Stratospheric sudden warmings (SSW) are a ubiquitous feature of the wintertime flow in the northern hemisphere. These events take their name from a rapid temperature increase of several tens of Kelvin over a few days in the high northern latitudes. These images from December 24, 2012 through January 22, 2013 are data models from NASA's Goddard Earth Observing System Model, Version 5 (GEOS-5) showing Ertel's Potential Vorticity (EPV) in potential vorticity units (PVU). The shading shows regions of high EPV, typical of the polar vortex, in white, and regions of low EPV, typical of tropical air masses, in darker shades. In the first image (top left) the deep-white regions represent high EPV and depict a strong, elongated polar vortex over northern Europe and Asia, with a tail of air adverted from the main vortex, which wrapped around the northern latitudes and circumscribed the pole. Over the next few days, the GEOS-5 analyses showed the vortex.
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