924 BULLETIN OP THE BUREAU OP PISHEEIES 



VERTICAL STABILITY AS AFFECTING THE CIRCULATION OF THE 



GULF 



A clue to the relative strength of vertical currents in different parts of the 

 gulf during the warm months is afforded by the relative degree of vertical stabiUty 

 of the water that opposes them. 



The relationship between vertical circulation and stability is simple. When- 

 ever or wherever the water is so nearly homogeneous as to the density that it has 

 little or no vertical stability (as is the case in the coastwise belt of the Gulf of Maine 

 in winter), vertical mixings or upwellings freely follow the tidal circulation and the 

 disturbing effects which the wind exercises on the surface; but if the superficial 

 stratum be made much lighter than the underlying stata by freshening or by solar 

 warming, it requires a considerable expenditure of force to drive the light surface 

 water down or to bring heavy water up from below. It is conceivable, also, that 

 the column might become so stable as to effectually insulate the deeps from any 

 influence from above. 



The activity of vertical circulation at any time or place in the gulf, therefore, 

 depends on the momentary balance between the mixing tendency of the tides, etc., 

 and the degree of vertical stability by which this is opposed. 



It is important to bear in mind that any given particle of water has no stabilitj'' 

 fer se, but only relative to the water above and below it. It is usual, therefore, 

 in hydrodynamic calculations, to state the stability for strata of convenient thick- 

 ness.^" Being strictly a function of the density of the water, a simple visual measure 

 of its relative value is afforded by the usual curves for density, plotted against depth, 

 remembering that the more the curves depart from the vertical, the higher the stabil- 

 ity, and that it is zero throughout any stratum where the curve is vertical. 



Regional variations in this respect may be represented graphically by plotting 

 the differences in density between the surface and some underlying stratum chosen as 

 a base, as in Figure 185. The greater the difference, the the more stable the water. 



In the Gulf of Maine the tidal currents are strong enough at all depths to effect 

 an active mixing of the water, were they unhindered; and the consumption of slope 

 water that takes place in the inner part of the basin (p. 941), with its constant replen- 

 ishment from offshore, is unmistakable evidence of some interchange between surface 

 and bottom. The prevalence of a decided contrast in saUnity between the superfi- 

 cial and deep strata throughout the year proves this interchange a slow process, how- 

 ever, wherever the water is more than 100 meters or so deep. The limiting factor 

 here is the stability of the water, for the specific gravity of the slope water in the 

 bottom of the gulf is always considerably higher than that of the superficial stratum, 

 even in whiter, when the latter is heaviest and itself has little or no stability. 



The gulf as a whole, then, is always in a state of stable equHibrium, whatever 

 may be the state of the water near its surface; and while not sufliciently so to pre- 

 vent vertical mixing from taking place constantly, we have no record of slope water 

 welling up to the surface from the bottom of the trough, nor is such an event to be 

 expected. 



«' The unit of stability usually employed is the number of surfaces of equal specific volume per 10 meters of depth, repre- 

 sented graphically by vertical Ihies varying in breadth according to the stability of the water in the several strata. (Sandstrom, 

 1919, p. 283.) 



