Interpretation¶
At the radar station at Rio Grande in Argentina the zonal wind speeds tend to decrease in magnitude (in both directions) after the onset of the SSWs. The easterlies in around 80 km altitude become less easterly and the westerlies between 90 and 100 km become less westerly. The mesospheric change in around 80 km altitude is exactly the zonal wind decrease which is expected by the theory of inter-hemispheric coupling. But all in all the winds are still stronger than in the climatological mean for the corresponding calendar day as the anomalies of the zonal wind show.
Remarkable is the strong PW activity in 85 to 95 km altitude around the SSW onset, which corresponds to a zonal wind increase in the same altitude range. Considering the SSW events individually, both can only be found for half of the events (2 out of 4).
The meridional wind component shows multi-day fluctuations appearing as stripes, which decrease in magnitude with the SSW onset. These may indicate a quasi-multiday wave propagation. To identify a possible periodicity further investigation needs to be accomplished. Again, these quasi-two-day-fluctuations in the meridional wind component only appeared in the analysis of one of the events around 3 to 1 week before the SSW onset.
Around 10 days after the SSW onset the meridional wind reverses to a southward direction. This reversal is associated with the negative anomalies in the meridional wind component which were expected by the inter-hemispheric coupling theory as well.
At Davis these inter-hemispheric coupling impacts are less pronounced. There are only very slight changes in the zonal wind component and the anomalies are of small magnitude. In general, Davis shows the least changes of the 5 stations. This seems logic as it is situated in the Antarctic and therefore is the southernmost station with the greatest distance to the northern hemispheric polar vortex where the SSW events occur.
The findings according to the anomalies of GW kinetic energy at Davis and Rio Grande seem to be consistent with the theoretical expectation from the inter-hemispheric coupling theory. One can see negative anomalies in around 90 km altitude before or around SSW events and positive anomalies below. This enhanced GW activity (compared to the climatological mean) - initially reaching up to 85 km - is also travelling downward with time (especially after the SSW onset), which agrees with the downward shift of GW drag stated by Körnich and Becker (2010) as a result of inter-hemispheric coupling. The decreasing kinetic energy of GWs in the upper mesosphere around the SSW onset is visible in the climatology plot as well. Especially at Davis this is well pronounced.
This agreement with the theory is an advance compared to the study of Yasui et al. (2016) as their obtained results were inconsistent with the inter-hemispheric coupling hypothesis.
To examine periodicities in GWs and PWs also individual events were investigated for the stations on the SH. An enhanced GW activity in around 100 km 40-60 days after SSW onset only appears for one SSW event for each station(2010 for Rio and 2013 for Davis). The quasi-two-day-fluctuations in the meridional wind component at Rio Grande only appeared in the 2010 event analysis around 3 to 1 week before the SSW onset. The strongly increased PW activity arround the SSW onset was only apparent for half of the SSW events at Rio Grande (2 out of 4).
An SSW event on the SH was investigated, too. The analysis was done for the SSW event with the onset on 18 September 2019 from 40 days before to 40 days after the onset (09.08.2019 to 28.10.2019). The results can be found in the notebooks for each station. The northernmost stations (Sodankyla and Kiruna) show an increase in southward wind around the SSW onset corresponding to a peak in the PW activity at the same time.
The NH stations at lower latitudes (CMA and Collm) reproduce the course of the zonal wind of the SH stations during an SSW event on the NH, albeit with different sign.
The SH stations do not show a clear peak of PW activity around the SSW onset how it was the case at the NH for events on the NH. But this may be attributed to the fact, that the regarded event is no major one and therefore the forcing may not reach the altitudes above 80 km, which are examined here.
Another interesting feature can be seen in the analysis of the momentum fluxes. While the climatology of the meridional component remains negative, the anomalies are positive in the time before the SSW onset for Davis and Rothera. Maybe due to latitudinal differences the situation is different at Rio Grande: The climatology of the meridional momentum flux is mainly positive (northward fluxes), but turns to southward direction at around 10 days before the SSW onset, corresponding to strong negative anomalies at this time.