Collaborative Research: Understanding Atlantic Decadal-to-Multidecadal Variability and Predictability

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Research under this award will examine the variability and predictability of Atlantic sea surface temperatures (SSTs), with a particular focus on the role of "weather noise" in producing decadal and multidecadal SST fluctuations. Weather noise refers to the variability of weather patterns which is independent of SSTs and occurs owing to the chaotic nature of air movements. Despite their short duration, weather systems produce air-sea thermodynamic fluxes which can be a significant source of low-frequency SST variability, through the reddening effect of the long thermal relaxation time of the ocean mixed layer. A key tool in the research, developed during the PIs' previous NSF grant on this topic, is a modified version of the NCAR Community Climate System Model (CCSM) referred to as the Interactive Ensemble CCSM (IE-CCSM). The IE-CCSM addresses the role of weather noise by dramatically reducing it, so that the weather noise mechanism can be seletively enabled or disabled. The procedure used in IE-CCSM is to integrate multiple copies of the atmospheric component model of the coupled climate model, and use the average of the surface fluxes from all copies to force the ocean component model. Each atmospheric component model will have different weather noise, thus the weather noise will be removed by averaging over all the copies. The ocean component model interacts only with the average of the atmospheric component models, and thus does not "see" the weather noise. Experiments conducted with the IE-CCSM will examine the importance of weather noise for four major patterns of Atlantic SST variability: a tripole SST pattern in the northern North Atlantic which is associated with the North Atlantic Oscillation, a monopole SST anomaly in the North Atlantic associated with Atlantic multidecadal variability, and which may be related to variability of the Atlantic meridional overturning circuation, and two patterns of tropical Atlantic variability, a zonal structure resembling the Pacific El Nino phenomenon, and a meridional structure characterized by changes in the cross-equatorial SST gradient. The research also include experiments to understand the impact of weather noise on the predictability of Atlantic SST variability.The work has eduational broader impacts through its support for three graduate students, which will help to develop the next generation of scientists in this research area. In addition, the research may lead to insights into the variability and predictability of the climate system over long time periods, which would be of interest to policy and decision makers concerned with the impacts of climate variability and change.