SURFACE WATERS OF THE NORTH ATLANTIC OCEAN. 
101 
rises and falls quickly in spring and autumn, and the changes of temperature are 
slow near the minimum and maximum. The waters in areas enclosed l)y land are 
abnormally warmed in summer, forming a Sprungschich t, and cold water tends to 
sink below the surface in winter on account of its greater specific gravity. 
Salinity has practically no seasonal variation (38), except perhaps in regions of 
permanent winds, where the evaporation is great, as in the region of the Trades, and 
possibly also on the north side of the Atlantic anticyclone. Thus while changes of 
temperature of water are due both to actual warming or cooling and to admixture 
with other water, changes of salinity are almost wholly due to the latter. 
2. In the North Atlantic surface water in low latitudes is normally warmer and 
salter than water to the north of it or l)elow it; hence an intrusion of water from tlie 
north, or a mixture with waters from l)elow, reduces both temperature and salinity at 
the surface; and a movement of this water northward is indicated by an increase of 
temperature and salinity. This holds good at all seasons, 1)ut it is specially true of 
temperature in summer. 
3. Surface water in high latitudes and near land is normally relatively fresli, on 
account of the large admixture of water derived from the land or from ice in propor¬ 
tion to the amount of evaporation. During autumn, winter, and early spring tlie low 
temperature of this water is strongly marked, l^ut in the hotter months of the year 
this is not so characteristic, e.specially where land influences are strong, either in the 
way of direct heating or addition of large quantities of wai’in land water. Similarly 
the surface layer is normally fresher than that underlying it, colder than it in winter, 
and warmer in summer. 
A southward movement of this water is therefore indicated by the extension of 
relatively low salinity at all seasons, and this is accompanied l)y a fall of temperature, 
which is in general well marked in winter, Init not in summer. It is to he iioted that 
any considerable southward fresliening is a certain indication ofsoutliward m(')vement, 
for the freshening by mixture with underlying layei'S or Ijy heavy rainfall is sliglit, 
even in low latitudes (39). 
4. A mixture of the surface water with the underlying layer produces the same 
apparent effect, in [a) low latitudes as an intrusion of water from the north, and in 
(Jj) high latitudes as an intrusion of water from the south. _ 
5. In the case of wider moving in an easterly or westerly direction, inequalities of 
temperature tend to disa})})ear, tlir(.>ugh prolonged exposure to uniform conditions, 
and such movements can frequeidly Ije traced on the salinity maps after they have- 
ceased to appear on those of tenq^erature. In spring and autumn, when the distril)u- 
tion of temperature on land and .sea tends to great local irregularities of heating 
and cooling, the isothermals give no reliable information al)out such movements. 
Applying these as general principles, and keeping es})ecial]y in mind the constant 
danger of misinterpretation due to No. 4, the rate and amomdof mixing of surface 
and under layers being practically an unknown quantity, tlie following seems a fair 
