SPECIAL PUBLICATION NO. 43
National Tidal Datum Epoch. northwest winter winds blow the water away from shore, and 2) water contracts as
Mean Sea Level (MSL) the arithmetic mean it cools. He notes that winds are more of hourly heights observed over the National important in shallow water where tide Tidal Datum Epoch. Shorter series are gauges are located, but that contraction specified in the name; e.g., monthly mean becomes important in deeper waters. sea level and yearly mean sea level. Swanson (1974) also notes *... seasonal changes resulting from changes in direct
Mean Tide Level (MTL) a plane midway barometric pressure, steric levels, river between Mean High Water and Mean Low discharge, and wind affect the monthly Water that may also be calculated as the variability." Cole (1997) notes that seasonal arithmetic mean of Mean High Water and variation in tides is usually attributed to two Mean Low Water. MTL and MSL planes harmonic constitutents: one with a period of approximate each other along the open coast one year termed the solar annual tidal (Swanson, 1974, p. 4). constituent, and the other with a period of six months termed the solar semiannual
Mean Low Water (MWL) the average of all constituent. Some consider these to be the low water heights observed over the meteoroligical in nature, rather than National Tidal Datum Epoch. astronomic. However, because the root cause of cyclic seasonal weather is the
Mean Lower Low Water (MLLW) the changing declination of the sun, they should average of the lower low water heights of more nearly be astronomical in origin. each tidal day observed over the National Harmonic analysis of the annual tidal record Tidal Datum Epoch. can easily determine the amplitude and phase of each of these constituents, thereby
Meanastronomicaltideelevationsexhibit providing a mathematical definition of the cyclic seasonal variability (Marmer, 1951; seasonalvariation. (George M. Cole, personal Swanson, 1974; Harris, 1981) and are communications.) Shorter-term changes included in tide predictions. Marmer (1951) occur bi-weekly and monthly; longer-term notes that seasonal variation in terms of changes occur in the relative levels of land monthly mean sea level for the U. S. can be and sea that are of eustatic or isostatic as much as one foot. Based on the many origins (e.g., Embleton, 1982). It is years of monthly data, researchers (Mariner, apparent, therefore, that there is natural 1951; Harris, 1981) note slight variations in variability associated with any average the seasonal cycle from year-to-year, but representation of tidal datums. Given these also recognize the periodicity in peaks and natural insensitivities associated with troughs over the years. For much of our averages, it is important that we do not coast, lower mean sea levels occur during exacerbate them through improper the winter months and higher mean sea manifestations of our own making when levels during the fall. Harris (1981) applying tidal datums as references. inspected the record to determine if storm
and hurricane occurrence was in any way At this point it is necessary to define responsible for the seasonal change, but certain terms. If one is interested in merely found "... no systematic variability". Galvin referencing a vertical distance without a (1988) reports that seasonal mean sea level requirement of spatial comparability, the changes are not completely understood, but result is termed a mnnst appcation. suggests that there appears to be two That is, the result of the application is good primary causes for lower winter mean tide only for that particular location. If, however, levels for the U. S. east coast: 1) strong in addition to a vertical datum, one has a
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