Physical Geography Of The Sea 1855/14
CHAPTER XIV. — THE CLIMATES OF THE OCEAN.
Gulf Stream likened to the Milky Way, — 492. — March and September the hottest Months in the Sea, 496. — How the Isothermal Lines move up and down the Ocean, 498. — A Line of invariable Temperature, 508. — How the western Half of the Atlantic is heated up, 509. — The Relation between a Shore-line in one part of the World and Climates in another, 512. — The Climate of Patagonia, 516. — The Summer of the northern Hemisphere warmer than the Summer of the southern, indicated by the Sea, 521. — How the cold Waters from Davis’s Straits press upon the Gulf Stream, 522. — How the different Isotherms travel from North to South with the Seasons, 523. — The Polar and Equatorial Drift, 524.
492. THERMAL charts, showing the temperature of the surface of the Atlantic Ocean by actual observations made indiscriminately all over it, and at all times of the year, have been published by the National Observatory. The isothermal lines which these charts enable us to draw, and some of which are traced on Plate IV., afford the navigator and the philosopher much valuable and interesting information touching the circulation of the oceanic waters, including the phenomena of the cold and warm sea currents; they also cast light upon the climatology of the sea, its hyctographic peculiarities, and the climatic conditions of various regions of the earth; they show that the profile of the coast-line of intertropical America assists to give expression to the mild climate of Southern Europe; they also increase our knowledge concerning the Gulf Stream, for it enables us to mark out, for the mariner’s guidance, the “Milky Way” in the ocean, the waters of which teem, and sparkle, and glow with life and incipient organisms as they run across the Atlantic. In them are found the clusters and nebulae of the sea which stud and deck the great highway of ships on their voyage between the Old World and the New; and these lines assist to point out for the navigator their limits and his way. They show this via lactea to have a vibratory motion that calls to mind the graceful wavings of a pennon as it floats gently to the breeze. Indeed, if we imagine the head of the Gulf Stream to be hemmed in by the land in the Straits of Bemini, and to be stationary there, and then liken the tail of the Stream itself to an immense pennon floating gently in the current, such a motion as such a streamer may be imagined to have — very much such a motion do my researches show the tail of the Gulf Stream to have. Running between banks of cold water (§ 1), it is pressed now from the north, now from the south, according as the great masses of sea matter on either hand may change or fluctuate in temperature.
493. In September, when the waters in the cold regions of the north have been tempered, and made warm and light by the heat of summer, its limits on the left (Plate VI ) are as denoted by the line of arrows; but after this great sun-swing, the waters on the left side begin to lose their heat, grow cold, become heavy, and press the hot waters of this stream within the channel marked out for them.
494. Thus it acts like a pendulum, slowly propelled by heat on one side and repelled by cold on the other. In this view, it becomes the chronograph of the sea, keeping time for its inhabitants, and marking the seasons for the great whales; and there it has been for all time vibrating to and fro, swinging from north to south and front south to north, a great self-regulating, self-compensating pendulum.
495. In seeking information concerning the climates of the ocean, it is well not to forget this remarkable contrast between its climatology and that of the land, viz.: on the land, February and August are considered the coldest and the hottest months; but to the inhabitants of the sea, the annual extremes of cold and heat occur in the months of March and September. On the dry land, after the winter “is past and gone,” the solid parts of the earth continue to receive from the sun more heat in the day than they radiate at night, consequently there is an accumulation of caloric, which continues to increase until August. The summer is now at its height; for, with the close of this month, the solid parts of the earth’s crust and the atmosphere above begin to dispense with their heat faster than the rays of the sun can impart fresh supplies, and, consequently, the climates which they regulate grow cooler and cooler until the dead of winter again.
496. But at sea a different rule seems to prevail. Its waters are the Store-houses in which the surplus heat of summer is stored away against the severity of winter, and its waters continue to grow warmer for a month after the weather on shore has begun to get cool This brings the highest temperature to the sea in September, the lowest in March. Plate IV. is intended to show the extremes of heat and cold to which the waters — not the ice of the sea are annually subjected, and therefore the isotherms of 40º, 50º, 60º, 70º, and 80º have been drawn for March and September, the months of extreme heat and extreme cold to the inhabitants of the “great deep.” Corresponding isotherms for any other month will fall between these, taken by pairs.Thus the isotherm of 70º for July will fall nearly between the same isotherms (70º) for March and September.
