利用海氣耦合模式探討太平洋年代際變異

Simulated Pacific Decadal Variability in an Ocean-Atmosphere Coupled Model

江柏君(Po-Chun Chiang)

指導教授 ： 隋中興

國立臺灣大學/理學院/大氣科學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103637

摘要

本研究藉由分析第二代通用地球系統模式(Community Earth System Model Version 2)參與第六次耦合模式比對計畫(Coupled Model Intercomparison Project Phase 6)之前工業化控制組(pre-industrial control)實驗的模擬結果來探討太平洋年代際變異(Pacific decadal variability)的特徵及變異的可能機制。我們首先分析模式資料的太平洋海表面溫度和副熱帶經向環流(subtropical cells)，發現與觀測的熱帶太平洋年代際變異和海洋通道有近似的特徵。接著，我們選擇訊號清楚的主要年代際事件並進行合成分析。合成的熱帶太平洋年代際變異呈現顯著的熱帶特徵，包括在由海表面溫度暖異常所定義之暖相位下，熱帶區域(10°N-10°S)的斜溫層緯向梯度和信風強度減弱，以及太平洋副熱帶經向環流和赤道潛流(equatorial undercurrent)減緩。模擬的副熱帶年代際變異顯示冷海表溫度橫跨熱帶暖海表溫度南北兩側，與聖嬰南方振盪(El Niño-Southern Oscillation)相近。模擬結果除了有比聖嬰南方振盪更寬的經向尺度外，大致與觀測的熱帶太平洋年代際變異一致。而冷相位下的合成特徵與暖相位接近，但符號相反。另外，熱帶太平洋年代際變異展現明顯的南北不對稱，海洋隧道的特徵在南太平洋比較明顯，而北太平洋則有太平洋經向模態(Pacific meridional mode)的特徵。我們更進一步比較數個選取的指數，顯示熱帶太平洋海表面溫度的變異與暖水體積、內部副熱帶經向環流及赤道潛流的傳送呈負相關，而與熱帶風應力旋度呈正相關。這些指數都表現出相近的年代際震盪週期，約15至20年。最後，熱帶太平洋的熱收支分析指出內部的經向熱傳送與垂直熱傳送在暖相位和從暖轉冷相位時排出熱帶太平洋暖水，調控熱帶太平洋年代際變異的相位轉換，而在冷相位和從冷轉暖相位時反向也成立。對此二收支項做尺度分離，發現到皆被年代際異常流所傳送的平均溫度所主導。雖然整體的合成熱帶太平洋年代際變異在熱帶太平洋區域(~10°N-10°S)與聖嬰南方振盪相似，但熱帶太平洋年代際變異的表現比起後者的振盪性更加不規律，且熱帶太平洋年代際變異的相位轉換機制也不如聖嬰南方振盪清晰，暗示非線性熱帶動力與隨機風場強迫的重要性。

關鍵字

太平洋年代際變異 ； 太平洋年代際振盪 ； 熱帶太平洋年代際變異 ； 遙相關 ； 海洋隧道 ； 太平洋副熱帶經向環流 ； 通用地球系統模式 ； 第六次耦合模式比對計畫

並列摘要

In this thesis, we analyze the simulated results of a pre-industrial control experiment (piControl) by the Community Earth System Model Version 2 (CESM2) available from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to study the features of Pacific decadal variability and to explore the possible mechanisms for the variability. First, we analyzed the pattern of Pacific sea surface temperature (SST) and subtropical cells (STCs) in the model that resemble the observed characteristics of tropical Pacific decadal variability (TPDV) and oceanic passageways. Next, we selected major decadal events with clear signals and performed a composite analysis. The composite TPDV shows distinct tropical features with the warm stage characterized by anomalous warm SST, weakened zonal thermocline gradient and trade winds in tropical zone (10°N-10°S) along with weakened Pacific STCs and equatorial undercurrent (EUC). The simulated subtropical decadal variability is similar to El Niño-Southern Oscillation (ENSO) with anomalous cold SST straddling tropical warm SST except with a broader meridional scale than that of ENSO, in general agreement with the observed TPDV. The composite features during the cold stage are similar to that of the warm stage but of opposite sign. The TPDV exhibits clear north-south asymmetry with the oceanic tunnel featuring more dominant in the Southern Pacific, and the Pacific meridional mode (PMM) more evident in the Northern Pacific. A further comparison of selected indices shows that the variation of tropical Pacific SST is negatively correlated with warm water volume, interior STCs and EUC transport, while positively correlated with tropical wind stress curl. These indices all display similar decadal oscillations at periods about 15-20 years. Finally, a heat budget analysis of tropical Pacific indicates that the interior part of meridional heat transport and the vertical heat transport discharge tropical Pacific warm water during the warm and warm-to-cold stages, which modulate the phase transition of TPDV, while the opposite is true in the cold and cold-to-warm stages. A scale separation of the two budget terms shows that both are dominated by transports of mean temperature by anomalous decadal flow. While the overall composite features of TPDV in tropical Pacific (~10°N-10°S) are ENSO-like, yet the TPDV appears more episodic than oscillatory and the phase transition mechanism of TPDV is not as clear as that of ENSO, implying the importance of nonlinear tropical dynamics and stochastic wind forcing.

並列關鍵字

Pacific decadal variability ； PDO ； TPDV ； teleconnection ； oceanic tunnel ； Pacific STCs ； CESM ； CMIP6

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