Trends in Extreme Geopotential Heights over North America: Another Perspective to Investigate Extreme Weather Events

The large-scale atmospheric circulation aloft is the dominant driver of surface weather conditions, including extreme weather events. Many previous studies have related extreme weather to the large-scale atmospheric circulation. Here, we analyzed the behavior of mid-tropospheric geopotential heights in order to understand trends in extreme weather events. Daily fields of 500 hPa geopotential height, surface air temperature and precipitable water from the NCEP-NCAR Reanalysis I were used over the greater North American study region (20°N – 90°N, 140°W – 40°W). For each grid point and each day of the year, an extreme high (low) event is defined when the geopotential height is above (below) the 95th (5th) percentile of values for that day during the time period from January 1948 to December 2014. Extreme temperature and precipitable-water events were defined the same way. The temporal and spatial variability of the area covered by extremely high and low geopotential heights were then analyzed.

Generally, the area covered by extremely high (low) geopotential heights increases (decreases) with time, and the strongest trend is in summer (JJA). The time series of extremely high geopotential heights also shows a clear regime shift around years 1963, 1977 and 1999, while that of extremely low geopotential heights shows clear regime shifts around 1977 and 1999.

The time series of extreme geopotential height extent correlates significantly with that of extreme temperature and precipitable water. For example, the recent trend toward drought conditions in California is consistent with more extreme ridging and less extreme troughing along the West Coast since 1999. We also find that the recent increase in the coverage of extremely inflated heights is most pronounced over the mid-latitude (south of 40oN), which consistent with a positive regional trend in hot, moist weather conditions.