OBX Summer Fun Waves – Dr. Jeff Hanson
Wave Man’s Notebook – July 1, 2016
I have been riding my bike to the beach lately. This morning I was greeted with a light offshore (southwest) breeze, near glassy conditions, a clear and cool ocean, a lone osprey quietly searching overhead, a regiment of pelicans skimming the troughs, and playful dolphins just offshore. And there were the summer fun waves – patiently ready for another day of play.
I am talking about the tranquil and faithful little swells that grace our beaches all summer long. They are typically knee to waist high, 8-10 s in period and steadily arrive from the southeast. They are perfect for the kids, adults wanting to learn to surf, and even experienced riders just looking for some easy fun. What is up with these waves? Since the wind is blowing offshore, where do they come from? Let’s explore…
Our summer fun waves are essentially trade-wind generated swell. The easterly trade winds over the ocean are enhanced by an atmospheric pressure gradient set up by the Azores or ‘Bermuda’ High pressure system that settles in the vicinity of Bermuda this time of year. As winds rotate clockwise around high pressure systems in the northern hemisphere, the winds to the southwest of the Bermuda High are directed towards the outer banks and thus send swells our way. The intensity and fetch of the winds are just right for generating the gentle 8-10s period swells that grace our beaches all summer.
Note that OBX summer winds are also part of the Bermuda High system, with predominant wind directions from the southwest, resulting in an offshore flow that is actually opposing the trade swell. These winds also push our warm water offshore to drive upwelling of cooler Mid-Atlantic Bight Water (https://en.wikipedia.org/wiki/Mid-Atlantic_Bight) along the northern beaches. During years that the Bermuda High moves closer to the US we have stronger winds and hence colder water temperatures. Since the cold Mid-Atlantic Bight Water only extends south to the vicinity of Cape Hatteras, water temperatures stay warm down south regardless of the wind conditions.
To better understand the role of trade swell in our overall wave climate, we look to the 35-year wave record collected by the US Army Corps of Engineers Field Research Facility in Duck. Averaging the total wave power observed for each month of the year confirms what locals already know so well – that our overall wave energy is lowest during June through August of each year. In fact, summer trade swell accounts for approximately 10% of our total annual wave power, the rest coming from nor’easters, tropical storms, southern ocean swell, and local weather events. Summer trade swell is occasionally interrupted by extra-tropical or tropical storms and hurricanes. Furthermore the occurrence of southern ocean swells (from below South Africa!) can add a new dimension to your summer rides (see http://obxsurfinfo.wpengine.com/2012/07/long-slow-ride-southern-hemisphere-swells/).
So grab the kids, rent a board, take a lesson, or just go for a day of play in our beautiful summer waves. And don’t mind the dolphin swimming next to you – it also enjoys a little summer fun.
For more information on OBX wave climate have a look at our wave energy study at http://www.waveforcetechnologies.com/assets/Hanson%20and%20Hathaway_WaveHindcastingWorkshop2015.pdf
Dr. Jeff Hanson
WaveForce Technologies / AcuSea
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