Defining Sudden Stratospheric Warmings

The classification and detection of Sudden Stratospheric Warmings (SSWs) are largely based on a definition developed by the World Meteorological Organization (WMO) in the decades after their discovery in 1952. An examination of literature suggests that pinpointing an exact reference for the SSW definition is elusive, but in general the definition involves the reversals of the meridional temperature gradient and of the zonal circulation polewards of 60° latitude, at or below the 10 hPa level. This definition was likely based on the observational system available during the 1970s, largely comprised of radiosondes and rocketsondes.

Versions of this definition are still commonly used to detect SSWs (despite an improved observational system and understanding of these events), but the details of the definition and its calculation are ambiguous. In addition, other SSW definitions have been proposed in the last few decades. Different interpretations and diagnostics result in inconsistent classifications of SSW events, which is problematic both for understanding the observed frequency and impacts of SSWs and for maintaining a robust metric with which to assess SSWs in climate models. To provide a basis for community-wide discussion, we examine how the SSW definition has changed over time and how sensitive the detection of SSWs is to the definition used. We argue that a standardized definition for SSWs is both necessary and useful for certain applications, but that the so-called WMO definition should be clarified and re-assessed to ensure that it serves current research and forecasting purposes.