Numerical Study of the Relationships between Climate and the Carbon Dioxide Cycle on a Regional Scale

Abstract

Numerical simulations were used to investigate the mechanisms of the physical and biological interactions between the terrestrial ecosystems and climate on a regional scale. Also, a study was conducted on how the regional interactions influence seasonal and interannual variations of regional atmospheric 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 physical and biological interactions between terrestrial ecosystems and the atmosphere using a short time step (a few minutes), and a fine grid scale (about 30km). 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, and also reasonably well reproduced the heat and water budgets for the land surface for each year. The interannual variations of atmospheric carbon dioxide concentrations calculated by the model exhibited a characteristic phenomenon of stepwise increase in the lower troposphere similar to that found in data observed over Japan. The model results suggested that the characteristic phenomenon in the lower tropospheric carbon dioxide concentrations over 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 activity, it was suggested that interannual variations of downward short-wave radiation over the land surface during the experimental period were mainly responsible for the interannual variations of vegetation activity. Interannual variations of vegetation activity influenced the interannual variations of the net carbon dioxide flux between the land surface and the atmosphere. In turn, these variations influenced the interannual variations of carbon dioxide concentrations in the lower troposphere over Japan.