August 1963 · one swell, followed from station to station
On 1 August 1963 a storm north of New Zealand raised a swell. Instruments on four shores recorded it as it went by. The last was a beach in Alaska, 11,000 kilometres away.
1 August 1963
A cyclone moving east across New Zealand reaches its full strength. Under it, at about 35 degrees south and 175 degrees west, the wind piles the sea into steep, crowded waves of every size, running into one another.
This is where the energy goes in. From here on, nothing adds to it.
Day 1
Once the waves are out from under the wind they sort themselves, because in deep water long waves travel faster than short ones. Waves that pass every 20 seconds carry their energy forward at 56.2 kilometres an hour. Waves 12.5 seconds apart manage 35.1.
The jumble of the storm draws out into smooth, even lines, the longest in front. This is swell.
A wave moves energy, not water. As each crest passes, the water under it goes round in a circle and ends up almost where it began. The circles get smaller with depth and soon fade out.
In swell with a 20-second beat the crests are 625 metres apart and run at 112 kilometres an hour, twice as fast as the energy they belong to. Each crest rises at the back of its group, runs through it and dies out at the front.
Day 1.7 · 2,342 km
The front of the swell reaches Tutuila, in Samoa, 2,342 kilometres from the storm. A pressure recorder on the sea bed registers it.
Here the whole train of waves from a storm like this goes by in about two days.
Summer 1963
The recorder was there because of an experiment. In 1963 Walter Munk and his colleagues at the Scripps Institution of Oceanography set up six stations along a single great circle from New Zealand to Alaska and kept them running for two and a half months, taking a record twice a day.
One of the six was a ship built to tip on end and float upright like a buoy. It stayed vertical in mid-ocean for 28 days.
Day 3.5 · 4,743 km
North of the equator, at Palmyra atoll, the swell is still there, and nobody on the beach could point to it. At these distances its amplitude was between one and ten centimetres.
The instruments measured the sea surface to a hundredth of a centimetre. The swell shows in their records as a peak at one exact beat.
Day 4.8 · 6,516 km
At Honolulu the peak is sharp. From how the beat of the waves changed from day to day, the researchers could work back to when the swell had set out and how far it had come, and the answer matched the storm on the weather charts.
They noted that the same method would have placed Palmyra to within 50 miles if nobody had known where it was.
The North Pacific
This was what the experiment was built to measure. For the long waves, the loss of energy between New Zealand and Alaska was less than two decibels: after crossing the largest ocean on Earth they still carried well over half of what they started with.
They thin out because they spread, not because anything stops them.
Day 8.2 · 11,002 km
The 20-second waves reach Yakutat, on the Gulf of Alaska, a little over eight days after the storm and 11,002 kilometres from it. The station stood a few hundred feet from a straight sand beach.
The shorter waves are still on their way. The train that passed Samoa in two days takes about a week to pass here, and the last of it comes in some 13 days after the storm.
In shallow water the bottom slows the waves down. They crowd together, grow taller and steeper, and fall over.
What breaks on this sand left a storm on the other side of the equator more than a week ago.
30 August 1963
It was not the longest journey of that summer. Several of the swells the stations recorded came from storms in the Indian Ocean, about 180 degrees away, as far as it is possible to be. One began south of Madagascar on 30 August.
In two and a half months the stations followed twelve swells from one end of the line to the other. Once or twice a week, the researchers wrote, a severe storm in the far south sends out a train of waves that can be traced across the entire ocean.
Some of the waves on any open beach began that way.
The storm, the stations and the findings are from the 1966 paper below. The distances are great-circle distances from the centre of the storm given there (35° S, 175° W) to each station. The travel times are not read from the paper: they are calculated from the deep-water rule the authors themselves used to trace swell back to its source, in which a wave’s energy moves at g × period ÷ 4π. They are for waves of one period; a real swell is a spread of periods and arrives over days. The arcs on the globe are drawn from the same rule. The sea and the beach are drawn, not filmed.