Climatology of upper- and lower-tropospheric closed thickness highs and lows

Previous work has shown that closed, defined as at least one closed geopotential height contour on a standard synoptic chart, cyclones and anticyclones at 500-hPa are an important aspect of the midlatitude and subtropical weather and climate. Closed cyclones have been shown to be associated with high-impact heavy rain and severe weather events, while closed anticyclones can be associated with extreme heat waves. Additionally, high-impact severe weather events can occur along the periphery of closed anticyclones where environments with high convective available potential energy (CAPE) values (>3000 J kg–1) and large 0–6-km wind shear values (>20 m s–1) are prevalent. In addition to closed cyclones and anticyclones, lesser studied closed thickness highs (warm) and lows (cool) are also an important aspect of weather and climate. These features can provide an indication of the genesis and lysis regions for warm and cold air masses and, if studied on the decadal timescale, they can provide a first-order approximation of climate change. Variations of the spatial and temporal distribution of thickness highs and lows as a function El Nino/Southern Oscillation (ENSO) is also of interest.

Given the importance of these upper- (defined as the 500–200-hPa layer) and lower- (1000–500-hPa) tropospheric closed thickness features to the weather and climate, the results of a global climatology of closed upper- and lower-tropospheric thickness highs and lows will be presented. The climatology was constructed for the period 1948–2009 using the NCEP/NCAR Reanalysis on a 2.5°×2.5° latitude-longitude grid. Preliminary findings indicate that upper- and lower-tropospheric thickness highs occur preferentially in continental regions during the warm season, while thickness lows are more evenly distributed spatially and can occur in all seasons over continents and oceans. Thickness lows tend to occur farther poleward relative to their thickness high counterpart, especially in the cold season (defined as December, January, and February). Specifically, this presentation will (i) show the results of a global climatology of thickness highs and lows, (ii) compare and contrast the Northern and Southern Hemisphere, (iii) compare and contrast El Nino and La Nina years, (iv) show the decadal spatial variations of the thickness climatology, and (v) investigate the connection of thickness highs and lows with high-impact weather events.