11:00 〜 11:15
[AAS08-08] Seasonality of MJO decay over the Maritime Continent based on the moist static energy budget
キーワード：マッデン・ジュリアン振動、海洋大陸、湿潤静的エネルギー、季節性
The Madden–Julian oscillation (MJO) is characterized by eastward-propagating (approximately 5 m/s) cloud clusters that extend thousands of kilometers across the Indo-Pacific warm pool. Some MJOs propagate beyond the Maritime Continent (MC) into the western Pacific (Propagating type), while others decay around the MC (Decaying type). The propagation and evolution of the MJO are understood through moisture dynamics, which evolve due to advection in front of the MJO. However, the current understanding lacks consideration of seasonality in the MJO-associated moisture field. Moreover, existing MJO theories do not explicitly account for seasonal variations.
In this study, we analyzed the moist static energy (MSE) budget using reanalysis data, examining seasonality by classifying events by month for both MJO types. Our analysis revealed three key findings. First, we observed monthly changes in MSE, which increased in front of the MJO. In November, the MSE increased sequentially from the Indian Ocean to the western Pacific, with a quarter-wavelength lag at each location. From December onwards, the MSE increase over the MC began 5–10 days earlier, with the increased MSE area extending wider zonally in front of the MJO. Second, the Decaying-type events exhibited weaker and delayed MSE accumulation than the Propagating type. In the Propagating type, MSE accumulated from the lower troposphere ahead of the MJO and gradually spread to the upper free troposphere. In contrast, the Decaying type showed minimal MSE accumulation throughout the troposphere. This difference appears to be caused by horizontal advection patterns. The Decaying-type events exhibited weaker meridional advection in November and December than the Propagating type. Additionally, in the Decaying type in January and February, the phase of the meridional advection was offset from the zonal advection, preventing simultaneous MSE accumulation. Third, previous studies overemphasized the characteristics of a specific period as MJO pictures, but we found that the MJO manifests differently across seasons. From January to February, anomalous zonal winds enhanced MSE by advecting mean moisture, consistent with previous findings. However, from November to early December, mean zonal winds advected anomalous moisture and decreased MSE. This seasonal variation occurs because the seasonal meridional shift in the dominant path of the MJO exposes it to different mean zonal wind directions. These findings underscore the importance of considering seasonal diversity when studying MJO mechanisms.
In this study, we analyzed the moist static energy (MSE) budget using reanalysis data, examining seasonality by classifying events by month for both MJO types. Our analysis revealed three key findings. First, we observed monthly changes in MSE, which increased in front of the MJO. In November, the MSE increased sequentially from the Indian Ocean to the western Pacific, with a quarter-wavelength lag at each location. From December onwards, the MSE increase over the MC began 5–10 days earlier, with the increased MSE area extending wider zonally in front of the MJO. Second, the Decaying-type events exhibited weaker and delayed MSE accumulation than the Propagating type. In the Propagating type, MSE accumulated from the lower troposphere ahead of the MJO and gradually spread to the upper free troposphere. In contrast, the Decaying type showed minimal MSE accumulation throughout the troposphere. This difference appears to be caused by horizontal advection patterns. The Decaying-type events exhibited weaker meridional advection in November and December than the Propagating type. Additionally, in the Decaying type in January and February, the phase of the meridional advection was offset from the zonal advection, preventing simultaneous MSE accumulation. Third, previous studies overemphasized the characteristics of a specific period as MJO pictures, but we found that the MJO manifests differently across seasons. From January to February, anomalous zonal winds enhanced MSE by advecting mean moisture, consistent with previous findings. However, from November to early December, mean zonal winds advected anomalous moisture and decreased MSE. This seasonal variation occurs because the seasonal meridional shift in the dominant path of the MJO exposes it to different mean zonal wind directions. These findings underscore the importance of considering seasonal diversity when studying MJO mechanisms.