Atlantic Multidecadal Variability and its Climate Impacts
The Atlantic multi-decadal variability (AMV, also known as Atlantic multi-decadal Oscillation, AMO), is a North Atlantic basin-wide sea surface temperature fluctuation on multi-decadal time scales. AMV has wide-ranging climate impacts and is linked to variability of Sahel rainfall, North and South American hydroclimate, and the Indian monsoon. The complexity in understanding the impacts of the observed AMV stems, in part, from its concurrence with a century-long warming of North Atlantic SST likely caused by anthropogenic forcing. A central research problem is therefore to separate out any naturally-occurring multi-decadal Atlantic SST variability from the radiatively forced SST change. Ting et al. [2009, J. Climate] did this by using CMIP3’s multi-model/multi-ensemble 20th Century simulations to estimate the radiatively-forced (both anthropogenic and natural) North Atlantic SST trend and confirmed the existence of natural multi-decadal SST variability over the North Atlantic in 20th Century observations. They further showed that the spatial structures of the forced and internal North Atlantic SST variability patterns are distinct and are each tied to unique world-wide precipitation anomalies. However, given the short observational record for SST (~130 years) and the lack of reliable century long observations of global precipitation and atmospheric circulation, it is difficult to establish the robustness of the AMV spatial and temporal structure and its impacts on precipitation.
In a recent paper submitted to Geophysical Research Letters, Ting et al.,  showed that there is a well-defined spatial pattern for AMV in the North Atlantic that is consistent in 20th Century observations as well as the climate model simulations of the 20th, 21st and pre-industrial conditions (See left panels of the figure), despite the differing temporal scales of the phenomenon between model and observations. Furthermore, models and observations agree on the important precipitation impacts of AMV over the tropical Atlantic basin. These follow from meridional shifts of the Atlantic ITCZ (as shown on the right panels of figure). For example, we are able to show that the observed robust drying trend over the Sahel from the 1960s to 1980s, associated with the negative AMV trend, and the wetting since the 1980s, associated with the positive AMO trend are not statistical artifacts despite the short duration of the instrumental record. However the models do not reproduce the observational associations of a positive AMV with drying over North America and Australia and wetting in the Indian monsoon region raising questions about the robustness of these connections. This disagreement potentially follows from differences between models and observations in ho AMV impacts tropical Pacific precipitation (and, as a result, precipitation around the world). More research is needed to untangle the links between the Pacific and Atlantic basins and how the two are coupled together via both atmosphere and ocean.
|Global surface temperature regression onto the AMV indices for (a) observations, (b) 20th Century, (c) 21st Century, and (d) Preindustrial CMIP3 model simulations. (e)-(h) same as (a)-(d), but for precipitation. Stippling in (a) and (e) indicates 95% confidence level based on Monte Carlo test, in (b), (c), (f), and (g) indicates 18 out of 23 models showing the same sign regression coefficients, in (d) and (h) indicates 16 out of 20 models showing the same sign regression coefficients. Contours in right panels are for climatological precipitation contoured at 2 mm/day intervals.|