Understanding Sea Level

Astronomical tides are fluid motions caused by gravity and the predictable orbits of the Earth, Moon, and Sun. Familiar tides include the diurnal and semi-diurnals (once- and twice-daily) tides influenced by the revolution of the Moon about the Earth. Storm surge refers to the increase in water level due to strong wind blowing over the ocean surface and low pressure allowing the sea to rise up. As their name suggests, storm surges are often caused by passing weather systems such as tropical and extratropical storms like hurricanes and nor’easters. Together, the sum of mean sea level, tide, and surge represent the still water level. Still water level is what you’d experience at the beach on a calm, sunny day without waves. Waves impact water level both through swell generated far off and through wind waves produced more locally. Wave setup refers to the increase in coastal water level above mean sea level, tide, and surge that results from waves breaking offshore. Swash indicates the difference between wave setup and the ultimate level up to which water rushes or washes up onto the beach following breaking onshore waves. Together, wave setup and swash are called wave runup. The sum of the still water level and wave runup represents the total water level.

Tides, storm surges, and waves can negatively influence the coast. It’s partly for this reason that these water-level changes are sometimes called sea-level extremes or sea-level extreme events. Waves, swell, and surge can destroy shoreline defenses and erode coastlines. Changes in the tides can alter the flow of water in harbors and estuaries as well as modify the shape of the seafloor and coastline. Unusually high tides can also cause high-tide floods, which is when the still water level exceeds a threshold related to minor flooding, often overtopping barriers and inundating roadways, with water sometimes bubbling up from storm drains. Since they can happen on calm, sunny days without winds or waves, high-tide floods are also known as sunny day floods, nuisance floods, and “king tides.”

Catastrophic flooding can occur when these forces—mean sea level, tides, storms, and waves—collide. Superstorm Sandy provides a sobering example. The surge from the storm struck New York City at high tide, and the maximum still-water level observed at the southern tip of Manhattan was 9 feet above the normal high-tide level. Had the surge arrive a few hours earlier or later, maximum water levels would have been much lower.

Sea-level extremes have been changing worldwide over the past few decades. The number of high-tide floods each year has increased or accelerated along the U.S. Atlantic Coast. In cities like New York, Norfolk, Virginia, and Galveston, Texas, the number of high-tide flooding days per year has more than doubled over the past 30 years. On the U.S. Pacific Coast, the frequency of high-tide flooding has varied from year to year, partly due to Pacific Ocean climate effects like the El Niño-Southern Oscillation. During El Niños, water temperatures in the Eastern Pacific are warmer and sea levels are higher than usual, making high-tide flooding more probable, while the reverse happens during La Niñas, making high-tide floods less likely. Changes in sea-level extremes and high-tide flooding are often thought to result from changes in mean sea level. For example, mean sea-level rise increases the background level on which tides, storms, and waves act, making it that much easier for an otherwise normal tide, storm, or wave to exceed coastal flood defenses. However, in some cases, changes in extremes and flooding can arise from changes in the tides, which can be due to modifications to harbors or dredging of estuaries, as well as changes in overall storminess. For example, the occurrence of the strongest landfalling hurricanes (Category 4 or 5) in the U.S. has increased over the past four decades, and models predict that hurricanes will even grow stronger in the future due to climate change.

As the ocean warms and ice melts, mean sea level will continue to rise into the future along most coastlines worldwide. The future could also bring stronger storms. Through better knowledge of tides, storms, and waves, and how they interact and influence one another, scientists hope to more confidently anticipate the vulnerability and risk of coastal communities in the face of climate change.