Seasonal jet stream controls on surface climate influence the timing of plant growth: a mechanistic perspective on landscape phenology shifts in a warming world

Global warming modulates the jet stream, strong hemispheric air currents that connect surface climates and drive weather patterns. In turn, plants respond to such changes in surface climate through physiological mechanisms that regulate seasonal timing of the start, peak, and end of the growing season. Here, we identify the spatial extent of landscapes responding to such jet stream shifts at key points in the growing season. We used phenology products derived from satellite observations of greenness, including the start of season (SOS), end of season (EOS), and day of peak (DOP) phenophases (1981-2012; Didan and Barreto, 2016). We then compared surface climate and phenology composite maps for the 10 northernmost versus southernmost jet stream positions, based on the seasonally and regionally explicit Northern Hemisphere jet stream indices (NHJ; Belmecheri et al., 2017). We characterized pixels based on their land cover type and Köppen-Geiger climate classification scheme, to generalize which landscapes' phenology are influenced by jet stream shifts and how. We found that SOS and EOS phenophases were more spatially clustered in their temporal response to NHJ shifts and corresponding changes in climate than DOP. Northern (southern) NHJ shifts typically corresponded with higher (lower) geopotential heights at 500hPa, warmer (cooler) temperatures over the climatological NHJ latitude, earlier SOS (e.g. deciduous forests, croplands and grasslands in eastern Europe and western Russia; the arid-steppe-cold grasslands of the eastern Mongolia-China border; the arid, cold-no dry season, closed shrubland and croplands in the western US), and later EOS (e.g. croplands in central US and southern Canada). Surprisingly, a northern spring NHJ led to warmer temperatures and a later SOS for croplands south of the Great Lakes in the US - which may be due to higher snowmelt and field saturation, delaying the use of heavy machinery in planting crops. The connectedness of the NHJ to atmospheric pressures at other latitudes is visible in some regions- for example, a northern spring NHJ across the Pacific Ocean clusters around 40N, coinciding with high pressure at 40N (earlier SOS in Korean Peninsula) and low pressure field anomalies at 50-70N (later SOS in eastern Russia). By characterizing phenoregions that are sensitive to shifts in atmospheric circulations, we are beginning to contextualize where and how vegetation will respond to future circulation variability and blocking frequency in a warming world.