces different responses in neutral ENSO and in ENSO years.

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