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Winter jet stream trends over the Northern Hemisphere

Trends in the speed and probability of winter jet stream cores over the Northern Hemisphere were measured for 1958–2007, and related changes in the thermal structure of the troposphere were identified. Eddy‐driven jet (EDJ) core speeds and probabilities increased over the midlatitudes (40–60°N), with changes as large as 15% (speed) and 30% (probability). These increasing trends are collocated with increases in baroclinicity driven by a spatially heterogeneous pattern of height change consisting of large‐scale warming with cooling centres embedded poleward of 60°N. The cooling centres reduced high‐latitude baroclinicity, making jet cores poleward of 60°N less frequent and weaker. Over the west and central Pacific, subtropical jet stream (STJ) core probabilities remained relatively constant while core speeds increased by as much as 1.75 m/s decade−1 in association with Hadley cell intensification. The STJ shifted poleward over the east Pacific and Middle East, and an equatorward shift and intensification of the STJ were found over the Atlantic basin–contributing to an increased separation of the EDJ and STJ. Copyright © 2007 Royal Meteorological Society