The Circulation Of The Earth, Marine And Atmospheric

( Originally Published 1915 )

ONE of the characteristics of life is a continuous circulation in the body of the living being; animals and plants have such a circulation inseparable from their very existence. If we consider that the Earth "lives" and evolves it also ought to have its own circulation.

We have already seen that an electric circulation exists in the crust and its nucleus, and we have mentioned the convection movements which occur in the liquid superficial mass of its interior magma. We shall find in the course of this chapter that the two media which envelop it, the hydrosphere and the atmosphere, are both the seat of a continuous circulation, the importance of which, from the point of view of the exterior aspect of the terrestrial relief, is very great. We shall first consider the atmospheric circulation and then the oceanic circulation, and we shall see that these two phenomena are connected by such a direct relationship that they are inseparable from each other.

Although the atmosphere does not possess the fixity of composition of a chemical compound, yet, at any rate in its lower layers, the composition is found to be very nearly invariable, viz.: 21% of oxygen, 78% of nitrogen, and i% of argon, besides infinitesimal traces of other rare gases, such as xenon, neon, and krypton and also of hydrogen and helium. This percentage composition is by weight. We speak here only of the simple gases that are chemical elements in the accepted use of the term; that is, chemical elements in the sense in which the word was used prior to the discovery of radioactive phenomena, and in which it is still used to elements in the customary sense of the word, whether they may be transformed into simpler elements by radioactive process or not. Two of the gaseous compounds, carbonic acid and water-vapour, play an important part in the economy of our Earth. In speaking of the first stages of the Earth's existence, we have dwelt on the protective function which they fulfil in regard to the surface temperature of the Earth, constituting a thermal mantle. The remaining ones such as ammonia, nitrogen, and sulphur compounds, ozone, etc., are present in only very small and variable quantities. The lower layers, in contact with the land and the seas, contain almost the same proportion of simple gases. The pro-portion of hydrogen and helium increases with height, and in the highest regions of the terrestrial atmosphere, the little air which remains is composed of 99% of hydrogen and I% of helium.

These gaseous substances are subjected to two forces, first, the centrifugal force, in consequence of the Earth's rotation, and secondly, the Earth's attraction. Furthermore, they are constantly exposed to the thermal action of the solar radiation. Water-vapour is the best absorbent of these rays and so constitutes the principal agent in the heating of the air under the influence of the Sun's rays.

If the terrestrial globe were a surface without relief, wholly covered with a homogeneous sub-stance such, for example, as sand, and if its axis of rotation were perpendicular to the plane of its orbit instead of being inclined, every point on the Earth would be subject to constant temperature conditions, save for the little variations in the distance of the Earth from the Sun in accordance with the law of Kepler. For every place on the globe throughout the entire year the days and nights would have an equal duration of twelve hours. The temperature, which would be at a maximum in the region of the equator, upon which the Sun's rays would fall normally, would diminish regularly up to the poles, where these rays, grazing the surface, would have no heating power at all, since this depends on the sine of the obliquity. Consequently, there would be no seasons, and the different terrestrial climates, that is to say, the sum-total of meteorological conditions at each point of the globe, would vary from one place to another in a continuous way.

But in reality things are not nearly as simple as this. In the first place the axis of rotation of the Earth is not erect; the plane of the terrestrial equator makes an angle of 231/2° with the plane of the ecliptic. From the resultant inclination, and, therefore, the existence of seasons and the inequality of the days and nights, the Earth is divisible into Geographical Zones: first, the Torrid Zone, through the centre of which the equator passes, and which comprises all places between the two tropics, those circles of latitude corresponding to 23 1/2° North and South latitude respectively; secondly, the two Frigid Zones, the centres of which are occupied by the respective poles, comprising the regions between the pole and the latitude circle of 66 1/2 ° North or South respectively; lastly, the two Temperate Zones, which include all the regions between the Frigid Zones and the Torrid Zone.

Another factor making for complexity in the superficial heating of the Earth is the lack of homogeneity of its surface which is covered with water over nearly three-fourths of its area, while the dry part is characterised by a varied relief including mountains and valleys, high plateaux, depressions, and deserts. The nature of the soil, and therefore its absorbing power for heat radiation, varies from one place to another. Consequently, the heating of the atmospheric layer resting upon the soil will thus also vary from place to place.

