Tropopause-level planetary wave source and its role in two-way troposphere–stratosphere coupling

Abstract

Abstract. Atmospheric planetary waves play a fundamental role in driving stratospheric dynamics, including sudden stratospheric warming (SSW) events. It is well established that the bulk of the planetary wave activity originates near the surface. However, recent studies have pointed to a planetary wave source near the tropopause that may play an important role in the development of SSWs. Here we analyze the dynamical origin of this wave source and its impact on stratosphere–troposphere coupling, using an idealized model and a quasi-reanalysis. It is shown that the tropopause-level planetary wave source is associated with nonlinear wave–wave interactions, but it can also manifest as an apparent wave source due to transient wave decay. The resulting planetary waves may then propagate deep into the stratosphere, where they dissipate and may help to force SSWs. Our results indicate that SSWs preceded by both the tropopause and the surface wave-source events tend to be followed by a weakened tropospheric zonal flow several weeks later. However, while in the case of a preceding surface wave-source event this downward impact is found mainly poleward of 60∘ N, it appears to be the strongest between 40 and 60∘ N for SSWs preceded by tropopause wave-source events. This suggests that tropopause wave-source events could potentially serve as an additional predictor of not only SSWs but also their downward impact as well.

Variation in the Holton–Tan effect by longitude

“…This SC‐WACCM4 result supports the finding that increased planetary wave activity flux convergence in the polar stratosphere under QBOE is driven by the QBO‐MMC and associated poleward shift of the tropospheric jet (White et al ., 2015). An interesting question to investigate going forward is what mechanism associated with the QBO may promote upward wave activity flux from the tropopause (e.g., Boljka and Birner, 2020).…”

“…Synoptic scale waves interact with the lower frequency mean flow, blocks, and stationary wave trains included (Nakamura et al ., 1997; Lorenz and Hartmann, 2001; 2003; Eichelberger and Hartmann, 2007) and thus may affect stationary waves. So, perhaps interaction between waves of various scales is necessary for changes in wave activity (e.g., Boljka and Birner, 2020). A final hypothesis is that zonal asymmetry in the QBO teleconnection arises because the QBO itself is zonally asymmetric (Hitchman and Huesmann, 2009; Tegtmeier et al ., 2020).…”

Sudden Stratospheric Warmings

“…Strong westerly winds in the polar night jet inhibit all but the largest, planetary‐scale waves from propagating into the stratosphere (Charney & Drazin, 1961). While planetary‐scale waves can spontaneously be generated by baroclinic instability (Domeisen & Plumb, 2012; Hartmann, 1979) or via upscale cascade from synoptic‐scale waves (Boljka & Birner, 2020; Scinocca & Haynes, 1998), they are chiefly forced by planetary‐scale features at the surface: topography and land‐sea contrast (Garfinkel et al, 2020). The relative zonal symmetry of the austral hemisphere explains why SSWs are almost exclusively a boreal hemispheric phenomena, but this does not imply that the stratosphere just passively responds to wave driving from the troposphere.…”

Seasonal prediction of the boreal winter stratosphere

“…On seasonal and interannual time scales the winter variability of the SPV is induced by vertical propagation of planetary Rossby waves from the troposphere (Andrews et al 1987;Newman et al 2001); wave generation internal to the stratosphere also occurs (Scott and Polvani 2004;Birner and Albers 2017;Boljka and Birner 2020). It is mainly lowfrequency, low-wavenumber planetary waves that propagate through the stratosphere, break in the strong mid-stratospheric zonal flow and slow down the vortex by transferring easterly momentum to the westerly winds (Andrews et al 1987;Haklander et al 2007).…”