次表層溫度異常在熱帶太平洋年代際變率中的作用

Abstract

對於引起熱帶太平洋年代際變化的機制以及相位轉換的過程，至今仍然存在爭議。其中有一種假設認為，由中緯度大氣強迫海洋次表層後的溫度異常及其後續傳至赤道的現象可能是主導機制之一，我們首先運用觀測和再分析數據，證明了北太平洋海洋-大氣耦合造成的年代際次表層鮮度的確能夠傳至接近赤道太平洋區域。具體來說，大氣阿留申低壓驅動的太平洋年代際振盪在中緯度和副熱帶東部太平洋次表層分別產生了正、負異常的鮮度分佈。這些鮮度異常分佈藉由平均加速勢定義的北太平洋路徑向赤道傳播，起源於中緯度東太平洋的正異常經過七年後可到達14°N，而起源於副熱帶東太平洋的負異常經過三年可達10°N。我們的結果證實，北太平洋藉由大氣橋驅動的熱帶-溫帶相互作用以及海洋通道的確可以產生潛在的年代際振盪。其次，我們使用觀測、再分析數據以及模式敏感度實驗，確認了赤道外次表層溫度異常對熱帶太平洋年代際變化的相位轉換有很關鍵的作用，在熱帶太平洋年代際變化的正位相期間，赤道變暖引起了赤道西太平洋的異常大氣響應，導致次表層存在負異常的海溫，這些溫度異常從西北太平洋，沿著北赤道反流路徑向赤道中太平洋方向移動，大約在三年後開始反轉赤道海溫的相位。同時熱帶太平洋年代際變化正相位時也能夠通過熱帶-溫帶相互作用調節數年後黑潮延伸流的變化，藉由北太平洋經向模態的海氣交互作用，加強上述赤道次表層海溫的反應。最終，五年後完全轉成冷相位。這個過程能夠完整的解釋赤道太平洋年代際振盪的機制與相位轉換的過程。

The mechanisms and processes underlying the phase transition of Tropical Pacific Decadal Variability (TPDV) remain incompletely understood. One hypothesis suggests that the equatorward propagation of remotely generated subsurface temperature anomalies and their subsequent equatorial emergence may play a dominant role. In the first part of the thesis, we utilize observation and reanalysis data to demonstrate the existence of a decadal subsurface spiciness (density-compensated temperature and salinity anomalies) mode that is intrinsic to the ocean-atmosphere coupling in the North Pacific. Specifically, the strengthening of the Aleutian Low, which is the dominant atmospheric forcing of the Pacific Decadal Oscillation, drives a dipole pattern of positive and negative spiciness anomalies in the eastern midlatitude and subtropics, respectively. These anomalies then propagate equatorward along the North Pacific pathway defined by the mean acceleration potential. The positive anomaly of midlatitude origin can reach 14°N after seven years of propagation while the negative anomaly of subtropical origin can arrive at 10°N after three years. In addition, a negative anomaly emerges in the midlatitude after the occurrence of the positive signal two years later, then follows the pathway that the positive takes to reach the tropics. Furthermore, we find that equatorial sea surface temperature variability can also contribute to the generation of the subsurface spiciness mode. Our results thus suggest a potential decadal oscillation in the North Pacific involving extratropical-tropical interaction via oceanic tunnel and atmospheric bridge. In the second part of the thesis, we use observation, reanalysis data, and model experiments to identify the key role of off-equatorial subsurface temperature anomalies in the phase reversal of TPDV. During the positive phase of TPDV, equatorial warming induces anomalous atmospheric responses in the off-equatorial western Pacific, leading to negative subsurface temperature anomalies. These temperature anomalies with the center of action residing in the northwestern region then follow the North Equatorial Countercurrent pathway toward the central basin to reverse the sign of the anomalies at the equator around three years later. The TPDV positive phase also possibly modulates the change of the Kuroshio Extension state a few years later through the tropical-extratropical interaction, which subsequently projects on the footprinting of Pacific Meridional Mode to strengthen the abovementioned negative subsurface disturbance 0-12 months before the peak cold phase. Eventually, the cold phase is completely established after five years. The same dynamic is also held with an opposite sign for the reversed phase, leading to a preferred decadal oscillation via off-equatorial subsurface temperature anomalies and extratropical-tropical ocean-atmosphere interaction.