Cold-Season Subtropical Air Mass Intrusions into North American Extratropical Latitudes: Dynamic and Thermodynamic Impacts on Extreme Precipitation

During the past several decades, extreme weather events in middle latitudes, particularly extreme precipitation, have been increasingly publicized in the media. Our objective in this study is to examine a possible thermodynamic basis for these events, namely subtropical airmass intrusions into higher latitudes. Our focus is on such intrusions that occur during the Northern Hemisphere winter (December, January and February) during the years from 1979 through 2023.

We first define a subtropical airmass based upon a combination of temperature and water vapor, more precisely the maximum value of equivalent potential temperature in the 900-700 hPa layer that is at least 310 K. This threshold value of theta-e is associated with the climatological values observed in subtropical regions.

We next identify such large values of theta-e that are observed in the latitudinal range of 40 to 50 deg N latitude from the operational radiosondes of North America that exist in these latitudes. The virtue of using the equivalent potential temperature is its conservation in the absence of friction and convective processes. Additionally, identifying the maximum value of theta-e in the 900-700 hPa layer also identifies the base of coupling between the free atmosphere’s lower level and the dynamic tropopause. This coupling may be characterized as the convective stability in the troposphere.

Our analyses of particularly extreme intrusions of subtropical air into extratropical latitudes reveal for eastern North America an anomalously strong subtropical that transports water vapor from the Gulf of Mexico and the subtropical regions of the North Atlantic. Such strong water vapor transports often resemble an atmospheric river.

Perhaps the most compelling result of this study is the identification of especially extreme values of lower-tropospheric equivalent potential temperature also identifies the most extreme amounts of 24-h precipitation.

Therefore, this purely thermodynamic metric, the equivalent potential temperature maximum in the lower troposphere, represents a significant diagnostic metric for extreme precipitation without explicit consideration of any dynamically based metric, such as vertical motion.

Case-study examples of the physical linkages between subtropical air mass intrusions and the associated dynamics of extreme precipitation events will be presented.