Coastal Climate Extension

In the past two weeks, both North and South Carolina have had significant hurricane anniversaries.  On September 16, 1999, Hurricane Floyd came ashore at Cape Fear, NC, as a category 2 hurricane with winds of 105 miles per hour.  20 years ago tonight – midnight, September 22 - the eye of Hurricane Hugo passed directly over the Isle of Palms, bringing with it category 4 strength winds around 135-140 miles per hour.  So the past couple of weeks have been times of reflection, with those of us who had some experience reliving the tragedy and hoping that the stories instill a dose of caution into all of the folks who have moved to our shores in the last 10-20 years.  But the one thing that has been on everyone’s mind is the all-important question: could it happen again?

Category 3 hurricanes that passed within 75 miles of North Carolina (NOAA CSC, 2009).

The short answer is that yes, if you live on the coast long enough, you will experience a hurricane.  The  Florida State University Center for Ocean-Atmospheric Prediction Studies (COAPS) hosts an online tool called Return Frequency for Hurricane Landfall based on ENSO Phase Based on Landfalling Hurricanes from 1851-2006. It allows you to make your own maps of the frequency with which hurricanes makes landfall in a given region, on average.  For the SC coast, category 1 or higher hurricanes make landfall between once every 17 years at the SC/GA border to  once every 31 years in northern Horry County.  North Carolina sees category 1 hurricanes much more frequently – once every 9 years in the Outer Banks (not surprising, given how far eastward this region extends into the Atlantic Ocean).  Of course, remember that the COAPS tool calculates their period based on where the very center of the storm passes – so even though Currituck County’s return period for category 1 or higher storms is mapped as once every 156 years, we know that this part of the coast feels the direct impacts of hurricanes making landfall to the south much more often than that!

Category 3 hurricanes that passed within 75 miles of South Carolina (NOAA CSC, 2009)

Hurricanes have happened before, and they’ll happen again.  Check out the maps of past category 3 storms, created using the NOAA Coastal Services Center’s Historical Hurricane Tracks viewer (above, and right).  These storms varied in size, so remember that the tracks themselves may not reflect the damage each storm did to either side.  Likewise, remember there are always other factors, too – even though Floyd was a category 2, a combination of the storm’s extremely heavy precipitation and the recent passage of Hurricane Dennis weeks before resulted in catastrophic flooding.

So what happens to hurricanes when the climate changes – will we get more of them?  Will they be stronger?  To be honest, scientists don’t know.  There are a lot of factors on a day to day basis that influence how strong a storm will be and where it will go – but models of climate change deal with much, much larger time scales than a few days.  This makes it challenging, but not impossible, to make some statements about how hurricanes will change in the future.  For a while, scientists thought that hurricanes would be more frequent and more severe, based on having warmer oceans.  However, sea surface temperatures aren’t the only important hurricane ingredient – one that is vital, but not yet easily modeled under climate change, is wind shear, or the differences in speed and direction of winds at different heights in the atmosphere.  Hurricanes are heat engines that transport heat from the surface of the ocean up into the storm.  If the wind shear is too strong, then that engine gets blown off-kilter – the heat transport is interrupted, and more or less, the storm gets blown apart.  Determining how climate change impacts wind shear, then, is crucial.  The most recent findings from several scientists (e.g. Emanuel et al., Knutson et al., Nolan and Rappin), suggests the wind shear will be less favorable for hurricane formation, reducing the frequency of the smaller storms that can’t stand up to it.  Consequently, storms that form could be slightly more intense – think of it as survival of the fittest.  There are still lots of questions to be answered, though, so the jury is still out. 

Bottom line: we don’t know if hurricanes will happen more often in the future.  Storms could be slightly more intense, but even then, we can’t say that human-induced climate change caused a particular stronger storm.  What we can say is that the long scale patterns of climate change may load the dice, changing the odds that the conditions will be more favorable for stronger storms to develop.  Regardless, none of this says anything about where future hurricanes will make landfall.  We can’t predict when the next Hugo or Floyd will come along.  But nature doesn’t schedule its storms based on the hurricane return frequency maps.  Rather, it depends on the day to day, week to week weather conditions.  And if those conditions are right for steering a storm toward the Carolinas, that’s all it takes for us to experience disruption and destruction.  Then, to us, that one roll of the dice will be all that matters.
Want more info? NC Sea Grant’s latest edition of Coastwatch is dedicated to observing the Floyd anniversary. SC Sea Grant’s quarterly publication Coastal Heritage doesn’t address Hugo, but the issue’s focus on adapting to sea level rise is important in an environment that’s also prone to hurricanes!