TRANSPORT-INDUCED INTERANNUAL VARIABILITY OF CARBON-MONOXIDE DETERMINED USING A CHEMISTRY AND TRANSPORT MODEL

Transport-induced interannual variability of carbon monoxide (CO) is s tudied during 1989-1993 using the Goddard chemistry and transport mode l (GCTM) driven by assimilated data. Seasonal changes in the latitudin al distribution of CO near the surface and at 500 hPa are captured by the model. The annual cycle of CO is reasonably well simulated at site s of widely varying character. Day to day fluctuations in CO due to sy noptic waves are reproduced accurately at remote North Atlantic locati ons. By fixing the location and magnitude of chemical sources and sink s, the importance of transport-induced variability is investigated at CO-monitoring sites. Transport-induced variability can explain 1991-19 93 decreases in CO at Mace Head, Ireland, and St, David's Head, Bermud a, as well as 1991-1993 increases in CO at Key Biscayne, Florida, Tran sport-induced variability does not explain decreases in CO at southern hemisphere locations. The model calculation explains 80-90% of intera nnual variability in seasonal CO residuals at Mace Head, St. David's H ead, and Key Biscayne and at least 50% of variability in detrended sea sonal residuals at Ascension Island and Guam. Upper tropospheric inter annual variability during October is less than 8% in the GCTM. Excepti ons occur off the western coast of South America, where mixing ratios are sensitive to the strength of an upper tropospheric high, and just north of Madagascar, where concentrations are influenced by the streng th of offshore flow from Africa.