These results show (although they have reference to only one stream) that the general influence of ocean-currents on the distribution of heat over the surface of the globe must be very great. If the quantity of heat transferred from equatorial regions by the Gulf-stream alone is nearly equal to all the heat received from the sun by the arctic regions, then how enormous must be the quantity conveyed from equatorial regions by all the ocean-currents together!
Influence of the Gulf-stream on the Climate of Europe.—In a paper read before the British Association at Exeter, Mr. A. G. Findlay objects to the conclusions at which I have arrived in former papers on the subject, that I have not taken into account the great length of time that the water requires in order to circulate, and the interference it has to encounter in its passage.
The objection is, that a stream so comparatively small as the Gulf-stream, after spreading out over such a large area of the Atlantic, and moving so slowly across to the shores of Europe, losing heat all the way, would not be able to produce any very sensible influence on the climate of Europe.
I am unable to perceive the force of this objection. Why, the very efficiency of the stream as a heating agent necessarily depends upon the slowness of its motion. Did the Gulf-stream move as rapidly along its whole course as it does in the Straits of Florida, it could produce no sensible effect on the climate of Europe. It does not require much consideration to perceive this. (1) If the stream during its course continued narrow, deep, and rapid, it would have little opportunity of losing its heat, and the water would carry back to the tropics the heat which it ought to have given off in the temperate and polar regions. (2) The Gulf-stream does not heat the shores of Europe by direct radiation. Our island, for example, is not heated by radiation from a stream of warm water flowing along its shores. The Gulf-stream heats our island indirectly by heating the winds which blow over it to our shores.
The anti-trades, or upper return-currents, as we have seen, bring no heat from the tropical regions. After traversing some 2,000 miles in a region of extreme cold they descend on the Atlantic as a cold current, and there absorb the heat and moisture which they carry to north-eastern Europe. Those aërial currents derive their heat from the Gulf-stream, or if it is preferred, from the warm water poured into the Atlantic by the Gulf-stream.
How, then, are these winds heated by the warm water? The air is heated in two ways, viz., by direct radiation from the water, and by contact with the water. Now, if the Gulf-stream continued a narrow and deep current during its entire course similar to what it is at the Straits of Florida, it could have little or no opportunity of communicating its heat to the air either by radiation or by contact. If the stream were only about forty or fifty miles in breadth, the aërial particles in their passage across it would not be in contact with the warm water more than an hour or two. Moreover, the number of particles in contact with the water, owing to the narrowness of the stream, would be small, and there would therefore be little opportunity for the air becoming heated by contact. The same also holds true in regard to radiation. The more we widen the stream and increase its area, the more we increase its radiating surface; and the greater the radiating surface, the greater is the quantity of heat thrown off. But this is not all; the number of aërial particles heated by radiation increases in proportion to the area of the radiating surface; consequently, the wider the area over which the waters of the Gulf-stream are spread, the more effectual will the stream be as a heating agent. And, again, in order that a very wide area of the Atlantic may be covered with the warm waters of the stream, slowness of motion is essential.
Mr. Findlay supposes that fully one-half of the Gulf-stream passes into the south-eastern branch, and that it is only the north-eastern branch of the current that can be effectual in raising the temperature of Europe. But it appears to me that it is to this south-eastern portion of the current, and not to the north-eastern, that we, in this country, are chiefly indebted for our heat. The south-west winds, to which we owe our heat, derive their temperature from this south-eastern portion which flows away in the direction of the Azores. The south-west winds which blow over the northern portion of the current which flows past our island up into the arctic seas cannot possibly cross this country, but will go to heat Norway and northern Europe. The north-eastern portion of the stream, no doubt, protects us from the ice of Greenland by warming the north-west winds which come to us from that cold region.
Mr. Buchan, Secretary of the Scottish Meteorological Society, has shown[19] that in a large tract of the Atlantic between latitudes 20° and 40° N., the mean pressure of the atmosphere is greater than in any other place on the globe. To the west of Madeira, between longitude 10° and 40° W., the mean annual pressure amounts to 30·2 inches, while between Iceland and Spitzbergen it is only 29·6, a lower mean pressure than is found in any other place on the northern hemisphere. There must consequently, he concludes, be a general tendency in the air to flow from the former to the latter place along the earth’s surface. Now, the air in moving from the lower to the higher latitudes tends to take a north-easterly direction, and in this case will pass over our island in its course. This region of high pressure, however, is situated in the very path of the south-eastern branch of the Gulf-stream, and consequently the winds blowing therefrom will carry directly to Britain the heat of the Gulf-stream.
As we shall presently see, it is as essential to the heating of our island as to that of the southern portion of Europe, that a very large proportion of the waters of the Gulf-stream should spread over the surface of the Atlantic and never pass up into the arctic regions.
Even according to Mr. Findlay’s own theory, it is to the south-west wind, heated by the warm waters of the Atlantic, that we are indebted for the high temperature of our climate. But he seems to be under the impression that the Atlantic would be able to supply the necessary heat independently of the Gulf-stream. This, it seems to me, is the fundamental error of all those who doubt the efficiency of the stream. It is a mistake, however, into which one is very apt to fall who does not adopt the more rigid method of determining heat-results in absolute measure. When we apply this method, we find that the Atlantic, without the aid of such a current as the Gulf-stream, would be wholly unable to supply the necessary amount of heat to the south-west winds.