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ABSTRACT:The effects of El Niño/Southern Oscillation (ENSO) under negative Arctic Oscillation (AO) phase on the Asian dust activity are investigated for springs of the period 1961-2002. The spring dust index (DI) describing the monthly frequencies of three types of dust events (e.g. dust storm, blowing dust, and floating dust) exhibits a significant increase in the years of negative AO phase (hereafter AO−) and El Niño, compared with that in the years of AO− and La Niña. Averaged over all observation stations, the spring DI (49.7) during the El Niño/AO− years is higher by 11.4% or 29.8% than that (38.3) during the La Niña/AO− years. We suggest possible physical mechanism that the anomalous large-scale environments associated with AO− and El Niño are more effective to provide favourable conditions to enhance Asian dust activity. During the El Niño/AO− years, meridional gradients of pressure and temperature over the dust source regions are significantly enhanced by decreasing the geopotential height and warming air temperature that originated from the north and south of source regions, respectively, under the influence of AO− and El Niño. These also intensify the zonal wind shear and atmospheric baroclinicity, thereby producing enhanced cyclogenesis and dust occurrences over the major source regions. At the same time, dust transport paths with the stronger westerly winds are developed by the combined constraints of anomalous cyclone over the Siberia and the Mongolia and anomalous anticyclone over the western North Pacific, and thus strengthen dust transport to the downwind regions.

Asian dust events in Korea recorded between the second and eighteenth centuries exhibited the same seasonal aspect as those experienced in the last 90 years.

Effects of cloud, air temperature, and specific humidity on downward longwave irradiance and their long-term variabilities are examined by analyzing the measurements made at the King Sejong Station in the Antarctic Peninsula during the period of 1996-2006. It has been shown that the downward longwave irradiance (DLR) is significantly correlated with three variables: air temperature, specific humidity, and cloudiness. Based on the relationship of the three variables with DLR, a multiple linear regression model has been developed in order to evaluate the relative contribution of each of the variables to the variation of DLR. The three variables together explained 75% of all the variance in daily mean DLR. The respective contribution from specific humidity and cloudiness to the variation of DLR was 46% and 23%; thus most of the DLR variability can be explained by the variations in the two variables. The annual mean of longwave cloud forcing shows 52 W m Ϫ2 with no remarkable seasonal cycle. It is also noted that the overcast cloud effect gives an increase by 65 W m Ϫ2 with respect to clear-sky flux throughout the year. It is suggested that the multiple regression model can be used to estimate the radiative forcings of variables influencing the DLR variability.A highly significant decrease in DLR with an average of Ϫ1.52 W m Ϫ2 yr Ϫ1 (Ϫ0.54% yr Ϫ1 ) is found in an analysis from the time series of the deseasonalized monthly mean values. Accordingly, the atmospheric flux emissivity, air temperature, and specific humidity have also decreased in their time series, while the cloudiness has increased insignificantly. Consequently, it may be concluded that the recent decrease in DLR is mainly attributed to the net cooling effect due to the decrease in air temperature and specific humidity, which overwhelm the slight warming effect in cloudiness. Analysis of mean monthly trends for individual months shows that, as for the annual data, the variations in DLR are mostly associated with those of air temperature, specific humidity, and cloudiness.