Effect of latitudinally displaced gravity wave forcing in the lower stratosphere on the polar vortex stability

Abstract

Abstract. In order to investigate the impact of a locally confined gravity wave (GW) hotspot, a sensitivity study based on simulations of the middle atmosphere circulation during northern winter was performed with a nonlinear, mechanistic, global circulation model. To this end, for the hotspot region we selected a fixed longitude range in the East Asian region (120° E–170° E) and a latitude range from 22.5° N–52.5° N between 18 km and 30 km, which was then shifted northward in steps of 5°. For the southernmost hotspots, we observe a decreased stationary planetary wave (SPW) 1 activity in the upper stratosphere/lower mesosphere, i.e. less SPWs 1 are propagating upwards. These GW hotspots are leading to a negative refractive index inhibiting SPW propagation at midlatitudes. The decreased SPW 1 activity is connected with an increased zonal mean zonal wind at lower latitudes. This in turn decreases the meridional potential vorticity gradient (qy) from midlatitudes towards the polar region. A reversed qy indicates local baroclinic instability which generates SPWs 1 in the polar region, where we observe a strong positive Eliassen-Palm (EP) divergence. Thus, the EP flux is increasing towards the polar stratosphere (corresponding to enhanced SPW 1 amplitudes) where the SPWs 1 are breaking and the zonal mean zonal wind is decreasing. Thus, the local GW forcing is leading to a displacement of the polar vortex towards lower latitudes. The effect of the local baroclinic instability indicated by the reversed qy also produces SPWs 1 in the lower mesosphere. The effect on the dynamics in the middle atmosphere by GW hotspots which are located northward of 50° N is negligible because the refractive index of the atmosphere is strongly negative in the polar region. Thus, any changes in the SPW activity due to the local GW forcing are quite ineffective.

Effects of missing gravity waves on stratospheric dynamics; part 1: climatology

“…S2), which is flanked by two patches of enhanced EPfd. This can be attributed to a missing impact of OGWD above the subtropical jet, because similar anomalies of the EPfd have been documented in idealised model studies by Šácha et al (2016) and Samtleben et al (2019). In those studies, artificially injected GWD in this region inhibits the upward and equatorward PW propagation from the mid latitudes and focuses the PWs poleward creating an EPfd anomaly in the polar region.…”

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 S3a-S3c). 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, 2019Samtleben et al, , 2020. 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".…”

“…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, 2019Samtleben et al, , 2020 or the vortex recovery (Limpasuvan et al, 2012). Cohen et al (2013) showed that perturbations to OGW forcing become balanced by changes in resolved wave drag in a mechanistic model, and Cohen et al (2014) showed further that the response fully develops on a time-scale of days.…”

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

“…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, 2019Samtleben et al, , 2020 or the vortex recovery (Limpasuvan et al, 2012). Cohen et al (2013) showed that perturbations to OGW forcing become balanced by changes in resolved wave drag in a mechanistic model, and Cohen et al (2014) showed further that the response fully develops on a time-scale of days.…”

“…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(Samtleben et al, , 2020. Shepherd (2014) highlighted the interaction between parameterized GWs and the large-scale circulation as one of the most uncertain aspects of climate modeling.…”