By 1940 the U.S. Navy Radio and Sound Laboratory at Point Loma in San Diego, California was working intensively on problems of anti-submarine warfare (ASW). It was a matter of great urgency; German U-boats were taking an increasing toll of allied shipping. The Scripps Institution of Oceanography involvement, starting informally as early as 1940, was part of a rapidly growing effort by the civilian science community to work with the uniformed Navy under the umbrella of the University of California Division of War Research (UCDWR). Collaboration between the civilian oceanographers and the uniformed Navy proved to be extraordinarily productive at Point Loma and elsewhere. The oceans are opaque to light and transparent to sound, and most of the progress in submarine detection had to do with ocean acoustics. The formation of a shadow zone in which submarines could hide, called the "afternoon effect," was the result not of lethargic sonar operators after a heavy luncheon at the Chief's mess, but could be traced to the refraction of sound by surface warming. Noisy shrimp beds could provide a haven for hiding submarines. A false shallow bottom indicated on fathometers at night was immediately identified by Scripps Professor Martin Johnson as related to a deep scattering layer associated with the diurnal migration of copepods.

Munk, W., and D. Day. 2002. Harald U. Sverdrup and the war years. Oceanography 15(4):7–29, https://doi.org/10.5670/oceanog.2002.02.

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