As regards the oceans, which form the chief part of the Earth's surface, the matter is simpler, for their surface is homogeneous and without relief. The molecules of the fluid masses, water and air, can thus freely obey the laws which govern them that is to say, those of attraction, centrifugal force, and equilibrium of gaseous substances. We should, therefore, find regular atmospheric conditions established over the great oceans, such as the Atlantic, the Indian Ocean, the Southern Seas, and especially, the Pacific. Now this is what actually occurs and we shall commence by saying a few words as to these conditions.

The tropical regions, viz.: those which belong to the Torrid Zone, are those most exposed to the solar radiation, for twice annually the Sun passes through the zenith of each of the places in this zone, at the moment when it is true noon at the place in question. The rays, therefore, fall perpendicularly on to the ground, and so have the maximum possible heating effect. As a consequence, especially along the equator, the atmosphere in contact with a warmer substratum is heated to the greatest degree in its lower layers. Furthermore, it is the lower layers which contain the most water-vapour and dust particles, and so absorb more completely the heat radiated from the Sun. There are, thus, two reasons why the equatorial atmosphere is relatively strongly heated. By means of instruments called bolometers the quantity of heat thus received annually by the equatorial belt has been measured. It has been found that it is sufficient to vaporise a layer of water four metres [12.2 ft.] deep covering the same area. Now meteorologists, on the other hand, have determined by means of observations ex-tending over numerous years the average yearly quantity of rain which falls on the equatorial belt; it is represented by a layer of water two metres [7.5 ft.] in depth. Even admitting that the whole of this water was vaporised by the solar heat, and that none of it soaked into the soil on which it fell, it is obvious that after complete vaporisation there would remain a surplus quantity of heat sufficient to vaporise as much again.

Nothing is ever lost in Nature's admirable economy. This surplus heat is employed for some other purpose, and that purpose is the further heating of the lower layers of the equatorial atmosphere. Thus superheated, and consequently of less density like the air in a fire balloon, these lower layers rise upwards to considerable heights. Also, this process is continuous, since the cause of the convection current, viz.: the equatorial heating, is always continuous. This is the fundamental movement of vertical circulation in the terrestrial atmosphere, and it gives rise to others, for in consequence of the uprising of masses of warm air towards the upper atmosphere, there is a rarefaction near the surface of the ground, with the result that masses of cold, dense air from the Temperate and Frigid regions flow towards the equator to take the place of that which has left it. If the Earth were at rest there would thus be northerly winds in the Northern Hemisphere and southerly winds in the Southern Hemisphere blowing, in the direction of the meridians, towards the equatorial regions.

But the Earth is not at rest; its movement of rotation, as we have seen, produces a deviation of the path of any moving body in the Northern Hemisphere towards the right, and in the Southern Hemisphere towards the left. These winds, which in the imaginary case flowed in the north-south direction, are actually deviated and become north-easterly winds to the north of the equator and south-easterly winds to the south of that line. These are the trade winds, which carried the caravels of Christopher Columbus to the New World. To-day, the track of these regular winds is known; Maury, the father of Oceanography, was the first to draw up monthly maps which indicated to sailors routes shortening by half the duration of long voyages made by sailing vessels.

As regards the ultimate destination of the masses of warm air which leave the equator and rise into the upper atmosphere, they travel towards the poles and gradually sink down as they cool, replacing the air of the Temperate and Frigid regions which has moved towards the equator. These winds are called the anti-trade winds. Their existence is rendered manifest by the movement of the cirrus clouds, which are in the form of delicate filaments and are more elevated than any other kind of cloud formation. Cirri always travel from the south-west to the north-east, in the Northern Hemisphere, driven by the upper returning air currents.

These anti-trade winds, which are also deviated by the Earth's rotation, gradually become westerly winds and finally merge into a current of air turning around the poles, adding their velocity of progression to the velocity which the Earth's rotation would impart to their molecules if they were originally at rest. The masses of air that they displace will thus turn around the poles with a velocity greater than that of the Earth itself, and a considerable centrifugal force is thus produced tending to throw the air outwards from the axis of rotation. Consequently, in the neighbourhood of this that is to say, around the poles, there is a rarefaction or atmospheric depression, this time of mechanical origin.

Thus, there is a thermal depression at the equator and a mechanical one at the poles; between these two minima the principle of continuity necessitates the existence of a maximum of pressure. By calculation, this should occur at latitude 300 North and South, and observation confirms the permanent existence of this high-pressure condition over the oceans in these latitudes.

We, therefore, have a complete circulatory motion of the air masses enveloping the Earth : the direct current from the Pole to the equator along routes inclined to the north-south direction on account of the Earth's rotation, and the return current in the upper regions of the atmosphere from the equator towards the poles.

