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天文台进行的海洋声学实验记录了声音从澳大利亚珀斯到其对极百慕大的传播时间。旅行时间为13,382 S，是对1960年海洋温度的测量，在横跨南印度洋、南大西洋和北大西洋的路径上沿着声道轴平均。以前关于这个主题的工作一直集中在确定声音所遵循的声学路径上。声音穿过全球海洋时的水平折射，特别是南极绕极洋流，给出了令人困惑的结果，即声音被非洲大陆阻挡在百慕大群岛。根据声速和水深测量的原始数据，以前的工作确定了两条声学路径，这两条路径需要与南印度洋和巴西北海岸的赫德岛进行声相互作用。从现代的角度来看，这种解释并不令人信服。现代高分辨率海洋模型将被用来：(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，是否可以解释对脚声传播的成功，包括百慕大声波到达模式的细节，以及(B)建立对脚声传播的当今旅行时间。从1960年到现在，旅行时间的变化是半个世纪以来海洋气候变化的一个衡量标准。这样的测量是有价值的，因为声波路径穿越了半个世纪前很少被观测到的南印度洋和大西洋地区。根据最近对海洋变暖的估计，今天的旅行时间预计将比1960年快约10秒。准确描述声传播对于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具有重要意义。具有挑战性的南大洋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保留了几个监测水听器。海洋气候变化的测量是当前海洋学和社会最重要的课题。这项拟议的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量化测量方法。

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.