Rape r 5 



and expanded with data from other sources 

 verted to centimeters. 



ALL data were con- 



For this rep or t/^ tide stations most representative of open ocean 

 conditions were seLected. ALthough many tide gage stations 

 examined in tnis report are Located on piers in sheltered coastal 

 harbors/' stations subject to highly variable local tidal 

 conditions common to Large river mouths ^such as Astoria^ OR)r 

 large shallow bays (such as Alameda^ CA), and straits or sounds 

 (such as Ketchikan^ AK)/^ were generally not included. Additional 

 criteria for station selection were a Long/^ continuous data 

 records a constant tidal reference datum/^ and an even 

 distribution of stations with distance along the coast. The 

 stations selected and their Locations are shown in Figure 5.1. 



The processes affecting sea Level are complex. In addition to 

 the well-understood tidal or astronomic forces^ sea Level is 

 a f f ect ed by : 



1. Changes in the average density of the water column. 



2. Changes in distribution of atmospheric pressure over the 

 ocean surface (resulting/' in partr in variations in Large 

 scale wind patterns). 



3. Variations in speed of alongshore components of ocean 

 currents. 



4. Changes in total mass of ocean 

 accretion or melting of glaciers. 



water resulting from 



5. Subsidence or emergence of the Land upon which the 

 is loc ated . 



gage 



The relative importance of these processes varies from station to 

 s t at i on. 



For ocean monitoring in support of fisheries assessment/ we are 

 interested in fluctuations with periods of months to years. For 

 this reason the data are presented in terms of monthly means 

 which remove the principal diurnal and semidiurnal periodicities 

 from the data. Fluctuations with periods longer than months or 

 years can be reduced by comparing the data with a 19-yr mean. 

 This compensates for the nodal tide/ which results from the 

 changing declination of the moon over a period of 18.61 years. 

 The nodal tide has a much greater "potential/" or effect/ than do 



'Permanent Service for Mean Sea Level/ Berkenhead/ 

 National Ocean Survey/ NOAA/ Washington/ DC 20852. 



U.K./ 



and 



52 



