The effects of ENSO under negative AO phase on spring dust activity over northern China: an observational investigation

Abstract

The effects of El Nino/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 Nino, compared with that in the years of AO- and La Nina. Averaged over all observation stations, the spring DI (49.7) during the El Nino/AO- years is higher by 11.4% or 29.8% than that (38.3) during the La Nina/AO- years. We suggest possible physical mechanism that the anomalous large-scale environments associated with AO- and El Nino are more effective to provide favourable conditions to enhance Asian dust activity. During the El Nino/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 Nino. 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.