Antipodal acoustic propagation and a half-century of ocean warming

1960年3月，哥伦比亚大学拉蒙特地质观测站进行了一项海洋声学实验，记录了声音从澳大利亚珀斯到对映点百慕大的传播时间。传播时间为13382秒，是1960年海洋温度的测量值，它沿声道轴在穿越南印度洋、南大西洋和北大西洋的路径上平均。之前关于这个问题的研究主要集中在确定声音的声路径上。声音在穿越全球海洋，特别是南极绕极流时的水平折射，给出了一个令人困惑的结果，即声音被非洲大陆从百慕大阻挡了。基于声速和测深的原始数据，之前的工作确定了两条声波路径，需要与南印度洋赫德岛和巴西北海岸的声波相互作用。从现代的角度来看，这种解释是没有说服力的。对1960年对映实验结果的解释仍然难以捉摸。现代高分辨率海洋模型将用于(a)确定强烈的小尺度特征，特别是好望角附近的阿古拉斯环，是否可以解释成功的对跖传播，包括声学到达百慕大模式的细节，以及(b)建立对跖声传播的当今传播时间。从1960年到现在旅行时间的变化是半个世纪海洋气候变化的度量。这种测量是有价值的，因为声波路径穿过了半个世纪前观测不足的南印度洋和大西洋区域。根据最近对海洋变暖的估计，今天的旅行时间预计将比1960年快10倍左右。声波传播的准确描述对于诸如从声学上跟踪或观察海洋哺乳动物，或探测秘密核爆炸或常规爆炸等目的都是重要的。南大洋充满挑战的环境与这两个例子特别相关。全面禁止核试验条约组织在印度洋设有几个监测水听器。海洋气候变化的测量是目前海洋学和社会中最重要的课题。这项工作为过去半个世纪海洋温度的净变化提供了一种独特的定量测量方法。

In March 1960, an ocean acoustics experiment conducted by the Lamont Geological Observatory of Columbia University recorded the travel times of sound from Perth, Australia to its antipode at Bermuda. The travel time, 13,382 s, is a measure of the ocean temperature in 1960, averaged along the sound channel axis over a path that traverses the southern Indian Ocean, the South Atlantic, and the North Atlantic. Previous work on this subject has been focused on determining the acoustic path followed by the sound. The horizontal refraction of the sound as it traversed the global ocean, and the Antarctic circumpolar current in particular, gives the perplexing result that the sound was blocked from Bermuda by the African continent. Based on primitive data for sound speed and bathymetry, previous work determined two acoustic paths that required acoustic interactions with Heard Island in the southern Indian Ocean and the north coast of Brazil. Viewed from a modern perspective, that explanation is unconvincing. An explanation for the results of the 1960 antipodal experiment remains elusive.Modern high-resolution ocean models will be employed to (a) determine if intense, small-scale features, Agulhas rings near the Cape of Good Hope in particular, can account for the successful antipodal propagation, including the details of the acoustic arrival pattern at Bermuda, and (b) establish a present-day travel time for the antipodal acoustic propagation. The change in travel time from 1960 to the present day is a measure of a half-century of ocean climate change. Such a measurement is valuable because the acoustic path traversed regions of the southern Indian and Atlantic Oceans which were poorly observed a half-century ago. Based on recent estimates of ocean warming, travel times today are expected to be about 10s faster than in 1960.The accurate description of acoustic propagation is of importance for such purposes as tracking or observing marine mammals acoustically, or for the detection of clandestine nuclear or conventional explosions. The challenging Southern Ocean environment is particularly relevant to both these examples. The Comprehensive Nuclear Test Ban Treaty Organization maintains several monitoring hydrophones in the Indian Ocean. The measurement of oceanic climate change is a subject of primary importance in oceanography and to society at the present time. The proposed work offers a unique quantitative measure of the net change of ocean temperature over the past half-century.