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Length of growing season is modulated by Northern Hemisphere jet stream variability

Atmospheric circulations create regions of similar climate at the land surface across the Northern Hemisphere's mid‐latitudes, while climate controls on plant phenology phases (phenophases) can vary. Here, we quantify and classify the seasonal impacts of the Northern Hemisphere jet stream (NHJ) on vegetation phenophases using novel NHJ indices, designed to characterize latitudinal jet stream position, and satellite‐based vegetation phenology products of length, start, and end of season (LOS, SOS, EOS). We composited years of northern and southern NHJ anomalies in April–May and October–November from LOS, SOS, and EOS and classified ecosystem response based on: latitudinal location relative to the climatological mean (CM) latitude of the NHJ; climate; and land cover type. For nearly a third of the NH, LOS was modulated by NHJ position. The magnitude and direction of NHJ influence on LOS was dependent on the season of NHJ position anomaly. In spring, a northern NHJ, which typically corresponded with warmer temperatures, led to earlier SOS and longer LOS, especially in cold systems north of the NHJ's CM. Surprisingly, in fall, a northern NHJ led to shorter LOS and earlier EOS for most systems, which may be due to the interaction of temperature and water availability in this season when water is critical for sustained growth. Southern NHJ corresponded with similar but opposite effects on phenophases. Crop land cover was mixed in LOS response to NHJ, reflecting the added impact of land‐use and management. The NHJ can synchronize phenology across apparently disparate regions of the Northern Hemisphere. Rather than simply lengthening or shortening seasons as the NHJ moves northward or southward, the influence of the NHJ on phenology at a given location depends on whether the NHJ is limiting resources—which vary relative to the climatological mean. We find that complex patterns in phenology emerge from a relatively simple contingency.