[A-AS10] Stratosphere-troposphere (Atmospheric) Processes And their Role in Climate

[AAS10-07] The Mechanism of the Interhemispheric Coupling Revealed by a Gravity-Wave-Permitting General Circulation Model

★Invited Papers

*Haruka Okui¹, Kaoru Sato¹, Shingo Watanabe² (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:Middle atmosphere, Gravity wave, Planetary wave, General circulation model

Interhemispheric coupling (IHC) addressed in this study is the positive correlation between temperatures in the polar winter stratosphere and polar summer upper mesosphere and lower thermosphere (MLT). Since its discovery through observations in the early 2000s, several mechanisms for the IHC have been proposed in previous studies. To date, studies on the IHC mechanism utilized general circulation models (GCMs) which employ gravity wave (GW) parameterizations. In most GW parameterizations, it is assumed that GWs are generated only in the lower atmosphere and do not propagate horizontally. Recently, however, there has been growing recognition of the importance of GWs whose generation and propagation processes do not conform to these assumptions. We investigated the role of GWs in the IHC mechanism using a high-resolution GCM without GW parameterizations. The GCM we used, known as the Japanese Atmospheric GCM for Upper Atmosphere Research (JAGUAR), covers from the surface to the lower thermosphere. We performed hindcast simulations of seven boreal winters with a high-resolution (T639L340) version of the JAGUAR model, whose vertical resolution is 300 m. Detailed analysis of anomalies from the 7-year climatology unveiled that GWs play a key role in the IHC through a successive interplay with larger-scale waves and interaction with the mean flows. The elucidated mechanism is outlined below, in association with a warm winter stratosphere (Fig. 1). When a warm anomaly appears in the polar winter stratosphere, 1) a cold anomaly emerges in the equatorial stratosphere. Keeping the thermal-wind balance with the cold anomaly, a westward anomaly of zonal-mean zonal winds forms in a height (z) range of z= 50–60 km in the low-latitudes of the summer hemisphere. 2) A positive (i.e., eastward) anomaly in GW forcing emerges in z= 60–70 km at ~20° S just above the westward wind anomaly. This anomaly is consistent with GW filtering. 3) A negative latitudinal gradient of zonal-mean potential vorticity (PV) is enhanced in z= 60–70 km at 40°–50° S on the high-latitude side of the positive GW forcing anomaly. This seems to be attributable to the enhanced northward PV flux that is equivalent to the positive GW forcing anomaly. 4) A positive anomaly of the EP flux divergence (EPFD) associated with quasi-two-day waves (QTDWs) appears in the region of the negative anomaly of meridional PV gradient. These positive EPFD and negative PV gradient anomalies suggest that QTDWs are generated by barotropic and/or baroclinic instability. 5) The QTDWs exert strong negative forcing in z= 85–95 km at 40°–70° S. 6) A positive temperature anomaly appears in z=80–90 km at ~60° S owing to adiabatic heating associated with the downwelling induced by the negative QTDW forcing. 7) On the polar side of the warm anomaly, an eastward anomaly is observed in zonal winds in the polar summer upper MLT. 8) Above the eastward wind anomaly, GW forcing is anomalously negative. This seems to be the direct cause of 9) the downwelling and thus warming in the polar summer upper MLT. Compared with resolved GW forcing, GW parameterizations in medium-resolution reanalysis data significantly underestimated GW forcing anomalies at Steps 2 and 8 of the above mechanism. This fact suggests that GWs not conforming to the assumptions in GW parameterizations, such as secondary GWs and laterally propagating GWs, make a large contribution to the IHC.