4:45 PM - 5:00 PM
[AAS10-12] Gravity waves and Rossby waves during the stratospheric sudden warming in the Southern Hemisphere in 2019
Keywords:Middle atmosphere, Sudden Stratospheric Warming, Atmospheric waves
It is known that major Stratospheric Sudden Warming (SSW) rarely occur in the Southern Hemisphere because of smaller planetary-wave amplitudes (e.g., VanLoon et al., 1973). In September 2019, however, a minor but strong SSW in the Southern Hemisphere (hereafter called the SSW-SH 2019) occurred (e.g., Rao et al., 2019). In this study, we examined the time evolution of the dynamical properties of gravity waves (GWs) and Rossby waves (RWs) during the SSW-SH 2019. Numerical simulations using a high-resolution (T639L340) Japanese Atmospheric General circulation model for Upper Atmosphere Research (JAGUAR) were performed to investigate the dynamical properties of GWs and RWs, especially quasi-6-day waves (Q6DWs) below the lower thermosphere. Some previous studies (e.g., Yamazaki et al., 2020) reported that Q6DWs in the mesosphere and above were enhanced during the SSW-SH 2019, but the dynamical characteristics and excitation mechanism of Q6DWs are not fully understood.
We found the GWs with largely negative zonal momentum vertical fluxes (u'w') in the stratosphere above the Andean Mountains and the Antarctic Peninsula before the SSW onset (7 September) and above and leeward of the Ross Sea after the onset. On the other hand, above z= 75 km, GWs with largely negative u'w' were observed to the south of 40°S before 3 September. On 3–10 September, when the polar vortex became weak, GWs with positive u'w' were found in the easterly wind region around 60°S. When the polar vortex reappeared after 10 September, GWs with negative u'w' were observed to the south of 40°S, although their magnitude was smaller than those before 3 September.
We showed that there were two types of Q6DWs (Figure 1), one with eastward phase velocity (Q6DW-E) and the other with westward phase velocity (Q6DW-W). The Q6DW-Es were dominant to the south of 50°S before 10 September, and Q6DW-Ws were dominant in both hemispheres after that in the mesosphere. They have a baroclinic structure in the vertical, which is different from a barotropic structure of normal mode 5-day Rossby wave (Hirota and Hirooka, 1984). It was suggested that the Q6DW-E is an unstable wave due to baroclinic instability conditioned by a pair of the meridional gradient of potential vorticity: a negative gradient in the high-latitude mesosphere and a positive gradient in the stratosphere. On the other hand, the Q6DW-W is likely to be an internal RW generated from barotropic/baroclinic instability in the upper stratosphere over 60°–80°S. This dynamical instability was associated with the MPV maximum due to the maximum of the static stability broadly existing over 40°–70°S in the upper stratosphere. We suggested that the dynamical instability was excited by the wave forcings associated with the Q6DW-Es and RWs propagating from the mid- and high-latitudes troposphere. In addition, GWs having positive forcing in the polar mesosphere and negative forcing in the stratosphere also contribute similarly to the RWs.
We found the GWs with largely negative zonal momentum vertical fluxes (u'w') in the stratosphere above the Andean Mountains and the Antarctic Peninsula before the SSW onset (7 September) and above and leeward of the Ross Sea after the onset. On the other hand, above z= 75 km, GWs with largely negative u'w' were observed to the south of 40°S before 3 September. On 3–10 September, when the polar vortex became weak, GWs with positive u'w' were found in the easterly wind region around 60°S. When the polar vortex reappeared after 10 September, GWs with negative u'w' were observed to the south of 40°S, although their magnitude was smaller than those before 3 September.
We showed that there were two types of Q6DWs (Figure 1), one with eastward phase velocity (Q6DW-E) and the other with westward phase velocity (Q6DW-W). The Q6DW-Es were dominant to the south of 50°S before 10 September, and Q6DW-Ws were dominant in both hemispheres after that in the mesosphere. They have a baroclinic structure in the vertical, which is different from a barotropic structure of normal mode 5-day Rossby wave (Hirota and Hirooka, 1984). It was suggested that the Q6DW-E is an unstable wave due to baroclinic instability conditioned by a pair of the meridional gradient of potential vorticity: a negative gradient in the high-latitude mesosphere and a positive gradient in the stratosphere. On the other hand, the Q6DW-W is likely to be an internal RW generated from barotropic/baroclinic instability in the upper stratosphere over 60°–80°S. This dynamical instability was associated with the MPV maximum due to the maximum of the static stability broadly existing over 40°–70°S in the upper stratosphere. We suggested that the dynamical instability was excited by the wave forcings associated with the Q6DW-Es and RWs propagating from the mid- and high-latitudes troposphere. In addition, GWs having positive forcing in the polar mesosphere and negative forcing in the stratosphere also contribute similarly to the RWs.