The trade winds blow continuously, since their cause, the solar heating, is continuous. They gradually produce a similar movement of the molecules of water at the surface of the sea, since neither air nor water being perfect fluids, friction is exercised between their respective molecules; along the equator the molecules of the water being influenced in two ways at the same time, viz.: by the north-east trade winds of the Northern Hemisphere and the south-east trade winds of the Southern Hemisphere, follow the resultant direction of these movements which is from east to west. Limiting ourselves to the current traversing the North Atlantic Ocean, the direction is from the coast of West Africa to that of Brazil. This is the origin of the equatorial current which meets Cape St. Roque and, because of the form of the American coast, there divides into two parts. One branch goes towards the south, which we will pass over for the time being; the other, which we shall follow up, goes northwards along the coast of Guiana.

One portion cf this branch, always composed of very warm water, passes outside the chain of the Antilles and then along the American coast, and being deflected towards the right by the Earth's rotation traverses the Atlantic Ocean in a slant-wise direction from south to north. The other part enters the Gulf of Mexico and accumulates there under the thrust of the water which continues to flow in, the Gulf being almost closed. There it bathes shores heated by the tropical sun, and the temperature of the water consequently in-creases. Under the influence of the mass of water from the equatorial current that is continually entering the bay, this heated water leaves it by the only possible exit, viz.: the Strait of Florida, through which it escapes with a velocity of 4 1/2 knots per hour, or in other words about eight kilometres. It, thus, enters the Atlantic again, and rejoins and reinforces the first northward-moving branch, giving to that current additional mass, greater velocity, and a higher temperature.

This current is called the Gulf-Stream, and it constitutes a river of warm water flowing between two banks of cold water, as Maury has described it. On leaving the Gulf of Mexico, its depth is about 400 metres [1300 ft.] and its breadth 60 kilometres [37 miles]. In the latitude of Cape Hatteras, its depth is not more than 300 metres [1000 ft.], but, on the other hand, it is larger, and the width of its surface reaches 120 kilometres [75 miles]. It supplies 33,000,000 cubic metres [8720 millions of gallons] per second that is to say, 2000 times more water than the Mississippi at its outlet, by means of which the ancient geographers formerly but erroneously sought to ex-plain its origin. These warm waters carry along an enormous quantity of heat. This quantity has been calculated and is expressed by 39,500,-000,000,000,000,000 calories 1 daily. This is equal to the whole of the heat which falls on one of the Frigid Zones during the six months when it is lighted and warmed by the Sun.2

This warm current forms the beginning of an oceanic circulation which completes itself by cold return currents, serving to replace the water which left the equatorial regions when warmed, and which becomes finally cooled near the poles. The most important of these cold currents is that of Labrador, which comes down the Baffin Sea and follows the coast of North America, the climate of which is the result of the Labrador current and is consequently very cold in winter. The current then plunges under the Gulf-Stream and reappears at the surface of the Atlantic again near the coast of Africa, thus favouring the abundance of fish along the African coast in the neighbourhood of the Walfish Bay, by cooling the high temperature of the sea in that part of the Atlantic.

Another cold current also descends along the east coast of Greenland, which is always fringed by pack-ice, rendering it inaccessible. The west coast of Greenland is bathed by a branch current derived from the Gulf-Stream, and is open to sail-ors during several months in the year; the Danes have established settlements on this coast. Floating wood from the tropics reaches as far as Disko Island, plainly showing that a current of equatorial origin has brought it there. It brings to the north-ern regions of the Atlantic, where the waters are always several degrees warmer than those which surround them, an enormous quantity of water-vapour which causes the persistent fogs which occur over Iceland, Newfoundland, and the neighbouring ocean. These fogs lie in the path of transatlantic liners during the winter, and constitute a serious and permanent danger to navigation between Europe and America and also to the fisheries.

Such, in its broad outlines, is the oceanic circulation of the North Atlantic. An analogous case is found in the South Atlantic, which possesses a circulation of a similar kind. In the North Pacific there exists an important current, the Kuro-Siwo, the "black river" of the Japanese, which although less rapid and less warm than the Gulf-Stream presents in its entirety the same characteristics. The South Pacific and the Indian Ocean have also their circulation, that of the Southern Hemisphere being always the inverse sense of rotation to that of the Northern one.

Finally, in the southern seas all the southerly branches of the circulation of the three great oceans, South Pacific, South Atlantic, and Indian Ocean, give a tangential impulsion to the liquid masses, and so thrust them eastwards in a general movement, often accentuated by the anti-trade winds, which in these latitudes are low down and blow also from west to east. The Antarctic Ocean is thus characterised by an exclusively west-east direction both as regards the movement of the water and by the air which surrounds them.

