熱帶太平洋環流的動力: 演化、調節與解釋

Abstract

本研究使用海洋模式資料，分析太平洋低緯度區域洋流的演變過程，並探究其背後機制。由於聖嬰–南方振盪現象(El Niño-Southern Oscillation，ENSO)在太平洋熱帶區域扮演了極其重要的角色，本研究主要著重於ENSO在赤道區域對洋流的影響，以及ENSO和其他氣候因子交互作用之下，熱帶區域洋流所產生的變化。 模式資料顯示，中太平洋聖嬰發生時，赤道地區的溫躍層產生了東西向振盪的變化，且南赤道洋流和赤道潛流明顯減弱，這和前人研究的結果是一致的；東太平洋聖嬰的情況完全不同：赤道區域的溫躍層產生了東西向及南北向的振盪，且自聖嬰的發展期開始，北赤道反流不斷增強，到了成熟期，南赤道洋流不僅未減弱，還持續增強了六個月。本研究進一步發現，風應力旋度透過艾克曼作用而帶動了溫躍層的變化，是造成兩種聖嬰現象之下太平洋環流有如此差異的關鍵。另外，模式資料顯示，中太平洋位於10°N–15°N , 160°E–170°E的風應力旋度場，激發了羅士培波(Rossby waves)，間接影響了北赤道洋流在菲律賓沿岸分支點的變化。1976~1992年，剛從負相位轉為正相位的太平洋十年期振盪(Pacific Decadal Oscillation，PDO)和ENSO勢均力敵，效力中和，使得中太平洋風場失去了激發Rossby waves的能力，間接導致北太平洋分支點緯度偏南；1993~2009年，PDO主導了中太平洋的風場並激發了Rossby waves，使得北太平洋分支點緯度偏北，其效力約為來自ENSO效力的13倍。而在1976年以前，當PDO仍為負相位時，中太平洋的風場由ENSO所主導。本研究發現，唯有PDO和ENSO兩者效力相差甚遠時，方能激發Rossby waves並使得北赤道洋流分支點往北偏移；當PDO初經歷相位轉換，與ENSO效力互相削弱，中太平洋的風場就失去了遠端影響北太平洋分支點的能力。Abstract Outputs from ocean models are adopted to analyze the ocean circulations in the low-latitude Pacific and explain the responsible mechanisms behind them. Due to the prominent role that El Niño-Southern Oscillation (ENSO) plays in the tropical climate system, the analyses focused on ocean circulation patterns under the influence of ENSO in equatorial Pacific and the interaction between ENSO and other decadal variability in the extra-tropical region. Simulation outputs show that results for the central Pacific El Niño (CP-El Niño) corresponded well with previous studies which suggested that thermocline variations in the equatorial Pacific contain an east-west oscillation. The eastern Pacific El Niño (EP-El Niño) experienced an additional north-south seesaw oscillation between approximately 15N° and 15°S. Moreover, the same outputs show that the flow patterns in the CP-El Niño are consistent with previous El Niño studies that both the eastward Equatorial Undercurrent and westward South Equatorial Current (SEC) weaken. On the other hand, the EP-El Niño displays a significantly distinct circulation pattern. The North Equatorial Counter-Current strengthens in the developing phase and persists into the peak of the warm event, while the northern branch of the SEC also intensifies during the mature phase and lasts for about half a year. The South Equatorial Counter-Current strengthens during the decaying phase of EP-El Niño. It is found that the shifting of the wind stress curl associated with the thermocline variability is chiefly responsible for the unique performance of EP-El Niño. Simulated current velocity shows that wind stress curl anomaly (WSCA) in the region of 10°N–15°N and 160°E–170°E generates Rossby waves and affects the North Equatorial Current (NEC) bifurcation along the Philippine coast. From 1976 to 1992, following a regime shift to the positive Pacific Decadal Oscillation (PDO) phase, PDO and ENSO match each other in strength and have a neutralized effect on the WSCA. From 1993 to 2009, WSCA matches PDO well, and its correlation with ENSO is lower. A linear regression model demonstrates that the influence of PDO has nearly thirteen times weight over that of ENSO. Prior to the 1976 regime shift, WSCA is closely related to ENSO from 1961 to 1975, and it does not correlate significantly with PDO. It is suggested that Rossby waves are preferentially generated in either the negative PDO phase when the ENSO signal dominates, or in the positive PDO phase when the ENSO signal is overshadowed. In the phase when the positive PDO counteracts with the ENSO signal, neither ENSO nor PDO has a significant influence on Rossby wave generations through the WSCA.