和風情況下日暖層物理過程的觀測

Abstract

在和風(≤8m/s)及晴朗的天候下，上層海洋吸收短波輻射後可形成日暖層(Diurnal Warm Layer, DWL)，在大洋中通常深達大約10公尺，日暖層的底部為日暖斜溫層(Diurnal thermocline)，在日暖斜溫層之下至混合層底部則稱作餘留層(Remnant layer)。為能更好地了解日暖層，本研究分析了2022年4月在臺灣西南海域及2021年5月在綠島渦流區佈放的船基海氣通量/交換觀測系統(Ship-based Air-sea Flux & Exchange System, SAFE)資料。SAFE搭載1200 kHz都卜勒流剖儀(Acoustic Doppler Current Profiler, ADCP)及約50支溫度計，以0.3-2公尺的高解析度測量上層20公尺海洋的海流和溫度。觀測結果中發現了對稱不穩定性及平流效應等過程，而最值得注意的是兩次實驗中都觀察到日暖層加深階段顯著的等溫線振盪。在小於4 m/s的微弱風速下，綠島迴流(recirculation)中的白天日暖層仍可達到約20公尺，遠超出一般情況約為4公尺內的厚度，分析顯示該處的日暖層加深主要受到迴流內部混合所影響，而非由風應力所驅動，且溫度振盪可能是由近N震盪的內波(Internal waves, IW)所引起，但有些波動具有KH不穩定(Kelvin-Helmholtz instability)的捲成(roll-up)結構。在臺灣西南海域，直接觀測到內波列結構及可能是由風引起的成熟KH波(billow)，其特徵是捲成(roll-up)和一些破碎波組合，其為過去文獻中較罕見之觀測結果。上述兩組實驗的波動都滿足KH不穩定的基本定理。本研究探討了日暖層加深的過程，結果顯示，除了多數文獻探討的大氣因素外，內波及KH不穩定都扮演著關鍵的角色。

Under moderate wind conditions (≤8 m/s) and clear sky, the penetration of insolation into the upper ocean could form a diurnal warm layer (DWL), which can reach ~10 m in the open ocean. To better understand DWL, this study analyzed the data collected by the Ship-based Air-sea Flux & Exchange System (SAFE) deployed in the Green Island wake in May 2021 and in the southwest of Taiwan in April 2022. SAFE has about 50 temperature sensors and a 1200 kHz Acoustic Doppler Current Profiler (ADCP) to measure currents and temperature variations in the upper 20 m ocean with high resolution. The observations revealed processes such as symmetric instability and advection effects, but the most noteworthy finding was the significant temperature oscillation observed during the deepening stage of the DWL in both experiments. Under wind speed ~2 m/s, the DWL in the Green Island recirculation zone can reach about 20 m, suggesting strong wake flow mixing instead of wind-induced mixing, leading to the deepening of DWL. The temperature oscillation was likely caused by near-N internal waves(IWs), with some fluctuations exhibiting a roll-up structure associated with Kelvin-Helmholtz(KH) instability. In the southwest of Taiwan, we directly observed the internal wave train and the mature KH billow (roll-up and breaking) that was likely caused by wind forcing, which is relatively seldom directly observed in previous studies. Both observations of temperature oscillation satisfy the basic theorem of KH instability. Our observations unveil the detailed processes in response to the DWL deepening – in addition to well-known atmospheric factors, internal waves, and KH instability play pivotal roles.