北太平洋上層熱力結構之聖嬰現象三種不同相位變化與其對颱風引起的冷渦之影響探討

Abstract

目前的許多文獻討論了許多年際變化對於海水溫度(SST)的影響，相對來說對於海洋的上層熱力結構的年際變化則比較少詳細探討，所以在本研究中會從1997~2010詳細逐年的討論太平洋上層結構對於三種不同相位的聖嬰現象所產生的環境條件改變。這裡我們利用多重測高衛星的海平面高度異常(SSHA)資料再藉由簡單的兩層海洋模式(two-layer reduced gravity model)推算出26度C等溫線的深度(D26)，而後再搭配由TMI(TRMM Microwave Imager)的海表面溫度測量衛星所得的SST就可知道從26度C向上至海表面的上層海水熱含量，稱為TCHP(Tropical Cyclone heat Potential)，為探討熱帶氣旋加強的重要因子之一，接下來本研究將先討論年際變化對太平洋上層海洋熱力結構的影響，再者分析這些海洋初始條件的改變對颱風所引發冷渦(cold wake)的影響。 上層海洋年際變化的結果顯示主要有三個具有明顯聖嬰現象訊號的海域，分別為西北太平洋(130~180°E ;0~15°N)，中太平洋赤道以東(180°E~140°W ;5°S~5°N)和東太平洋(120~90°W ;0~20°N)地區。對於D26和TCHP來說，在傳統聖嬰年東太平洋(西北太平洋)地區距有明顯的正(負)相位，但是在非傳統聖嬰年時正相位在中太平洋地區比較顯著，在反聖嬰年時則相反，西北太平洋(中太平洋、東太平洋)地區呈現明顯的正(負)相位，就海洋的觀點上這些上層海洋條件(如D26)的年際改變進而對在東太平洋及西北太平洋的颶風或颱風的加強(intensification)具有相當影響。另外因為年際變化而有所改變的海洋上層結構，亦會影響颱風經過海洋時所留下的冷渦的強度以及回復時間，分析2010年三個個案發現對於海洋表層來說其冷渦強度跟海洋初始條件有關，越好的初始場(較深的D26)海表面溫度降溫的作用越小，而回復時間大約是14至30天不等，而海洋上層結構(SSHA,D20,D26,TCHP)的強度與回復時間較不一定，主要與周圍紊流(eddy)的配置和移動速度有關。

In existing literature, there are many studies discussing the impact of inter-annual variability on sea surface temperature (SST). In contrast, much less work has been done explore the impact of inter-annual variability to the subsurface ocean thermal structure. In this study, we aim to explore the impact of the three phases of ENSO on the ocean subsurface thermal structure in the Pacific and western North Pacific ocean. In this study, SSHA (Sea Surface Height Anomaly) data from multiple satellite altimeters are used as input to an 2-layer reduced gravity ocean model to derive the depth of the 26°C isotherm (D26) (Shay et al. 2000; Goni and Trinanes 2003; Pun et al. 2007). The altimetry-derived D26 is then used together with the TRMM (Tropical Rainfall Measuring Mission) and AMSR-E (Adavanced Microwave Scanning Radiometer) satellite-observed SST to estimate the Tropical Cyclone heat Potential (TCHP), an important parameter related to cyclone’s intensification (Shay et al. 2000; Goni and Trinanes 2003; Lin et al. 2005; 2008; 2009a; 2009b). This work first explores the impact of inter-annual variability to the upper ocean thermal structure in the North Pacific. Its subsequent impact on the typhoon-induced cold wake is also explore. The results in the three phases of ENSO of the subsurface ocean thermal structure from 1997 to 2010 show that there are three areas with significant signals: western north pacific(130~180°E;0~15°N), central to east equatorial pacific (180°E~140°W ;5°S~5°N) and eastern pacific (120~90°W ;0~20°N).For D26 and TCHP, there are obviously positive(negative) phase in eastern (western north) pacific in traditional El Nino periods, but the positive phase is significant in central equatorial pacific in non-traditional El Nino rather than eastern, in contrast with the positive (negative) phase in western (eastern and central) pacific in La Nina and for typhoon season (July to September), the above three areas have different characters in typhoon intensification in the view of ocean condition, in the cold(warm) phase year there is a better ocean pre-condition (deeper D26) for typhoon intensification in the western pacific (eastern pacific) basin. The other results for typhoon-induced cold wake strength and recovery show that a better ocean pre-condition (deeper D26) has a less significant SST dropped, and for the three cold wake cases in 2010, the ocean surface recovery (SST) is about 14 days least and 30 days most; the subsurface recovery (SSHA, D20,D26, TCHP) depend on the eddies surrounded the cold wake.