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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
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

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
,
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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Yunwei Yan
,
Lei Zhang
,
Xiangzhou Song
,
Guihua Wang
, and
Changlin Chen

Abstract

Diurnal variation in surface latent heat flux (LHF) and the effects of diurnal variations in LHF-related variables on the climatological LHF are examined using observations from the Global Tropical Moored Buoy Array. The estimated amplitude of the climatological diurnal LHF over the Indo-Pacific warm pool and the equatorial Pacific and Atlantic cold tongues is remarkable, with maximum values exceeding 20.0 W m−2. Diurnal variability of sea surface skin temperature (SSTskin) is the primary contributor to the diurnal LHF amplitude. Because the diurnal SSTskin amplitude has an inverse relationship with surface wind speed over the tropical oceans, an inverse spatial pattern between the diurnal LHF amplitude and surface wind speed results. Resolving diurnal variations in the SSTskin and wind improves the estimate of the climatological LHF by properly capturing the daytime SSTskin and daily mean wind speed, respectively. The diurnal SSTskin-associated contribution is large over the warm pool and equatorial cold tongues where low wind speeds tend to cause strong diurnal SSTskin warming, while the magnitude associated with the diurnal winds is large over the highly dynamic environment of the intertropical convergence zone. The total diurnal contribution is about 9.0 W m−2 on average over the buoy sites. There appears to be a power function (linear) relationship between the diurnal SSTskin-associated (wind-associated) contribution and surface mean wind speed (wind speed enhancement from diurnal variability). The total contribution from diurnal variability can be estimated accurately from high-frequency surface wind measurements using these relationships.

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