The influence of surface versus free-air decoupling on temperature trend patterns in the western United States

We analyzed temperature trends from 460 GHCNv2 weather stations in the western United States for 1948-2006 to determine whether the extent of decoupling of surface temperatures from the free atmosphere influences past change. At each location we derived monthly indices representative of anticyclonicity using NCEP/NCAR 700 hPa reanalysis pressure fields. The number of anticyclonic days minus cyclonic days (A-C) is positively correlated with temperature anomalies at exposed convex sites and in the north of the domain where the free atmosphere controls temperature, anticyclonic months being warmer. In topographic concavities, and in the south of the domain where the influence of upper air ridges and troughs is muted, the relationship is much weaker. We use the gradient of the A-C index-temperature relationship to represent a coupling index, highest at exposed free-air locations. On a mean annual basis there are no strong relationships between temperature trend magnitude, elevation, topographic incision, or coupling index. However, in winter, warming is weaker at decoupled locations, especially when snow cover is present. Where snow is absent in winter, and in fall, the relationship is reversed. Circulation changes (increased cyclonicity) can explain the disparity in warming between decoupled and exposed locations in fall and to a certain extent in winter (increased anticyclonicity), although winter results are also regionally sensitive. Thus, future climate change may be different (amplified or muted dependent on season and/or surface characteristics) in locations prone to surface decoupling, compared with locations exposed to the free atmosphere. Understanding such processes will aid downscaling of future climate change.