Ocean current

“ This current receives volume and velocity from another cause, which is thus explained: "Immediately under the sun, or where the beams of that luminary are direct, a vacuum is produced, into which the circumambient air rushes; and as this vacuity is carried westward along the equator, upwards of 1,035 miles hourly, an atmospheric current follows, which, acting on the ocean waters, impels them westward, and adds force and mass to the tropic current. In the Atlantic Ocean, from the peculiar structure of its shores, a very remarkable phenomenon—the Gulf Stream—is produced. ”

“ Now we may judge how exceedingly small a current this would produce when we consider that there is a rise of about two feet in the open ocean at one place and a fall of the same amount at another every six hours, caused by the tides, and yet the flowing of the water from the one place to the other place to produce this rise at one place, and fall at the other, it is well known does not produce any sensible currents in the open sea. ”

“ This is much less than the opposing force arising from the great specific gravity of the water in the northern part of the Atlantic on account of its lower temperature, as we have shown by calculations. The same reasoning may be applied to any other part of the ocean; for, if the salt of the ocean has any influence in producing currents, it must be to produce an undercurrent from the torrid zone, where evaporation is supposed to be in excess, toward the poles, and consequently, a counter-current at the surface from the poles toward the equator. ”

“ In some other parts of the ocean, instead of there being organic life capable of changing, by animal or vegetable secretions, the specific gravity of the supposed salt and heavy and hot water at 90°, there may be none such, as in a "Desolate Region." This water then may go off as an under-current freighted with heat to temper some hyperborean region or to soften some extra-tropical climate, for we know that such is among the effects of marine currents. ”

“ Furthermore, the current in the great ocean could not have been maintained to run continually one way from the beginning of the world unto this day, had there not been some through passage by the strait aforesaid, and so by circular motion be brought again to maintain itself, for the tides and courses of the sea are maintained by their interchangeable motions, as fresh rivers are by springs, by ebbing and flowing, by rarefaction and condensation. ”

“ Hence it is, that the ocean, which covers so large a portion of the earth’s surface, produces the effect of making the alternations of heat and cold much less violent than they would be if it were absent. The different temperatures of its upper and lower parts produce a current which draws the seas, and by means of the seas, the air, towards the mean temperature. ”

“ Also of a low barometer.—But, be that as it may, the air which these vapour-bearing winds—vapour-bearing because they blow over such an immense tract of ocean—pour into this stopping-place has to ascend and flow off as an upper current, to make room for that which is continually flowing in below. In ascending it expands and grows cool, and, as it grows cool, condensation of its vapour commences; with this, vast quantities of latent heat, which converted the water out at sea into vapour for these winds, are set free in the upper air. ”

“ Yet this is a field of inquiry of very great [662] importance in physical geography, since an accurate determination of the temperature at different depths is certainly the best, and frequently the only means, for determining the depth, the width, the direction and general path of the warm ocean currents, which are the chief agents in diffusing the equatorial heat; and more especially of those deeper currents of cold water which return from the poles to supply their places, and complete the watery circulation of the globe. ”

“ This is the essential cause for the difference in strength between the currents observed in salt and fresh waters, for, of all the current-producing causes which act in fresh waters, only the one resulting from variations of temperature can sensibly affect the specific gravity, while the specific gravity of sea water, besides being much more affected by variation of temperature, is still further influenced by the fresh water which rains upon the surface of the ocean. ”

“ The Red Sea current.—The currents which run from the Atlantic into the Mediterranean, and from the Indian Ocean into the Red Sea, are the reverse of this. Here the bottom of the current is probably a water-level, and the top an inclined plane, running down hill. Take the Red Sea current as an illustration. That sea lies, for the most part, within a rainless and riverless district. It may be compared to a long and narrow trough. Being in a rainless district, the evaporation of it is immense ”

“ One ignorant of the currents of the ocean might be puzzled at times in observing that an iceberg floats southward at the same time that pieces of wood are floating northward, both apparently acted upon by the same current. This may be explained by recalling that warm water is lighter than cold and hence is found as the upper layer when a cold and a warm current are flowing in different directions, one upon the other. ”

“ OCEANIC CURRENT PHENOMENA 3110. Ocean Eddies and Rings Eddies with horizontal diameters varying from 50-150 km have their own pattern of surface currents. These features may have either a warm or a cold core and currents flow around this core, either cyclonically for cold cores or anticyclonically for warm cores. The most intense of these features are called rings and are formed by the pinching off of meanders of western boundary currents such as the Gulf Stream. Maximum speed associated with these features is about 2 knots. ”

“ The movements of the waters of the ocean, it will be remembered, are largely 178 under the control of the winds, so that the essential factor in the transfer of heat from place to place is atmospheric circulation. The primary causes of movements in the air, as is a matter of current knowledge, are the differences that arise in temperature at various localities. In regions where the air becomes more highly heated than over adjacent areas it expands, and in consequence becomes lighter, volume for volume, than the air over neighbouring areas, and is forced upward and overflows aloft. ”

