Fewer Troughs, Not More Ridges, Have Led to a Drying Trend in the Western United States

Wei Zhang, Vittal Hari, Simon S-Y Wang, Matthew D. LaPlante, Gregg Garfin, Grace Affram, Rohini Kumar

Research output: Contribution to journalLetterpeer-review

15 Scopus citations


High-amplitude ridges can enforce heat-trapping systems that persist through an entire season, contributing to drought events. However, the impacts of day-to-day weather system changes at the meso- and synoptic-scale are also important, albeit less well studied. We identify five weather types (WTs) from 500-hPa geopotential height anomalies. These categories include WT4, which represents ridges and associated dry and warm anomalies, and WT3, which characterizes troughs and associated wetter- and colder-than-normal conditions and is the largest contributor to precipitation in the western U.S. While no discernable trend is evident for WT4 conditions, WT3 conditions have undergone a distinctive decrease in frequency at both annual and seasonal scales, thus a general drying trend across the west during this time period may be more significantly associated with fewer troughs rather than more ridges. Attribution analysis reveals that this decrease in wetter and colder weather is a likely result of anthropogenic forcing.

Original languageEnglish (US)
Article numbere2021GL097089
JournalGeophysical Research Letters
Issue number1
StatePublished - Jan 16 2022


  • Western U.S.
  • drought
  • ridge
  • trough
  • weather types

ASJC Scopus subject areas

  • Geophysics
  • General Earth and Planetary Sciences


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