The thermohaline circulation (THC) is an important regulator of the meridional heat transport in the North Atlantic. By ventilating the deep ocean, the THC regulates oceanic uptake of CO2 from the atmosphere by mixing it downward, therefore retarding climate change (e.g. Sarmiento and Le Quere, 1996). The THC is associated with deep water formation, most of which occurs in the north North Atlantic, in close proximity to the Arctic Ocean. With its ice cover, the Arctic Ocean is a large reservoir of freshwater (Aagaard and Carmack, 1989) and the North Atlantic THC is known to be sensitive to freshwater input (Weaver et al. 1998).
Climate models show different sensitivity of their simulated THC to increasing atmospheric CO2 concentration. Typically, the THC weakens as the radiative forcing increases and then may recover after the radiative forcing is held fixed, at least for a doubled level of CO2 (Manabe and Stouffer, 1994). The response of THC has been found to be highly dependant on the rate of CO2 increase (e.g. Stocker and Schmittner, 1997; Stouffer and Manabe, 1999). However, there is no agreement on the primary causes of the THC changes in response to increasing greenhouse gases. Some studies (Dixon et al., 1999; Stouffer and Manabe, 1999) point out the dominant role of surface freshwater fluxes, whereas others suggest that thermal forcing has the strongest contribution (Mikolajewicz and Voss, 2000). The former is due to the overall intensification of the hydrological cycle and an enhanced atmospheric poleward moisture transport, which supplies freshwater to the regions of deep water formation; while the later is due to the effects of oceanic surface heating on the density and circulation.
The influence of flux adjustments and how they affect the response of a model under climate change is also an important question. Some of the areas which require large flux adjustments are near the regions of deep water formation in the North Atlantic. One of the aims of this proposal is to examine the impact of individual components of the air-ice-sea fluxes, thermal versus haline, on the North Atlantic THC and climate. Unlike the studies by Dixon et al. (1999), Stouffer and Manabe (1999) and Mikolajewicz and Voss (2000) we will separate and analyse the effects associated with surface freshwater fluxes from Arctic sea ice. Since both of these previous studies used flux adjustments, we will also investigate the sensitivity of the results to the use of adjustments. Numerical experiments will look at the separate response under different rates of increase and different stabilisation values of atmospheric CO2 for both the transient and equilibrium phases of climate change.
Another goal of this proposal is to investigate the role of sea ice dynamics on climate sensitivity to increased atmospheric CO2. Stouffer and Manabe (1999) have shown that the response of the THC in their model depends of the rate of CO2 increase. Sea ice is simply driven by ocean currents in their model, while the model used by Mikolajewicz and Voss (2000) had no sea ice dynamics at all. It has been shown (e.g. Mauritzen and Hakkinen, 1997; Holland et al., 2001a; Saenko et al., 2002) that the North Atlantic THC can be greatly affected by the freshwater input due to sea ice export from the Arctic. We propose to investigate the sensitivity of the results obtained by Stouffer and Manabe (1999) by looking at the different responses from models that: exclude ice dynamics, have ocean driven ice or have full elastic-viscous-plastic ice dynamics. This issue has been in part addressed by Holland et al. (2001b) but we will perform much wider range of experiments, using the most recent version of the model described in Weaver et al. (2001).
Most global coupled climate models do not allow flow of water through the Canadian Archipelago (e.g. Stouffer and Manabe, 1999; Mikolajewicz and Voss, 2000; Boer et al., 2000). However, this flow may play a significant role in controlling the freshwater budget of the Arctic Ocean, affecting the outflow or formation of NADW (Goosse et al., 1997). An objective is to examine the effects of allowing flow through the Canadian Archipelago under the different climate change scenarios.
Outflow of dense deep water created in the Arctic Ocean or the Greenland Iceland Norwegian Seas also has an influence on the THC of the North Atlantic. Most models do not represent Demark Strait outflow waters very well and one reason is thought to be the poor representation of the bottom boundary layer flow, especially over steep topography. We propose to improve the representation of density-driven downsloping flows in our model, using a simple parametrisation of the ocean bottom boundary layer like that of Campin and Goosse (1999). It is hoped that such a parametrisation will improve the model's performance in simulating flows across Denmark Strait. The importance of such a parametrisation will be tested for different forcing scenarios and ocean mixing schemes.
We seek funding for two years in order to carry out numerical experiments, under different future forcing scenarios, to investigate:
In order to achieve our objectives, we will use a coupled climate model of intermediate complexity, the most recent version of which is described in Weaver et al. (2001). The basic model components include the GFDL ocean model, an energy-moisture balance model of atmosphere with advective-diffusive representation of heat and moisture transport, and a dynamic-thermodynamic sea ice component.
We will run this coupled model for a variety of transient and equilibrium future forcing scenarios, varying the rate of atmospheric CO2 increase and its stabilisation value. Particular emphasis will be given to the analysis of different (positive/negative) feedbacks operating in the climate system. In accordance with our proposal, an intensive analysis of budgets of heat and freshwater for the Arctic-North Atlantic region will be performed. Particular emphasis will be directed to an analysis of the ocean's ability to take up and vertically transport heat under different future climate change scenarios and for different time scales.
The proposed work in targeted at the "Processes related to climate model improvements", "Processes related to Arctic climate research" and "Additional topics of importance to the Arctic". It will contribute to our understanding of different physical processes which are responsible for stabilising or destabilising climate under enhanced global warming.
The collaborators in this proposal bring the required expertise for this project. O. Saenko has experience in both regional Arctic and global sea ice modelling, as well as in the development and implementation of coupling schemes. A. Weaver and M. Eby have a great deal of experience in the analysis of the results from and development of coupled climate models.
This project is aimed at understanding the separate roles of different feedback processes in the Arctic-North Atlantic system under enhanced global warming. By the end of the second year of the project we will submit at least three manuscripts to peer reviewed journals drawn from:
|Salary support for O. Saenko||$25 000|
|Salary support for M. Eby||$25 000|
|Travel for O. Saenko to CMOS conference||$1 500|
|Materials (publication costs for 1 paper)||$1 500|
|Administration and Overhead (10 %)||$5 300|
|Salary support for O. Saenko||$25 000|
|Salary support for M. Eby||$25 000|
|Travel for M. Eby to CMOS conference||$1 500|
|Materials (publication costs for 2 papers)||$3 000|
|Administration and Overhead (10 %)||$5 450|
Leveraging is as follows:
|2002 - 03||$177,500|
|2003 - 04||$177,500|
The project will be coordinated by A. Weaver with funds deposited in a University of Victoria account. The proximity of A. Weaver's lab to the CCCMa makes daily information sharing an easy task. Daily interactions will continue through the term of the CCAF project
|UVic / SEOS / Climate Group / Research Funding / CCAF Weaver, Saenko, Eby||Last updated: Monday, 30-Sep-2002 09:44:54 PDT|