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Why is ocean water so cold in the northern Outer Banks?

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If you’ve been to the Outer Banks beaches to cool off recently, the ocean water may have been colder than expected.

Ocean temperatures at the beaches of Virginia and the Carolinas generally make for pleasant swimming and wading by the end of July. But there is a thorny exception along the northern Outer Banks.

South of Cape Hatteras, the Gulf Stream ocean current keeps the water relatively warm, with a regular supply of tropical water moving in from the southwest.

That current continues to travel northeastward into the Atlantic, even though the North Carolina coastline begins to turn slightly west when traveling north of the Cape Hatteras National Seashore. The result is relatively cooler water at the beaches north of Nags Head.

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The last couple of mornings, the water temperature at the Duck pier has been in the lower 60s. At the same time, Nags Head ocean temperatures were in the 70s, and Oregon Inlet ocean temperatures were in the 80s.

Wind direction also plays a big role in water temperatures this time of year north of Nags Head.

During prolonged periods when the wind is from the south and southwest, the near-surface ocean water along the coast is pulled out to sea. But because Earth is curved, the water is not pulled in the same direction as the wind; it is pulled at an angle of about 45 degrees to the right of the wind direction. This water movement with respect to the coastline is known as Ekman transport, named for the Swedish scientist who first described it in 1905.

To compensate for the near-surface water being pushed away from the coast, deeper water moves upward from below. That water is much colder than the surface water, so the water temperature near the coast drops substantially. Lighter winds or slightly onshore winds stop the process and bring the water temperature back up.

So, when making a guess about the water temperature at the northern Outer Banks, pay attention to the winds. You can find real-time information on tides and water temperatures from the NOAA Tides and Currents page.

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