全球暖化趨緩下的黑潮

Abstract

黑潮為北太平洋副熱帶環流的西方邊界流，對中低緯度之間的物質與能量交換扮演著重要的角色。黑潮是由北太平洋風場驅動及維持，然而近二十年來，在全球暖化趨緩期間大氣條件產生了變異，黑潮會如何反應此氣候變化仍不清楚。藉由分析多組獨立資料，此研究確認了位於呂宋東側外海的黑潮上游強度存在著增強的趨勢，這個改變歸因於菲律賓海的氣旋式風旋度異常以及赤道信風的增強。黑潮上游增強有利於其北向通過呂宋海峽、伴隨較少的西向入侵南海事件，因此減少了這段期間從太平洋輸送到南海的熱量。同一時期，黑潮位於東臺灣以北至東海的主流域呈現減弱且增暖的趨勢，原因歸因於氣旋式大洋風場趨勢及暖池暖化。黑潮強度減弱已知伴隨陸棚方向的位移，而暖池暖化則加強了黑潮傳輸的熱量，兩個因素導致更多熱量經由黑潮輸送到東海。太平洋的氣候狀態在暖化趨緩期間之轉變，已知與太平洋年代際震盪(Pacific Decadal Oscillation, PDO)有關。PDO的相位狀態在這個時期遷移至負相位，貢獻了許多趨勢上的風場特徵，例如信風的增強及大洋風場的氣旋式傾向。PDO除了在趨勢上與黑潮的變異高度相關外，前人研究認為在年際變化上，PDO也能藉由影響大洋風場來調控黑潮。然而此研究結果指出，這個可預報性在1998年之後被擾亂，原因在於PDO的影響力被北太平洋第二模態取代。在1990年後期，PDO遷移至負相位所引起的黑潮長週期變化，調節了氣候系統的能量平衡，而黑潮在年際變化上的未來推估則變的困難。The Kuroshio part of the North Pacific subtropical gyre is vitally important to balance heat and mass between tropical and extratropical. The current is driven and maintained by the Pacific atmospheric circulation. However, pronounced changes in the wind field have been observed widely during the recent global warming hiatus. What has happened to the Kuroshio concurrently? According to independent data sets, a strengthening upstream of the Kuroshio in east Luzon has been confirmed, and attributed to the anomalous cyclonic wind in the Philippine Sea and enhanced trade wind. The strengthening Kuroshio tends to bypass the Luzon strait and reduces the westward intrusion into the South China Sea (SCS), which decreases the heat transport from the warm pool into the SCS. In the main stream from east Taiwan to the south of Japan, weakening but warming of the Kuroshio has been attributed to the basin wind stress curl (WSC) which tends to be cyclonic and warming in the warm pool. The weakening Kuroshio favors shelf-ward migration, and the warming warm pool injects more heat into the current, both give rise to more heat onto the East China Sea (ECS) shelf. The changing Pacific wind system during the hiatus has been characterized by the Pacific Decadal Oscillation (PDO), whose climatic condition has been shifted into negative phase and shares many traits to long−term changes of basin wind such as strengthening trade wind and cyclonic tendency of basin WSC. Besides long−term time scale, previous studies suggest that the PDO also controls the interannual variability of the Kuroshio main stream via basin wind system. However, this study confirmed that the predictable interannual variability of the Kuroshio has been disturbed after the late 1990s. Since the PDO shifted into negative phase during the recent hiatus, the associated long−term change of the Kuroshio will be modified, such as the energy balance of climate system, and its interannual variability will become hard to speculate.