東亞季風春夏轉換期降雨和環流特徵及南海與周圍區域氣候季內震盪之辨識

Abstract

東亞季風系統有很顯著的年循環，特徵為每年的夏季偏溼而冬季偏乾。此系統也有很明顯的氣候季內震盪，其特徵為每年在相同時期會固定出現的季內震盪，為瞬時季內震盪與年循環相位鎖定的結果，也被稱作為快速年循環。本研究主要探討春夏轉換期東亞季風的轉變，並辨識在南海區域的氣候季內震盪，了解南海氣候季內震盪與南海夏季季風肇始以及東亞季風演變的關係。 本研究運用數據導向方法分析印太季風區1979至2023年3至7月逐日850-hPa風場類型，運用天氣類型能客觀描述印太亞澳季風演變的規律。由於4至7月是從南海到日本韓國夏季季風快速建立的時期，850-hPa風場類型提供了以大尺度季風演變進程為主軸判斷夏季季風肇始發生時間與位置的客觀指引。南海地區由東風轉為西風的時間與氣候季內震盪發生的時間相近，將向外長波輻射資料進行諧波分析後發現氣候季內震盪的特徵為在五月上旬有一峰值（乾）而在五月下旬有一谷值（溼）。將每年特徵與氣候季內震盪類似的季內震盪稱為準氣候季內震盪，以進一步闡明東亞夏季季風的季節演變與南海風場降水變化之間的關係。結果發現在41年中有16年呈現出季節演變的典型序列，表示這16年為南海向外長波輻射的變化具有明顯快速年循環的年份。然而，這16年只有一半的年份與馬登-朱利安振盪(MJO)高度相關，將這8年定義為MJO-CISO-U850-SCSSM一致的年份。從合成分析發現，如果準氣候季內震盪谷值發生的前十天內MJO位於太平洋上，則此MJO會影響南夏季季風肇始與東亞季風的發展。之後會進一步了解南海的快速年循環在沒有明顯MJO的情況下是如何發生，以及MJO-CISO-U850-SCSSM一致的年份與其他年份之間的根本差異是什麼。需要更多的研究來理解東亞季風從春季到夏季的季節演變以及南海及周圍地區的快速演變。

The East Asian monsoon system (EAM) shows strong annual cycle with the wet season in summer and dry in winter. The monsoon system exhibits distinct climatological intraseasonal oscillations (CISOs), also known as the "fast annual cycle," which are characterized by ISOs occurring consistently during the same period each year. This study explores the evolution of the EAM during spring to summer transition, identifies the CISO over the South China Sea (SCS) using the OLR data, and examines its relationship with the SCS summer monsoon onset process and the broad EAM low-level circulation and precipitation features. Using a data-driven approach, the spring-to-summer transition of the EAM is objectively described based on the low-level wind weather types (WTs) obtained from k-means cluster analysis of the March-July daily 850-hPa wind data from 1979 to 2023. The monsoon evolution of a large-scale circulation over the Indo-Asia-Pacific region is represented by the chronological sequence of the occurrence frequency of the WTs. The spring to summer transition of the WTs shows clear wind direction shift over the SCS that signals the beginning of a summer season. The timing of the prevailing wind changes from the easterlies to westerlies over the SCS coincides with the CISO. The CISO is positive (dry) peak of ISO component of the OLR in early May and negative (wet) valley in late May. For individual years, we identify the ISO that resembles the CISO as quasi-CISO to further clarify the relationship between the seasonal transition of the huge EAM system and the changes over the SCS. It turns out that 16 among 41 years show prototypical sequence of seasonal transition. These 16 years are identified as the years with clear “fast annual cycle” over the SCS in terms of the OLR variations. However, only a half of the 16 years are identified as highly correlated with the Madden-Julian Oscillation (MJO). These 8 years are labeled as the MJO-CISO-U850-SCSSM consistent years. The composite maps of the MJO-CISO-U850-SCSSM years suggest that if the MJO is over the Pacific Ocean two pentads before the quasi-CISO valley, this MJO will influence the onset of the SCSSM and the development of the EAM. The unanswered question is: how the SCS fast annual cycle with no clear relationship with the MJO are triggered? What are the fundamental differences between the MJO-CISO-U850-SCSSM years and other years? More studies are needed to understand the spring to summer seasonal transition of the EAM and the sharp transition over the SCS and its surrounding region.