The sea moves. That will not be news to you. But waves and tides are only the most ephemeral of the sea's dynamic motions. The wind, weather and currents also carve longer-lasting patterns onto the oceans, raising or lowering parts of its surface for days, weeks, years or even millennia. As low-pressure weather systems move over the ocean they suck up a bump in the sea surface, though these broad domes are rarely more than about 30 centimetres high. Strong winds can pile up storm surges of several metres, which can have devastating effects - as Superstorm Sandy and Hurricane Katrina showed.
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