Surf's up: Learning to forecast the waves

IN THE summer of 1942, Walter Munk went to the beach. It wasn’t a holiday; Munk worked for the Pentagon and he was there to watch American troops practise for an amphibious landing in north-west Africa. The Allies were losing the war, and the invasion would be their attempt to retake the initiative.

The troops were using boats called LCVPs (Landing Craft, Vehicle, Personnel) – smaller versions of the drop-bow boats that would later storm beaches from Normandy to Iwo Jima. They were not the most seaworthy of vessels. “When the waves exceeded five feet, the LCVPs would swamp,” recalls Munk, now emeritus professor at the Scripps Institution of Oceanography in La Jolla, California. “They would call it a day, and wait for another that was a little calmer.”

Munk was concerned. “I went back and learned about waves at the landing beaches in north-west Africa. In the winter they exceeded six feet most of the time. I thought a terrible catastrophe was about to happen.”

His commanding officer dismissed his objections. “They”, the officer said, must have figured this out. Today, the 91-year-old Munk is convinced there never was a “they”.

Everyone knew that waves were generated by distant storms, but no one knew where they came from or why, and no one had tried to make surf forecasts. Munk decided to tackle the problem on his own, puzzling out the physics of how storms generate swells and what happens as they hit a beach after crossing thousands of kilometres of open water. A month later, he took his findings back to his superiors. Again they brushed him off.

Luckily, he was stubborn. Before the war, Munk had been studying oceanography, so he took his concerns to his mentor, Harald Sverdrup, then director of Scripps and widely regarded as the top oceanographer in the US. Previously, the two had worked on anti-submarine warfare, looking for ways to help defend North Atlantic convoys from German U-boats. Munk, however, had been born in Austria and Sverdrup had relatives in German-occupied Norway. Partly because of rumours spread by rival scientists, both had trouble maintaining security clearances in the super-clandestine field of anti-submarine warfare.

“Ironically, the US may have inadvertently won the war by denying [them] clearances and getting them out of anti-submarine warfare and into surf forecasting,” says Peter Neushel, a historian at the University of California, Santa Barbara, and an avid surfer.

Despite his security problems, Sverdrup had the clout to get military chiefs to recognise the need for forecasts. He and Munk developed a model but they still needed to test it. “We were desperate to see whether the method was working,” Munk says.

Then they learned that before the war Pan Am had been flying seaplanes on an Atlantic test route between Bermuda and the Portuguese archipelago of the Azores. On each trip, the pilots had recorded the height of the surf. It was a treasure-trove of data.

Sverdrup and Munk hunted down old weather maps, then plugged the weather data into their model to “hindcast” wave heights for each Pan Am landing. To their joy, they found a close correlation with what the pilots had recorded – with one puzzling exception. “Once in a while, the observations showed a big spike which we missed,” says Munk.

Then they noticed that these spikes occurred at evenly spaced intervals – always on Saturday nights, Munk remembers. “We decided they were related more to Portuguese wine than meteorological conditions.”

Reassured, the oceanographers were able to give the thumbs-up for a landing in north Africa on 8 November 1942. This time the army listened, and 35,000 US troops went ashore in calm surf. “I’d say we were tremendously lucky with not-so-good weather maps and not-so-good beach charts,” says Munk. This time the army listened, and 35,000 US troops went ashore in calm surf.

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