Written by Erica Madonna from the Bjerknes Centre and the Geophysical Institute at the University of Bergen.
Jet streams are narrow bands of strong winds that shape midlatitude weather and climate. For example, the North Atlantic jet exhibits frequent north-south shifts that profoundly affect precipitation and temperature over Europe.
Occasionally, the jet may stay fixed in one position for longer periods. For example, the 2009/2010 winter saw a remarkably stable southern North Atlantic jet. This coincided with a strongly negative North Atlantic Oscillation (NAO) index, meaning that the pressure difference between the Icelandic low and the Azores high was smaller than normal.
Previous studies argue that persistent southward shifts in the jet reflect changes in dynamics linked to convection in the tropical Pacific, while frequent shifts on daily timescales arise from the action of upper-level eddies in the jet region.
In this study, we bridge the climate (year-to-year variability) and weather (day-to-day variability) perspectives to demonstrate that, whenever the North Atlantic jet (black contours in the schematic) shifts south, it consistently shows evidence of a tropical influence (stronger heating, in red) and weaker eddies (in blue).
Winter 2009/2010 was, in fact, the result of consecutive episodes of southern jet shifts each lasting several weeks.
Madonna, E.; Li, C.; Wettstein, JJ. (2019): Suppressed eddy driving during southward excursions of the North Atlantic jet on synoptic to seasonal time scales. Atmos Sci Lett. 2019;e937. https://doi.org/10.1002/asl.937