海洋羅士培波穿過菲賓律群島的動力過程研究

Abstract

羅斯貝波的出現是由於地球自轉和球形結構。在海洋中，它們由浮力強迫和風應力產生，借助衛星高度計（經過適當濾波後），我們可以在全球海洋中觀測到它們，它們表現為向西傳播的海平面訊號。儘管羅斯貝波在全球海洋中普遍存在，但人們對熱帶地區的羅斯貝波研究最為深入，尤其是在厄爾尼諾-南方濤動（ENSO）相關研究中。 ENSO期間異常的西風會產生赤道外羅斯貝波，以及沿著東邊界反射的沿岸捕獲波。據報道，羅斯貝波會加深（抬升）溫躍層，造成下沉流（上升流），減弱或增強表層地轉流，並導致沿岸海平面上升。在西部邊界，羅斯貝波反射為向東傳播的赤道捕獲波，同時透過印尼貫穿流（ITF）傳播到印度洋。迄今為止，尚未有觀測或建模工作研究羅斯貝波在菲律賓群島的傳播。菲律賓群島地形複雜，擁有眾多海峽/水道和邊緣海，這些水域在連接南海和西北太平洋的該區域表層環流中發揮著至關重要的作用。邊緣海海洋生物多樣性豐富，就魚類物種數量而言，菲律賓被認為是珊瑚三角區（海洋生物多樣性中心）的中心。然而，菲律賓群島正受到氣候變遷的顯著影響，海平面和海表溫度上升導致風暴潮和強烈颱風頻繁。鑑於羅斯貝波在影響海平面變化、水文特徵和表層環流模式以及管理漁業和應對氣候變遷相關風險方面的潛在作用，有必要研究羅斯貝波在菲律賓群島的傳播。本研究假設羅斯貝波能夠穿越菲律賓南部群島，並影響邊緣海的海平面變化、水文特徵、初級生產力和環流。為進行本研究，我們處理並分析了2017年衛星反演的每日海面高度異常（SLA）數據，並輔以驗潮站記錄、衛星反演的葉綠素a濃度、海表溫度（SST）、海表鹽度（SSS）以及三寶顏半島西海岸的季度和月度沙丁魚捕撈量數據。我們採用過程導向的數值模型來研究羅斯貝波的傳播路徑和能量學。透過模型實驗，我們考察了不同情境下的傳播過程。此外，我們還進行了模擬中尺度渦旋（同樣向西傳播）衝擊菲律賓群島的模型實驗，以研究羅斯貝波穿過島嶼的傳播以及可能出現的上升流/下沉流。 2017年經去趨勢處理和30天低通濾波的海平面異常（SLA）數據以及沿岸海平面記錄的分析表明，羅斯貝波可以傳播到蘇拉威西海，然後進入蘇祿海，最終到達南海中部。對葉綠素a、海表溫度（SST）和海表鹽度（SSS）的分析顯示，2017年10月中旬羅斯貝波波峰傳入蘇祿海，增強了初級生產力，表現為從沙巴東北海岸延伸至蘇祿海中部的異常葉綠素a濃度舌狀區域。將營養物質引入蘇祿海中部的可能驅動機制是，與羅斯貝波傳播相關的地轉流將營養物質從沙巴沿岸輸送過來。沙丁魚捕撈量數據不支持2017年10月羅斯貝波傳播期間沙丁魚捕撈量可能增加的觀點。三維原始方程式模型表明，模型海面高度異常（SLA）訊號能夠穿過菲律賓群島傳播至南海中部，其傳播時間與觀測結果一致。在邊緣海域，瞬時能量通量路徑為：從西里伯斯海西邊界出發，經錫布圖海峽，沿蘇祿海西邊界，最終穿越民都洛海峽。在正（負）模型SLA訊號傳播期間，20公尺深度處的相應流速沿蘇祿海西邊界向北（向南），沙巴沿岸溫度相對較低（較高），而蘇祿海的低溫區域則較為局限（分佈廣泛）。模型實驗表明，來自西北太平洋的入射能量約有2%至5%傳播至南海中部。模型結果表明，當中尺度渦旋（反氣旋和氣旋）衝擊菲律賓東部海域時，渦旋會發生形變（即扁平化），並在未穿過島嶼的情況下消散。在氣旋渦旋衝擊菲律賓東部海域期間，觀測到相對較高的（較低的）海面溫度。 本研究首次利用觀測資料揭示了羅斯貝波在菲律賓群島的傳播及其相應的生物學影響。儘管模型配置理想化，但它支持觀測得到的傳播路徑，表明蘇祿海盆地溫度降低，並顯示正（向北）和負（向南）模型海面高度異常（SLA）信號衝擊期間表層流向發生逆轉。這些結果增進了我們對海洋大陸（MC）動力學的理解，表明羅斯貝波可以在菲律賓南部邊緣海域傳播，並提高了人們對其潛在影響的認識。在菲律賓群島，值得注意的潛在影響包括：對蘇祿海漁業的正面影響、沿海洪災、颱風強度增強、風暴潮加劇以及氣候變遷（海平面上升和海表溫度升高）的疊加效應。

