Physical Geography of the Sea and its Meteorology/Chapter 17

by Matthew Fontaine Maury

CHAPTER XVII.

§ 720-735.—THE CLIMATES OF THE SEA.

720. A "milky way" in the ocean.—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 a few 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 their cold and warm currents; these lines disclose a thermal tide in the sea, which ebbs and flows but once a year; they also cast light upon the climatology of the sea, its hyetographic peculiarities, and the climate 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 they enable us to mark out, for the mariner's guidance, that "milky way" in the ocean, the waters of which teem, and sparkle, and glow with life and incipient organisms as they flow across the Atlantic. In them are found the clusters and nebulae of the ocean 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 in the sea 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 (§ 71), it is pressed now from the north, now from the south, according as the great masses of sea water on either hand may change or fluctuate in temperature.

721. The vibrations of the Gulf Stream.—In September, when the waters in the cold regions of the north have been tempered, and been made warm and light by the heat of summer, its limits on the left are as denoted by the line of arrows (Plate VI.); 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 into the channel marked out for them. 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 a chronograph for 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, once every year, swinging from north to south, and from south to north again, a great self-regulating, self-compensating pendulum, beating time in the sea to the seasons of the year.

722. Sea and land climates contrasted.—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, namely: 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 contimie 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 the winter again. But at sea a different rule seems to prevail. Its waters are the store-houses^[1] 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 40°, 50°, 60°, 70°, and 80° 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 70° for July will fall nearly midway between the same isotherms (70°) for March and September.

723. Plate IV.—A careful study of this plate, and the contemplation of the benign influence 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 marvellous adaptations. How, let us inquire, does the isothermal of 80°, 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 80°, 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 its 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 towards 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 break in them, like that which we see in the isotherm of 80° 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 80° in September. Moreover, this isotherm of 80° 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 list. In this fact we have another link in the chain of proof (Chap. XXII.), going to show that the sea receives more heat than it radiates off again. Being now left to the gradual process of cooling by evaporation, atmospherical contact, and radiation, this isotherm 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 inquired to restore it so rapid. Hence the slow rate of movement at which this line travels on its march south. Between the meridians of 25° and 30° west, the isotherm of 60° in September ascends as high as the parallel of 56° N. In October it reaches the parallel of 50° north. In November it is found beneath the parallels of 45° and 47°, 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 40°. It is all the rest of the year in returning northward to the parallel whence it commenced its flow to the south in September. 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 are 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. 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, are therefore, though colder, yet lighter (§ 426) than the warmer waters of the ocean. And thus we have repeated here, though on a smaller scale, the phenomena as to the flow of cold waters from the north, which force the surface isotherm of 60° from latitude 56° to the parallel of 40° during three or four months. Changes in the colour or depth of the water, and the shape of the bottom, etc., are also calculated to 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, assists to bend or produce irregular curves in the isothermal lines. After a careful study of this plate, and the Thermal Charts of the Atlantic Ocean, from which the materials for it are derived, I am led to infer that from January to August the mean temperature of the atmosphere between the parallels of 56° and 40° north, for instance, and over that part of the ocean in which we have been considering the fluctuations of the isothermal line of 60°, is at least 60° of Fahrenheit, and upward, 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 60° from its January to its September parallel. 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 60°. 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 60° is higher than the mean dew-point along the isotherm of 50°, and this, again, higher than that of 40°, this than 30°, and so on. Now suppose, merely for the sake of illustration, that the mean dew-point for each isotherm be 5° lower than the mean temperature, we should then have the atmosphere which crosses the isotherm of 60°, with a mean dew-point of 55°, gradually precipitating its vapours until it reaches the isotherm of 50°, with a mean dew-point of 45°; by which difference of dew-point the total amount of precipitation over the entire zone between the isotherms of 60° and 50° 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 vapour deposited by any given volume of atmosphere, as it is blown from the vicinity of the tropical towards that of the polar regions, is greater than that which is taken up again. This is an interesting and important fact.

724. The effects of night and day upon the temperature of sea water.—Having, therefore, more precipitation in. high than in low latitudes at sea, we should have more clouds; and therefore it requires 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 60° from latitude 56° down to the parallel of 40°, than it did for those cool surface currents to float it down. After this southwardly motion of the isotherm of 60° has been checked in December by the cold, and after the sources of the current which have 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, with increased powers both as to intensity and duration. Thus, in studying the physical geography of the sea, we must take cognizance of its actinometry also, for here 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.

725. A belt of uniform temperature at sea.—Now, returning towards the south: we may, on the other hand, infer that the mean atmospherical temperature for the parallels between which the isotherm of 80° fluctuates is below 80°, 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 80° in August and the isotherm of 60° 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 belt or band may have its cycles also, but they are probably of a long and uncertain period.

726. The western half of the Atlantic warmer than the eastern.—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 (§ 131) by the great equatorial caldron to the west of longitude 35°, and to the north of Cape St. Roque, in Brazil. The lowest reach of the 80°isotherm for September—if we except the remarkable equatorial flexure (Plate IV.) which actually extends from 40° 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, how beautiful, and striking is the cause! Cape St. Roque is in 5° 30' south. Now study the configuration of the Southern American Continent from this cape to the Windward 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 35° 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: 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 30° and 40° 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 dispenses its overheated waters up towards 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.

