Transient Responses of a Coupled Ocean-Atmosphere Model to Gradual Changes of Atmospheric CO2. Part II: Seasonal Response

This study investigates the seasonal variation of the transient response of a coupled ocean-atmosphere model to a gradual increase (or decrease) of atmospheric carbon dioxide. The model is a general circulation model of the coupled atmosphere-ocean-land surface system with a global computational domain, smoothed geography, and seasonal variation of insolation. It was found that the increase of surface air temperature in response to a gradual increase of atmospheric carbon dioxide is at a maximum over the Arctic Ocean and its surroundings in the late fall and winter. On the other hand, the Arctic warming is at a minimum in summer. In sharp contrast to the situation in the Arctic Ocean, the increase of surface air temperature and its seasonal variation in the circumpolar ocean of the Southern Hemisphere are very small because of the vertical mixing of heat over a deep water column. In response to the gradual increase of atmospheric carbon dioxide, soil moisture is reduced during the June-July-August period over most of the continents in the Northern Hemisphere with the notable exception of the Indian subcontinent, where it increases. The summer reduction of soil moisture in the Northern Hemisphere is relatively large over the region stretching from the northern United States to western Canada, eastern China, southern Europe, Scandinavia, and most of the Russian Republic. During the December-January-February period, soil moisture increases in middle and high latitudes of the Northern Hemisphere. The increase is relatively large over the western portion of the Russian Republic and the central portion of Canada. On the other hand, it is reduced in the subtropics, particularly over Southeast Asia and Mexico. Because of the reduction (or delay) in the warming of the oceanic surface due to the thermal inertia of the oceans, the increase of the moisture supply from the oceans to continents is reduced, thereby contributing to the reduction of both soil moisture and runoff over the continents in middle and high latitudes of the Northern Hemisphere. This mechanism enhances the summer reduction of soil moisture and lessens its increase during winter in these latitudes. The changes in surface air temperature and soil moisture in response to the gradual reduction of atmospheric CO2 are opposite in sign but have seasonal and geographical distributions that are broadly similar to the response to the gradual CO2 increase described above.