[A-CG33] Multi-scale ocean-atmosphere interaction in the tropics

[ACG33-05] Tropical Intraseasonal Variability as a Linear Leading Moisture Dynamic Mode of the Warm-Pool Background State

*林 未知也¹、Fei-Fei Jin² (1.国立研究開発法人国立環境研究所、2.University of Hawaii at Manoa)

キーワード：熱帯季節内変動、マッデン・ジュリアン振動、エルニーニョ南方振動、湿潤線型傾圧モデル

Tropical intraseasonal variability (ISV) is dominated by the Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO), and its spatiotemporal characteristics vary with the Indo-Pacific warm-pool background on seasonal and longer timescales. Previous works have aimed to reveal ISV dynamics in various frameworks, whereas a unifying view for the ISV change characteristics remains elusive. Motivated by the recent advance in moisture mode theory for the MJO, we proposed a view for the ISV as a linear moisture mode modulated by varying background states, which was elucidated using a moist linear baroclinic model (mLBM) improved with a simple convective scheme relating convective precipitation to tropospheric and boundary-layer moisture anomalies and simple cloud–radiation feedback representations in atmospheric heating and surface fluxes. Under a boreal winter background state, this improved mLBM yielded a large-scale eastward-propagating mode with a phase speed of 3–5 m/s over the warm-pool region, resembling the MJO. Under a boreal summer background state, the leading mode involved east–northward propagation, resembling the BSISO but with too slow phase speed relative to the observed value. Background lower-tropospheric winds and thermodynamic fields are important in determining the growth rate and periodicity of the leading mode, whose stability strongly depends on cloud–radiation feedback and background state variations. For example, adding El Niño anomalies onto the background state causes eastward displacement of the wintertime MJO-like ISV activity, reminiscent of the observed MJO modulations by El Niño events.