Meteorological indices for Asian outflow and transpacific transport on daily to interannual timescales -: art. no. D18308

We examine the variability of Asian pollution outflow to the western Pacific and its associated long-range transport to the northeast (NE) Pacific between 1992 and 2002, using a global simulation of carbon monoxide (CO). We develop three new sea-level pressure indices, which capture variations in transpacific transport on daily to interannual timescales. Our first index, LRT1 (standardized daily pressure anomalies in NE China), is related to the passage of mid-latitude cyclones in East Asia and captures 14-22% of the variability in the outflow of Asian pollution to the western Pacific. A second index, LRT2 (standardized daily pressure anomalies North of Hawaii), captures 25-32% of the variability in transpacific transport during winter-spring. Enhanced transpacific transport is characterized by a strong Pacific High displaced to the west of 140 degrees W and a strong low over Alaska, resulting in strong zonal winds in the NE Pacific. The last index, LRT3 (based on monthly SLP anomalies over the North Pacific), identifies stronger than normal Pacific High and Aleutian Low pressure systems, which together result in unimpeded transport across the Pacific during winter and spring. LRT3 captures 42% and 55% of the monthly and interannual variance in transpacific transport, respectively. We find that 1992-1996 and 2002 were below-average transpacific transport years, while 1998 2000 had unusually strong transpacific transport. By extending our LRT3 time series back to 1948, we find that the strength of transpacific transport projects strongly upon the Pacific/North American (PNA) index, the leading mode of month-to-month variability over the Pacific. There appears to be decadal variability in LRT3, with below-average transpacific transport for 1948-1976 and above-average transport since 1977.