| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
from Gene Whitt's flying site
..........................................................
The Mean Level of the Sea
Water level measurements at primary tide stations show that
the problem is somewhat more complex than that. For one thing,
mean sea level measured relative to the land - is changing.
Today, global sea level rise is occurring everywhere due to
increases in water volume in the world's oceans. But the chief
reason for the difference in trend shown above lies within
the earth's crust.
Along with most of the U.S. East Coast, falls within a region
where the crust is sinking , adding to the effect of a global
rise in sea level. However, in many parts of the U.S. West Coast,
and especially in Alaska, the crust is actually emerging faster
than the global sea level rise rate.
Tidal Factors
Nothing is going to prevent sea level from changing in response
to these and other factors. However, we can take sea level
averages over several years to obtain a tidal datum - a vertical
reference based on some phase of the tide - to slow the process
if only temporarily. This is a workable idea because, in addition
to sinking crusts and melting ice, tidal variations also have their
effect on sea level.
One such effect is the 18.6-year cycle of the lunar nodes ? a cycle
accompanied by variations in tidal range. Another force for change
is the annual variation in solar declination that modulates solar
heating and density of ocean waters.
To account for both, a 19-year period of water level averaging,
the National Tidal Datum Epoch (NTDE) ? has been established
in the United States. NTDEs have included the years 1924-1942,
1941-1959, 1960-1978, and most recently, 1983-2001. NTDEs
thus are being updated roughly every twenty years.
1) Why nineteen years and not twenty?
"We can take sea level averages over several years to obtain
a tidal datum - a vertical reference based on some phase of
the tide - to slow the process if only temporarily. This is a
workable idea because, in addition to sinking crusts and
melting ice, tidal variations also have their effect on sea level.
One such effect is the 18.6-year cycle of the lunar nodes ;
a cycle accompanied by variations in tidal range. Another
force for change is the annual variation in solar declination
that modulates solar heating and density of ocean waters.
Basic definitions commonly used in the U.S. and its territories:
Mean Sea Level (MSL) Arithmetic mean of hourly water
levels observed during current NTDE.
Mean Higher High Water (MHHW) Mean of higher high water
heights during current NTDE.
Mean High Water (MHW) Mean of all high water heights
observed during current NTDE.
Mean Low Water (MLW) Mean of all low water heights
observed during current NTDE.
Mean Lower Low Water (MLLW) Mean of lower low water
heights during current NTDE.
Mean Tide Level (MTL) ? A datum located midway
between MHW and MLW
All tidal datums have elevations above some arbitrary but
well-protected reference usually the station datum
or staff zero
Tidal Datum Transfers
1) Tidal datum elevations vary from place to place as
dictated by tide wave hydrodynamics
2) Establishing the datum by direct means (19-year
series every 20 years) is an exacting and expensive
operation conducted at a relatively small number of
primary tide stations. Fortunately, there?s an easier
way called simultaneous comparisons.:
At a primary tide station, get the NOS tidal datum
elevations for the current NTDE.
--Obtain a month of tidal observations at station B
matched by simultaneous readings at station A.
--Calculate monthly mean tide level at both stations
--Calculate the monthly mean range and the range
ratio between the stations
--If stations A and B are connected by a tidal
waterway and we assume that both experience
similar monthly deviations from mean tide level.
There are 175 stations all over the world. Here's
a link that allows you to see each station:
http://co-ops.nos.noaa.gov/usmap.html
http://co-ops.nos.noaa.gov is a very cool web site!
Check it out!