11:15 AM - 11:30 AM
[AAS09-08] Contribution of gravity waves and quasi-two-day waves to the interhemispheric coupling in austral winter
★Invited Papers
Keywords:middle atmosphere, interhemispheric coupling, reanalysis
It is known that when strong warming occurs in the winter polar stratosphere, the summer polar upper mesosphere also becomes warmer than usual with a time lag of 2–10 days. This event is called interhemispheric coupling (IHC), which was first suggested by model studies (e.g., Becker & Schmitz, 2003). In the boreal winter stratosphere, sudden stratospheric warming (SSW) often occurs. Thus, several previous studies are made for the IHC during the SSWs in the northern hemisphere (NH, e.g., Körnich & Becker, 2010). However, few studies have been made on the IHC initiated by SSW in the southern hemisphere (SH) in the austral winter, which rarely occurs. In the present study, the global structure and dynamical mechanism of the IHC initiated by stratospheric warming events, not limited to the major SSW, in the SH during the austral winter are examined using the JAGUAR-DAS reanalysis data (Koshin et al., 2020; 2022).
JAGUAR-DAS is a data assimilation system with a four-dimensional local ensemble transform Kalman filter (4D-LETKF), which was developed by our group to make a global reanalysis data for the atmosphere up to the lower thermosphere (z= ~115 km). The model resolution is T42L124, which can express phenomena at synoptic and larger scales. Assimilated observation data sets in JAGUAR-DAS are the NCEP PREPBUFR (https://rda.ucar.edu/datasets/ds337.0/), temperature from the Aura microwave limb sounder (MLS; Livesey et al., 2020), that from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) Sounding of the Atmosphere using Broadband Emission Radiometry (SABER; Remsberg et al., 2008), and radiation from the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager/Sounder (SSMIS; Swadley et al., 2008). Using the JAGUAR-DAS, reanalysis data over about 15 years of 2004–2019 was produced. Cold equatorial stratosphere (CES) events, most of which are associated with strong stratospheric warming in the austral winter, are extracted to examine the IHC, following the method by Yasui et al. (2021), who made an analysis for the boreal winter.
In the austral winter, a strong CES associated with stratospheric warming in the SH occurs about once every two years, while major warming rarely occurs. The mechanisms of the IHC initiated by the SH stratospheric warming in the austral winter are qualitatively similar to those initiated by the NH one in the boreal winter. However, there are two significant differences in the IHC characteristics between the austral and boreal winters. One is that SH stratospheric warmings in the austral winter occur mainly at midlatitudes, whereas NH stratospheric warmings in the boreal winter are observed mainly at high latitudes. A warm temperature anomaly simultaneously appears in the southern polar mesosphere, which is not observed in the NH for the boreal winter. This temperature anomaly is likely related to the dominant latitude of the stratospheric warming and the zonal wind above. The other is related to the difference in the intraseasonal variation of the quasi-two-day wave (QTDW) activity in the summer mesosphere: The amplitude of the QTDWs is large only for about a month around the solstices, and quite small during other seasons. For the CES events occurred in July in the austral winter, the contribution of QTDWs to the IHC is large, as is similar to the CES events in the boreal winter. In contrast, the contribution of QTDWs to the IHC was quite small in late August and September when the QTDW activity is low. These results suggest that the time lag, altitude, and magnitude of the temperature anomaly in the summer lower thermosphere associated with the IHC are significantly affected by the intraseasonal variation of the QTDW activity.
JAGUAR-DAS is a data assimilation system with a four-dimensional local ensemble transform Kalman filter (4D-LETKF), which was developed by our group to make a global reanalysis data for the atmosphere up to the lower thermosphere (z= ~115 km). The model resolution is T42L124, which can express phenomena at synoptic and larger scales. Assimilated observation data sets in JAGUAR-DAS are the NCEP PREPBUFR (https://rda.ucar.edu/datasets/ds337.0/), temperature from the Aura microwave limb sounder (MLS; Livesey et al., 2020), that from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) Sounding of the Atmosphere using Broadband Emission Radiometry (SABER; Remsberg et al., 2008), and radiation from the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager/Sounder (SSMIS; Swadley et al., 2008). Using the JAGUAR-DAS, reanalysis data over about 15 years of 2004–2019 was produced. Cold equatorial stratosphere (CES) events, most of which are associated with strong stratospheric warming in the austral winter, are extracted to examine the IHC, following the method by Yasui et al. (2021), who made an analysis for the boreal winter.
In the austral winter, a strong CES associated with stratospheric warming in the SH occurs about once every two years, while major warming rarely occurs. The mechanisms of the IHC initiated by the SH stratospheric warming in the austral winter are qualitatively similar to those initiated by the NH one in the boreal winter. However, there are two significant differences in the IHC characteristics between the austral and boreal winters. One is that SH stratospheric warmings in the austral winter occur mainly at midlatitudes, whereas NH stratospheric warmings in the boreal winter are observed mainly at high latitudes. A warm temperature anomaly simultaneously appears in the southern polar mesosphere, which is not observed in the NH for the boreal winter. This temperature anomaly is likely related to the dominant latitude of the stratospheric warming and the zonal wind above. The other is related to the difference in the intraseasonal variation of the quasi-two-day wave (QTDW) activity in the summer mesosphere: The amplitude of the QTDWs is large only for about a month around the solstices, and quite small during other seasons. For the CES events occurred in July in the austral winter, the contribution of QTDWs to the IHC is large, as is similar to the CES events in the boreal winter. In contrast, the contribution of QTDWs to the IHC was quite small in late August and September when the QTDW activity is low. These results suggest that the time lag, altitude, and magnitude of the temperature anomaly in the summer lower thermosphere associated with the IHC are significantly affected by the intraseasonal variation of the QTDW activity.