熱帶太平洋年際震盪

Abstract

本研究透過分析太平洋年際變異去討論聖嬰現象的相位轉換機制。在聖嬰發展過程中，赤道變異透過和溫躍層深度距平相關的緯向及經向海洋質量傳輸，與離赤道過程密切相關。與熱帶風應力旋度相關的經向海洋傳輸在決定赤道緯向平均溫躍層變化中扮演了很重要的角色，其通過溫躍層反饋過程反轉聖嬰現象的相位。與經向傳輸相比，緯向傳輸則會通過與東太平洋海溫距平相關的平流反饋影響聖嬰現象的發展。 太平洋的經向海洋質量傳輸主要由密度躍層的副熱帶經向環流（pycnocline Subtropical Cells）、海表層艾克曼傳輸（Ekman transport）和西方邊界流所組成。本研究利用1960至2010年期間的SODA海洋同化資料，分析副熱帶經向環流的變化及其與熱帶氣候變化的因果關係。研究結果表明，從南北緯九度輻合進入赤道的副熱帶經向海洋傳輸距平變化，由西半部的經向環流（160°E至130°W）為主導，與赤道海溫距平變化關係非常良好。從副熱帶經向環流、赤道熱含量、風應力旋度以及熱帶海溫的相關分析，可以給定聖嬰現象熱能累積和釋放（recharge-discharge）過程中，累積中（recharging）、最大累積（recharged）、暖海溫相位（warmest SST）、釋放中（discharging）及最大釋放（discharged）五個階段的時距，分別是8、10、2和8個月。聖嬰及反聖嬰事件的合成分析也可以求出與相關分析一致的時距，分別為4至12、6、2和4個月。本研究闡明聖嬰發展中次表層傳輸過程以及各過程中各階段的時距，對於了解聖嬰現象的相位轉換機制和改進模式中太平洋年際變化的模擬有很大的幫助。 除了太平洋經向海洋傳輸會透過次表層過程影響聖嬰現象外，緯向海洋傳輸也會透過平流反饋機制影響聖嬰的發展。本研究發現，在聖嬰現象達到峰值階段後，赤道緯向海洋傳輸的快速反向將迅速的終止聖嬰事件。赤道上的緯向傳輸主要由溫躍層深度的南北曲狀結構所決定。當聖嬰正在發展的時候，赤道上西太平洋及中、東太平洋分別存在相反方向的緯向環流，兩邊的環流都是由赤道緯向風場引發的溫躍層距平所產生，而東邊的緯向環流在此時主要會透過海溫平流的方式使聖嬰增長。當聖嬰在成熟時期，從東邊界反射的溫躍層深度距平訊號，使赤道外的溫躍層深度距平在東太平洋變得更加重要，並且讓溫躍層深度距平的南北曲狀結構反轉。此現象使赤道緯向海洋環流快速從由西向東轉變成由東向西的傳輸，並很快地延伸到整個赤道太平洋範圍。反向的緯向傳輸不只會透過海溫平流的方式去減弱聖嬰現象的強度，更重要的是會透過降低赤道溫躍層本身的東西傾斜率和減弱與風的反饋作用去終止聖嬰事件。該赤道緯向海洋環流反轉的現象不管在暖事件或是冷事件中都會存在，但環流反轉在暖事件中更能有效的減弱聖嬰現象的強度。 進一步將緯向海洋傳輸相關的過程分為低頻聖嬰過程和高頻海洋波動過程。這兩個過程都由溫躍層深度距平的南北曲狀結構所決定，並透過緯向海洋環流在峰值階段後的快速反轉終止聖嬰事件。對於低頻過程而言，緯向傳輸呈現較慢並且為整個太平洋海盆地的演變。在聖嬰（反聖嬰）的發展階段，由西向東（由東向西）的海洋傳輸在中、東部太平洋盛行，並通過緯向傳輸造成的海溫平流增強海溫距平。聖嬰現象達到峰值階段後，由熱能累積和釋放（recharge-discharge）過程導致的反轉緯向傳輸，透過海溫平流減弱了海溫距平。高頻緯向傳輸呈現向東傳的赤道凱爾文波（Kelvin wave）過程。在聖嬰（反聖嬰）發展過程中，明顯的西風爆發（WWBs）/東風爆發（EWSs）發生在北半球夏、秋季，並伴隨沉降（上升）凱爾文波向東傳。當聖嬰現象在北半球冬季達到峰值時，凱爾文波的信號到達太平洋東邊界，並反射為離赤道的羅士比波（Rossby wave），造成緯向傳輸快速的反轉，削弱東太平洋的海溫距平。聖嬰衰減階段的緯向傳輸距平主要由低頻過程所主導；然而，反聖嬰衰減階段則是由低頻和高頻過程共同控制。本研究結果闡明聖嬰現象的相位轉換機制，並針對聖嬰鎖相現象（phase-locking）提供更進一步的討論。

