Meteorological analysis of long range transport of mineral aerosols over the North Pacific

Abstract

Meteorological data are used to investigate the importance of source, transport, and removal factors in the temporal and areal variation of mineral dust concentrations observed in the SEAREX (Sea/Air Exchange Program) Network of aerosol sampling stations over the open Pacific Ocean. Earlier studies established that Asian desert dust is carried out over the ocean by the mid‐latitude westerlies and may be present over the entire North Pacific area. A 6‐year climatology of routine surface meteorological reports of dust in Asia is used to examine the broad maximum in mineral dust concentrations in the months February‐May observed at all the North Pacific sites. The gross features of this seasonal variation are controlled by the occurrence and areal extent of dust outbreaks. Secondary peaks in the frequency of dust storms, in October‐December, correspond to isolated peaks in the dust concentration at the sites closest to the mid‐latitudes. Isentropic trajectory analyses are presented for individual events of peak concentrations supporting earlier indications that the dust transport begins at high elevations over the arid regions and gradually subsides for those parcels which move toward the south. The average transport path for high concentration periods, obtained by superposing trajectory patterns for events over a 6‐year period, is shown to explain the latitudinal variation in the concentrations. Removal by local orographic showers at Oahu is shown to be unimportant, while removal by the intense tropical rainfall near Fanning Island is shown to be a dominant influence on the seasonal variation of concentrations there. Rainfall data at the remote Pacific sites indicate that the 1981–1986 period was generally representative, despite the strong El Niño/Southern Oscillation event which occurred in 1982–1983.