“ All the principal currents of the globe are in fact moving in the exact direction in which they ought to move, assuming the winds to be the sole impelling cause. In short, so perfect is the agreement between the two systems, that, given the system of winds and the conformation of sea and land, and. . . the system of oceanic circulation might be determined a priori. Or, given the system of the ocean-currents, together with the conformation of sea and land ”

“ Thus all the water carried forward by any current must have the place it left immediately occupied by water from another place, so that only a complete system of circulation can exist in the ocean. Further, all water particles when moving undergo a deviation from a straight path due to the forces set up by the rotation of the earth deflecting them towards the right as they move in the northern hemisphere and towards the left in the southern. ”

“ As ocean currents, like the Gulf Stream of the Atlantic and the Kuro Siwo of the Pacific, carry heat from the warmer to the colder regions of the earth, so air currents arise out of those differences of heat and of barometrical pressure which it is obviously their function, as far as possible, to remove. ”

“ Thus the ocean currents not only help to vary the earth by producing changes in the climate of both sea and land, breaking up the uniformity which would otherwise characterize regions at the same distance from the equator, but they induce, by the consequences of the migrations which they enforce, changes in the organic tenants of the sea. ”

“ Land as a Barrier to Ocean Currents.—We have seen already the great importance of elevated land to serve as condensers and ice-accumulators; but there is another and hardly less important effect that may be produced by an extension of land in high latitudes, which is, to act as a barrier to the flow of ocean currents. In the region with which we are more immediately interested it is easy to see how a comparatively slight alteration of land and sea, such as has undoubtedly occurred, would produce an enormous effect on climate. ”

“ Even the study of the floating ice broken away from the main mass may lead to important conclusions as to its mode of origin and the nature of the land from which it comes. Other problems to be investigated are the origin of the cold ocean currents that take their rise in the south, the conditions of the atmospheric pressure and temperature in that region, and the questions relating to terrestrial magnetism, which have a very important bearing on the practice of navigation. ”

“ The prevalent winds, dependent on the fairly constant distribution of atmospheric pressure over the great oceans, are the most important determinant of currents. As at most points in the ocean the temperature, salinity and chemical composition of the water are mainly determined by the currents—that is, by the condition at the place whence the water came—it is obvious that a study of currents must precede any general view of the distribution of marine forms. ”

“ Despite superficial similarities in the primitive equations underlying both atmospheric and oceanic models--both are based heavily on fluid dynamics--differences in surface area, mass, specific heat, density, and a myriad of other factors lead to very different responses to environmental inputs. Perhaps most importantly, the ocean and atmosphere have temperature response and equilibrium times that differ by several orders of magnitude. ”

“ Winds exercise but little influence constant currents.—More water cannot run from the equator or the pole than to it. If we make the trade-winds to cause the Gulf Stream, we ought to have some other wind to produce the Polar flow; but these currents, for the most part, and for great distances, are submarine, and therefore beyond the influence of winds. Hence it should appear that winds have little to do with the general system of aqueous circulation in the ocean. ”

“ THOUGH the currents of the ocean have long occupied the attention of scientific men, no general theory has yet been found to answer under all circumstances. It may, I think, be assumed that oceanic currents depend upon principles as fixed as those to which we refer the currents of air; and also, that heat and cold operate in like manner upon both; to these causes may be added the influence of the heavenly bodies, and it is therefore to be regretted, that navigators have never thought of comparing with accuracy the changes and courses of currents with the revolutions of the sun and planets. ”

“ We thus see actually existing in nature a force perhaps quite sufficient to give rise to just such a system of currents as that which mariners find in the Pacific (§ 401) —currents which appear to rise in mid ocean, run at unequal rates, sometimes east, sometimes west, but which always lose themselves where they rise, viz., in mid ocean. ”

“ Heat the top of an iron plate, and the bottom becomes warm; but heat the top of a sheet of water, as in a pool or basin, and that at the bottom remains cool. The heat passes through iron by conduction, but to get through water it requires to be conveyed by a motion, which in fluids we call currents.
235. Therefore the study of the climates of the sea involves a knowledge of its currents, both cold and warm. They are the channels through which the waters circulate, and by means of which the harmonies of old ocean are preserved. ”

“ The polar origin of the cold deep water found in low latitudes has long been considered probable, and has given rise to a theory of a general oceanic circulation in a vertical and horizontal direction, produced by differences of temperature and density. Recent theoretical investigations, however, seem to indicate that these causes alone are incapable of producing currents, and, to-day, the theory that the winds are mainly responsible for all current movements very largely predominates. ”

“ Local causes produce local currents. A clear sky in one region allows the sun’s rays to pour upon, let us say, the ocean, producing great heat; the result of which is evaporation. Aqueous vapour is very light, therefore it rises; and in doing so the aqueous particles carry the air up with them, and the wind necessarily rushes in below to supply its place. The falling of heavy rain, in certain conditions of the atmosphere, has the effect of raising wind. Electricity has also, in all probability, something to do with the creation of motion in the atmosphere. ”