How a phenomenon known as “king tides” is intensifying hurricane flooding
As we sit in the thick of the Atlantic Ocean's hurricane season, each storm brings its own threats to island nations scattered throughout the body of water and cities sitting on the coast. With climate change at least partially to blame for the increase in the number and severity of extreme weather events, it's time to get familiar with some of the effects that may come with storms this year and in the future. Enter: king tides, a natural occurrence that has a hand in determining the intensity of floods.
You may have already heard about king tides in conversations about Hurricane Dorian — the Category 5 storm that devastated the Bahamas earlier this year, doing more than $8 billion in damages and resulted in more than 70 deaths. As the storm approached the coast of Florida, it had lost some of its strength and was downgraded to a Category 3 storm, but still carried the threat of significant damage. It was also accompanied by the threat of king tides. The weather phenomenon requires the perfect conditions to produce, but when the right confluence of events come together, these massive waves can put the coasts at risk of serious damage.
What are king tides?
According to the United States National Oceanic and Atmospheric Administration's (NOAA) National Ocean Service, king tides actually don't have a scientific definition. It's a commonly used non-scientific term that describes exceptionally high tides, especially those created by massive storms and hurricanes. Tides themselves are a completely normal occurrence where waves come in, rolling along the shore, driven by the gravitational pull of the moon and sun as they interact with the earth. Higher tides typically occur during new or full moons. These conditions play a role in creating king tides, which are an extreme version of those typical high tides.
What conditions are required for king tides?
Like standard high tides, king tides usually come to fruition during a new or full moon. What ends up creating this weather event, though, is when the moon is at its perigee. At that point, the moon is the closest it will ever be to the Earth. This event happens once every 28 days, according to NOAA. At that point, the moon's gravitation pull is at its strongest, producing higher tides. But, three or four times per year, the moon reaches its perigee during a new or full moon. The result is something often referred to as a spring tide, which results in a tide that is slightly higher than would usually be experienced on a coast. These spring tides don't refer to the season, though they do typically happen in the spring and the fall. In most cases, spring tides can produce increase tides levels of several inches — perhaps not enough to notice much of a difference under standard circumstances, though they have been known to be associated with coastal flooding in particularly low-lying areas of land. According to the CBC, spring tides can be 20 percent larger than a standard tide.
What is also required to bring about these king tides is the right weather conditions. On the Atlantic coast of the U.S., where these tides present a particular threat, it just so happens that conditions are right during the fall. That is typically when tides are at their highest because water is at its warmest point in the region — in part the same reason that fall is considered hurricane season. These storms and tides both thrive off the warm weather and can essentially feed into one another, creating bigger storms accompanied by higher king tides.
What kind of damage can king tides do?
Like normal tides, king tides present the threat of flooding. When they line up with storms, king tides become particularly dangerous because they can push a lot of water inland in a very short period of time. During Hurricane Dorian, the combination of the storm and king tides produced record levels of flooding in coastal South Florida, according to AccuWeather. Measurements at Virginia Key in Miami saw water reach 2.94 feet above sea level, which was the second-highest water level of the year. Other areas saw water surpass three feet above sea level during that same time period. Even without storms involved, king tides still rise sea levels and cause flooding by producing bigger waves along coastal shorelines.
How does climate change affect king tides?
The good news about king tides is that they are actually a pretty predictable phenomenon. NOAA tracks tides all year long and has a very good idea of when levels will be at their highest, and because the moon moves in such a predictable pattern, it's not hard to figure out when king tides will occur. What makes this weather occurrence more of a threat than it once was is the ongoing effects of climate change.
As mentioned, king tides become more of a threat when timed up with a major storm or hurricane. Climate change and the increasing global temperature increases the likelihood of conditions that produce storms and may increase the severity of them. A 2013 study projected that the Atlantic Ocean could see a 45 to 87 percent increase in the frequency of Category 4 and 5 hurricanes as a result of climate change. There is also the risk of an increase in the number of storms that occur, which would increase the likelihood of a storm timing up perfectly with a king tide to create more devastating effects on the Atlantic coast.
Climate change is also causing the sea levels to rise, which makes the effects of king tides more dangerous and more likely to cause damage. According to NOAA, sea levels are rising at a rate of about one-eighth of an inch per year. The government agency estimates that flooding is estimated to be about 300 to 900 percent more frequent in coastal communities in the U.S. than it was just 50 years ago — a significant problem given that about 40 percent of the country's population lives directly on or near a shoreline. As temperatures continue to increase, storms continue to get more powerful and water levels continue to rise, the natural occurrence that is the king tide will start to present a bigger threat with potentially more devastating results associated with it.