In this study, we investigate the Pacific interannual variability to clarify the phase transitions of El Nino-Southern Oscillation (ENSO) phenomenon. During the ENSO evolution, the equatorial variations are closely connected to off-equatorial processes through zonal and meridional mass transports associated with the thermocline anomalies. The meridional transport associated with tropical wind stress curl plays a role in determining the equatorial zonal-mean thermocline variations which could transit the phase of ENSO via thermocline feedback. Compared with meridional transport, the zonal transport plays an important role in affecting the ENSO evolution through advection feedback associated with SST anomalies in the eastern Pacific. The meridional transport in the Pacific is composed of equatorward geostrophic flow within the interior pycnocline subtropical cells (STCs), surface Ekman transport and western boundary current. The Simple Ocean Data Assimilation (SODA 2.2.4) analysis for the period of 1960-2010 is used to study the Subtropical Cells (STCs) variability and its causal relation with tropical climate variability. Result shows that the interior STCs transport into the equatorial basin through 9°S and 9°N is well connected with equatorial SST (9°S-9°N, 180°-90°W). The highest correlation at interannual timescales is contributed by the western interior STCs transport within 160°E and 130°W. It is known that the ENSO recharge-discharge cycle experiences five stages, i.e., the recharging stage, recharged staged, warmest SST stage, discharging stage and discharged stage. A correlation analysis of interior STCs transport convergence, equatorial WWV, wind stress curl and SST identifies time interval between the five stages, which are 8, 10, 2 and 8 months, respectively. A composite analysis for El Nino and La Nina developing events is also performed. The composited ENSO evolutions are in accordance with the recharge-discharge theory and the corresponding time lags between the above denoted five stages are 4~12, 6, 2, and 4 months. Those results clarify subsurface transport processes and their time intervals, which are useful for refinement of theoretical models and for evaluating couple ocean-atmosphere general circulation model results. Zonal transport affects the ENSO evolution through advection feedback associated with SST anomalies in the eastern Pacific. Our result shows the sudden basin-wide reversal of anomalous equatorial zonal transport above the thermocline at the peaking phase of ENSO triggers rapid termination of ENSO events. The anomalous equatorial zonal transport is controlled by the concavity of anomalous thermocline meridional structure across the equator. During developing phase of ENSO, opposite zonal transport anomalies form in the western-central and central-eastern equatorial Pacific, respectively. Both are driven by the equatorial thermocline anomalies in response to zonal wind anomalies over the western-central equatorial ocean. At this stage, the anomalous zonal transport in the east enhances ENSO growth through zonal SST advection. In the mature phase of ENSO, off-equatorial thermocline depth anomalies become more dominant in the eastern Pacific due to the reflection equatorial signals at the eastern boundary. As a result, the meridional concavity of the thermocline anomalies is reversed in the east. This change reverses zonal transport rapidly in the central-to-eastern equatorial Pacific, joined with the existing reversed zonal transport anomalies further to the west and forms a basin-wide transport reversal throughout the equatorial Pacific. This basin-wide transport reversal weakens the ENSO SST anomalies by reversed advection. More importantly, the reversed zonal transport reduces the existing zonal tilting of equatorial thermocline and weakens its feedback to wind anomalies effectively. Further, the oceanic processes associated with zonal transport are separated into low-frequency ENSO cycle and high-frequency oceanic wave process. Both processes can be represented by the concavity of meridional thermocline anomalies and generate the reversal of equatorial zonal current at the peaking phase which be a trigger to the rapid termination of ENSO events. For low-frequency process, the zonal transport presents slower and basin-wide evolution. During the developing phase of El Nino (La Nina), the eastward (westward) transport prevails in the central-eastern Pacific and enhances the ENSO through zonal SST advection by anomalous zonal current. At the peak of ENSO, a basin-wide reversal of transport resulted from recharge-discharge process is occurred and weakens the SST anomalies through advection damping. The high-frequency zonal transport presents obvious eastward propagation related to the Kelvin wave at equator. The major wester wind bursts (WWBs)/easterly wind surges (EWSs) occur in boreal summer and fall with coincident downwelling (upwelling) Kelvin waves for El Nino (La Nina) events. After the peak of El Nino (La Nina), the signal of Kelvin waves reaches eastern boundary in boreal winter and reflect as off-equator Rossby waves, then the zonal transport just switches from eastward (westward) to westward (eastward). The high-frequency equatorial zonal transport can be definitely represented by equatorial wave dynamics captured by first three EOFs based on high-pass filtered equatorial thermocline. The transport anomaly during decaying phase is more dominated by low-frequency process in El Nino events; however, the transport anomaly is caused by both low- and high-frequency process during La Nina decaying phase. Those results clarify the sudden phase transition of ENSO and provide an additional remark of phase-locking