利用環境噪訊干涉法擷取太平洋亞重力波之經驗格林函數

Abstract

亞重力波是一種海洋重力波，週期較長，範圍從20到300秒，其形成機制是來自於風場動力學、海浪傳播、海岸地形、海床地形和各種波過程之間複雜的非線性相互作用。在這項研究當中，我們應用環境噪訊干涉法分析了在太平洋收集為期十年的Deep-ocean Assessment and Reporting of Tsunami (DART)資料，得到了對應於亞重力波週期的經驗格林函數。經驗格林函數所展示的傳遞行為與理論波的色散現象一致。功率譜密度和頻譜圖分析揭示了北太平洋和東南太平洋站點能量的季節性變化，前者在冬季達到高峰，後者則在夏季達到高峰，與WAVEWATCH III中的亞重力波觀測相似。我們結合Fast Marching Method (FMM)的波傳路徑，徹底探索了亞重力波強度和傳遞方向的季節變化，目的是建立與氣候變遷的潛在的聯繫，並辨認亞重力波的能量來源。我們的結果顯示，在冬季盛行西風加劇的時期，氣旋活動主要是亞重力波的能量來源。我們觀察到亞重力波由西向東傳遞，與以阿留申群島為中心的氣旋活動移動路徑一致。相反地，當盛行西風減弱時，無論在冬季還是夏季，沿岸反射都將成為亞重力波的主要能量來源。

Infra-gravity waves (IGWs), characterized by gravity waves with longer periods ranging from 20 to 300 seconds, originate from the intricate interplay of nonlinear interactions involving wind dynamics, wave dispersion, coastal features, seabed topography, and various wave processes. In this study, we applied ambient noise interferometry to analyze cross-correlation functions (CCFs) derived from a 10-year dataset collected by the Deep-ocean Assessment and Reporting of Tsunami (DART) system in the Pacific Ocean, yielding empirical Green's functions (EGFs) corresponding to IGWs periods. The EGFs demonstrated notable propagating behavior, aligning with empirical wave dispersion relationships. Power Spectral Density (PSD) and spectrogram analysis unveiled seasonal patterns in North and Southeast Pacific Ocean stations, with winter intensity peaking in the former and summer in the latter, resembling IGWs observations from WAVEWATCH III. We combined the ray path from Fast Marching Method (FMM) to thoroughly explore the seasonal variation of IGWs in intensity and propagation direction, aiming to establish potential links to climate changes and to identify the sources of IGWs. Our results reveal that during periods of heightened winter Westerlies, storm activity predominantly fuels the source energy of IGWs. This inference is supported by the west-to-east propagation direction of IGWs along the Aleutian Islands, aligning with the movement of storms. Conversely, when Westerlies weaken, whether in winter or summer, shoreline reflection emerges as the primary energy source for IGWs.