Haiming Xu

Seasonal variations in atmospheric responses to oceanic eddies in the Kuroshio Extension

Analyses using high-resolution satellite observations reveal distinct seasonal variations in atmospheric responses to oceanic eddies in the Kuroshio Extension (KE) region, characterised by much stronger surface wind speed and heat flux responses in the cold seasons (winter and spring) than in the warm seasons (summer and autumn). Cloud liquid water and rain rate also display seasonally dependent characteristics, with more deficit (surplus) in winter than in summer over the cold (warm) oceanic eddies. CFSR (Climate Forecast System Reanalysis) data can well reproduce these seasonal variations in surface atmospheric responses to the eddies in the KE region, albeit with much weaker responses in surface wind speed and with stronger responses in latent heat flux in comparison with the results based on satellite observations. In addition, the CFSR data also reveal remarkable seasonal variations in tropospheric responses, with eddy-induced wind speed (vertical velocity) anomalies reaching as high as 900 hPa (800 hPa) in winter, while they only occur near the sea surface in summer. The Weather Research and Forecast (WRF) model is applied to study the seasonal variations in atmospheric responses to idealised oceanic eddies. The model successfully simulates the seasonal variations in atmospheric responses to an idealised warm eddy in terms of wind speed, heat flux, marine atmospheric boundary layer (MABL) height and vertical velocity in both seasons. Both the CFSR data and the model simulations indicate that the seasonal variations in atmospheric responses to oceanic eddies can be attributed to the variations in background atmospheric stability during different seasons.

Responses of the East Asian jet stream to the North Pacific subtropical front in spring

This study concerns atmospheric responses to the North Pacific subtropical front (NPSTF) in boreal spring over the period 1982–2014. Statistical results show that a strong NPSTF in spring can significantly enhance the East Asian jet stream (EAJS). Both transient eddy activity and the atmospheric heat source play important roles in this process. The enhanced atmospheric temperature gradient due to a strong NPSTF increases atmospheric baroclinicity, resulting in an intensification of transient eddy and convection activities. On the one hand, the enhanced transient eddy activities can excite an anomalous cyclonic circulation with a quasi-baraotropical structure in the troposphere to the north of the NPSTF. Accordingly, the related westerly wind anomalies around 30°N can intensify the component of the EAJS over the Northeast Pacific. On the other hand, an enhanced atmospheric heat source over the NPSTF, which is related to increased rainfall, acts to excite an anomalous cyclonic circulation system in the troposphere to the northwest of the NPSTF, which can explain the enhanced component of the EAJS over the Northwest Pacific. The two mechanisms may combine to enhance the EAJS.

Impacts of northern Tibetan Plateau on East Asian summer rainfall via modulating mid-latitude transient eddies

Roles of the Tibetan Plateau (TP) in forming and changing the seasonal Asian climate system have been widely explored. However, little is known about modulation effects of the TP on extratropical transient eddies (TEs) and subsequent synoptic responses of the East Asian rainfall. In this study, the Community Atmospheres Model version 5.1 coupled with a slab ocean model is employed to highlight the important role of the TP in regulating the upper-tropospheric transient wave train. Comparison between sensitivity experiments with and without the TP shows that the northern TP excites a strong anomalous anticyclone, which shifts the upper-level East Asian westerly jet northward and helps transfer barotropic and baroclinic energy from the mean flow to the synoptic TE flow. The transient wave train is primarily shifted northward by northern TP, and is forced to propagate southeastward along the eastern flank of the TP until reaching eastern China. Before the strengthening of monsoonal southerlies, the TP-modulated transient wave train cools the troposphere, which decreases the static stability over northern China. Meanwhile, the associated anomalous warm advection induces ascending motion, leading to excessive rainfall by releasing unstable energy as the southerly strengthens. Due to the southeastward propagation of the wave train, anomalous heavy rainfall subsequently appears over eastern China from north to south, which increases day-to-day rainfall variation in this region. Additionally, occurrence of this upper-tropospheric transient wave train associated with low-level southerly peak is substantially increased by northern TP.

Features of Rossby Wave Propagation Associated with the Evolution of Summertime Blocking Highs with Different Configurations over Northeast Asia

AbstractThis study categorized blocking high (BH) episodes during the boreal summer in northeast Asia (40°–70°N, 100°–150°E) into four types according to their wave-breaking features at the dynamic tropopause on the initial day: anticyclonic warm, cyclonic warm, anticyclonic cold, and cyclonic cold. Based on the results of a statistical analysis, it was shown that 1) the anticyclonic-warm type tended to occur in eastern Russia (55°–70°N, 127.5°–142.5°E), whereas the other three types preferentially occurred in the vicinity of Lake Baikal; 2) the two cold types generally were more common than the two warm types; and 3) the average life spans of the two anticyclonic types were longer than those of the two cyclonic types. According to a composite analysis, the four BH types were preceded by different wave train–like anomalies over the Eurasian continent over approximately one week. Correspondingly, each BH type was characterized by distinct Rossby wave propagation features. Interestingly, a northeastward pro...

Nonlinear effects of anthropogenic aerosol and urban land surface forcing on spring climate in eastern China

Anthropogenic aerosols and urban land cover change induce opposite thermal effects on the atmosphere near surface as well as in the troposphere. One can think of these anthropogenic effects as composed of two parts: the individual effect due to an individual anthropogenic forcing and the nonlinear effects resulting from the coexistence of two forcing factors. In this study, we explored the role of such nonlinear effects in affecting East Asian climate, as well as individual forcing effects, using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Atmospheric responses were simulated by including anthropogenic aerosol emission only, urban cover only, or the combination of the two, over eastern China. Results showed that nonlinear responses were different from any effects by an individual forcing or the linear combination of individual responses. The nonlinear interaction could generate cold horizontal temperature advection to cool the troposphere, which induced anomalous subsidence along the Yangtze River Valley (YRV). This anomalous vertical motion, together with a weakened low-level southwesterly, favored below-normal (above-normal) rainfall over the YRV (southern China), shifting the spring rain belt southward. The resultant diabatic cooling, in turn, amplified the anomalous descent and further decreased tropospheric temperature over the YRV, forming a positive feedback loop to maintain the nonlinear effects. Consequently, the nonlinear effects acted to reduce the climate anomalies from a simple linear combination of two individual effects and played an important role in regional responses to one anthropogenic forcing when the other is prescribed.