GULF OF MEXICO 



115 



In the above-mentioned article, Grace studied 

 the Ki component and constructed a cotidal chart 

 for that component from which "the motion ap- 

 pears to be that of a wave entering through 

 Florida Channel, progressing round the basin in 

 the positive sense, suffering reflexion at the north- 

 westerly and southerly coasts, and leaving by 

 Yucatan Channel; the time occupied by this pro- 

 gression is about six hours." 



SEA LEVEL 



The term "sea level" is used to designate the 

 level or elevation of the sea from which the tide 

 rises and falls. In other words, it is the level of 

 the sea freed from the rise and fall of the tide and 

 is derived by averaging the hourly heights of the 

 tide for a period of one or more days. It is con- 

 venient, too, to use the expressions daily sea level, 

 montlily sea level, and yearly sea level to denote, 

 respectivelv, the sea level derived by averaging 

 the hourly heights of the tide for the period of a 

 day, month, or year. 



If the hourly heights of the tide at any place 

 are averaged for each day over a number of days, 

 it is found that sea level fluctuat'es from day to 

 day by amounts varying from less than a tenth of 

 a foot to more than a foot. During a single month 

 sea level on one day may differ by several 

 feet from that on another day. These relatively 

 large fluctuations are obviously to be ascribed to 

 the effects of wind and weather which vary from 

 day to day. 



If now we derive sea level for a month, we find 

 that from one month to another sea level may 

 fluctuate by as much as half a foot, and within a 

 year, two monthly values of sea level may differ 

 by a foot. Since the seasonal change in wind 

 and weather is largely periodic, the question arises 

 whether the change in monthly sea level likewise 

 is periodic. On investigation, this is found to be 

 the case, and in figure 32 are shown the average 

 heights of montlily sea level at nine tide stations 

 in the Gulf of Mexico based on three or more 

 years of observations. 



It is customary to explain the annual variation 

 in sea level as due primarily to the effects of wind 

 and weather, but studies giving quantitative rela- 

 tionships in the Gulf of Mexico between the annual 

 variation in sea level on the one hand and wind 

 and weather on the other hand are wanting. In 



259534 O — 54 9 



general, it is seen that throughout the Gulf sea 

 level is low in the first half of the year and high 

 in the autumn. It is of interest to note, too, on 

 the United States coast, the marked increase in 

 the secondary minimum in the summer at the 

 stations west of the Mississippi River. 



The curves of annual variation for Coatzacoal- 

 cos, Progreso, and Habana must be considered as 

 approximate only, being based on a few years of 

 observations. 



In connection with the determination of mean 

 sea level at any place, it is clear that in view of 

 the seasonal variation in sea level, a month of ob- 

 servations can give only an approximate value. 

 Within a year, however, the seasonal variation is 

 eliminated, and the fluctuations due to the dis- 

 turbing effects of wind and weather tend to 

 balance out. In figure 33 are shown the yearly 

 values of sea level at the four stations for which 

 20 years or more of observations are available. 



Disregarding for the moment the dashed-line 

 curve associated with each of the diagrams, it is 

 seen that sea level from one year to another may 

 differ b}' amounts varying from several hundredths 

 of a foot to several tenths of a foot. The larger 

 fluctuations must obviously be ascribed to the 

 disturbing effects of wind and weather which do 

 not repeat themselves exactly from year to year. 

 These fluctuations can be eliminated by smooth- 

 ing by moving means, and a more or less smooth 

 curve derived. In that manner, the dashed-line 

 curve for each of the yearly sea level diagrams was 

 derived. 



For Key West, there appears to have been little 

 change in sea level from 1913 to 1930, after which 

 there appears to be a more or less regular rise 

 amounting to 0.37 foot by 1950, or a little less 

 than 0.02 foot per year. In this connection, it is 

 of interest to note that the change in sea level at 

 this station approximates the change found at the 

 tide stations along the Atlantic coast of the 

 United States from Florida to Massachusetts 

 (Marmer 1949). 



At Cedar Keys the break in the observations 

 from 1925 to 1938 allows only tentative conclu- 

 sions, but the evidence is for little change in sea 

 level between 1915 and 1925 and a gradual rise 

 from 1939 to 1950 of about 0.3 foot. This gives 

 a rise at the rate of a little over 0.02 foot per year 



