If conditions are right, the high waves produced by a tropical storm can be focused like laser beams, travelling more than 1000 kilometres to hit distant coasts 鈥� though tracing such waves back to their source will be a challenge.
Normally, the waves produced by a tropical storm radiate out like those of a stone dropped in a pond, diminishing as their energy gets spread out over larger circles. But when at the Scripps Institution of Oceanography in California was investigating sea conditions using a computer model of cyclone Mekunu, which formed over the Arabian Sea in May 2018, he saw a pattern he didn鈥檛 expect: waves propagating in straight lines.
They turned out to be a phenomenon that has never been seen, either in real life or in models: circular waves turned into straight parallel beams by an ocean current. Mekunu made landfall on the coast of Oman on 25 May, but those beams barrelled towards the west coast of India and Pakistan under the influence of the monsoon current, which moves across the Arabian Sea.
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According to Sun鈥檚 model, one day later and over 1000 kilometres from the track of the cyclone, those beams brought alternating pulses of waves that were unusually high or extra low, raised or diminished in each case by up to a metre, that hit all along the coast from Karachi in Pakistan to Mumbai in India. Each pulse was separated by about 100 kilometres.
Sun says this happened because the current refracted the waves similar to the way glass refracts light. According to his model, eddies in the monsoon current acted as a fluid lens to gather Mekunu鈥檚 diverging waves into a parallel formation, allowing them to maintain strength over longer distances. He will present the work at the virtual on 4 March.
However, Sun can鈥檛 point to any examples of coastal damage that in hindsight must have been caused by a distant storm. That isn鈥檛 surprising, says at Heriot-Watt University, UK, because of the many uncertainties of real-life weather modelling and prediction. 鈥淭racking down the cause in each case unambiguously would be very difficult.鈥�
That said, Woolf thinks Sun鈥檚 modelling is sound. 鈥淏ut in my opinion the focusing will not be very sharp,鈥� he says. He compares the situation to the difference between a computer model of light waves that suggest an object is in sharp focus, but the same experiment done physically leads to a smeared image. 鈥淚n the case of ocean waves some instabilities may act against sharp focusing,鈥� he says. 鈥淣evertheless, focusing can occur and may be significant.鈥�
