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Large Wildfires in the Western United States Exacerbated by Tropospheric Drying Linked to a Multi‐Decadal Trend in the Expansion of the Hadley Circulation

Analyses of wildfire‐climate relationships in North America were conducted using diverse independent observation and reanalysis data sets for the period 1984–2014. Results show that the western United States (WUS) has experienced the most robust increase in burned area, even though Alaska and western‐central Canada have comparable warming trends. In addition to warming, the WUS has been under the influence of multi‐decadal trends in tropospheric relative humidity deficit, reduced cloudiness, increased surface net insolation, and enhanced adiabatic warming and drying from increased tropospheric subsidence, as well as drying from enhanced offshore low‐level flow. These trends are found to be associated with a widening of the descending branch of the Hadley circulation, consistent with climate model projections under greenhouse gases warming. Due to the relative short (~30 years) data record, the aforementioned trend signals are likely also be affected by phase changes of natural interdecadal variability during the data period.