727. The warmest sides of oceans and the coldest shores of continents in juxta-position.—Like the western half of the North Atlantic Ocean, the western half of every one of the three great oceans is the warmer. The great flow of warm water in the North Pacific is with the "Black Stream of Japan," on the Asiatic side; in the South Pacific it is with the Polynesian drift, on the Australian side: opposite to these warm Pacific currents and on its eastern side, are the Humboldt current in one hemisphere, and the California current in the other—cold currents both. In the South Indian Ocean, the warm water is with the Mozambique current on the African side, and the cold drift on the Australian; and in the South Atlantic, Plato IV. shows that, parallel for parallel, the littoral waters of Brazil are several degrees warmer than those on the African side. Thus at sea the climatic conditions of the land are reversed, for the coldest side of the ocean is next the warmest side of the continent, and vice versa. The winds from extra-tropical seas temper the climates of the shores upon which they blow, not so much by the sensible heat they convey as by the latent heat which is liberated from the vapour they bring. This being condensed, as upon the British Islands and Western Europe, sets free heat enough not only to soften the climate, but to rarefy the air to such an extent as to be observed in the mean barometric pressure.

728. The climates of Europe influenced by the shore-lines of Brazil.—Here we are again tempted to pause and admire the beautiful revelations which, in the benign system of terrestrial adaptations, 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 most intimate, but makes us acquainted with the arrangements by which such relations are established. 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, as far up as the parallel of 40° N., with a mantle of warmth above summer heat. 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 "fulfil 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, these winds and currents are to Lo regarded as the flues and regulators for distributing it at the right time, and at the right places, in the right quantities. By March, when "the winter is past and gone," the furnace which had been started by the rays of the sun in the previous summer, and which, by autumn, had heated up the ocean in our hemisphere, has cooled 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 expended 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 40° north, and which raised this immense area to the temperature of 80° and upward, is not to be found in early spring on this side of the parallel of 8° north. The isotherm of 80° in March, after quitting the Caribbean Sea, runs parallel with the South American coast towards 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. 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 from 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 and the tropics is again heated and brought into the state for supplying the vapour and the heat to maintain the needful temperature in Europe during the absence of the sun in the other hemisphere. Thus the equable climates of Western Europe are accounted for.

729. The Gulf of Guinea and the climate of Patagonia.—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 assisting to keep warm the extra-tropical regions of South America. Every traveller 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 seems to be no extremes of heat and cold, as at the north. Newport, Rhode Island, for instance, latitude 41° north, longitude 71° west, and Rio Negro, latitude 41° south, and longitude 63° 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 Island (latitude 51-2° south), thousands of bullocks, sheep, and horses are running wild over the country, gathering a living all through the winter." The water in the equatorial caldron of Guinea overflows to the south, as that of St. Roque does to the north; it carries to Patagonia and the Falkland Islands warmth, which, uniting with the heat set free by precipitation during the passage of the vapour-laden west winds across the Southern Andes, carries beyond latitude 50° into the other hemisphere the winter climate of South Carolina on one side of the North Atlantic, or of the "Emerald Island " on the other.

730. Shore-lines.—All geographers have noticed, and philosophers have frequently remarked upon the conformity as to the shore-line profile of equatorial America and equatorial Africa. It is true, wt cannot 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 in the Gulf of Guinea, on opposite sides of the Atlantic; but one of the purposes, at least, which this peculiar configuration was intended to subserve, is without doubt now revealed to us. 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, helps to temper 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, is a part of the wonderful mechanism, each having its offices to perform in the grand design. 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 at 80° and upwards in March, as we should find in the North Atlantic for September. But do we? By no means. The area that is covered on this side of the equator with water at 80° and upwards is nearly double that on the other. Thus we have the sea as a witness to the fact which the winds had proclaimed, viz., that summer in the northern hemisphere is hotter than summer in the southern.

731. Sudden changes in the water thermometer.—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' 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 15°, or 20°, and even of 30°, have been observed to take place in the temperature of the sea. The cause has puzzled navigators long, but how obvious is it 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 20°, 25°, and in rare cases even as much as 30° 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 40°, sometimes to the west of 60°. On my passage to England November, 1800, I passed over this horse-shoe; the water in it was 16° colder than the water at its side. It looked as though we might have been on soundings. 732. The fogs of Newfoundland.—By its discovery we have clearly unmasked the very seat of that agent which produces the Newfoundiand 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 lightning the influence of the Gulf Stream in the eastern half of the Atlantic as far up as the parallel of 55° Is., for there, in the dead of winter, a thunder-storm is not unusual.

733. Aqueous isothermal lines.—These isothermal lines of 50°, 60°, 70°, 80°, etc., 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.

734. The meeting of cool and warm waters.—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 Dr 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 simply with a free hand, merely for the purpose of illustration.

735. The direction of aqueous isotherms on opposite sides of the sea.—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 towards the eastern, in a north-eastwardly 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 (§ 726) 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.

1. Vide Chap. XXII., Actinometry of the Sea.