Living With the Sea
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Fall solstice occurred just a few days ago and winter will soon descend upon the island. One of the pleasures of living on Nantucket is the opportunity to become more attuned with the seasons, the night sky, the rise of the sun and moon, and with the tides. A review of some of the celestial and meteorological phenomena we've experience lately seems fitting as the days wind down and darkness creeps earlier and earlier into our daytime schedules.
Whether one is tying up a boat, surf casting, or planning a beach stroll, it pays to be aware of the tides. Tides are the product of the periodic motion of the waters of the sea due to changes in the attractive forces of the moon and sun upon the rotating earth. Water quite literally sloshes back and forth as the earth turns on its axis and a gravitational tug is exerted by the moon and the sun. During the new and full moons, the distance between the high and low tides increases, which means that high tides get higher and low tides get lower. These are called spring tides and they produce on average about twenty percent more amplitude in the high and low tides. On Nantucket, the typical maximum difference between high and low tides is about four feet.
In most parts of the world, the difference between high and low tide amounts to only a few feet. Nevertheless, this change in water level still is enough to be very important in some areas. This can be pretty obvious when attempting to launch a boat at low tide. Because of their effect on navigation, tides have been studied for many years, and today physical oceanographers know quite a bit about their causes and characteristics. As we will learn, sometimes anomalies occur that force us to rethink some of our assumptions and may encourage us to do a better job of recording our local tides.
This past summer you might have noticed that Nantucket experienced higher than normal high tides in June and July. Many of the tidal cycles we saw were of the magnitude one would expect for the winter time, when the earth is tilted closer to the moon. Here at the UMASS Boston Nantucket Field station, we have the opportunity to observe the tides on a daily basis, and I was especially surprised to see the abnormally high tide levels. They were extremely obvious during our late night horseshoe crab inventories. Well, fortunately we were not imagining things; the National Ocean and Atmospheric Administration (NOAA) and many Atlantic coast scientists, harbormasters, and fishermen recorded the same phenomenon. From (www.tidesandcurrents.noaa.gov/press/EastCoastWaterLevelAnomaly.shtml Accessed September 22, 2009):
Starting in early June 2009, observed tides have been increasingly elevated above predicted tidal elevations along the entire U.S. East Coast from Maine to the east coast of Florida. During the period from June 19 through June 24 for instance, these water levels were running between 0.6 to 2.0 feet above normal depending upon location. As of July 1, these anomalies continue, but running lower at 0.3 to 1.0 ft. above normal. It is not unusual for smaller regions and estuaries along the U.S. East Coast to experience this type of anomalous event at this time of year; however the fact that the geographic extent of this event includes the entire East Coast event is anomalous.
NOAA released a technical report (Accessed Sept 27th at www.noaanews.noaa.gov/stories2009/20090831_tides.html) that addresses some of the theories for why this anomaly might have occurred. "'The ocean is dynamic and it's not uncommon to have anomalies,' said Mike Szabados, director of NOAA's Center for Operational Oceanographic Products and Services. 'What made this event unique was its breadth, intensity and duration.'"
The report notes that some of the highest atypical sea levels occurred closer to where the anomaly was thought to have formed in the Mid-Atlantic, where cities like Baltimore, Maryland experienced extreme high tides as much as two feet higher than normal. After observing water levels six inches to two feet higher than originally predicted, NOAA scientists began analyzing data from select tide stations and buoys from Maine to Florida and found that a weakening of the Florida Current Transport—an oceanic current that feeds into the Gulf Stream—in addition to steady and persistent Northeast winds, contributed to this anomaly. Whether this is a decadal or cyclic phenomenon or is somehow related to the El Nino/La Nina cycles remains to be seen. Scientists are certainly going to be paying more attention to the East coast tides.
One of the items mentioned in the report is that the impacts of the event were amplified by the occurrence of a perigean-spring tide, which occurs close to the vernal equinox when the moon is closest to the Earth and its gravitational pull heightens the elevation of the water. The combined effects of this tide with the sea level anomaly produced minor flooding on the coast.
In the language of science, an equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. For the rest of us, it's one of two times a year when the Sun crosses the equator, and the day and night are of approximately equal length. For many people the autumnal equinox is a sign of fall and a harbinger of cool nights and approaching holidays.
"Equinox" means literally, "equal night." As the angle of the earth's inclination toward the sun changes throughout the year, lengthening or shortening the days according to season and hemisphere, there are two times annually when day and night are of roughly equal duration: the spring and autumnal equinoxes. These celestial "tipping points" have been recognized for thousands of years and have given rise to a considerable body of seasonal folklore.
At the autumnal equinox (Sept 22, 2009; 5:18 P.M. EDT), the Sun appears to cross the celestial equator, from north to south; this marks the beginning of autumn in the Northern Hemisphere. The vernal equinox, also known as “the first point of Aries,” is the point at which the Sun appears to cross the celestial equator from south to north. This occurs about March 21, marking the beginning of spring in the Northern Hemisphere. The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 25,800 years. This motion is called the precession of the equinoxes.
Hopefully, now that fall is here, the island can begin to dry out. According to the Northeast Regional Climate Center (NRCC; www.nrcc.cornell.edu/), Massachusetts received 152% of normal rainfall since January, resulting in the third wettest summer in Massachusetts. Although rain can be a damper, it is rarely a significant factor in local tides. Large rainfall events can set up more stratified conditions in our ponds and harbor and is often the culprit in causing huge algal blooms as the excess water washed more material into our ponds. After heavy rainfall, the harbors often must be closed to shell fishing because of a relationship found between this runoff and increased bacterial production. Once in a while, an excess of fresh water can form a lens on salt water if mixing by winds does not occur. That lens can allow algae to grow faster simply through the amplification of sunlight in the water column which is exacerbated by the extra nutrients washed into the harbor.
Our next stop will be winter, when we brace for storms, higher tides, and eroding winds. For now, I am going to enjoy my favorite time on the island and try to remember to pay attention to the constantly changing forces of the land and sea.