497. A careful study of this plate, and the contemplation of the benign influences of the sea upon the climates which we enjoy, suggest many beautiful thoughts; for by such study we get a glimpse into the arrangements and the details of that exquisite machinery in the ocean which enables it to perform all its offices, and to answer with fidelity its marvelous adaptations.
498 How, let us inquire, does the isotherm of 80º, for instance, get from its position in March to its position in September? Is it wafted along by currents, that is, by water which, after having been heated near the equator to 80º, then flows to the north with this temperature? Or is it carried there simply by the rays of the sun, as the snow-line is carried up the mountain in summer? We have reason to believe that it is carried from one parallel to another by each of these agents acting together, but mostly through the instrumentality of currents, for currents are the chief agents for distributing heat to the various parts of the ocean. The sun with his rays would, were it not for currents, raise the water in the torrid zone to blood heat; but before that can be done, they run off with it to the poles, softening, and mitigating, and tempering climates by the way. The provision for this is as beautiful as it is benign; for, to answer a physical adaptation, it is provided by a law of nature that when the temperature of water is raised, it shall expand; as it expands, it must become lighter, and just in proportion as its specific gravity is altered, just in that proportion is equilibrium in the sea destroyed. Arrived at this condition, it is ordained that this hot water shall obey another law of nature, which requires it to run away, and hasten to restore that equilibrium. Were these isothermal lines moved only by the rays of the sun, they would slide up and down the ocean like so many parallels of latitude — at least there would be no breaks in them, like that which we see in the isotherm of 80º for September. It appears from this line that there is a part of the ocean near the equator, and about midway the Atlantic, which, with its waters, never does attain the temperature of 80º in September. Moreover, this isotherm of 80º will pass, in the North Atlantic, from its extreme southern to its extreme northern declination — nearly two thousand miles — in about three months. Thus it travels at the rate of about twenty-two miles a day. Surely, without the aid of currents, the rays of the sun could not drive it along that fast.
499. Being now left to the gradual process of cooling by evaporation, atmospherical contact, and radiation, it occupies the other eight or nine months of the year in slowly returning south to the parallel whence it commenced to flow northward. As it does not cool as rapidly as it was heated, the disturbance of equilibrium by alteration of specific gravity is not so sudden, nor the current which is required to restore it so rapid. Hence the slow rate of movement at which this line travels on its march south.
500. Between the meridians of 25º and 30º west, the isotherm of 60º in September ascends as high as the parallel of 56º. In October it reaches the parallel of 50º north. In November it is found between the parallels of 45º and 47º, and by December it has nearly reached its extreme southern descent between these. meridians, which it accomplishes in January, standing then near the parallel of 40º. It is all the rest of the year in returning northward to the parallel whence it commenced its flow to the south in September.
501. Now it will be observed that this is the season — from September to December — immediately succeeding that in which the heat of the sun has been playing with greatest activity upon the polar ice. Its melted waters, which arc thus put in motion in June, July, and August, would probably occupy the fall months in reaching the parallels indicated. These waters, though cold, and rising gradually in temperature as they flow south, are probably fresher, and if so, probably lighter than the sea water; and therefore it may well be that both the warmer and cooler systems of these isothermal lines are made to vibrate up and down the ocean principally by a gentle surface current in the season of quick motion, and in the season of the slow motion principally by a gradual process of calorific absorption on the one hand, and by a gradual process of cooling on the other.
502. We have precisely such phenomena exhibited by the waters of the Chesapeake Bay as they spread themselves over the sea in winter. At this season of the year, the charts show that water of very low temperature is found projecting out and overlapping the usual limits of the Gulf Stream. The outer edge of this cold water, though jagged, is circular in its shape, having its centre near the mouth of the Bay. The waters of the Bay, being fresher than those of the sea, may, therefore, though colder, be lighter than the warmer waters of the ocean. And thus we have repeated here, though on a smaller scale, the phenomenon as to the flow of cold waters from the north, which force the surface isotherm of 60º from latitude 56º to 40º during three or four months.
