[A-AS01] 大規模な水蒸気場と組織化した雲システム

[AAS01-05] Can Mixed Rossby-gravity waves make building blocks of MJO convection in the Indian Ocean?

*高須賀 大輔¹、佐藤 正樹² (1.お茶の水女子大学理学部情報科学科、2.東京大学大気海洋研究所)

キーワード：マッデン・ジュリアン振動、赤道波、スケール間相互作用

This work examines if mixed Rossby-gravity waves (MRGs) can be an effective controlling factor of MJO initiation and propagation over the Indian Ocean (IO) in general, which is aimed at extending a case-study-based idea about the cross-scale interaction between the MJO and MRGs (Takasuka et al. 2019, GRL). We detect 47 MJO events initiated in the IO during boreal winters of 1982-2012 using outgoing longwave radiation data. In a statistical sense, we find that MJO convection begins to develop in the southwestern IO (SWIO), where convectively coupled MRGs with tropical depression-type circulations (MRGTDs) robustly exist. Further classification of MJO cases suggests that 26 of 47 cases are related to more predominant MRGTD activities, although some secondary roles of Kelvin waves cannot be dismissed. The convective initiation of those MRGTD-enhanced MJOs is prompted by the development of low-level MRGTD circulations as a result of amplified downward energy dispersion associated with uppertropospheric baroclinic conversion. This continuous MRGTD-convection coupling before MJO initiation is supported by (i) MRG–TD transition with zonal wave contraction due to background upper-tropospheric convergence, and (ii) plentiful moisture advection into the western IO by intraseasonal winds during an MJO-suppressed phase. After this MRGTD-induced MJO triggering, low-level MRGTD winds with eastward group velocity start to trigger convection to the east successively, which is responsible for MJO propagation over the IO. These results indicate that we can regard MRGTDs as influential contributors to making building blocks of MJO convection at least in the IO.