Why Bermuda and Hawaii Observe High Waves but Relatively Low Storm Surges

Hurricane Gonzalo slammed Bermuda with damaging winds on Friday and Saturday.  The strong category-2 hurricane generated winds of 110 mph is it hit the island nation. At the L.F. Wade International Airport, sustained winds reached 93 mph, with gusts to 113 mph, as the southern eyewall hit (Masters 2014).

Gonzalo generated massive waves, which were forecast to reach as high as 30-40 ft (9.1 - 12.2 m). However, storm surge levels were relatively low for such a large, intense storm. As of Saturday, the highest storm surge observation available online was a 6-ft (1.83-m) storm tide that inundated portions of L.F. Wade International Airport (See Saturday’s blog post).

Hurricane Ana generated massive waves along south-facing shores of the Hawaiian Islands

Image: http://www.nbcnews.com/watch/nbc-news/surfers-enjoy-hurricane-anas-big-waves-344900675578.

Meanwhile, Hurricane Ana churned in the Pacific this weekend, passing just south of the Hawaiian Islands. The category-1 hurricane produced massive surf, forecast at 12-20 ft (3.88-6.10 m), which was highest along the south-facing shores of O’ahu and Kauai. However, maximum surge levels along south-facing Honolulu harbor remained less than one-half foot high.

Storm surge in south-facing Honolulu harbor remained below one-half foot. This NOAA Tides and Currents graph depicts predicted tide levels in blue and actual water levels in red. Storm surge is the difference between red and blue. 

Source: http://tidesandcurrents.noaa.gov

Why did these two hurricanes generate massive waves, but relatively low storm surge/ storm tide? These coastal profile illustrations provide the answers.

Graphic created by Hal Needham

The image above is a diagram of coastal flooding on mid-oceanic volcanic islands. Although there are many differences between the geology of Bermuda and Hawaii, both island regions trace volcanic origins, as Bermuda sits on the rim of an ancient caldera, and Hawaii owes its origins to a plate moving over a hotspot, which created this island chain. Although the islands have many geological differences, both are isolated island regions, surrounded by relatively deep water.

As strong winds blow over the ocean and push water towards such islands, the energy transported across the ocean remains largely intact until coming close to shore, because the bathymetry, or offshore water depth is relatively deep. This means that waves do not dissipate much energy until they are close to shore, when they finally “feel the bottom” and break. Such coastal areas observe very high waves. However, the deep bathymetry also serves to suppress storm surge levels because underwater currents efficiently redistribute excess water.

This system can be contrasted to shallow river deltas, where offshore bathymetry is shallow (see graphic below). In these areas, hurricanes generate much higher storm surges, but lower waves. Also, the waves tend to start breaking farther from shore, serving to dissipate more wave energy in these areas. Surges are higher in these areas because the shallow bathymetry forms a "small container," so displaced water has nowhere to go but "up." The Mississippi and Louisiana coastline provides a good example of shallow water near a river delta that is vulnerable to hurricanes.

 Graphic created by Hal Needham

The bigger picture of these recent coastal flood events is that coastal profile, such as shape of coastline and bathymetry, greatly affects the height of waves and storm surges. This explains why locations like Bermuda and Hawaii typically observe high waves, but relatively low storm surges.

REFERENCES

Masters, J., 2014: Gonzalo Brushes Newfoundland; Ana Drenching Hawaii. Dr. Jeff Masters Wunderblog. Available on the Web at: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2835.