503. Changes in the color or depth of the water, and the shape of the bottom, &c., would also cause changes in the temperature of certain parts of the ocean, by increasing or diminishing the capacities of such parts to absorb or radiate heat; and this, to some extent, would cause a bending, or produce irregular curves in the isothermal lines.
504. After a careful study of this plate, and the Thermal Charts of the Atlantic Ocean, from which the materials for this plate were derived, I am led to infer that the mean temperature of the atmosphere between the parallels of 56º and 40º north, for instance, and over that part of the ocean in which we have been considering the fluctuations of the isothermal line of 60º, is at least 60º of Fahrenheit, and upward, from January to August, and that the heat which the waters of the ocean derive from this source — atmospherical contact and radiation — is one of the causes which move the isotherm of 60º from its January to its September parallel.
505. It is well to consider another of the causes which are at work upon the currents in this part of the ocean, and which tend to give the rapid southwardly motion to the isotherm of 60º. We know the mean dew-point must always be below the mean temperature of any given place, and that, consequently, as a general rule, at sea the mean dew-point due the isotherm of 60º is higher than the mean dew — point along the isotherm of 50º, and this, again, higher than that of 40º — this than 30º, and so on. Now suppose, merely for the sake of illustration, that the mean dew-point for each isotherm be 50 lower than the mean temperature, we should then have the atmosphere which crosses the isotherm of 60º, with a mean dew-point of 55º, gradually precipitating its vapors until it reaches the isotherm of 50º, with a mean dew-point of 45º; by which difference of dew-point the total amount of precipitation over the entire zone between the isotherms of 60º and 50º has exceeded the total amount of evaporation from the same surface. The prevailing direction of the winds to the north of the fortieth parallel of north latitude is from the southward and westward (Plate VIII.); in other words, it is from the higher to the lower isotherms. Passing, therefore, from a higher to a lower temperature over the ocean, the total amount of vapor deposited by any given volume of atmosphere, as it is blown from the vicinity of the tropical toward that of the polar regions, is greater than that which is taken up again.
506. The area comprehended on Plate VIII. between the isotherms of 40º and 50º Fahrenheit is less than the area comprehended between the isotherms 50º and 60º, and this, again, less than the area between this last and 70º, for the same reason that the area between the parallels of latitude 50º and 60º is less than the area between the parallels of latitude 40º and 50º; therefore, more rain to the square inch ought to fall upon the ocean between the colder isotherms of 10º difference, than between the warmer isotherms of the same difference. This is an interesting and an important view, therefore let me make myself clear: the aqueous isotherm of 50º, in its extreme northern reach, touches the parallel of 60º north. Now between this and the equator there are but three isotherms, 60º, 70º, and 80º, with the common difference of 10º. But between the isotherm of 40º and the pole, there are at least five others, viz., 40º, 30º, 20º, 10º, 0º, with a common difference of 10º. Thus, to the north of the isotherm 50º, the vapor which would saturate the atmosphere from zero, and perhaps far below, to near 40º, is deposited, while to the south of 50º the vapor which would saturate it from the temperature of 50º up to that of 80º can only be deposited. At least, such would be the case if there were no irregularities of heated plains, mountain ranges, land, &c., to disturb the laws of atmospherical circulation as they apply to the ocean.
507. Having therefore, theoretically, at sea more rain in high latitudes, we should have more clouds; and therefore it would require a longer time for the sun, with his feeble rays, to raise the temperature of the cold water, which, from September to January, has brought the isotherm of 60º from latitude 56º to 40º, than it did for these cool surface currents to float it down. After this southward motion of the isotherm of 60º has been checked in December by the cold, and after the sources of the current which brought it down have been bound in fetters of ice, it pauses in the long nights of the northern winter, and scarcely commences its return till the sun recrosses the equator, and increases its power as well in intensity as in duration. Thus, in studying the physical geography of the sea, we have the effects of night and day, of clouds and sunshine, upon its currents and its climates, beautifully developed. These effects are modified by the operations of certain powerful agents which reside upon the land; nevertheless, feeble though those of the former class may be, a close study of this plate will indicate that they surely exist.
