All Researches

This is my Doctoral Dissertation about discussing the characteristics and formation of low frequency eddy energies applying Reanalysis and hierarchical models

Future projection of diffusion conditions associated with extreme haze events over East China is of great importance to government emission regulations and public human health. Here the diffusion conditions and their changes under future warming scenarios are examined. The relative strength of haze events in Northern China Plain (NCP) have increased from 150% during 2006-2015 to 190% during 2090-2099 under RCP8.5 scenarios, induced by stronger and longer-lingering anti-cyclone anomaly in Eastern China. The strengthened anticyclone anomaly is mainly induced by increased north wave train convergence emanating from Barents-Kara Sea, and the longer duration of anticyclone anomaly is mainly induced by stronger local feedback which can extract more energy from the basic state to maintain the anticyclone anomaly in Eastern China. Aerosol reduction is found to play a dominant role in strengthening the upstream wave train near Barents-Kara Sea and the downstream anti-cyclone in Eastern China, while the effects from greenhouse gases increase are small. Results in this study indicate that aerosol reduction east of Scandinavia Peninsula induced radiation effects can strengthen upstream wave train by baroclinic conversion which is conducive to haze formation in Eastern China., suggesting more stringent regulations on aerosol emissions in China are needed to meet air quality standards.

The East Asian jet has evident interannual variability in strength and position and has broad impacts on weather and climate in the Asian-Pacific-American region. In this study, we quantify the relative contributions of distinct dynamical processes responsible for the wintertime interannual variability of the East Asian jet core. The quantification is based on analyzing the zonal momentum budgets. Our budget analyses show distinct up- and downstream controls for the interannual intensification of the jet. Over the upstream, the jet intensification is predominantly driven by enhanced upper-level angular momentum transport associated with local convective-driven Hadley-cells. In addition to convections over the Tropical Pacific, those over the Bay of Bengal–South China Sea are found responsible for a distinct local Hadley cell over 80◦–100◦ E, which accelerates the jet from its very upstream. Over the downstream, angular momentum advected by the mean flow from the upstream is the first-order term responsible for the jet intensification, whereas synoptic eddies play a secondary role. For the interannual meridional displacement of the jet, synoptic eddy activities over the North Pacific are found to contribute predominantly. For both the intensification and the meridional displacement, the upper-tropospheric jet covaries closely with changes in the Subtropical Oceanic Frontal Zone in the North Pacific. Such a strong covariation implies the contributions of the air–sea interactions to the downstream jet variability.

Low-frequency (LF) transient eddies (intra-seasonal eddies with time scales longer than 10 days) is more and more found important in large-scale atmospheric circulations, high-impact climate events and subseasonal-to-seasonal forecast. In this study, features and the maintenance of available potential energy of LF eddies (LF EAPE), which denotes LF temperature fluctuations, have been investigated. Our study shows that wintertime LF EAPE exhibits much greater amplitude than that of the extensively studied high-frequency (HF) eddies, with distinct horizontal and vertical structures. Different from HF eddies, whose action centers are over midlatitude oceans, the LF EAPE is most active in the populated continents in midlatitude as well as the subpolar region. By diagnosing the derived energy budget of LF EAPE, we find that the LF perturbations with the upstream strong LF EKE in ocean basins and the strong background temperature gradient in mid- and high-latitude continents (e.g. the costal regions along Greenland-Barents-Kara sea) together result in strong baroclinic generation, which is the major source of LF EAPE. The generated LF EAPE in the subpolar region is transported downstream and southward to midlatitude continents via background flow, which can result in persistent temperature fluctuations (i.e. cold surge) in Eurasia and North America. The generated LF EAPE is also transferred to HF EAPE via cross-frequency eddy-eddy interactions and converted to LF EKE via vertical motions. The above energy budget suggests that, in addition to the classical Lorenz energy cycle between eddy and zonal mean flow, there is an energy cycle between LF and HF eddies, indicating a linkage between eddies in midlaittude and polar regions.

Previous studies have examined the projected climate types in China by 2100. This study identified the emergence time of climate shifts at a 1◦ scale over China from 1990 to 2100 and investigated the temporal evolution of Ko ̈ppen–Geiger climate classifications computed from CMIP5 multi-model outputs. Climate shifts were detected in transition regions (7%–8% of China’s land area) by 2010, including rapid replacement of mixed forest (Dwb) by deciduous forest (Dwa) over Northeast China, strong shrinkage of alpine climate type (ET) on the Tibetan Plateau, weak northward expansion of subtropical winter- dry climate (Cwa) over Southeast China, and contraction of oceanic climate (Cwb) in Southwest China. Under all future RCP (Representative Concentration Pathway) scenarios, the reduction of Dwb in Northeast China and ET on the Tibetan Plateau was projected to accelerate substantially during 2010–30, and half of the total area occupied by ET in 1990 was projected to be redistributed by 2040. Under the most severe scenario (RCP8.5), sub-polar continental winter dry climate over Northeast China would disappear by 2040–50, ET on the Tibetan Plateau would disappear by 2070, and the climate types in 35.9% and 50.8% of China’s land area would change by 2050 and 2100, respectively. The results presented in this paper indicate imperative impacts of anthropogenic climate change on China’s ecoregions in future decades.