11:20 AM - 11:40 AM
[AAS06-09] Seasonal predictability of the persisted negative Antarctic oscillations after sudden stratospheric warming in the Southern Hemisphere in 2019
★Invited Papers
Keywords:sudden stratospheric warming, Antarctic Oscillation, Seasonal predictability
In the early September 2019, a sudden stratospheric warming (SSW) occurred in the southern hemisphere for the first time in 17 years. The impact extended downward to the Earth’s surface, and the negative-phase of the Antarctic Oscillation (AAO) in the troposphere persisted from mid-October to the end of December. In this study, we investigate the relationship between the persistence of the negative AAO and SSW, and show that wave activity anomalies was formed in which the negative AAO is easily maintained. We also examine the seasonal predictability of the negative AAO using a seasonal prediction system of the Japan Meteorological Agency. It is shown that the persistence of the negative AAO is predictable, but that SST anomalies in the tropical Indian Ocean have little effect on this predictability.
The data used for the analysis of the circulation field is the JMA long-term reanalysis data JRA-55 (Kobayashi et al., 2015). The forecast data are the results of an ensemble 6-month forecast experiment with initial values of September 13 and 28, 2019, by an in-service seasonal forecast model (the coupled global atmosphere-ocean model CPS2, Takaya et al., 2018). The mass-weighted average method (MIM, Iwasaki 1989, 1992) on the isothermal surface is applied to these data to analyze the wave activity anomalies.
The evolution of the strength of the polar vortex from the stratosphere to the troposphere associated with SSW in the SH shows that the weakness of the polar vortex persisted in the upper stratosphere after the SSW, and the center of the anomaly descended to the lower stratosphere to the troposphere after mid-October. This was well predicted by the seasonal prediction experiment.
The zonal mean field during the period of persisted negative AAO is confirmed in terms of wave-mean flow interaction. The stratospheric polar night jet seen in the climatology is absent in the 3-month averaged zonal mean zonal winds from October to December 2019.
Corresponding to the zonal mean zonal winds, there is no waveguide along the climatological polar night jet, making it more difficult for planetary waves from the troposphere to propagate vertically than normal. In terms of wave activity, there was a strong convergence of E-P flux from 50S to 70 S in the middle and upper troposphere. As diagnosed by the spatial structure of the convergence-divergence anomalies of E-P fluxes, the direct meridional circulation in the extratropics in the troposphere showed enhanced anomalies, which suggest intensification anomalies of the cold air outflow to the mid-latitudes in the lower troposphere. This meridional circulation anomaly was also predicted in the seasonal prediction experiment.
From October to December 2019, SST anomalies in the tropical Indian Ocean were distributed high in the west and low in the east, and high temperature anomalies were also observed across the tropical Indian Ocean. In order to confirm the effect of the tropical SST on the negative phase of AAO, a sensitivity experiment was conducted by nudging the Indian Ocean tropical SST to the climatological SST.
Similar to the seasonal prediction experiment in the previous section, the results of this sensitivity experiment predicted downward propagation of the weakness of the polar vortex and the persistence of the negative phase AAO in the troposphere after mid-October. The anomalies in the zonal-mean wave activity, EP flux, also show similar characteristics to those of the seasonal prediction experiment. This indicates that the persistent negative AAO can be predict even without the Indian Ocean tropical SST anomalies.
The results of the experiment suggest that SST anomalies in the tropical Indian Ocean has little effect on the persistence of the negative phase of the AAO. As future work, we would like to analyze what contributed to the persistence of the negative phase of the AAO and what was the source of the seasonal predictability. It is also interesting to note that the seasonal prediction experiment was able to predict the timing of the onset of negative AAO. Investigating the reasons for the predictions and improving our understanding of stratosphere-troposphere coupling are future tasks.
The data used for the analysis of the circulation field is the JMA long-term reanalysis data JRA-55 (Kobayashi et al., 2015). The forecast data are the results of an ensemble 6-month forecast experiment with initial values of September 13 and 28, 2019, by an in-service seasonal forecast model (the coupled global atmosphere-ocean model CPS2, Takaya et al., 2018). The mass-weighted average method (MIM, Iwasaki 1989, 1992) on the isothermal surface is applied to these data to analyze the wave activity anomalies.
The evolution of the strength of the polar vortex from the stratosphere to the troposphere associated with SSW in the SH shows that the weakness of the polar vortex persisted in the upper stratosphere after the SSW, and the center of the anomaly descended to the lower stratosphere to the troposphere after mid-October. This was well predicted by the seasonal prediction experiment.
The zonal mean field during the period of persisted negative AAO is confirmed in terms of wave-mean flow interaction. The stratospheric polar night jet seen in the climatology is absent in the 3-month averaged zonal mean zonal winds from October to December 2019.
Corresponding to the zonal mean zonal winds, there is no waveguide along the climatological polar night jet, making it more difficult for planetary waves from the troposphere to propagate vertically than normal. In terms of wave activity, there was a strong convergence of E-P flux from 50S to 70 S in the middle and upper troposphere. As diagnosed by the spatial structure of the convergence-divergence anomalies of E-P fluxes, the direct meridional circulation in the extratropics in the troposphere showed enhanced anomalies, which suggest intensification anomalies of the cold air outflow to the mid-latitudes in the lower troposphere. This meridional circulation anomaly was also predicted in the seasonal prediction experiment.
From October to December 2019, SST anomalies in the tropical Indian Ocean were distributed high in the west and low in the east, and high temperature anomalies were also observed across the tropical Indian Ocean. In order to confirm the effect of the tropical SST on the negative phase of AAO, a sensitivity experiment was conducted by nudging the Indian Ocean tropical SST to the climatological SST.
Similar to the seasonal prediction experiment in the previous section, the results of this sensitivity experiment predicted downward propagation of the weakness of the polar vortex and the persistence of the negative phase AAO in the troposphere after mid-October. The anomalies in the zonal-mean wave activity, EP flux, also show similar characteristics to those of the seasonal prediction experiment. This indicates that the persistent negative AAO can be predict even without the Indian Ocean tropical SST anomalies.
The results of the experiment suggest that SST anomalies in the tropical Indian Ocean has little effect on the persistence of the negative phase of the AAO. As future work, we would like to analyze what contributed to the persistence of the negative phase of the AAO and what was the source of the seasonal predictability. It is also interesting to note that the seasonal prediction experiment was able to predict the timing of the onset of negative AAO. Investigating the reasons for the predictions and improving our understanding of stratosphere-troposphere coupling are future tasks.