Thus, there is a great superficial oceanic circulation and also, doubtless, a vertical oceanic circulation maintaining an interchange of water between the warm upper layers of equatorial origin and the deep waters, of polar origin, occupying the great depths. Below 6000 metres [3.75 miles] the temperature of the water at the bottom of the seas is always between 0° C. [32° F.] and 1° C. [33.8° F.].

Even the polar regions themselves, where the sea is covered over with ice-fields, are not exempted from the general law of the circulation of water. The currents traversing these regions displace the ice itself and it was by means of this ice-drift that Nansen, voluntarily imprisoning his ship, was able to effect his journey to the neighbourhood of the North Pole. The mountains of floating ice, the icebergs, which are fragments from the glaciers covering the circumpolar lands, are carried by cold currents even as far south as the regions where transatlantic liners cross. They finally disappear in the warm waters of the Gulf-Stream, after constituting a great danger to navigation in the course of their drift.

The temperature difference between equatorial waters and the cold waters of the polar regions also causes a flow of water from the Pole to the Equator, because of the resulting difference of density. In that way is produced a superficial oceanic circulation which in magnitude and direction enhances the circulation that is primarily due to the trade winds. There is, thus, a certain relation between the aerial and the marine circulation. We shall see later that this relation is still more direct, and that marine currents, produced by the movement of regular winds, react in turn upon the aerial currents, and produce, by a wonderful interconnection, the general circulation of the atmosphere, even above the continents.

If we look at a map of ocean currents, it is apparent that the general circuits of oceanic circulation lie around the regions of high pressure which exist over the great oceans in the latitudes of 3°. Above these warm currents are masses of air to which the current communicates both part of its heat and part of its movement.

Let us consider in particular the Gulf-Stream; it engenders above it an aerial "Gulf-Stream" which is warm and humid, being rich in water-vapour, since its elevated temperature enables it to contain a greater quantity. When the marine Gulf-Stream meets the continental plateau, and then the shores of the European continent, it is checked by the obstacle and compelled to alter its route, but the atmospheric stream above it is not so stopped and continues its direction unaltered. The warm and moist air masses constituting it first meet the western shores of Europe and bring to these the warmth which helps to produce their pleasantly temperate climate, and the humidity which leads to their actual rainfall conditions. Always deviated to the right by the Earth's rotation, they condense their vapour over Sweden, Finland, and Russia, the great lakes of which are thus fed, then over the Ural Mountains, and down across the steppes and deserts of Central Asia. The warmth and humidity has been lost in passing over Europe, and it is as dry winds that they pass over Asia in completing their return journey towards the equator. Now when a country is swept by dry winds only, it never rains there, and consequently vegetation cannot flourish. Hence the deserts which mark the route of this atmospheric return current, the desert of Turkestan, the desert of Arabia, and the Sahara Desert. Thus, by a very curious reciprocal effect the Gulf-Stream is indirectly the cause of the desert-forming climates of the Old World.

It also produces many other results. As we have already said, it produces an atmospheric stream over it which passes on and circulates over the land mass of the Old World. But this air current is subject to the laws which govern all gaseous currents. In particular we know that when a chimney is in draught the pressure is always less great in the interior than outside; there is always more or less of a rarefaction in the central line of an air current, and the rarefaction is greater in proportion as the current is more rapid. Thus, all along the course of the atmospheric stream we observe the rotatory storms, known as cyclones or cyclonic depressions, which constitute the storms of wind and rain which come upon us in Europe nearly always from the westwards. It is for this reason that English sailors have called the Gulf-Stream the father of storms. We thus see what action the marine circulation exerts upon the circulation and the vicissitudes of the atmosphere, even over the continents far away from the sea.

What we have described in the North Atlantic region with regard to the Gulf-Stream applies equally to the North Pacific, with the Kuro-Siwo. This marine current determines the formation of an atmospheric current above its tepid waters, which envelops it and travels with it. Just as in the case of the Gulf-Stream, its course is in the same sense of rotation as that of the hands of a watch. Similarly also, in the Southern Hemisphere, the three warm currents give rise to three aerial currents travelling above them, both turning in the contrary direction to the hands of a watch.

