Response of the North Atlantic Thermohaline Circulation and Ventilation to Increasing Carbon Dioxide in CCSM3

Abstract

The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO 2 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 CO 2 . 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 CO 2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO 2 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 CO 2 , 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 CO 2 are well predicted by the corresponding differences in the water-mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid-to high latitudes with increasing CO 2 also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation time scale in response to increasing CO 2 . In the subpolar North Atlantic south of the Greenland-Scotland ridge system, there is a significant increase in subsurface ages as open-ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In middle and low latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming.

Variations of the Atlantic meridional overturning circulation in control and transient simulations of the last millennium

“…However, this generally applies to RF changes that are stronger than the ones during the last millennium. Extending our transient simulations to the 21st century under the A2 scenario, a decrease of the AMOC strength of 17% is found similar to the reduction reported in Bryan et al (2006) (not shown). Thus, we conclude that the RF changes in the last millennium are too weak to cause a direct AMOC response.…”

“…As for the volcanic forcing, there is no clear relationship between the RF variations and the AMOC intensity in our simulations. Other studies report a negative impact of strong RF changes on the AMOC, both for changes of the solar irradiance (e.g., Goosse and Renssen, 2006) and GHGs (e.g., Bryan et al, 2006). However, this generally applies to RF changes that are stronger than the ones during the last millennium.…”

“…A model intercomparison study indicates that the weakening is mainly caused by changes in the surface heat flux (Gregory et al, 2005). Also for the CCSM3, which is used in this study, simulations with an idealized CO 2 increase of 1% per year (Bryan et al, 2006) and scenario based projections of the 21st century (Meehl et al, 2006) show a weakening of the AMOC. However, the less pronounced GHGs increases in the 20th century do not result in a significant change of the AMOC (Meehl et al, 2006).…”

“…Compared to observations the simulated climate exhibits some major biases in the North Atlantic basin which will likely have an impact on the AMOC: the path of the NAC is too zonal, the northward heat transport is reduced, and the sea ice coverage is overestimated (Yeager et al, 2006). In addition, some deep water formation regions are not correctly represented, i.e., the convection region in the Labrador Sea is displaced to the southeast and the mixing in the GreenlandIceland-Norwegian (GIN) Seas is unrealistically low (Bryan et al, 2006). Nevertheless, the low-resolution CCSM3 has proven to provide useful information about the mechanisms of AMOC variations (D'Orgeville and Peltier, 2009;Yoshimori et al, 2010).…”

The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation

“…This holds in global climate models under strong external forcing, as shown by the weakening of the MOC in response to a large freshwater input at high latitude (e.g., Vellinga and Wood 2002) or to increased high latitude precipitation and temperature in global warming conditions (e.g., Bryan et al 2006). Whether the smaller salinity changes that occur in the absence of strong external forcing significantly impact the MOC is more difficult to establish.…”