It’s High Tide Along the East Coast Side

by Bill Chameides | August 3rd, 2009
posted by Erica Rowell (Editor)

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It’s been a very strange summer for East Coasters. Lots of rain and, while the globe has experienced record warm temperatures, it’s been relatively cool on the East Coast. In fact New York City is on track to have the coolest summer on record.

As if that wasn’t enough, in June tides began running anywhere from a half to two feet higher than had been predicted all along the East Coast and nobody knows why.

We tend to think of sea level like the water level in a bathtub, flat and uniform everywhere. And we think of rising sea level just like water rising in a tub with the spigot turned on — the water along the coasts rises evenly just like it rises along the porcelain walls.

It turns out the bathtub analogy is not good for sea level rise. The high tides being experienced by coastal dwellers along our eastern seaboard are a case in point.

Certainly the key determinant in the height of seawater along a given coast is the amount of water in the ocean. Add more water to the ocean, for example by melting glaciers, and sea level will rise and the ocean’s reach inland will increase.

But other factors also come into play. Winds are one such factor. An onshore wind pushes the water onshore, increasing the height of the tides. Offshore winds do just the opposite.

Scientists reportedly suspect changes in off-shore winds might be the cause.

Can Winds Have That Much Effect on Ocean Tides?

You bet. Just ask a surfer. But the effects can go way beyond tides.

Consider the so-called El Nino/Southern Oscillation (ENSO) in the South Pacific. Periodic shifts in wind patterns over the Pacific Ocean help contribute to a sloshing back and forth of the entire ocean. Here we can use a bathtub analogy that works: think of water sloshing back and forth in a tub. The ocean water sloshes in one direction during El Nino and it piles up along the South American coast; it sloshes in the other direction during La Nina and it piles up near Asia and Australia.

The shifts from El Nino to La Nina have far-reaching effects — influencing rainfall patterns and temperatures around the globe.

It has been speculated by some that the current high tide conditions along the East Coast are related to a similar, but not nearly as globally important, phenomenon known as the North Atlantic Oscillation (NAO). During the positive phase of the NAO there is a large difference between high pressure over the Atlantic in the subtropics and low pressure in the polar regions. During the negative phase the pressure difference is relatively small. As the pressure difference between those two regions increases and decreases, air sloshes to the north and then to the south, and the westerly winds over the ocean intensify and weaken.

Is the NAO responsible for the high tides? Maybe. We have been in a negative phase of the NAO since June and this would favor weaker offshore winds and thus higher tides along the coast, but the intensity of this negative phase does not look to be especially unusual so it’s difficult to explain the current tidal situation using the NAO alone.

A more interesting question in my opinion is whether whatever is causing the tidal anomalies is also contributing to the strange weather this summer in the eastern United States.

Sea Level Rise and Global Warming

The high tides along the East Coast provide an object lesson for preparing for the effects of global warming.

In 2007, the Intergovernmental Panel on Climate Change predicted that sea levels will rise an average of seven to 23 inches by the end of the century without fully accounting for ice sheet melting.

More recent estimates that incorporate an ice-sheet thaw tend to predict larger rises: at the upper end of the range, about three feet or more by 2100.

But like this summer’s anomalous high tides in the East, the increase in sea level will not necessarily be uniform around the globe, because along with sea level rise, global warming will cause changes in the atmospheric circulation.

Changes in atmospheric circulation mean changes in winds and thus potentially tidal changes. In fact, climate models predict that one of the globe’s regions that will experience an unusually large rise in sea levels will be the northeastern coast of the United States. Two recent papers out this spring estimate that the Northeast might see an additional rise of about eight inches or 12-20 inches on top of the global average.

And if all this has you confused, try experimenting in the bathtub. It’s scientific, it’s fun, and if you’re grown up, your Mom won’t scold you if you make a mess.

filed under: climate change, El Nino, El Nino-Southern Oscillation, ENSO, faculty, global warming, oceans, science, temperatures
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