The relationship between the North Atlantic Oscillation(NAO) and the tropical cyclone frequency over the western North Pacific(WNPTCF) in summer is investigated by use of observation data. It is found that their linkage appears to have an interdecadal change from weak connection to strong connection. During the period of 1948–1977, the NAO was insignificantly correlated to the WNPTCF. However, during the period of 1980–2009, they were significantly correlated with stronger(weaker) NAO corresponding to more(fewer) tropical cyclones in the western North Pacific. The possible reason for such a different relationship between the NAO and the WNPTCF during the former and latter periods is further analyzed from the perspective of large-scale atmospheric circulations. When the NAO was stronger than normal in the latter period, an anomalous cyclonic circulation prevailed in the lower troposphere of the western North Pacific and the monsoon trough was intensified, concurrent with the eastward-shifting western Pacific subtropical high as well as anomalous low-level convergence and high-level divergence over the western North Pacific. These conditions favor the genesis and development of tropical cyclones, and thus more tropical cyclones appeared over the western North Pacific. In contrast, in the former period, the impact of the NAO on the aforementioned atmospheric circulations became insignificant, thereby weakening its linkage to the WNPTCF. Further study shows that the change of the wave activity flux associated with the NAO during the former and latter periods may account for such an interdecadal shift of the NAO–WNPTCF relationship.
A nuclear accident involving the leaking of radioactive pollutants occurred at the Fukushima Nuclear Power Plant in Japan, following an earthquake and subsequent tsunami on March 11,2011. Using official Japanese data on pollutant emissions during the accident, this study simulates the dispersion of nuclear pollutants. The source term of the nuclear leakage of radioactive material is designed using PM2.5 as the tracer of radioactive pollutants, and the study considers dry and wet deposition processes. A coupled-model system is constructed from the air-quality model Models-3/CMAQ and the Weather Research and Forecasting atmospheric model. The transport path and distribution of radioactive pollutants over long and short distances are simulated with different model horizontal resolutions of 30 and 4 km respectively. The long-distance simulation shows that, following the Fukushima nuclear accident, under the effect of westerly winds, radioactive pollutants are transported generally towards the eastern Pacific and reach the American continent after 5 days, but their concentration is only about 10-7 times the concentration near the Fukushima Nuclear Power Plant. The time required for pollutants to reach the United States is basically consistent with measurements made in California on March 18. Because the upper westerly wind is faster than the lower westerly wind, the distribution of pollutants tilts eastward in terms of its vertical structure. The short-distance （local） highresolution simulation indicates that strong winds and precipitation associated with a cyclone can accelerate the deposition, dif- fusion and transport of pollutions, and local cyclonic circulation can change the transport path of pollutants, even resulting in repeated effects of pollution in some areas. Pollutants disperse to southeastern Honshu, Japan, on March 14, 2011, agreeing well with the timing of local observations of increases in the absorbed dose rate. Results also show that radioactive pollutants from the Fukushima nuclear accident ar
This paper analyzes the large-scale atmospheric circulation characteristics of anomalous cases of January temperatures that occurred in Northeast China during 1960-2008 and precursory oceanic conditions.The January monthly mean surface air temperature(SAT) anomalies and the duration of low temperature are used to define temperature anomaly cases.The anomalous cyclonic circulation over northeast Asia strengthens the northerly flow in cold Januarys,while the anomalous anticyclonic circulation weakens the northerly flow in the warm Januarys.The negative(positive) North Pacific sea surface temperature anomaly(SSTA) and increased(decreased) sea ice concentration in the Barents-Kara seas in the preceding month are probably linked to the cyclonic(anticyclonic) circulation pattern over northeast Asia in the cold(warm) cases.Further analyses indicate that the preceding oceanic conditions play distinct roles in the SAT anomalies over Northeast China on different time scales.Strong relationships exist between North Pacific SSTA and the SAT in Northeast China on the interannual time scale.On the other hand,the sea ice concentration is more closely associated with the interdecadal variations of SAT in Northeast China.
