THE GULF STREAM.*

By ALEXANDER AGASSIZ.

The Gulf Stream is the best known and at the same time the most remarkable example of the effect of oceanic circulation upon the distribu tion of temperature in connection with the currents of the North Atlantic. It has long been known to geographers that a cold current coming from Greenland joins the Labrador current, and extends in a southerly direction along the eastern coast of the United States, while a warm current pouring through the Straits of Florida flows in the opposite direction along the coast of the southern Atlantic States, and is deflected from the banks of Newfoundland crossing the Atlantic diagonally. This body of warm water makes itself felt along the west coast of the British Islands, penetrating even as far as the coast of Spitzbergen, and perhaps beyond, to Nova Zembla. It is impossible to discuss the results of the more recent investigations of the Gulf Stream carried on by the Blake, without including the general questions of oceanic circulation, and of the thermal conditions of the Atlantic in particular. I shall therefore briefly state such points, derived from the explorations of the Challenger and other expeditions, as will assist us in understanding the history and physics of this great oceanic current.

Sir Charles Lyell has called attention to the fact that in the present epoch the most marked physical feature of the surface of the globe is its subdivision into a land and an oceanic hemisphere. Thomson, like him, looks upon the oceans as continuous, and has happily styled the Atlantic, the Pacific, and the Indian oceans as great gulfs of the Southern Ocean.

The striking hydrographic character of the North Atlantic is its comparative isolation from the Arctic Ocean; the South Atlantic, on the contrary, is fully open to the circulation of cold water coming from the

* From the Bulletin of the Museum of Comparative Zoology, at Harvard College, in Cambridge, Mass., vol. XIV: chap. ix, pp. 241-259.

Along the American coast the sudden transition from the green, cold, and more or less turbid water found along the coast and continental shelf, into the deep blue waters of the warm Gulf Stream, is one which has been noticed by all who have passed from the shore seaward. This cold green water, which has such a chilling influence on the climate of the New England States, follows the line of the Atlantic coast of the United States far towards the base of the peninsula of Florida.

Antarctic Ocean. The South Atlantic is shut off from its northern area by the ridge extending from St. Paul's Rocks to Ascension, at a depth of about 2,000 fathoms. The Challenger Ridge runs nearly north and south, leaving a free communication between the Antarctic Ocean and the eastern and western basins of the South Atlantic. The North Atlantic is subdivided into an eastern and western basin at a depth of about 1,500 fathoms by the Dolphin Rise, which follows in a general way the course of the S-shaped Atlantic basin. Ridges separating the Atlantic from the Arctic Ocean extend across Denmark Straits, probably at a shallow depth. From Greenland to Iceland the depth has an average of 500 fathoms; from Iceland to the Færöes, an average of about 300 fathoms, and from there to the Orkneys, of not more than 220 fathoms. From the configuration of the bottom it is evident that a larger amount of cold water must reach the tropics from the Antarctic than from the Arctic regions,* which are shut off from the Atlantic by submarine ridges. Over these and through the channels of Baffin's Bay but a limited amount of cold water can find its way south. In the eastern Atlantic the principal cooling agent must be the cold water slowly flowing northward from the Antarctic between the Challenger Ridge and Africa.

The shape of the northern extremity of South America, together with the action of the southerly trades, is such as to split the southern equatorial current, and to drive a considerable part of this southern current northward to join the westerly drift which flows to the northward of the Greater Antilles and Bahamas. The phenomena of oceanic circulation in their simplest form are here seen to consist of westerly currents impinging upon continental masses, deflected by them to the northward and eastward, and gradually lost in their polar extension.

There is on the west side of the North Atlantic an immense body of warm water, of which the Gulf Stream forms the western edge, flowing north over a large body of cold water that comes from the poles and flows south. The limits of the line of conflict between these masses are con

*The temperature line run diagonally across the Atlantic from Madeira to Tristan da Cunha by the Challenger brings out the remarkably shallow stratum of warm water of that part of the equatorial regions which corresponds to the regions of the tradewinds both north and south of the equator. The temperatures of the belts of water between 200 and 500 fathoms north and south of the line plainly show that the colder water found south of the equator can not come from the warmer northern belt of the same depth, but must come from the colder helt adjoining the equatorial region. In other words, the cold water may be said to rise towards the surface near the equator; and from the temperature of the two sides of the North Atlantic it is also evident that the supply of cold water flowing from the Antarctic into the Atlantic is greater than that coming from the Arctic regions. This vertical circulation, characteristic of the equatorial belt, is insignificant, however, when compared with the great horizontal oceanic currents.

In the Pacific the amount of cold water flowing into it through the narrow and shallow Bering Strait is infinitesimal compared with the mass of cold water creeping northward into the Pacific gulf from the depths of the Southern Ocean,

stantly changing, according to the seasons.

At one time the colder water from Davis's Straits spreads like a fan near the surface, driving the Gulf Stream to the east,* and at another, large masses of warm water extend towards the Faroe Islands, with branches toward Iceland and the coast of Portugal.

An examination of an isothermal chart of the Atlantic clearly shows the effect of the isolation of the Northern Atlantic, the area of maximum temperature (820) extends over a far greater space in the North than in the South Alantic. The Gulf of Mexico and the Caribbean become greatly superheated in September (to above 86°), the effect of this superheating in conjunction with the westerly equatorial drift being seen clearly in the northerly extension of the isothermal lines. In the South Atlantic, owing in part to the greater regularity in the shape of the basin, the difference in the extension of the isothermal lines is but little marked.

The temperature sections of the Challenger, from Teneriffe to Sombrero, show remarkably well the great contrast in temperature between the eastern and western basins of the Atlantic, which are separated by the Dolphin Rise. In the eastern basin the cold water on the bottom is supplied by the indraft from the South Atlantic, while the warmer surface water of the western basin is due to the westerly equatorial currents. We seem, therefore, to have masses of water of different temperatures accumulated at certain points by surface or bottom curents, to be distributed again, either north or south, into the general oceanic circulation, thus restoring the equilibrium disturbed by the unequal distribution of heat and cold on the surface of the ocean.

Another temperature section (Fig. 1), which I shall borrow from the Challenger soundings, to complement the work of the Blake in the same regions, is that which extends from Halifax to the Bermudas, and thence to St. Thomas. The temperatures observed by these vessels show plainly the path of the warm surface water, which flows outside of the West India Islands, and joins the Gulf Stream proper, whose waters when united are banked against the cold Labrador current in its course along the American coast.

Undoubtedly, the early observations made upon the temperature of the ocean were defective, owing to the somewhat imperfect instruments at the disposal of the early explorers; yet they determined the general position of the cold and warm currents of the ocean along our shores.

*The direction from which the currents come is plainly shown by the nature of the bottom specimens, made up in part of globigerinæ brought by the warmer southerly surface currents, and in part of northern foraminifera and of volcanic sand derived from Jan Mayen and Spitzbergen. The dividing lines between these deposits may be considered as the boundaries of the arctic current where it passes under the Gulf Stream.

The parallelism of temperature is also very marked in the South Pacific, where there are no disturbing influences. (See J. J. Wild, Thalassa, (pl. xv.) and Challenger Temperatures.)