Assessment of the Long-Term Trends of Transient Inverted Troughs within the North American Monsoon Region: Mechanisms and Implications for Warm Season Precipitation

Abstract:

The North American Monsoon (NAM) season, which encompasses the months of July through September, is associated with an increase in severe weather throughout much of the southwest Contiguous US (CONUS). Transient inverted troughs (IVs) are upper tropospheric disturbances that result in both synoptic scale and mesoscale enhancement of convection in the NAM region. Thus IVs often result in organized convection, which takes the form of squall lines and mesoscale convective systems. Organized convection during the NAM results in severe weather hazards, including severe straight line winds, blowing dust, and flash flooding. Previous work has suggested that long-term changes in IV climatology are dependent on strengthening of the monsoon ridge, a semiperminant anticyclone that is climatologically centered over the southwest CONUS during the NAM season. This previous work considered IV trends from WRF dynamically downscaled NCEP/NCAR reanalysis I from 1951 to 2010. Using the same objective tracking methodology, where IVs are located and tracked as 250 hPa potential vorticity anomalies, long-term trends in IV track density climatology are analyzed from four dynamically downscaled GCMs. The impacts of these changes in IV track density climatology are considered through further dynamical downscaling of the WRF simulations to the convection resolving scale. The long-term precipitation trends from days with favorable thermodynamic regimes for organized convection are considered through these high-resolution simulations. We consider the consistency of the dynamically downscaled GCMs at reproducing the NAM ridge and the IV track density climatology during overlapping time periods from the reanalysis (1981-2010). 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. These findings are used to discern the relative roles of natural climate variability and anthropogenic climate change on IV climatology and subsequent effects on monsoon season precipitation in the southwest CONUS.