Polar vortex

polar vortex

A planetary-scale mid- to high-latitude circumpolar cyclonic circulation, extending from the middle troposphere to the stratosphere. The Northern Hemisphere vortex often features two centers—one near Baffin Island and the other over northeast Siberia—with analogous circumpolar asymmetry atypical in the Southern Hemisphere. The westerly airflow is largely a manifestation of the thermal wind above the polar frontal zone of middle and subpolar latitudes. The vortex is strongest during the winter in the upper troposphere and stratosphere when the pole-to-equator temperature gradient is strongest. The stratosphere component of the circulation may be referred to separately as the “polar stratospheric vortex.” In summer, the strongest westerly circulation is largely confined to the troposphere, and the polar stratospheric vortex reverses in the upper stratosphere because of solar heating during the polar day.

Reference: Simmons, A., M. Hortal, G. Kelly, A. McNally, A. Untch, S. Uppala (2005) ECMWF Analyses and Forecasts of Stratospheric Winter Polar Vortex Breakup: September 2002 in the Southern Hemisphere and Related Events. J. Atmos. Sci., 62, 668–689. [1]

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Breakup of ice covering a body of water at a site; depends on ice thickness, strength, flow velocity, and river geometry.
"Breakup connotes the end of winter to a resident of the north." (Glossary of Arctic and Subarctic Terms, Arctic, Desert, Tropic Information Center Pub. A–105, 1955).
See ice breakup.