Is the Tropospheric Wave Activity Anomalous before Sudden Stratospheric Warmings? (Invited Presentation)

Sudden stratospheric warmings (SSW) are one of the most dramatic phenomena in the climate system, with global impacts that range from alterations of the mesospheric response to particle precipitation, to influences on the midlatitude storm tracks in subseasonal and seasonal timescales. SSWs are mechanically driven by enhanced wave activity and its nonlinear interaction with the background flow. While the necessary roles of both the tropospheric and stratospheric circulation triggering SSWs have long been recognized, there is still uncertainty as to whether the troposphere needs to be in an anomalous state for an SSW to occur. In this presentation, we will address this question using a combination of reanalysis products and 240 years of output from the Whole Atmosphere Community Climate Model to provide statistical robustness to our results. To make more emphasis on dynamical events, we will define sudden stratospheric deceleration (SSD) events, of which SSWs are a subset; and also anomalous tropospheric wave events.

Reanalysis and model provide very similar statistics: Around 1/3 of the SSD events identified are preceded by anomalous wave events in the troposphere, while 2/3 of the SSDs are not preceded by a tropospheric wave event. In addition, only ~1/6 of all anomalous tropospheric wave events are followed by an SSD in the stratosphere. We will also provide evidence that cautions the interpretation of the upward wave activity flux at 100 hPa as a proxy of the tropospheric injection of wave activity into the stratosphere.