592 KOSSiTER [chap. 16 



to a monthly mean and an annual mean of the order of 0.003 ft and 0.002 ft 

 respectively. If the gauge record can be read to an accuracy of 0.05 ft, random 

 errors in reading the charts will give standard deviations for monthly and 

 annual means of about 0.0019 and 0.0005 ft respectively. Much larger errors 

 can quite easily be introduced, however, through inadequate maintenance of 

 the gauge as, for example, by incorrect setting of the recording pen by 0.05 ft 

 for a few months at a time. It therefore appears that, unless the most careful 

 attention is paid to the maintenance of tide gauges and the reduction of their 

 records, monthly and annual means can only be considered accurate to about 

 0.01 ft (3 mm) and 0.003 ft (1 mm) respectively. 



3. Causes of Variations in Sea-Level 



By definition sea-level is the height of the boundary between sea and air, 

 measured relative to a fixed point (bench mark) on land, and hence is susceptible 

 to changes with time in 



A. The distribution of oceanographical factors, 



B. The distribution of climatological factors, 



C. The distance of the fixed point from the centre of the earth. 



To these must be added 



D. The long-period astronomical tides. 



Let us consider these four main causes separately, where possible. 



A. Oceanographical Factors 



It is well known that surface gradients must exist in all bodies of water on a 

 rotating earth as a result of the fundamental dynamical relationships existing 

 between gradient currents, density currents and isobaric surfaces, but the time 

 variations of these phenomena in the open oceans cannot be studied in much 

 detail in the absence of adequate observations of current, temperature and 

 salinity in depth and in time. Only on continental shelves and in marginal seas 

 does this seem feasible at present. 



Nevertheless, within recent years much valuable work has been done on 

 seasonal variations in sea-level due to time changes in temperature and salinity, 

 by comparing observed values with computed "steric" levels. Steric changes 

 are defined as those resulting from changes in density of a water column without 

 change in its mass and are produced, for example, when wind-driven water of 

 low density replaces an equal weight of denser water. This is essentially a 

 hydrostatic concept, with the underlying assumption that a reference level 

 exists in the water at which density is independent of time, since the mathe- 

 matical equation used is 



Aa dp, 

 'Pa 



where z is the steric level, pa the atmospheric pressure, po the pressure at the 



1 p 



~ 9 Jp 