508. Now, returning toward the south: we may, on the other hand, infer that the mean atmospherical temperature for the parallels between which the isotherm of 80º fluctuates is below 80º, at least for the nine months of its slow motion. This vibratory motion suggests the idea that there is, probably, somewhere between the isotherm of 80º in August and the isotherm of 60º in January, a line or belt of invariable or nearly invariable temperature, which extends on the surface of the ocean from one side of the Atlantic to the other. This line or band may have its cycles also, but they are probably of long and uncertain periods.
509. The fact has been pretty clearly established by the discoveries to which the wind and current charts have led, that the western half of the Atlantic Ocean is heated up, not by the Gulf Stream alone, as is generally supposed, but by the great equatorial caldron to the west of longitude 35º, and to the north of Cape St. Roque, in Brazil. The lowest reach of the 80º isotherm for September — if we except the remarkable equatorial flexure (Plate IV.) which actually extends from 40º north to the line — to the west of the meridian of Cape St. Roque, is above its highest reach to the east of that meridian. And now that we have the fact, how obvious, beautiful, and striking is the cause!
Cape St. Roque is in 50 south. Now study the configuration of the Southern American Continent from this Cape to the Wind ward Islands of the West Indies, and take into account also certain physical conditions of these regions: the Amazon, always at a high temperature because it runs from west to east, is pouring an immense volume of warm water into this part of the ocean. As this water and the heat of the sun raise the temperature of the ocean along the equatorial sea-front of this coast, there is no escape for the liquid element, as it grows warmer and lighter, except to the north. The land on the south prevents the tepid waters from spreading out in that direction as they do to the east of 350 west, for here there is a space, about 18 degrees of longitude broad, in which the sea is clear both to the north and south.
510. They must consequently flow north. A mere inspection of the plate is sufficient to make obvious the fact that the warm waters which are found east of the usual limits assigned the Gulf Stream, and between the parallels of 30º and 40º north, do not come from the Gulf Stream, but from this great equatorial caldron, which Cape St. Roque blocks up on the south, and which forces its overheated waters up to the fortieth degree of north latitude, not through the Caribbean Sea and Gulf Stream, but over the broad surface of the left bosom of the Atlantic Ocean.
511. Here we are again tempted to pause and admire the beautiful revelations which, in the benign system of terrestrial adaptation, these researches into the physics of the sea unfold and spread out before us for contemplation. In doing this, we shall have a free pardon from those at least who delight “to look through nature up to nature’s God.” What two things in nature can be apparently more remote in their physical relations to each other, than the climate of Western Europe and the profile of a coast-line in South America? Yet this plate reveals to us not only the fact that these relations between the two are the most intimate, but makes us acquainted with the arrangements by which such relations are established.
512. The barrier which the South American shore-line opposes to the escape, on the south, of the hot waters from this great equatorial caldron of St. Roque, causes them to flow north, and in September, as the winter approaches, to heat up the western half of the Atlantic Ocean, and to cover it with a mantle of warmth above summer heat as far up as the parallel of 40º. Here heat to temper the winter climate of Western Europe is stored away as in an air-chamber to furnace-heated apartments; and during the winter, when the fire of the solar rays sinks down, the westwardly winds and eastwardly currents are sent to perform their office in this benign arrangement. Though unstable and capricious to us they seem to be, they nevertheless “fulfill His commandments” with regularity and perform their offices with certainty. In tempering the climates of Europe with heat in winter that has been bottled away in the waters of the ocean during summer, they are to be regarded as the flues and the regulators for distributing at the right time, and at the right places, in the right quantities.
513. By March, when “the winter is past and gone,” the furnace which had been started by the rays of the sun in the previo’us summer, and which, by autumn, had heated up the ocean in our hemisphere, has gone down. The caldron of St. Roque, ceasing in activity, has failed in its supplies, and the chambers of warmth upon the northern sea, having been exhausted of their heated water, which has been c—xpended in the manner already explained, have contracted their limits. The surface of heated water which, in September, was spread out over the western half of the Atlantic, from the equator to the parallel of 40º north, and which raised this immense area to the temperature of 80º and upward, is not to be found in early spring on this side of the parallel of S0 north.
