Seasonal Sea Level Change

Just like there are daily periodic variations in sea level on the scale of a few hours, there are monthly to seasonal variations that result from changes in alignment of the Earth, moon, and other celestial bodies and changes resulting from the Earth's position relative to the sun. When the gravitational forces of the moon, the sun, and other features align, it is possible to generate higher than normal, or even lower than normal tides depending on alignment.

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Figure 4.5: Image showing movement of the tidal bulges around the Earth in response to the rotation of the earth relative to the moon system. When the moon aligns with other celestial features, even more extreme water levels can result.

Credit: Image courtesy of NOAA

These monthly tides often flood areas with water that, on a normal day, are not wet. Seasonally, water levels can also change depending especially on changes in the prevailing wind direction, and meteorological phenomena - some of which were discussed above, but might also include changes in seasonal runoff, climate oscillations (i.e., El Nino, etc.), and even seasonal changes in the temperature and density of water. On the Mid-Atlantic Coast of the U.S., prevailing wind and coastal currents help to, on average, move water northward along the shore during the summer bringing warmer waters with them, but this pattern often reverses so that wind and water during the rest of the year moves southward along the shore resulting in substantive cooling of coastal waters. These can impact average water levels seasonally in the coastal bays, including the Chesapeake, Delaware, and the myriad of smaller bays along the Delmarva Peninsula, in part because of their geographic orientation, but also due to changes in water density. If you need to reinforce your mental map, a good way to visualize the interaction of different water masses along the east coast is to look at sea-surface temperatures for this region. NASA scientist Dr. Kuring has provided a graphic available at: http://oceanmotion.org/eastern-boundary-sst.htm.

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Figure 4.6: This image shows a strong cold water mass in place from Virginia northward, while a warm water mass (the Gulf Stream) is in place in the south and extends northward. There are many such maps that can show variation on a seasonal basis.

Credit: Image provided courtesy of NASA Goddard Space Flight Center, Dr. Norman Kuring.