Assessment of the long-term trends of transient inverted trough climatology over the North American Monsoon Region from four dynamically downscaled GCMs

Transient inverted troughs (IVs) are important for convective organization of storms in the North American Monsoon (NAM) region. This organized convection often takes the form of squall lines and mesoscale convective systems, which result in severe weather hazards, including severe straight line winds, blowing dust, and flash flooding. An objective tracking methodology to locate IVs as 250 hPa normalized potential vorticity anomalies has been applied to a Weather Research and Forecasting (WRF) regional climate model forced by NCEP/NCAR reanalysis I from the years 1951 to 2010. This previous work has shown that long-term changes of the climatology of IVs are largely dependent upon the strengthening of the monsoon ridge, a semipermanent anticyclone that is centered over the NAM region during the late summer. The present study builds upon this previous work by assessing the long-term climatological changes of IVs from four WRF dynamically downscaled GCMs. The downscaled CMIP3 solutions include the MPI-ECHAM and HadCM3, and the downscaled CMIP5 solutions include the MPI-ECHAM and HadGEM. The impacts of climate change on the NAM region are considered in the context of the long-term changes to IV track density climatology and precipitation on days with and without an inverted trough within the region. Convection resolving WRF simulations of precipitation from days with favorable thermodynamic environments for severe weather, based on the same forcing data, are considered in addition to the precipitation from the coarser regional model. These results also show how the downscaled GCM solutions differ from observed changes, based on dynamically downscaled reanalysis. The performance of these downscaled GCM solutions is highly dependent upon the ability of the forcing GCM to realistically simulate the climatology and position of the monsoon ridge. Due to uncertainties in the reanalysis data caused by the integration of satellite data after 1979, these findings help to verify the long-term trends already found in the data record. These findings are significant for stakeholders across the NAM region, as IVs are often associated with extreme weather events.