Equatorial current

“ And collecting the main points into a few general propositions, it may be laid down, that all steady winds are modified by the rotation of the earth, in such a manner that Equatorial currents of air receive a westerly deflection, and Polar currents an easterly deflection; but latitudinal currents suffer no change. The N.E. and S.E. Trade Winds are steady Polar currents. The Monsoons are alternations of a Polar and an Equatorial current, twice during the year. Therefore they are N.E. and S.W. in the Northern Hemisphere, and S.E. and N.W. in the Southern Hemisphere. ”

“ There is, therefore, a constant tendency with the air that these upper currents carry polarward to be crowded out, so to speak—to slough off and turn back. Thus the upper current is ever ready to supply the trade-winds, as they approach the equator, with air exactly at the right place, and in quantities just sufficient to satisfy the demand.
224. How is it drawn down from above?—This upper air, having supplied the equatorial cloud-ring (§ 514) with vapour for its clouds, and with moisture for its rains, flows off polarward as comparatively dry air. The dryest air is the heaviest. ”

“ The winds which persistently blow from the north-east on the north side of the equator, and from the south-east on the south side, drive the superficial waters onwards, and give rise to converging oceanic currents which unite to form the great westerly equatorial current. ”

“ As the sun advances northward in the spring, the southeast trades creep up toward and across the equator, the volume of that portion of the equatorial current which is diverted to the northward of Cape San Roque is gradually increased, and this increase is soon felt far to the westward, in the Yucatan and Florida straits. Figures fail utterly to give even an approximate idea of the amount of heat thus conveyed from the tropics to the north temperate zone by the ceaseless pulsations of this mighty engine of oceanic circulation. ”

“ Diurnal rotation.—We see by the plate that the two opposing currents of wind called "the trades," are so unequally balanced that the one recedes before the other, and that the current from the southern hemisphere is larger in volume; i. e., it moves a greater zone or belt of air. The south-east trade-winds discharge themselves over the equator—i. e., across a great circle—into the region of equatorial calms, while the north-east trade-winds discharge themselves into the same region over a parallel of latitude, and consequently over a small circle. ”

“ The most conspicuous and important aërial current on the globe is the permanent double vortex playing between the equator and the poles. The heated air of the equatorial belt, uplifted by expansion, overflows beneath the isothermal layer toward the north and south, thereby increasing the pressure in the higher latitudes sufficiently to generate a surface inflow along the earth, and thus maintaining a perpetual closed circulation which is felt all over the globe. ”

“ Polar waters always flow towards the equator, still partaking, however, of the westerly direction unless deflected by local causes; and, being cold, flow beneath the warmer currents, which have been expanded by equatorial heat, until, being warmed, they gradually rise and mingle with the equatorial currents, which, in addition to their westerly motion, also flow towards the Poles, till, becoming cooled, they sink and merge with the Polar waters, thus maintaining a continued circulation. ”

“ Influence of Winds on the Temperature of the Equator.—The distance from the northern to the southern tropics being considerably more than three thousand miles, and the area of the intertropical zone more than one-third the whole area of the globe, it becomes hardly possible for any currents of air to reach the equatorial belt without being previously warmed by contact with the earth or ocean, or by mixture with the heated surface-air which is found in all intertropical and subtropical lands. ”

“ But the large masses of land, of uneven surface, which occur between the tropics, and the consequent inequalities of temperature, check the tendency of the intertropical atmosphere to a regular course, introduce derangement in its movements, so that it is only in the great open seas that the trade-winds are experienced. Still, it has been observed that, in some countries under and near the equator, constant easterly winds are found, which are no doubt identical in their cause with those that distinguish the equatorial regions of the ocean. ”

“ From this equatorial zone, the heart of the globe, come forth two rivers of tepid water that heat the coasts of the north. They are the two currents that issue from the Gulf of Mexico and the Java Sea. Their enormous liquid masses, fleeing ceaselessly from the equator, govern a vast assemblage of water from the poles that comes to occupy their space, and these chilled and fresher currents are constantly precipitating themselves on the electric hearth of the equator that warms and salts them anew, renewing with its systole and diastole the life of the world. ”

“ Croll proves that the greater part of the vapor raised in the equatorial regions falls there as rain; that the upper winds are dry, and gather moisture only when they again reach the earth and become surface-winds in the temperate and arctic zones.
If all currents of the ocean and atmosphere should cease, no heat could be transferred from the equatorial to the arctic regions, and temperatures would depend on the solar heat falling in the respective latitudes. The equatorial and arctic regions would become uninhabitable, the mean temperatures rising to 135° Fahr. ”

“ Without going into meteorological details, some of which have been given in the preceding chapter, the main facts may be explained by the mode in which the great aerial currents are distributed. The trade winds passing over the ocean from north-east to south-west with an oblique tendency towards the equator, become saturated with vapour, and are ready to give out moisture whenever they are forced upwards or in any other way have their temperature lowered. The entire equatorial zone becomes thus charged with vapour-laden air which is the primary necessity of a luxuriant vegetation. ”

“ A strange fact is that when a new period opens, the spots appear first in high northern and southern latitudes, far from the solar equator, and as the period advances they not only increase in number and size, but break out nearer and nearer to the equator, the last spots of a vanishing period sometimes lingering in the equatorial region after the advance-guard of its successor has made its appearance in the high latitudes. Spots are never seen on the equator nor near the poles. ”

“ Since, moreover, the equator enjoys nearly constant insolation, while the polar regions suffer great variations of heat, with the varying altitude of the sun, the equatorial atmosphere is both much hotter and more equable than the poles, and high latitudes generally. 366Thus at the equator the frost level stands constantly at 18,000 feet, while in the middle latitudes it varies greatly in height from season to season. ”

“ Rising up at the equator, it did not leave all its moisture with the cloud-ring, but, retaining a part, conveyed it through the cloud region, above the north-east trades, to this calm belt, where there was a descent and a crossing. The fact that these dry places are all within or on the equatorial side of this calm belt, while countries abounding with rains and well watered with running streams are to be found all along its polar side, is clearly indicative of a crossing. Upon no other supposition can we account for the barrenness on one side, the fertility on the other. ”

“ It can, therefore, hardly be matter of doubt, that great permanent currents, caused by the unequal heating of the equatorial and polar regions, do exist in the higher strata of the atmosphere—an inference which is supported not only by the occurrence of the trade-winds and the monsoon, but by a variety of other facts and observations. ”

“ The heated air from the equator flows at first as an upper current towards the poles, but as it gradually loses its high temperature, it becomes mixed with the currents setting from the poles towards the equator, causing the atmospheric disturbances and variable winds characteristic of the cooler temperate zones. As a rule, bodies of air of different temperatures do not very quickly mix, but tend to arrange themselves in layers or strata in which masses of unequal temperature are superposed. ”

“ This view is not satisfactory, because the nucleus of a depression once formed on the equator, there would be intro-moving masses of air proportioned in violence to the amount of the depression and the steepness of the barometric gradient, down which they rush to reach the point of lowest barometer. The true reason that no great cyclone has ever been formed nearer the equator than the third parallels of latitude appears to be, that the equatorial belt is a belt of calms. ”