The Roles of Westward-Propagating Waves and the QBO in Limiting MJO Propagation

Kai Huang

Dept. Atmospheric, Oceanic and Earth Sciences, George Mason University

Tuesday, Nov 09, 2021, 2:00 pm

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Abstract

Madden-Julian Oscillation (MJO) is the dominant subseasonal variability in the tropics and it induces impacts on global weather and climate systems such as the tropical cyclone genesis, the phase shift of east Asian monsoon, and extreme precipitation events in the mid-latitudes through teleconnection. MJO also provides significant predictability for the subseasonal weather prediction, bridging the gap between short-term weather forecast and the long-term climate prediction.

A recent study categorized the MJO during boreal winter season into four types including stand, jump, slow and fast MJO. This study focuses on the stand and jump MJO. Based on whether their convection penetrates the Maritime Continent (MC), stand and jump MJO are seen as non-penetrating (NP) MJO, while the rest two are seen as eastward-penetrating (EP) MJO. Results reveal the relative roles of the westward-propagating wave (WPW), as well as the QBO and ENSO, in limiting MJO propagation. Lack of the pre-moistening over the southern sea surface of MC is responsible for NP MJO's failure to penetrate MC. The active convection of WPW hinders the NP MJO descending branch and therefore leads to the insufficient meridional advective moistening over the southern sea surface of the MC. The overall drying over the MC for jump MJO also comes from the intraseasonal dry anomalies induced by WPW over the western Pacific, which are then advected to MC by the superposed seasonal mean northeasterlies. The independent convection over the Pacific for jump MJO is influenced by a combined effect of the QBO and ENSO. Under the influence of the preferred QBO westerly (QBOW) phase, the decoupled tropopause instability of NP MJO propagates eastward into the Pacific and amplifies the local WPW convection for jump MJO. For stand MJO, however, the La Nina-like seasonal mean cool SST anomalies confine WPW within the western Pacific. Therefore, the decoupled tropopause instability of stand MJO is out phase of WPW over the central Pacific and fails to induce an independent convection there.