The temperature of sea water had already been observed by Ellis, in
1749, in the Atlantic, and subsequent expeditions have furnished a
great number of temperature observations in various seas and for
various depths. The diversity of instruments and of methods employed by
the earlier observers, and the faulty methods of recording, have made
the uniform reduction of many of these observations difficult or
impossible. The most complete and valuable collection of these older
observations up to 1868, with an account of the instruments and methods
used by each observer, was published by Prestwich, in 1876, in the
Philosophical Transactions, Vol. 165.
With the advent of the great scientific expeditions, which were
supplied with modern and refined instruments, our knowledge of the
thermal conditions of the sea has progressed immensely, and we are now
able to construct charts of all the oceans, showing the distribution of
the isotherms with considerable accuracy.
The annual average surface temperature has been found higher in the
Indian Ocean than in either the Atlantic or Pacific; the North Atlantic
is slightly warmer than the North Pacific, but the South Pacific is
warmer than the South Atlantic; this holds generally good also for the
temperatures between surface and bottom.
The temperature generally decreases more or less rapidly from the
surface down to about 500 fathoms, at which depth it is quite uniformly
between 39° and 40° F. From that depth it decreases slowly towards the
bottom: in the Polar seas to between 27° and 28° F.; in the middle and
higher latitudes of the northern hemisphere and at depths of 2,000 to
3,000 fathoms, to between 34° and 36° F.; at the equator and in
southern latitudes it remains in the neighborhood of 32° F.
The low temperatures at the bottom are thought to be due to a steady
but slow circulation of water from the Polar seas towards the equator,
and, where the circulation is most free and unobstructed, as in the
South Atlantic, South Pacific and Indian Ocean, the bottom temperature
is slightly lower than in the North Atlantic and North Pacific, both of
which are connected with the Polar Sea by comparatively narrow and
shallow straits.
The theory of this circulation from the Polar seas is greatly
strengthened by the facts appearing from the investigation of the
bathymetric isotherms in inclosed seas, i.e., seas which are separated
from the deep oceans by submarine barriers. In such seas the
temperature decreases slowly from the surface down to the depth of the
barrier, and from there on remains constant to the bottom.
The influence of currents on the surface temperature is very marked,
cold currents bending the isothermal lines towards the equator, and
warm currents bending them towards the poles. The seasonal changes in
surface temperatures are considerable, being the least in the tropical
zones.
In the _Atlantic Ocean_ the maximum surface temperature lies near the
coast of South America, between Para and Cayenne, and another maximum
occurs near the west coast of Africa, between Freetown and Cape Coast
Castle.
The _Pacific Ocean_ shows the peculiarity that the surface temperatures
on the western side are lower than those on the eastern side. Between
45° N. and 45° S. the temperature does not fall below 50°, but between
those parallels and the poles it remains most always below that figure.
The warmest water is found in the _Red Sea_ where the surface
temperature has been recorded as high as 90°. North of the equator the
mean annual temperature is considerably above 80°, but south of it, to
about the parallel of 25°, it varies from 80° to 70°.