[A-CG33] 中緯度大気海洋相互作用

[ACG33-14] Sensitivity of the Baroclinic Annular Mode to the Latitude of a Midlatitude Oceanic Frontal Zone

*中山 盛雄¹、中村 尚¹、小川 史明² (1.東京大学 先端科学技術研究センター、2.北海道大学大学院 理学研究院)

キーワード：中緯度海洋前線帯、ストームトラック、気候変動

Baroclinic annular mode (BAM) is a major mode of annular variability in the Southern Hemisphere, representing the hemispheric-scale pulsing of extratropical eddy activity. Recently, Nakayama et al. (2021; hereinafter cited as N21) showed that a midlatitude oceanic frontal zone, characterized by sharp gradient of sea-surface temperature (SST), significantly enhances and thereby anchors the BAM signature by effectively restoring a strong meridional gradient of near-surface baroclinicity. Their investigation was, however, limited to the case when an oceanic frontal zone is at 45º latitude as observed in the South Indian Ocean. As an extension of N21, the present study assesses the dependence of the BAM variability on the latitude of an oceanic frontal zone by analyzing a set of atmospheric general circulation model (AGCM) experiments with zonally-uniform SST profiles prescribed (i.e., aqua-planet experiments). The experiments where the latitude of the frontal SST gradient is systematically changed reveal that the strongest lower-tropospheric BAM signature is anchored systematically in the vicinity of the SST front. They also reveal that the strongest upper-tropospheric BAM signature is anchored systematically poleward by ~10º of the latitude of an oceanic frontal zone when it situates subtropics and midlatitude (equatorward of 45º). These tendencies are consistent with N21 and suggest the significant impacts of an oceanic frontal zone on the BAM. Meanwhile, when an oceanic frontal zone is situated at a subpolar latitude (poleward of 50º), the upper-tropospheric BAM signature maximizes equatorward of an oceanic frontal zone and the BAM variability becomes weaker. The BAM signature thus becomes similar to what is realized without frontal SST gradient. This suggests the significance of internal dynamics of the extratropical troposphere in the BAM dynamics that tends to dominate over the effect of an oceanic frontal zone when situated at a subpolar latitude.