西太平洋颱風季季內振盪年際變化之模擬

Simulations of the Interannual Variations of Intraseasonal Oscillation during Typhoon Season over the Western Pacific

徐邦琪(Pang-Chi Hsu) ； 鄒治華(Chih-Hua Tsou) ； 陳正達(Chen-Ta Chen)

《大氣科學》 35卷3期 (2007/09) Pp. 189-217

摘要

季內振盪(ISO)為熱帶大氣系統的重要訊號，西太平洋地區ISO的活動與熱帶天氣及短期氣候變化相關。本篇研究除了探討颱風季七－九月西太平洋ISO與SST之年際關連外，亦利用高解析度ECHAM4T106與ECHAM5/MPI-OM海氣耦合模式來探討ISO年際變化模擬的情形。觀測場分析顯示，颱風季季內振盪氣候降水變異量最大值出現於熱帶風暴(TS)活動十分活躍之西北太平洋地區，而渦度變異量中心位於降水變異量中心的北／西北方。西北太平洋ISO的年際變化與七-九月ENSO訊號具遙相關連，其中以西北太平洋東南區域(WNPSE，150°-180°E, 0°-15°N)ISO活動與ENSO相關最顯著。暖年（Nino3.4 SSTA高於0.8個標準差）時，西太平洋暖海溫向東延伸至中太平洋，可能有利30-60天振盪沿赤道的東移。當30-60天振盪東移至換日線時，30-60天正渦度位於30-60天降水之西北前緣，對流由中太平洋地區轉向西北移行至台灣附近。暖年沿ISO西北移行路徑上，30-60天對流活躍，有較多的TS發生。冷年（Nino3.4 SSTA小於0.8個標準差）時，熱帶地區ISO訊號微弱，WNPSE 30-60天振盪為負距平，TS發生頻率亦較低。
ECHAM4 T106及ECHAM5/MPI-OM耦合模式均可模擬颱風季季內降水及渦度變異量之空間分佈，但ECHAM4 T106模式在定量上低估30-60天及10-20天降水變異量，海氣耦合模式模擬之降水變異量較接近觀測。ECHAM4 T106可模擬暖年沿赤道東移之30-60天振盪，然而，自中太平洋向西北發展之30-60天ISO僅移行至155°E，10°N附近。ECHAM5/MPI-OM耦合模式模擬30-60天振盪移行特徵與觀測較為接近。推測海氣交互作用可能是維持30-60天振盪強度及持續向西北移行過程之重要關鍵。西太平洋地區10-20天振盪的年際變化與30-60天不同，推測造成此兩頻段年際變化的機制可能不完全相同。而模式模擬10-20天振盪的能力有限，ECHAM4 T106及ECHAM5/MPI-OM均低估10-20天變異量距平值。

關鍵字

颱風季季內振盪 ； 年際變化 ； 高解析度大氣環流模式 ； 海氣耦合模式

並列摘要

Intraseasonal oscillation (ISO) is an important signal in the tropical atmospheric system. The tropical weather and short-term climate variability over the western Pacific are related to the ISO activity. This study not only investigates the interannual correlation between ISO over the western Pacific and SST during typhoon season (July to September), but also examine the simulations of ISO interannual variations in high-resolution (T106) ECHAM4 atmospheric GCM and ECHAM5/MPI-OM air-sea coupled GCM. The analysis results from observation data show that the largest climatological variance of intraseasonal precipitation during typhoon season occurs over the western Pacific where the tropical storm (TS) activity is vigorous. The maximum variance of vorticity is located to the north/northwest of precipitation variance center. The interannual variation of ISO over the western Pacific has teleconnections with ENSO. The ISO shows the most significant correlation with ENSO signal in the southeastern region of western North Pacific （WNPSE，150°-180°E, 0°-15°N）. During warm years (Nino3.4 SSTA greater than 0.8 standard deviation), the warm SST over the western Pacific extends eastward that might be beneficial for the 30-60 day ISO propagation eastward along equator. Once the 30-60 day ISO arrives at dateline, the 30-60 day positive vorticity appears to the northwest of 30-60 day precipitation. Meanwhile, ISO turns to migrate northwestward from central Pacific toward Taiwan. Along the route of northwestward-propagating ISO, during warm years, the 30-60 day convection is active and the TS frequency is also increased. During cold years (Nino3.4 SSTA smaller than 0.8 standard deviation), the ISO signals are weak in the tropics, 30-60 day ISO shows negative variance anomalies in WNPSE region, as well as the TS frequency is decreased.
The spatial distributions of intraseasonal precipitation and vorticity during typhoon season can be adequately simulated by ECHAM4 T106 and ECHAM5/MPI-OM. However, ECHAM4 T106 underestimates the variances of 30-60 day and 10-20 day precipitation. The ECHAM5/MPI-OM produces a realistic intraseasonal precipitation variance. ECHAM4 T106 is able to simulate the eastward-propagating ISO along the equator. However, the northwestward-propagating ISO in ECHAM4 T106 can only propagate to 155°E, 10°N after it migrates from central Pacific. The propagation of 30-60 day ISO simulated by ECHAM5/MPI-OM is similar to observation. It suggests that the air-sea interaction is an important factor for maintaining the northwestward-propagating ISO. The mechanisms responsible for the interannual variability of 30-60 day and 10-20 day ISO might not be the same. Both ECHAM4 T106 and ECHAM5/MPI-OM models underestimate the variance anomalies of 10-20 day ISO.

並列關鍵字

Intraseasonal oscillation during typhoon season ； Tnterannual variability ； High-resolution ； Atmospheric GCM ； Air-sea coupled GCM