A Comprehensive Atlas of Mid-Winter Sudden Stratospheric Warmings and Associated Surface Climate Extremes

Sudden stratospheric warmings (SSWs) are extreme climate events that involve large and rapid temperature increases and complete reversals of the climatological westerly winds in the boreal polar stratosphere. These events can propagate downward into the troposphere where they have substantial impacts on wintertime surface climate, such as extreme cold outbreaks in North America and Eurasia and extreme warming over Greenland and other regions of rapid ice change. While various diagnostics of SSWs have been proposed in the literature, there is no definitive long-term database of individual events. The development of a comprehensive atlas of SSW events would address at least three key issues. The first is inconsistencies in the historical record. Whereas one diagnostic based on zonal winds at 10 mb and 60°N yields no major SSWs during the 1990s, other diagnostics detect several major events during that decade. The second issue is stratosphere-troposphere coupling. Current diagnostics based on zonal winds yield stratospheric warming events that do not always propagate to the surface. The third issue involves the type and timing of SSWs. Recent literature suggests that “displacement”-type and “split”-type SSWs may cause different surface climate responses. A comprehensive SSW atlas would clarify the differences between these two types of warmings, including their surface impacts and the timescales of their downward propagation.

In response to the Climate Program Office FY2014 competition for Climate Observations and Monitoring (COM)/Datasets and Indicators – Data Sets for Weather/Climate Extremes and considering NOAA’s long-term climate goals, we will address weather/climate extremes in the physical climate system, in the sense both that (a) SSWs themselves are one of the most impressive extremes in the climate system and that (b) they influence surface climate extremes with the potential for severe impacts on human and natural systems. Currently there is no consistent database of SSW events, and tabulations that exist in the scientific literature are inhomogeneous regarding the frequency and occurrence of events. We propose to: (1) examine the three-dimensional structure of stratospheric variability in atmospheric reanalyses to determine the key components of major SSW events; (2) reveal new insights about which events couple to the surface; and (3) using these findings, develop a comprehensive multi- decadal atlas of major warmings in the historical record, including their type, timing, and spatial structure. The expected utility of the resulting dataset would be: (1) as a research tool to advance understanding and modeling of extreme events in the stratosphere and their coupling to wintertime tropospheric extremes; (2) as an observational record for evaluating representation of SSWs in climate model simulations of historical and future climate variations; (3) as a basis for improvement of the formal SSW definition, as well as an international monitoring and forecasting system for SSWs, through efforts with the international stratospheric community; and (4) as an education and outreach tool.