2005: Response of the North Atlantic thermohaline circulation and ventilation to increasing carbon dioxide in CCSM3

• Main Authors: Frank O. Bryan, Gokhan Danabasoglu, Norikazu Nakashiki, Yoshikatsu Yoshida, Dong-hoon Kim, Junichi Tsutsui, Scott C. Doney
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: North Atlantic; North atlantic Thermohaline circulation
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.452.6945

The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1 % per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22%–26 % of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal time scales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water-mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep-water formation is to increase their density; that is, changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the correspond-