Impact of local gravity wave forcing in the lower stratosphere on the polar vortex stability: Effect of longitudinal displacement

Abstract

Abstract. The effects of gravity wave (GW) breaking hotspots in the lower stratosphere, especially the role of their longitudinal distribution, are evaluated through a sensitivity study by using a simplified middle atmosphere circulation model. For the position of the local GW hotspot, we first selected a fixed latitude range between 37.5 and 62.5° N and a longitude range from 112.5 to 168.75° E, as well as an altitude range between 18 and 30 km. This confined GW hotspot was then shifted in longitude by 45° steps, so that we created 8 artificial GW hotspots in total. Strongly depending on the location of the respective GW hotspot with regard to the phase of the stationary planetary wave of wavenumber 1 (SPW 1) generated in the model, the local GW forcing may interfere constructively or destructively with the modeled SPW 1. GW hotspots, which are located in North America near the Rocky Mountains lead to an increase of the SPW 1 amplitude and EP flux, while hotspots located near the Caucasus, the Himalayas or the Scandinavian region lead to a decrease of these parameters. Thus, the polar vortex is less (Caucasus and Himalayan hotspots) or more weakened (Rocky Mountains hotspot) by the prevailing SPW activity. Because the local GW forcing generally suppresses wave propagation at midlatitudes, the SPWs 1 are propagating into the polar region, where the refractive index turned to positive values for the majority of the artificial GW hotspots. An additional source of SPW 1 may be local instabilities indicated by the reversal in the meridional potential vorticity gradient in the polar region in connection with a positive EP divergence. In most cases, the SPWs 1 are breaking in the polar region and maintain the deceleration and thus, the weakening of the polar vortex. While the SPWs 1 that form when the GW hotspots are located above North America propagate through the polar region into the middle atmosphere, the SPWs 1 in the remaining GW hotspot simulations were not able to propagate further upwards because of a negative refractive index above the positive refractive index anomaly in the polar region. GW hotspots, which are located near the Himalayas influence the mesosphere/lower thermosphere region because of possible local instabilities in the lower mesosphere generating additional SPWs 1, which propagate upwards into the mesosphere.

Diverse Dynamical Response to Orographic Gravity Wave Drag Hotspots—A Zonal Mean Perspective

“…For the RM hotspot, a suppressed upward propagation of resolved waves throughout the stratosphere leads to significant positive EPFD anomalies, but only near the stratopause (and continuing to the mesosphere, see Figures ). The diversity of the resolved wave anomalies in the stratosphere dependent on the hotspot is in agreement with mechanistic model studies with artificial OGWD enhancements (see Šácha et al., 2016 for the EA hotspot and Samtleben et al., 2019, 2020 also for different regions). This indicates that the anomalous resolved wave propagation starting from the lower stratosphere is directly caused by the strong OGWD events and not by the “hotspot preconditioning”.…”

“…Samtleben et al. (2020) have shown that the OGWD influence on the zonal mean dynamics is mainly driven by changes in the propagation of planetary‐scale RWs with zonal wavenumber 1 (PW1). Decomposition of the EPF and EPFD anomalies according to the wavenumber (Figures 1d–1l) shows that the propagation of PW1 is suppressed for all three hotspots.…”

“…These conditions can be in general similar for each composited event of the particular hotspot and emerge in our results as significant anomalies, which we refer to as “hotspot preconditioning” further in the text. To distinguish in our results between significant anomalies caused by a causal response to the OGWD and imprints of the “hotspot preconditioning”, we rely on the fundamentals of OGW parameterization schemes, knowledge of OGWD dynamical impacts available from the rich body of literature on sensitivity simulations (Cohen et al., 2013; Eichinger et al., 2020; Šácha et al., 2016; Samtleben et al., 2019, 2020; Sigmond & Scinocca, 2010) and the correspondence between anomalies of different quantities.…”

“…Since the advent of the so‐called wave driving paradigm (Holton et al., 1995) the dynamical effect of OGWs in models has been increasingly evaluated as a contribution to the driving of the advective Brewer‐Dobson circulation (BDC; Butchart, 2014; Sato & Hirano, 2019). Another OGW impact that received considerable attention is connected to sudden stratospheric warming (SSW) events (Baldwin et al., 2020), either via the SSW preconditioning and triggering (Albers & Birner, 2014; Richter et al., 2010; Šácha et al., 2016; Samtleben et al., 2019, 2020) or the vortex recovery (Limpasuvan et al., 2012).…”

“…It includes RW sourcing by instabilities induced by OGW drag (OGWD) (McLandress & McFarlane, 1993; Sato et al., 2018; Smith, 2003), OGWD impact on RW breaking in the surf zone and the alteration of resolved wave propagation through modification of the background winds (Sigmond & Scinocca, 2010). Further, it has been demonstrated that the OGWD impact on resolved wave fields depends on the spatio‐temporal OGWD distribution (Boos & Shaw, 2013; Šácha et al., 2016; Shaw & Boos, 2012) and specifically on the location of OGWD relative to the stationary planetary scale RWs (Samtleben et al., 2019, 2020).…”