1998 Year of the Ocean Impacts of Global Climate Change 



comparison studies are still at an early stage and validation of this coupled mode requires 

 additional effort. 



Results from another numerical model suggest that on multidecadal time scales, the NAO 

 is coupled to the thermohaline circulation of the Atlantic. (Thermohaline circulation refers to 

 circulation driven by density differences in the water with density being determined by 

 temperature and salt content.) Recent observations also show that the decadal signal in upper 

 layer temperature and NAO index are both superimposed on longer term trends. In addition, 

 analysis of historical atmospheric data indicates that a strong decadal signal in NAO variability is 

 limited to the past 30 to 40 years. 



Recent model results from the Geophysical Fluid Dynamics Laboratory (GFDL) suggest 

 the existence of a mode of climate variability on a multidecadal time series involving interactions 

 between the Arctic and North Atlantic. This mode is characterized by variations in the export of 

 fresh water and sea ice from the Arctic. It has substantial impacts on the atmospheric circulation 

 and climate over parts of Europe, as well as on the intensity of the thermohaline circulation and 

 associated meridional heat transport in the North Atlantic. Analyses of observations and various 

 model runs with and without COj increases provide data for: (I) increased understanding of the 

 dynamics of the coupled ocean atmosphere system; (2) identification of climatically important 

 oceanic features to monitor; (3) model validation; (4) predictability studies; and (5) separation of 

 natural and anthropogenic climate change. For example, observations can be used to benchmark 

 decadal and multidecadal variability from model simulations. If the models provide realistic 

 results of recent variability, this will offer confidence to conduct long simulations and assess the 

 role of COj on the air-sea system. 



Recent observations in the Atlantic argue for hypothesis driven climate studies in this 

 basin. After decadal increases in the NAO index, a significant decrease in this pattern has just 

 occurred (with expected changes in European weather). Concurrent with the increase in the NAO 

 index, extrema in several properties of the thermohaline circulation in the Atlantic have been 

 observed (e.g., convection in the Labrador Sea and lack of convection in the Greenland Sea, 

 increased subsurface temperatures in the subtropical Atlantic, and recently ventilated waters 

 observed in the deep currents along the western boundary of the North Atlantic Ocean). 



Several data-sets have been collected and several observing networks are in place that 

 provide a critical foundation for detection of climate change. The NOAA/DOE/NSF supported 

 World Ocean Circulation Experiment hydrographic program and global carbon survey have 

 provided an accurate benchmark for the ocean inventory of CO, and other properties. 

 Comparisons of these data with historical records are revealing large-scale variability in ocean 

 properties over decadal time scales. Synthesis of these data will greatly improve estimates of 

 interior ocean fluxes. Analysis of the COj survey data will yield robust estimates of the oceanic 

 uptake of CO, to date thereby offering critical constraints on partitioning of anthropogenic CO, 

 between the atmosphere, ocean, and terrestrial biosphere. Continued direct measurements are 

 needed next to: (1) track the rate of change of oceanic CO, and estimate CO, fluxes (e.g., GCM's 

 suggest that ocean sequestration of CO, decreases in response to climate warming); (2) track 



G-21 



