西北太平洋地區的熱帶氣旋，約八成以上生成於季風環流圈中，過去 研究指出季風環流圈的發展和MJO (30-60 天震盪)是有關聯的。另外，MJO 也會影響熱帶氣旋路徑的轉向(Recurve)與直行(Straight)，而路徑轉彎的 熱帶氣旋在向北移動的過程，會受到中緯度西風帶的吹送，使得移動速度 明顯加快。因此，我們提出一些疑問：是否移速快的熱帶氣旋都是屬於會 轉向的？我們將1979-2012 年JTWC TC/TD 最佳路徑的資料做統計，發現確 實平均移速前10%的快速TC/TD 路徑幾乎是轉向的，而後10%移速慢的則多 位於南海一帶。而初步的分析結果也產生了2 個問題：(1)為何慢速的TC/TD 不會北上受到西風帶影響而加速，而快速的則有此情形？(2)為何快速的 TC/TD 在剛形成未進入西風帶時，初速就比慢的TC/TD 快上許多？ 為了找出原因，使用NCEP-DOE Reanalysis-2 資料將快、慢速TC/TD 的環境場做合成分析，我們發現快速的環境場中，位於台灣東方、日本南 方的區域，皆有一個擾動低壓以利TC/TD 向北移行並進入西風帶；而在慢 速的環境中，低壓擾動則位於南海、菲律賓一帶，而此擾動低壓東北方皆 伴隨著一個擾動高壓，使得慢速的TC/TD 僅能侷限在南方。接著，為了找 出快、慢速TC/TD 初速差異的原因，選取TC/TD 生成前2 天至後2 天共5 天，利用渦度方程式做進一步動力機制的合成分析。從這5 天趨勢項的渦 度收支變化，我們可以看到初速快的TC/TD 渦度收支隨著時間有明顯地增 加，而初速較慢的則逐漸減少。深入探討渦度方程式各項的貢獻，發現平 流項除了TC/TD 周圍正、負渦度分布和趨勢項極為相似外，渦度收支變化 也跟趨勢項很一致，因此說明快、慢TC/TD 的初始速度差異是由渦度的平 流效應所主導的。;Previous studies had shown that roughly more than 80% of tropical cyclones/ tropical depressions(TC/TDs) develop in the monsoon gyres over the western North Pacific and the activity of the monsoon gyres is associated with MJO (30-60 days oscillation) development. In addition, MJO also affects the track of TC/TDs, more recurving (straight moving) TC/TDs occur during MJO low (high). When the recurving TC/TDs go northward, they will be accelerated by the mid-latitude westerlies. Thus, one may wonder: Whether or not the fast moving TC/TDs are always recurving? Using the 1979-2012 JTWC TC/TD best track dataset, we found that the top 10% fast moving TC/TDs are recurving, while the last 10% TC/TDs which have slow moving speed cluster in the South China Sea. Based upon this preliminary result, two major issues are raised: (1)Why the slow moving TC/TDs do not go northward and eventually embed in the mid-latitude westerlies, but the fast moving TC/TDs do? (2)Why the initial speed is significantly higher for fast moving TC/TDs than those slow ones immediately after they are formed? In order to answer these two concerns, the NCEP-DOE Reanalysis-2 data are used for the composite analysis to examine the environmental differences between the fast and slow moving types of TC/TDs circulations. It appears that an anomaly low located between south of Japan and east of Taiwan is conducive to both the further development and northward migration of the fast moving TC/TDs. In contrast, the anomaly low is confined to the South China Sea and west of Philippines with an anomaly high toward its northeast side for the slow moving TC/TDs situation so that the slow moving TC/TDs are limited in this south region. To better understand the dynamic mechanism causing the initial speed differences between the fast and slow moving TC/TDs, the vorticity budget analysis is applied to evaluate the contribution of each term in the equation from two days before to two days after TC/TDs genesis. It is interesting that the time series of vorticity tendency for the fast moving TC/TDs show significant increase whereas the slow moving ones decrease gradually during these five-day initial stages. After careful investigation, it turns out that the advection term, not only the horizontal distribution but also the time variation, is similar to tendency term. Apparently, the differences in initial speed between fast and slow moving TC/TDs are dominated by the advection effect.
