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Development of the polar vortex in the 1999–2000 Arctic winter stratosphere

The 1999–2000 Arctic stratospheric vortex was unusually cold, especially in the early winter lower stratosphere, with a larger area near polar stratospheric cloud formation temperatures in Dec and Jan, and much lower temperatures averaged over Nov–Jan, than any previously observed Arctic winter. In Nov and early Dec, there was a double jet in the upper stratosphere, with the anticyclone cutoff in a region of cyclonic material. By late Dec, there was a discontinuous vortex, large in the upper stratosphere, small in the lower stratosphere; evolving to a strong, continuous, relatively upright vortex by mid‐Jan. This vortex evolution in 1999–2000 is typical of that in other cold early winters. Despite unusually low temperatures, the lower stratospheric vortex developed more slowly than in previous unusually cold early winters, and was weaker than average until late Dec.