This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.
This project will analyze the Coupled Model Intercomparison Project (CMIP5) models with a focus on the representation of Atlantic Multidecadal Oscillation (AMO) variability and its hydroclimate impacts. The AMO varies in phase on a timescale of 5-8 decades, and exerts considerable influence on North American hydroclimate, e.g. precipitation and droughts, surface air-temperature, and Atlantic hurricanes. Depending on its phase, the AMO impact can either offset or exacerbate the greenhouse gas forced warming signal over the continents in the Northern Hemisphere over a multidecadal period. Thus climate models undertaking decadal predictions and multidecadal projections cannot afford to misrepresent AMO variability.
The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. This project seeks in particular to evaluate the representation of the AMO in coupled climate models. Thus the research will lead to improved understanding of the role of multidecadal natural variability and secular change in the evolving climate of the 20th and 21st centuries. By providing a baseline on latest versions of the climate models, this study will directly address issues of model deficiencies on model credibility.
The Atlantic Multidecadal Oscillation (AMO) is an oscillation in North Atlantic sea-surface and sub-surface temperature and salinity with an oscillation period ranging from 5-8 decades. AMO exerts considerable influence on North American and African hydroclimate (precipitation and droughts, surface air-temperature) and on the Atlantic hurricanes making landfall over United States. In view of the multi-decadal AMO life-cycle and the substantial impact on precipitation and surface temperature on continents on both sides of the Atlantic Ocean, the climate system models undertaking decadal predictions and multidecadal projections of climate change driven by greenhouse gas increases cannot afford to misrepresent AMO variability and its regional climate impacts. This is because AMO's impact on surface temperature can both offset and exacerbate (depending on its phase) the greenhouse gas related warming signal over the Northern continents over a multidecadal period -- a signal that is the focus of the recent assessment of the Inter-Governmental Panel on Climate Change (IPCC). This research project evaluated the 20th century simulations of earth's climate produced by the leading climate system models, focusing on the space-time structure of AMO variabiltiy and its surface climate impacts. Note, model simulations can be evaluated only in the 20th century since earth observations are in hand for this period. Knowing how well the climate system models do in this past period is important for generating confidence in the projections of climate change produced by the same models; these projections constitute the backbone of the IPCC's fifth assessment report (AR5). Project analysis revealed that the four leading (widely used) climate system models underestimate the characteristic period of the AMO as well as its temporal structure. The models, for example, underestimated (overestimated) the amplitude of SST variablity on the 70–80 (10–20) year time scales. All of the four examined models were able to mimic the mid-latitude focus of the AMO SST anomalies and some features of the oceanic heat content signal. The models were however found quite deficient in the simulation of other AMO features. These included subsurface salinity and heat content variations, atmospheric circulation departures, and related continental precipitation anomalies. These deficiencies were estimated to be too significant to allow useful attribution and projection analysis of climate variability and change. The undertaken analysis suggests that rendering of the AMO spatiotemporal structure and related near-surface climate impacts has not benefited from the extensive development of climate models in the past 5 years (the CMIP3-to-CMIP5 phase), especially in the United States.