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The Mechanism of Interhemispheric Coupling Revealed by a Gravity Wave‐Permitting General Circulation Model.

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Title: The Mechanism of Interhemispheric Coupling Revealed by a Gravity Wave‐Permitting General Circulation Model.
Authors: Okui, H.1 (AUTHOR) okui@eps.s.u-tokyo.ac.jp, Sato, K.1 (AUTHOR), Watanabe, S.2 (AUTHOR)
Source: Journal of Geophysical Research. Atmospheres. 8/16/2025, Vol. 130 Issue 15, p1-20. 20p.
Subject Terms: *General circulation model, *Stratosphere, *Climatology, *Weather & climate change, *Polar climate, Gravity waves, Atmospheric waves
Geographic Terms: Antarctica
Abstract: Interhemispheric coupling (IHC) is the positive correlation between temperatures in the polar winter stratosphere and the polar summer upper mesosphere and the lower thermosphere. Over the past two decades, several mechanisms have been proposed to explain the IHC. However, a consensus on the mechanism is yet to be fully reached, particularly regarding the role of gravity waves (GWs). We conduct hindcast simulations for seven boreal winters using a GW‐permitting general circulation model encompassing the whole neutral atmosphere. The model is initialized through spectral nudging to state‐of‐the‐art reanalysis data of identical vertical coverage. Treating the 7‐year averages of the model outputs as a climatology, the IHC is investigated as a sequential evolution of anomalies relative to it. The results showed that consequential interplay of GWs and quasi‐two‐day waves in the summer mesosphere is the key driver of the IHC, as is consistent with Yasui et al. (2021, doi: 10.1175/jas‐d‐21‐0045.1) except for the final step toward the summer pole facilitated by primary GWs in the Antarctic. These waves are filtered by an anomalously weak westward jet due to a temperature gradient caused by planetary wave forcing, considered to be quasi‐two‐day waves. The mechanism is mostly applicable to both types of IHC events: those associated with sudden stratospheric warming and those with vortex intensification. Meanwhile, comparisons across the seven boreal winters indicate vulnerabilities in specific processes underlying the mechanism. In the reanalysis, it is suggested that GW parameterizations underestimate the GW forcing anomalies, but analysis increments reduce this discrepancy. Plain Language Summary: Interhemispheric coupling (IHC) describes a teleconnection between the winter polar stratosphere and the polar summer mesopause, where temperatures in these regions change in tandem. The mechanism of the IHC remains debated, particularly the role of gravity waves (GWs), which are small‐scale atmospheric waves. To investigate how IHC develops, we conduct simulations of seven Northern Hemisphere winters using a high‐resolution general circulation model that explicitly resolves GWs. The model is initialized with state‐of‐the‐art reanalysis data, both of which cover the entire neutral atmosphere, spanning from the Earth's surface to the lower thermosphere. The 7‐year output data set enables analysis of the climatology and anomaly of GWs in the entire neutral atmosphere. Analyzing the data set, we highlight the critical role of interactions between GWs and planetary‐scale waves. It is shown that Antarctic GWs influenced by planetary waves contribute to the final step toward the IHC‐like temperature anomaly around the polar summer mesopause. Key Points: Hindcasts of the whole neutral atmosphere over seven boreal winters are conducted using a gravity wave‐permitting general circulation modelAnomalies from the climatology capture consecutive interplay of gravity waves (GWs) and quasi‐two‐day waves, leading to interhemispheric coupling (IHC)GW parameterizations tend to underestimate GW forcing responsible for the IHC [ABSTRACT FROM AUTHOR]
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Abstract:Interhemispheric coupling (IHC) is the positive correlation between temperatures in the polar winter stratosphere and the polar summer upper mesosphere and the lower thermosphere. Over the past two decades, several mechanisms have been proposed to explain the IHC. However, a consensus on the mechanism is yet to be fully reached, particularly regarding the role of gravity waves (GWs). We conduct hindcast simulations for seven boreal winters using a GW‐permitting general circulation model encompassing the whole neutral atmosphere. The model is initialized through spectral nudging to state‐of‐the‐art reanalysis data of identical vertical coverage. Treating the 7‐year averages of the model outputs as a climatology, the IHC is investigated as a sequential evolution of anomalies relative to it. The results showed that consequential interplay of GWs and quasi‐two‐day waves in the summer mesosphere is the key driver of the IHC, as is consistent with Yasui et al. (2021, doi: 10.1175/jas‐d‐21‐0045.1) except for the final step toward the summer pole facilitated by primary GWs in the Antarctic. These waves are filtered by an anomalously weak westward jet due to a temperature gradient caused by planetary wave forcing, considered to be quasi‐two‐day waves. The mechanism is mostly applicable to both types of IHC events: those associated with sudden stratospheric warming and those with vortex intensification. Meanwhile, comparisons across the seven boreal winters indicate vulnerabilities in specific processes underlying the mechanism. In the reanalysis, it is suggested that GW parameterizations underestimate the GW forcing anomalies, but analysis increments reduce this discrepancy. Plain Language Summary: Interhemispheric coupling (IHC) describes a teleconnection between the winter polar stratosphere and the polar summer mesopause, where temperatures in these regions change in tandem. The mechanism of the IHC remains debated, particularly the role of gravity waves (GWs), which are small‐scale atmospheric waves. To investigate how IHC develops, we conduct simulations of seven Northern Hemisphere winters using a high‐resolution general circulation model that explicitly resolves GWs. The model is initialized with state‐of‐the‐art reanalysis data, both of which cover the entire neutral atmosphere, spanning from the Earth's surface to the lower thermosphere. The 7‐year output data set enables analysis of the climatology and anomaly of GWs in the entire neutral atmosphere. Analyzing the data set, we highlight the critical role of interactions between GWs and planetary‐scale waves. It is shown that Antarctic GWs influenced by planetary waves contribute to the final step toward the IHC‐like temperature anomaly around the polar summer mesopause. Key Points: Hindcasts of the whole neutral atmosphere over seven boreal winters are conducted using a gravity wave‐permitting general circulation modelAnomalies from the climatology capture consecutive interplay of gravity waves (GWs) and quasi‐two‐day waves, leading to interhemispheric coupling (IHC)GW parameterizations tend to underestimate GW forcing responsible for the IHC [ABSTRACT FROM AUTHOR]
ISSN:2169897X
DOI:10.1029/2025JD043763