Long Slow Ride – Southern Hemisphere Swells

WaveMan’s Notebook

It is that time of year again – the lull between spring nor’easters and late summer tropical storms – when some gentle yet powerful swells from the Southern Hemisphere make the long haul to the Outer Banks.  Let’s have a look at where these ‘grandfather’ waves come from, their journey to the Outer Banks, and how they impact our beaches and summer surfing…

To fully put southern swell into context, one needs an understanding of the Outer Banks wave climate.  The big drivers of our surf-able wavefield are (1) extra-tropical storms, or “nor’easters” that occur during winter and spring, and (2) tropical storms, including Hurricanes, during late summer and fall.  And yes, occasionally one of these storms occurs out of sequence, such as the mega-extratropical storm Nor’Ida during November 2009 and the early Tropical Storms Alberto and Beryl during spring 2012.  In late May through July, however, there is typically a lull during which we have no major storms for a couple of months.  This gives the entire Atlantic wavefield a chance to settle down, and paves the way for some great conditions for practicing the art of riding waves.

fig 1

Figure 1. Webcam image of Southern Hemisphere swell in Duck on 9 June 2009. Note the long distance (wavelength) between wave crests. Photo courtesy of US Army Corps of Engineers Field Research Facility, Duck, NC.

If there are no storms, where do our summer waves come from?  For starters, the semi-permanent Bermuda High, a high-pressure anticyclone which settles over the Atlantic Ocean in the vicinity of Bermuda during the summer, delivers a small but dependable 8-12 second southeast swell to our summer beaches.  At the right spots these waves can shoal up for a fun break that is great for beginner lessons as well as providing fun relaxing rides for old pros.  For more serious summer action, and the primary topic of this notebook entry, we are occasionally visited by some powerful yet gentle 15-18 second swells that make the trip up from far below the equator in the South Atlantic Ocean.  Although they are typically very small in height as they approach the Outer Banks, Southern Hemisphere swells carry an astonishing amount of energy, shoal up nicely and deliver an entirely different surfing experience (Figure 1).

So what is going on ‘down below’ to stir up these swells from afar?  While we are experiencing tranquil summer weather, the South Atlantic Ocean is churning winter in full bloom.  Fierce storms rage across long fetches, typically moving west to east from South America to below the southern tip of Africa.  As these storms are generally moving away from us, and furthermore rotating clockwise (as opposed to counter-clockwise storms in the northern hemisphere), it takes a carefully-positioned monster of a storm to send waves all the way up past South America towards the US east coast.

fig 2

Figure 2. Surface pressure and wind barbs for an intense winter storm below the southern tip of Africa on May 28, 2009. Red polygon shows the generation region for swells heading towards the Outer Banks. Meteorological display courtesy of Oceanweather, Inc.

One such event occurred on 28 May 2009 and is depicted by the surface pressure map of Figure 2.   The tightness of the pressure contours tells us this was a powerful storm.  The wind barbs show that the backside of the storm produced waves headed in our direction.  But how do those waves reach the Outer Banks, some 8000 miles away?  Oceanic storms have a similar effect as a series of rocks dropped in a pond.  Each rock produces a circular ring of waves that expand all the way to the shoreline.  As storms evolve, the circular winds continually push out packets of waves in all directions around the storm.  Like the rock in the pond, waves moving away from a storm continue to travel and expand across ocean basins until distant shorelines are met.  The waves travel along great-circle routes, similar to the route shown in Figure 3.  Note that the swell from this storm, taking 13 days to reach the US east coast, was felt from South Carolina to Maine.

It is interesting to look at how the Southern Hemisphere swells are ‘transformed’ along their long slow expedition.  Due to wave dispersion, wave packets stretch out along their path as they move away from a storm.  This occurs because, in deep water, the longer period waves travel faster than the shorter period waves, producing the clean swell experienced from distant storms.  The result is astonishing; southern ocean storm waves that are generated over the course of a few hours are continually sorted along an 8000 mile journey and result in a clean swell that can last for several days by the time it reaches our shores.

fig 3

Figure 3. Travel route for swells generated by the Southern Hemisphere winter storm on 28 May 2009 and traveling to the US East Coast.

A unique feature of Southern Hemisphere swell is the flux of wave energy, or ‘wave power’ arriving at our shorelines.  The amount of power in a deep water wave is a factor of the wave height squared times the wave period.  Wave period is the time (typically in seconds) between successive wave crests as they pass a fixed point on the ocean surface.  The longer the time between two passing wave crests, the more energy that wave is transporting.  For example, let’s assume we have a 2-ft 8-s period swell from the Bermuda High and a 2-ft 18-s period swell from the Southern Hemisphere.  Although both swells have the same height, the Southern Hemisphere waves carry 10 times more wave power than the Bermuda High waves!  This extra energy flux results is stronger longshore currents, frequent rips, and the movement of sand.  I have observed the beach literally transformed overnight by southern swell; from a smooth flat beach the day before to a highly 3-dimensional scalloped beach the day after.  As far as surfing; well you already know the answer:  Southern swell can deliver some nice long rides with a powerful push.  This unique phenomenon usually only happens once or twice per summer, so check our forecasts and buoy reports, be wary of rips, and have some summer fun on these amazing waves from afar…

For further exploration, try these links:

http://en.wikipedia.org/wiki/Dispersion_(water_waves) http://en.wikipedia.org/wiki/Wave_power

Dr. Jeff Hanson
Kill Devil Hills, NC

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