MJO和熱帶波動對亞澳季風區次季節降雨高峰事件的影響及展期預報應用

Modulation of the Asian-Australian Monsoon Region Subseasonal Peak Precipitation Events by MJO and CCEWs in Boreal Winter and Extended-Range Prediction Application

蔡元懷(Wayne Yuan-Huai Tsai)

指導教授 ： 盧孟明

共同指導教授 ： 隋中興(Chung-Hsiung Sui)

國立臺灣大學/理學院/大氣科學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202003122

摘要

印度－太平洋暖池區是北半球冬季 (十一月至四月) 熱帶區域降雨的中心之一，豐沛的雨量受到多重尺度氣候現象的影響，如聖嬰現象、馬登－朱利安震盪 (MJO)、熱帶波動 (CCEWs)等。本研究旨在了解不同時空尺度的氣候變異如何影響冬季的主要降雨事件，以及分析歐洲、美國和加拿大等先進預報作業單位次季節－季節 (S2S) 預報模式 (ECMWF、ECCC、NCEP) 對極端降雨事件的預報能力與產品應用範圍。
冬季的主要降雨事件以季內尺度降雨高峰事件為主要分析對象，也就是以每年十一月至翌年二月作為季節挑選出15天累積降雨最大值定義為「次季節降雨高峰事件 (簡稱高峰事件)」。研究MJO和熱帶波動對高峰事件的影響，發現 MJO 與赤道羅士比波 (Equatorial Rossby Wave, ER) 和羅士比重力混合波 (mixed Rossby- gravity tropical wave, MT) 影響了高峰事件的發生時間和降雨強度；但在南北緯 10 度以內的赤道海洋大陸 (Maritime Continent) 區域由於有複雜地形作用，大尺度波動對高峰事件的發生時間和強度影響不大。以 2017/18 年發生在南海和 2018/19 年在澳洲東北部的次季節降雨高峰事件為例，前者發生時間和降雨強度明顯受到從西太平洋移入的 ER 和 MT 波動的影響，以致在 2017 年 12 月有連續三個颱風侵襲菲律賓；後者受到從印度洋往澳洲移動的 MJO 以及從南太平洋往西移動的 ER 波動影響，以致在 2019 年 1 月下旬發生近 20 年最強次季節降雨高峰事件。
為瞭解S2S預報模式對澳洲季風肇始和次季節降雨高峰事件的預報能力，本研究分析了預報模式產出的 20年回溯性預報 (hindcast) 的40天展期預報資料。ECMWF和ECCC預報模式可以在二週前合理預測澳洲季風肇始的發生；ECMWF、ECCC和NCEP則可在一週前合理預測高峰事件的發生，然而更長期的預測模式沒有能力掌握。預測表現可歸因於模式對 MJO 和熱帶波動的預測能力。本研究揭示熱帶波動對熱帶地區極端降水的監測和預報有關鍵的角色，可作為未來改進與校驗模式的參考。

關鍵字

印太暖池熱帶對流 ； 季內降雨預報 ； 馬登－朱利安震盪 ； 熱帶波動 ； 次季節－季節預報

並列摘要

The Indo-Pacific warm pool region is one of the precipitation centers in the tropics during the extended boreal winter season (NDJFMA). The abundant winter rainfall amount over this region involves multiple-scale phenomena such as the monsoons, ENSO, Madden-Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs). This study aims at understanding how the climate variability on various temporal and spatial scales modulate the major precipitation events. The subseasonal-to-seasonal (S2S) prediction models provided by ECMWF, NCEP, and ECCC are also analyzed to understand the models’ ability on predicting extreme rainfall events and their application.
The boreal winter subseasonal peak precipitation event defined as an event with maximum 15-day accumulated rainfall amount during the period from November to February is the major precipitation events focused in this study. The results show noticeable modulation of MJO, Equatorial Rossby (ER) wave and Mixed Rossby-gravity wave/TD-type disturbances (MT wave) on the occurrence time and rainfall intensity of the peak events. However, the MJO and CCEWs modulation is not detectable in the equatorial belt between 10°S~10°N because of the complicated geographic and topography distribution straddling equatorial Maritime Continent region. The temporal and intensity modulation of tropical waves and the interaction of weather systems are further diagnosed in two cases of extreme rainfall events during the South China Sea Two Island Monsoon Experiment (SCSTIMX) winters, one occurred in December 2017 over the Philippines and the South China Sea (SCS) and another in 2018/19 at Townsville, Australia. The former is due to three successive typhoons associated with westward propagating ER and MT wave-train emanated from Western North Pacific, while the latter was modulated by MJO and ER wave, which contributed to the record-breaking rainfall events revealed in CPC MORPHing technique (CMORPH) precipitation data.
In order to understand the current forecast status of the S2S models, we analyzed 20 years of the hindcast data of the 40-day forecast generated by three S2S models. On the seasonal prediction skill, ECMWF and ECCC models can predict the Australian summer monsoon onset two weeks ahead. On the prediction of subseasonal peak precipitation events, the models perform reasonably well for the first week prediction, but not as well for prediction at longer lead time. The prediction skill can be related to the predictability of MJO and CCEWs. This study reveals that MJO and tropical waves play a crucial role for monitoring the tropical extreme rainfall and its prediction, which can be provided as an observational evidence to verify and improve the prediction models.

並列關鍵字

Tropical convection over Indo-Pacific warm pool ； Subseasonal precipitation prediction ； MJO ； Convectively coupled equatorial waves ； S2S prediction

參考文獻

Adler, R. F., and Coauthors, 2003: The Version-2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979–Present). J. Hydrometeorol., 4, 1147–1167, doi:10.1175/1525-7541(2003)004<1147:TVGPCP>2.0.CO;2.

Bazo, J., R. Singh, M. Destrooper, and E. Coughlan de Perez, 2019: Pilot Experiences in Using Seamless Forecasts for Early Action: The “Ready-Set-Go!” Approach in the Red Cross. Sub-Seasonal to Seasonal Prediction, A.W. Robertson and F. Vitart, Eds., Elsevier, 387–398.

Berrisford, P., and Coauthors, 2011: The ERA-Interim archive Version 2.0. ECMWF, 23.

Berry, G. J., and M. J. Reeder, 2016: The dynamics of Australian monsoon bursts. J. Atmos. Sci., 73, 55–69, doi:10.1175/JAS-D-15-0071.1.

Chang, C.-P., C.-H. Liu, and H. C. Kuo, 2003: Typhoon Vamei: An equatorial tropical cyclone formation. Geophys. Res. Lett., 30, 10–13, doi:10.1029/2002GL016365.