Dynamics of stratospheric wave reflection over the North Pacific

Abstract. Stratospheric wave reflection events involve the upward propagation of planetary waves, which are subsequently reflected downward by the stratospheric polar vortex. This phenomenon establishes a connection between the large-scale circulations in the troposphere and in the stratosphere. Here, we investigate wave reflection events characterised by an enhanced difference between poleward eddy heat flux over the Northwest Pacific and equatorward eddy heat flux over Canada. Previous research has pointed to a link between these events and anomalies in the tropospheric circulation over North America, with an associated abrupt continental-scale surface temperature decrease over the same region. In this study, we aim to elucidate the dynamical mechanisms governing this chain of events.

We find that anomalies of meridional eddy heat flux over the Northwest Pacific and Canada change sign before and after reflection events. A westward-propagating ridge, associated with a positive geopotential height anomaly, and the development of a trough downstream can explain this sign change. The trough advects colder-than-average air southwards in the lower troposphere over North America, leading to an abrupt temperature decrease close to the surface. This corresponds in the upper troposphere to negative and in the lower troposphere to positive anomalies of meridional eddy heat flux. The evolution of this large-scale pattern resembles the shift from a Pacific Trough to an Alaskan Ridge weather regime. Furthermore, stratospheric wave reflection events exert a far-reaching influence beyond North America on the tropospheric circulation across the Northern mid- and high latitudes. One example is the zonalisation and intensification of the North Atlantic eddy-driven jet stream resulting in more frequent occurrences of windy extremes over Europe a few days after the temperature decrease across North America.