The consideration of these atmospheric circuits which constitute the general circulation of the atmosphere is due to a French scientist Maurice de Tastes, whose work has been overlooked, and whose name is not even mentioned in certain treatises on meteorology. This conception more-over did not pretend to give details but only a general impression of the atmospheric circulation. The complete theory, which would enable us to predict, with all their most detailed circumstances, every meteorological phenomena without exception, has yet to be attained, but the outline given by Maurice de Tastes will none the less remain as the first exact general representation of the movements with which the air enveloping us is endowed.

What is noteworthy about this result is that it enables us to predict occurrences which formerly were difficult enough to explain after the event, viz.: the cyclones of the tropical regions the production of which follows naturally from the conception of aerial current circuits. Let us consider in fact the two aerial currents which carry air along, one above the Gulf-Stream in the North Atlantic, the other above Kuro-Siwo in the North Pacific. These two circuits are separated from each other by Texas and the warm lands of North America. At the period of the summer solstice when the solar temperature attains its maximum, this region becomes heated more rapidly than the neighbouring sea. Consequently, a movement of ascension will be produced in the air strata lying above it and thus it will be the seat of a depression. As a result of this depression, the neighbouring air masses move towards the region and the atmospheric circuits, Atlantic and Pacific, separate up to this point, become displaced, and so come into contact with each other. The molecules of air that, in the region between these two aerial cur-rents, are constrained to rotate in a circle in the opposite direction to the hands of a watch, are forced into the same direction of rotation both by the cyclonic movement due to the local depression and by the rotation couple due to the proximity of the portions of the two circuits travelling in opposite directions. A cyclone is thus produced and the phenomenon occurs each time the conditions which give rise to it recur. Consequently, it is in the warm season and in the regions where the two neighbouring circuits can meet that these rotary storms are engendered. We can, therefore, predict that cyclones will be local and seasonal, and observation verifies this exactly. An admirable confirmation of this theory has been afforded by the absence of cyclones in South America in spite of the proximity of the two circuits of the South Atlantic and the South Pacific, for the reason that between the two the Cordillera of the Andes forms an effective barrier. Cyclones are phenomena which do not attain any great heights in our atmosphere ; at 2000 or 3000 metres [i .25 to 2.50 miles], if not quite absent, they are at any rate greatly weakened. Therefore, the Cordillera, the summits of which rise to a height of 6000 and 7000 metres [3.75 to 4.25 miles] and the mean altitude of which in the Argentine region attains and surpasses 3500 metres [2.2 miles] oppose an insurmountable obstacle to the meeting of the Atlantic and the Pacific currents, and, consequently to the formation of cyclones between them.

This double circulation, atmospheric and marine, of which the two manifestations are so directly connected, is completed by a third circulation which is the consequence of the two first, viz.: the fluvial circulation which returns to the sea the waters that the solar heat has removed from it in the form of vapour, and which atmospheric cur-rents assisted by oceanic currents have trans-ported to colder continents, where it has been precipitated as rain or to high mountains where it has been condensed as snow.

Everyone knows how necessary water is. Where there is none, no animal or vegetable life is found. All the water that is indispensable to the existence of organised beings and indispensable also to that industry which is the accompaniment of life, arises from the condensation of atmospheric water-vapour in the form of rain and snow. If the total quantity of rain falling on the land surface in the course of the year were uniformly distributed, and if the land surface were everywhere quite level and so coincided with a surface con-centric with the geoid, the water so falling would, at the end of a year, form a layer 85 centimetres [33.464 in.] in thickness, which implies, considering the area of the continents, an equal quantity of rain occupying a volume of 122,500 cubic kilo-metres [27,400 cub. miles]. If we recall that the volume of the water of all the oceans is about 1300 million cubic kilometres [312 million cub. miles], it follows that the total annual rainfall represents about the eleven-thousandth part of this.

Only a portion of this rainfall is restored to the seas by means of rivers. These, in fact, carry annually 28,000 cubic kilometres [665o cub. miles] of water to the sea, scarcely one quarter of the total water precipitated on the land surface. As regards the rest of the rainfall, part is evaporated, and the remainder absorbed by the ground and by living beings. Rivers actually restore to the oceans only 1/48,000,000 part of the water that the Sun's heat had raised from them by evaporation. This fraction, viz.: the 28,000 cubic kilometres [665o cub. miles], is all that, by the double means of the atmospheric circulation and the pluvial circulation, executes a constant circuit between the seas and continents, taking back to the oceanic reserves, by the operation of gravity, the water they lose by evaporation. Rivers thus play in some measure the part in the Earth's economy that blood-vessels do in the living organism, vessels that lead back to its starting-point the blood which has been taken to all parts of the body and which enables it to live.