Stratospheric-Tropospheric Coupling and Extreme Cold: A case study of the January 1985 Cold Air Outbreak over the United States

Abstract:

Billion dollar disasters in the U.S. take a variety of forms, including cold air outbreaks (CAO) and freezes. Motivated by research that shows a linkage between sudden stratospheric warnings (SSW) and extreme CAOs over the North American and Eurasian continents, this study examines the connection between the Arctic stratosphere and a CAO over the eastern U.S. in January 1985. A SSW occurred roughly 18 days prior to the extreme CAO event, where the eastern U.S. experienced temperatures less than -4 standard deviations below normal. Using ECMWF-interim reanalysis data, three-dimensional animations show the evolution of wave transience in the lower stratosphere 10-days prior to the 18-21 January CAO.

A week prior to the CAO, easterly vertical shear of the zonal wind over the Pacific in the upper stratosphere at 60°N suggested the presence of a Rossby wave reflective layer. These reflecting layers have been linked to downward directed wave activity flux (WAF) and to locally enhanced ridging over Alaska. In this case, the ridging was phased with a positive Pacific/North American Pattern (PNA). A known precursor to CAOs over the eastern U.S. Successive cyclonic wave breaking upstream of the ridge lead to the rapid deterioration of the ridge one day before the CAO. The analysis examines the contribution from the vertical component of WAF in the amplification of the Alaskan ridge that facilitated the northerly flow in the CAO.

This case study is presented in the broader context of the three-dimensional synoptic to planetary scale evolution that can lead to extreme CAOs over the U.S. Identifying the physical mechanisms involved in stratosphere-troposphere coupling prior to CAOs could inform S2S forecasting of this type of event. The analysis includes a discussion of the subseasonal-to-seasonal (S2S) reforecast models struggled to capture the SSW and CAO at S2S lead times and similar events.