| Summary: | Mid-latitude mesoscale weather in the climatological summer is heavily influenced by fluctuations in synoptic-scale circulation patterns. Previous research has linked Arctic amplification to alterations in summer synoptic climatology, leading to more extreme weather events in the mid-latitudes. In this study we reconstruct seasonal (JJA) upper-level (500 hPa) atmospheric flow for four geographic locations in the mid-latitudes using an alpine larch (Larix lyallii Parl.) tree-ring chronology derived from western Montana. Our goal is to assess the long-term (400+ year) stability of upper-level flow to place the observed trends in a historical context. Spatial pattern correlations indicate that tree growth increases when meridional flow and zonal flow are strong west (r = 0.504, p = 0.001, n = 37) and north (r = 0.642, p < 0.001, n = 37) of the study site, respectively. Tree growth declines when meridional flow and zonal flow are strong east (r = -0.497, p = 0.001, n = 37) and south (r = -0.584, p < 0.001, n = 37) of the study site, respectively. Our 444-year climate reconstructions of 500 hPa flow show that ridging is becoming more intense in recent decades while troughs are declining in intensity.
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