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Using monthly data from the European Center for Medium-Range Weather Forecast 40-year reanalysis (ERA-40), we have revealed a teleconnection pattern over the extratropical Northern Hemisphere through the empirical orthogonal function analysis of summer upper-tropospheric eddy temperature. When temperature is higher (lower) over the Eastern Hemisphere (EH), it is lower (higher) over the Western Hemisphere (WH). The teleconnection manifested by this out-of-phase relationship is referred to as the Asian-Pacific oscillation (APO). The values of an index measuring the teleconnection are high before 1976 and low afterwards, showing a downward trend of the stationary wave at a rate of 4% per year during 1958-2001. The index also exhibits apparent interannual variations. When the APO index is high, anomalous uppertropospheric highs (lows) appear over EH (WH). The formation of APO is likely associated with a zonal vertical circulation in the troposphere. Unforced control runs of both the NCAR Community Atmospheric Model version 3 and the Community Climate System Model version 3 capture the major characteristics of the teleconnection pattern and its associated vertical structure. The APO variability is closely associated with sea surface temperature (SST) in the Pacific, with a significantly positive correlation between APO and SST in the extratropical North Pacific and a significantly negative correlation in the tropical eastern Pacific. Sensitivity experiments show that the anomalies of SST over these two regions influence the APO intensity, but their effects are opposite to each other. Compared to the observation, the positive and negative anomalous centers of the extratropical tropospheric temperature triggered by the SST anomalies have a smaller spatial scale.

A perturbed air-surface coupled model has been developed to simulate and predict perturbations of POPs concentrations in various environmental media under given climate change scenarios. By introducing the perturbations in air temperature and precipitation induced by climate change in the model, we have examined the corresponding perturbations in the concentration of POPs in the closed air-soil and air-water systems. Numerical experiments for several POPs have been conducted based on the possible future climate change scenarios. It was found that hexachlorobenzene (HCB), hexachlorocyclohexanes (HCHs), and a polychlorinated biphenyl (PCBs) congener, PCB-153, exhibit strong response to specified climate change scenarios as shown by their high concentrations perturbations in air. In the air-soil system the coupled model predicts 4-50% increases in the air concentrations of these chemicals corresponding to an increase of 0.05-0.1 K yr(-1) in the air temperature. Based on our simulations, a 20% increase/decrease in precipitation can result in a 53% and 4% decrease/increase in perturbed air concentration of γ-HCH and α-HCH, respectively. Also, the model can be used to determine the direction of air-surface exchange of POP perturbations induced by climate change.

Over the last two decades, there is an ever‐increasing frequency of severe hail events occurred in Ontario, Canada. This upward trend is robust as verified through the MK test with consideration of removing a lag one autoregressive process. It is shown through the composite analysis that the severe hail frequency is closely linked with the atmospheric convective instability and precipitable water. It is also demonstrated that the high‐hail‐event years are associated with warmer air temperature than the low‐hail‐event years, indicating that the severe hail events occur more frequently in the warming climate.

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