514. The isotherm of 80º in March, after quitting the Caribbean Sea, runs parallel with the South American coast toward Cape St. Roque, keeping some 8 or 10 degrees from it. Therefore the heat dispensed over Europe from this caldron falls off in March. But at this season the sun comes forth with fresh supplies; he then crosses the line and passes over into the northern hemisphere; observations show that the process of heating the water in this great caldron for the next winter is now about to commence.
515. In the mean time, so benign is the system of cosmical arrangements, another process of raising the temperature of Europe commences. The land is more readily impressed than the sea by the heat of the solar rays; at this season, then, the summer climate due these transatlantic latitudes is modified by the action of the sun’s rays directly upon the land. The land receives heat fromn them, but, instead of having the capacity of water for retaining it, it imparts it straightway to the air; and thus the proper climate, because it is the climate which the Creator has, for his own wise purposes, allotted to this portion of the earth, is maintained until the marine caldron of Cape St. Roque is again heated and brought into the state for supplying the means of maintaining the needful temperature in Europe during the absence of the sun in the other hemisphere.
516. In like manner, the Gulf of Guinea forms a caldron and a furnace, and spreads out over the South Atlantic an air-chamber for heating up in winter and keeping warm the extra-tropical regions of South America. Every traveler has remarked upon the mild climate of Patagonia and the Falkland Islands. “Temperature in high southern latitudes,” says a very close observer, who is co-operating with me in collecting materials, “differs greatly from the temperature in northern. In southern latitudes there seem to be no extremes of heat and cold, as at the north. Newport, Rhode Island, for instance, latitude 41º north, longitude 71º west, and Rio Negro, latitude 41º south, and longitude 63º west, as a comparison: in the former, cattle have to be stabled and fed during the winter, not being able to get a living in the fields on account of snow and ice. In the latter, the cattle feed in the fields all winter, there being plenty of vegetation and no use of hay. On the Falkland Islands ( latitude 51—20 south), thousands of bullocks, sheep, and horses are running wild over the country, gathering a living all through the winter.”
517. The water in the equatorial caldron of Guinea can not escape north — the shore-line will not permit it. It must, therefore, overflow to the south, as that of St. Roque does to the north, carrying to Patagonia and the Falkland Islands, beyond 50º south, the winter climate of Charleston, South Carolina, on our side of the North Atlantic, or of the “Emerald Island” on the other. All geographers have noticed, and philosophers have frequently remarked upon the conformity, as to the shore-line profile, of equatorial America and equatorial Africa.
518. It is true, we can not now tell the reason, though explanations founded upon mere conjecture have been offered, why there should be this sort of jutting in and jutting out of the shore-line, as at Cape St. Roque and the Gulf of Guinea, on opposite sides of the Atlantic; but one of the purposes, at least, which this peculiar con figuration was intended to subserve, is without doubt now revealed to us.
519. We see that, by this configuration, two cisterns of hot water are formed in this ocean; one of which distributes heat and warmth to western Europe; the other, at the opposite season, tempers the climate of eastern Patagonia. Phlegmatic must be the mind that is not impressed with ideas of grandeur and simplicity as it contemplates that exquisite design, those benign and beautiful arrangements, by which the climate of one hemisphere is made to depend upon the curve of that line against which the sea is made to dash its waves in the other. Impressed with the perfection of terrestrial adaptations, he who studies the economy of the great cosmical arrangements is reminded that not only is there design in giving shore-lines their profile, the land and the water their proportions, and in placing the desert and the pool where they are, but the conviction is forced upon him also, that every hill and valley, with the grass upon its sides, have each its office to perform in the grand design.
520. March is, in the southern hemisphere, the first month of autumn, as September is with us; consequently, we should expect to find in the South Atlantic as large an area of water of 80º and upward in March, as we should find in the North Atlantic for September. But do we? By no means. The area on this side of the equator is nearly double that on the other.
