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In the summer of 2010, western Russia experienced extreme heat, which was noted for its exceptional spatial spread, long duration, high intensity and impacts. We use an anomaly-based approach to decompose atmospheric variables into daily climatic components and anomalies from two reanalysis datasets. We show that a surface heat wave event results from a downward extension of an anomalously warm air column below a centre of positive geopotential height anomalies in the upper troposphere. Therefore, we use this approach to analyse all summer regional heat wave events with spatial scales larger than 2000 km and durations greater than 5 days over western Eurasia from 1980 to 2014. Our results demonstrate that the rapid increase in regional heat wave events over western Eurasia since 2010 is a direct response to the increasing frequency of large-scale, quasi-stationary positive centres of maximum height anomalies in the upper troposphere.

A B S T R A C TThe Arctic sea ice has undergone a substantial long-term decline with superimposed interannual sea ice minimum (SIM) events over the last decades. This study focuses on the relationship between atmospheric circulation and the SIM events in the Arctic region. Four reanalysis products and simulations of one climate model are first analysed to confirm the existence of the Arctic cell, a meridional circulation cell to the north of 808N, by visualising through the mean streamline and mean mass stream function in the Northern Hemisphere. Dynamical analyses of zonally averaged stationary eddy heat and momentum fluxes as well as the global precipitation rate data further confirm its existence. Finally, we found that the change in the Arctic sea ice concentration lags the variations of the descending air flow intensity associated with the Polar and Arctic cells, by about 2 months for the climatic annual cycle and about 10 months for the interannual anomaly. Five Arctic SIM events during the last three decades support this relationship. These results have implications for understanding the relationship between atmospheric circulation and sea-ice variations, and for predicting the Arctic sea ice changes.

The southwest vortex (SWV), a low-pressure system bringing severe rainfall in southwest China, is one of the most important synoptic systems in China. Using both the National Centers for Environmental Prediction Final (NCEP-FNL) operational global analysis dataset and the Weather Research and Forecasting (WRF) model simulation, a sophisticated SWV with dual-core structure (DCSWV) over the Sichuan Basin in 2010 was studied. The DCSWV system consisted of two cores, one near Leshan City (named “C1”) and another near Langzhong City (named “C2”). The high-resolution WRF model reproduced the life cycle of the DCSWV well. The diagnostic analysis of the vorticity budget indicated that the stretching and tilting terms played important roles in the development stage of “C1”, while the stretching and vertical advection of vorticity were the major contributors to the formation and development stage of “C2”, which implied the importance of moisture convergence and ascending motion. Sensitivity experiments showed that the DCSWV was closely associated with the release in latent heat as well as the effect of topography. The great release in latent heat provided significantly positive feedback to the DCSWV system, which was decisive to the formation and development stages of “C2”. The topography of the Tibetan Plateau and the Yun-Gui Plateau affected the location and duration of the DCSWV.

Previous studies indicate that the summer (July-August) rainfall over North China has decreased since the mid-1970s due to the weakening of East Asian summer monsoon (EASM). However, this study firstly discovers the new evidences that the summer rainfall over North China had a significant increasing tendency during 1979–1996; since 1997, this increasing tendency has halted while more summer droughts occurred over North China. One important cause for the halted increasing tendency over North China is the interdecadal decrease of the westerly water vapor transport during 1997–2016 in addition to the weakened EASM. The decrease of the westerly water vapor transport during 1997–2016 was due to the interdecadal warming over Lake Baikal. The interdecadal warming in the upper troposphere at 200 hPa forced the weakening of the upper-level zonal winds since 1997, which resulted in the anomalous descending flow over the north side of North China and the halted precipitation trend in North China.