2009年8月初，莫拉克颱風和科尼與艾陶颱風幾乎同時發生於西北太平洋的大尺度季風環流圈內。此環流圈南側的西南季風環流提供充足的水氣，形成一個大範圍的對流活躍帶，其空間尺度遠大於颱風主渦旋半徑。莫拉克颱風發生期間的大氣環流具多重尺度特性，包括颱風渦旋、東南往西北移動的10-30天擾動的氣旋環流、以及南往北移動的40-50天擾動氣旋環流。在莫拉克颱風侵襲台灣期間，10-30天擾動是大尺度水氣輻合的主要來源，並在颱風南側形成大範圍對流帶，莫拉克颱風則沿此一對流帶北緣移動。由於大範圍對流帶與西南季風環流位於颱風南側，加上南部陡峭地形對西南氣流的舉升作用，颱風最強的對流出現在南側，形成偏南的不對稱對流分佈，以致於近颱風中心的台灣北部雨量不如預期的多，強對流與超大豪雨反而發生於台灣南部。 2009年7-9月，菲律賓海的雨量、水氣量與海溫均高於90百分位 (即前百分之十)，屬於高度異常。季風槽偏南且偏強，台灣處於下沈區屬於偏乾旱的氣候型態，鄰近海域不利颱風生成與發展，且大多數颱風路徑偏南偏西。此一狀況顯示莫拉克颱風發生於一個乾旱的年份。此一異常氣候型態可歸因於赤道中太平洋偏暖海水引發的異常大氣環流。雖然處於偏乾的氣候型態，7月底-8月初月出現一個短暫的空檔，讓原本活躍於南菲律賓海的對流性環流透過10-30天與40-50天擾動移行至台灣附近，導致莫拉克颱風伴隨10-30天擾動挾帶豐沛水汽侵襲台灣，造成破紀錄的超大豪雨與災害。

Typhoon Morakot (2009) occurred almost simultaneously with Typhoon Goni and Etau within a monsoon gyre in the Western North Pacific. The southwesterly monsoon flow in the southern flank of the gyre supplied abundant moisture and formed a large-scale convection-active region, which is much larger than the size of typhoon vortex. The large-scale circulation during the lifetime of typhoon Morakot was characterized by multiscale circulations including typhoon vortex and the cyclonic circulation embedded in the northwestward-moving 10-30 day and northward-moving 40-50 day disturbances. During the influencing period of Morakot on Taiwan, the 10-30 day disturbance was the major moisture supplier and formed the large-scale convective region south of the typhoon, which moved westward along the northern flank of this convection belt. The combined effect of the strong monsoonal southwesterly/convection belt in the south and the lifting effect of the steep terrain in southern Taiwan resulted in the southward-shifted convection zone associated with the typhoon. As a result, the strong convection and heavy rainfall occurred in southern Taiwan, instead of in northern Taiwan where the typhoon center passed. During July-September 2009, sea surface temperature, low-level specific humidity, and rainfall were anomalously high by exceeding 90 percentile in most part of the Philippine Sea. The stronger-than-normal monsoon trough shifted anomalously southward. As a result, Taiwan was located in a region of anomalous subsidence and deficit rainfall. The typhoon tracks in the 2009 typhoon season shifted southward and moved westward. These anomalous features, which were likely the results of the anomalous warm SST in the central equatorial Pacific, indicate that Taiwan was actually in a severe drought condition during 2009 typhoon season. This dry and stable condition was temporarily interrupted by the move-in of the 10-30 day and 40-50 day perturbation from the tropical Philippine Sea during late July and early August and resulted in the record-breaking rainfall in Taiwan.