521. Thus we have the sea as a witness to the fact that the winds (§ 196) had proclaimed, viz., that summer in the northern hemisphere is hotter than summer in the southern, for the rays of the sun raise on this side of the equator double the quantity of sea surface to a given temperature that they do on the other side; at least this is the case in the Atlantic. Perhaps the breadth of the Pacific Ocean, the absence of large islands in the temperate regions north, the presence of New Holland, with Polynesia in the South Pacific, may make a difference there. But of this I can not now speak, for thermal charts of that ocean have not yet been prepared.
522. Pursuing the study of the climates of the sea, let us now turn to Plate VI. Here we see at a glance how the cold waters, as they come down from the Arctic Ocean through Davis’s Straits, press upon the warm waters of the Gulf Stream, and curve their channel into a horse-shoe. Navigators have often been struck with the great and sudden changes in the temperature of the water hereabouts. In the course of a single day’s sail in this part of the ocean, changes of 15º, or 20º, and even of 30º, have been observed to take place in the temperature of the sea. The cause has puzzled navigators long, but how obvious is it not now made to appear! This “bend” is the great receptacle of the icebergs which drift down from the north; covering frequently an area of hundreds of miles in extent, its waters differ as much as 20º, 25º, and in rare cases even as much as 30º of temperature from those about it. Its shape and place are variable. Sometimes it is like a peninsula, or tongue of cold water projected far down into the waters of the Gulf Stream. Sometimes the meridian upon which it is inserted into these is to the east of 40º, sometimes to the west of 50º longitude. By its discovery we have clearly unmasked the very seat of that agent which produces the Newfoundland fogs. It is spread out over an area frequently embracing several thousand square miles in extent, covered with cold water, and surrounded on three sides, at least, with an immense body of warm. May it not be that the proximity to each other of these two very unequally heated surfaces out upon the ocean would be attended by atmospherical phenomena not unlike those of the land and sea breezes? These warm currents of the sea are powerful meteorological agents. I have been enabled to trace, in thunder and light, ning, the influence of the Gulf Stream in the eastern half of the Atlantic, as far north as the parallel of 55º north; for there, in the dead of winter, a thunder-storm is not unusual.
523. These isothermal lines of 50º, 60º, 70º, 80º, &c., may illustrate for us the manner in which the climates in the ocean are regulated. Like the sun in the ecliptic, they travel up and down the sea in declination, and serve the monsters of the deep for signs and for seasons.
524. It should be borne in mind that the lines of separation, as drawn on Plate IX., between the cool and warm waters, or, more properly speaking, between the channels representing the great polar and equatorial flux and reflux, are not so sharp in nature as this plate would represent them. In the first place, the plate represents the mean or average limits of these constant flows — polar and equatorial; whereas, with almost every wind that blows, and at every change of season, the line of meeting between their waters is shifted. In the next place, this line of meeting is drawn with a free hand on the plate, as if to represent an average; whereas there is reason to believe that this line in nature is variable and unstable as to position, and as to shape rough and jagged, and oftentimes deeply articulated. In the sea, the line of meeting between waters of different temperatures and density is not unlike the sutures of the skull-bone on a grand scale — very rough and jagged; but on the plate it is a line drawn with a free hand, for the purpose of showing the general direction and position of the channels in the sea, through which its great polar and equatorial circulation is carried on.
525. Now, continuing for a moment our examination of Plate IV., we are struck with the fact that most of the thermal lines there drawn run from the western side of the Atlantic toward the eastern, in a northeastwardly direction, and that, as they approach the shores of this ocean on the east, they again turn down for lower latitudes and warmer climates. This feature in them indicates, more surely than any direct observations upon the currents can do, the presence, along the African shores in the North Atlantic, of a large volume of cooler waters. These are the waters which, having been first heated up in the caldron (§ 509) of St. Roque, in the Caribbean Sea, and Gulf of Mexico, have been made to run to the north, charged with heat and electricity to temper and regulate climates there. Having performed their offices, they have cooled down; but, obedient still to the “Mighty Voice” which the winds and the waves obey, they now return by this channel along the African shore to be again replenished with warmth, and to keep up the system of beneficent and wholesome circulation designed for the ocean.