Numerical study of the relationships between climate and the carbon dioxide cycle on a regional scale

Numerical simulations were used to investigate the mechanisms of the physic al and biological interactions between the terrestrial ecosystems and clima te on a regional scale. Also, a study was conducted on how the regional int eractions influence seasonal and interannual variations of regional atmosph eric carbon dioxide concentrations. The numerical simulation was performed using a physical regional climate model, including sophisticated biological land surface processes. The experimental area is the Japanese Islands and surrounding area. The regional climate model can estimate the nonlinear phy sical and biological interactions between terrestrial ecosystems and the at mosphere using a short time step (a few minutes), and a fine grid scale (ab out 30 km). Experimental time integrations were performed for six years and five months, with the results of the last six years being examined. The model reproduced the seasonal variations of meteorological elements, an d also reasonably well reproduced the heat and water budgets for the land s urface for each year. The interannual variations of atmospheric carbon diox ide concentrations calculated by the model exhibited a characteristic pheno menon of stepwise increase in the lower troposphere similar to that found i n data observed over Japan. The model results suggested that the characteri stic phenomenon in the lower tropospheric carbon dioxide concentrations ove r Japan was related to the interannual variations of vegetation activity on and around the Japanese Islands. The vegetation activity was influenced by the interannual variations of climate around the Japanese Islands. From examinations of the principal elements that influence the vegetation a ctivity, it was suggested that interannual variations of downward short-wav e radiation over the land surface during the experimental period were mainl y responsible for the interannual variations of vegetation activity. Intera nnual variations of vegetation activity influenced the interannual variatio ns of the net carbon dioxide flux between the land surface and the atmosphe re. In turn, these variations influenced the interannual variations of carb on dioxide concentrations in the lower troposphere over Japan.