Rossby waves exist due to the earth’s rotation and spherical shape. In the oceans, they are generated by buoyancy forcing and wind stress, and thanks to satellite altimeters, they can be observed in the world’s oceans (after applying appropriate filters) as westward-propagating sea level signatures. Despite being ubiquitous in the world’s oceans, they are well studied in the tropics, especially in relation to El Niño-Southern Oscillation (ENSO). The anomalous westerlies during ENSO create extraequatorial Rossby waves, as well as the reflection of coastally trapped waves along the eastern boundary. Rossby waves are reported to deepen (lift) the thermocline and cause downwelling (upwelling), weaken or strengthen surface geostrophic currents, and enhance coastal sea levels. In the western boundary, Rossby waves are reflected as eastward propagating equatorially trapped waves and at the same time transmitted to the Indian Ocean through the Indonesian Throughflow (ITF). To date, no observational or modelling work has been carried out to investigate the transmission of Rossby waves through the Philippine Archipelago. The Philippine Archipelago has a complex topography with several straits/passages and marginal seas which play a vital role in the surface circulation in the region, linking the South China Sea (SCS) and western North Pacific. The marginals seas are high in marine biodiversity, and in terms of number of fish species, the Philippines is considered as the center of the Coral Triangle (center of marine biodiversity). However, the Philippine Archipelago is significantly affected by climate change through rising sea level and sea surface temperature (SST), resulting in high storm surges and intense typhoons. Given the potential role of Rossby waves in affecting the sea level variability, hydrography, and surface circulation pattern, as well as managing fisheries and climate change related risks, there is a need to study the transmission of Rossby waves through the Philippine Archipelago. In this study, it is hypothesized that Rossby waves can transmit through the southern Philippine Archipelago and influence the sea level variability, hydrography, primary productivity, and circulation in the marginal seas. To carry out the study, satellite-derived daily SLA from 2017 were processed and analyzed, supplemented by tide gauge records, as well as satellite-derived chlorophyll a concentration, sea surface temperature (SST), sea surface salinity (SSS), and quarterly and month sardine catch off west coast of Zamboanga Peninsula. A process-oriented numerical model was employed to examine transmission pathways and energetics. Model experiments were performed to examine the transmission process under different scenarios. Furthermore, model experiments simulating mesoscale eddy (which are also westward propagating) impingement on the Philippine Archipelago were performed to examine transmission through islands and possible upwelling/downwelling. Analyses of the detrended and 30-day low-pass filtered sea level anomaly (SLA) data and coastal sea level records from 2017 suggest that Rossby waves can propagate into the Celebes Sea, then Sulu Sea, and eventually reach the central SCS. Analyses of the chlorophyll a, SST, and SSS reveal that the transmission of the Rossby wave crest into the Sulu Sea in mid-October 2017 enhanced primary production, in the form of anomalous tongue of chlorophyll a concentration extending from northeast coast of Sabah to central Sulu Sea. The plausible driving mechanism of introducing nutrients into the central Sulu Sea is the advection of nutrients from the coast of Sabah by the geostrophic current associated with the transmitted Rossby wave. The potential increase in sardine catch during transmission of Rossby wave in October 2017 is not supported by sardine catch data. The three-dimensional primitive equation model showed that model SLA signals can transmit through the Philippine Archipelago to the central SCS, with propagation timings that are consistent with observation. In the marginal seas, the instantaneous energy flux pathway is from the western boundary of the Celebes Sea, through the Sibutu Passage, along the western boundary of the Sulu Sea, and through the Mindoro Strait. During transmission of positive (negative) model SLA signal, associated velocity at 20-m depth is northward (southward) along the western boundary of Sulu Sea, temperature off the coast of Sabah is relatively low (high), while cooler temperature in the Sulu Sea is restricted (widespread). Model experiments reveal that approximately 2−5% of the incident energy from the western North Pacific transmitted into the central SCS. Model results on impingement of mesoscale eddies (anticyclonic and cyclonic) east of the Philippines reveal that eddies are deformed (i.e., flattened) and dissipated without transmission through the islands. Relatively high (low) surface temperature is observed east coast of the Philippines during cyclonic (anticyclonic) eddy impingement. This study reveals, for the first time, the transmission of Rossby waves through the Philippine Archipelago and its corresponding biological consequence using observations. The model, although with idealized configuration, supports the transmission route obtained from observation, suggests cooling of the Sulu Sea basin, and show reversal of surface currents during impingement of positive (northward) and negative (southward) model SLA signal. These results improve our understanding of the maritime continent (MC) dynamics, show that Rossby waves can transmit in the southern Philippine marginal seas, and give awareness about its potential influences. Notable potential influences in the Philippine Archipelago include positive impact to fisheries in the Sulu Sea, coastal flooding, intensification of typhoons, high storm surges, and compounding climate change effects (rising sea level and sea surface temperature).