Characteristics of Observed Atmospheric Circulation Patterns Associated with Temperature Extremes over North America

Abstract

Motivated by a desire to understand the physical mechanisms involved in future anthropogenic changes in extreme temperature events, the key atmospheric circulation patterns associated with extreme daily temperatures over North America in the current climate are identified. The findings show that warm extremes at most locations are associated with positive 500-hPa geopotential height and sea level pressure anomalies just downstream with negative anomalies farther upstream. The orientation, physical characteristics, and spatial scale of these circulation patterns vary based on latitude, season, and proximity to important geographic features (i.e., mountains, coastlines). The anomaly patterns associated with extreme cold events tend to be similar to, but opposite in sign of, those associated with extreme warm events, especially within the westerlies, and tend to scale with temperature in the same locations. Circulation patterns aloft are more coherent across the continent than those at the surface where local surface features influence the occurrence of and patterns associated with extreme temperature days. Temperature extremes may be more sensitive to small shifts in circulation at locations where temperature is strongly influenced by mountains or large water bodies, or at the margins of important large-scale circulation patterns making such locations more susceptible to nonlinear responses to future climate change. The identification of these patterns and processes will allow for a thorough evaluation of the ability of climate models to realistically simulate extreme temperatures and their future trends.

North American extreme temperature events and related large scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

“…Heuristically, the role of LSMPs in producing ETEs could be considered the result of either (1) a direct contribution to the large-scale circulation that facilitates the air mass excursion or (2) the indirect modulation of sub-scale variability, such as regional modulation of storm track behavior by blocking patterns. Besides such dynamically driven impacts, there exist possible local impacts related to the interaction of the LSMP with local topography or coastline features, leading to possible local symmetries in the response pattern (e.g., Loikith and Broccoli 2012). Current knowledge of the remote forcing, dynamics and local forcing of LSMPs associated with ETEs is summarized next.…”

“…The distortion of the flow by that southeastern ridge enables the cold air mass to cross the Rockies. Loikith and Broccoli (2012) capture a portion of this pattern in geopotential heights at 500 hPa using their 'grand composite' technique. More local to the area of the extreme, colder temperatures are associated with an adjacent surface high (e.g.…”

“…Midwest events Heat waves affecting the Midwest have anticyclonic flow at mid-levels, either as a closed anticyclone or as a strong ridge (Klein 1952;Karl and Quayle 1981;Namias 1982;Chang and Wallace 1987;Lyon and Dole 1995;Kunkel et al 1996;Livezey and Tinker 1996;Palecki et al 2001;Meehl and Tebaldi 2004;Lau and Nath 2012;Loikith and Broccoli 2012;Teng et al 2013), with associated clear skies allowing maximum solar heating of the surface as well as adiabatic warming from subsidence.…”

“…For at least some events, the continental anticyclonic flow at mid-levels is part of a larger pattern of anomalies with remote centers over the Pacific and Atlantic (Namias 1982;Chang and Wallace 1987;Livezey and Tinker 1996;Lau and Nath 2012;Loikith and Broccoli 2012;Teng et al 2013), with some suggestion that the anomalies are generated in the Pacific (Namias 1982; Lyon and Dole 1995;Livezey and Tinker 1996;Teng et al 2013), with possible predictability (Teng et al 2013). Teng et al (2013) have noted the similarity of the circulation pattern to a Rossby wave number 5 wave train in the jet stream waveguide.…”

“…ncep.noaa.gov/data/teledoc/telecontents.shtml) and California LSMPs and in assessing the PNA contribution to last winter's extreme cold in eastern North America (neither shown here), we do not find notable contribution from such modes. Several studies identified the LSMPs associated with specific extreme hot events (Grotjahn and Faure 2008;Loikith and Broccoli 2012) and CAO events (Konrad 1996;Carrera et al 2004;Grotjahn and Faure 2008;Loikith and Broccoli 2012). Parts of these LSMPs tend to be uniquely associated with the corresponding extreme weather and those parts have some predictability (Grotjahn 2011).…”