The surface waters of the whole Pacific have less salt than those in the corresponding regions of the Atlantic. For example, the salt content west of San Francisco is 33.2%0, but west of Lisbon over
; in the north-east trade region of the Pacific it measures only about
whereas, in the region of the Atlantic north-east trades, it is over
or 37%o. The reason for this is not yet known with certainty; probably evapora tion is less than in the Pacific because of the smaller velocity of the wind, and the rainfall is greater than in the Atlantic. The geographical disposition of the varying salt content is essentially the same in the Pacific as in the Atlantic and Indian oceans. In each hemisphere, in the region of the north-east and south-east trade-winds, appears a broad zone with relatively the highest salt content. In northern latitudes its centre lies in the middle of the ocean ; in southern latitudes it is in the eastern or South American half. Between these, mostly from
N. lat., is a small strip of water poor in salt, stretching from the Philippines to the coast of Central America; here the salt content amounts to less than 34%o in the west, less than 33%0 in the east, where, in the Gulf of Panama, 31%0 or even
have been observed. North and south also from the two first-named tropical maxima the salt content diminishes, particularly in the north Pacific. In the ocean off the Japanese islands, in spite of a warm current resembling the Gulf Stream, the salt content is only 34.5%0 or 34%o, almost
less than in the Atlantic. The Okhotsk and Bering seas, which perhaps can be compared with Baffin bay and the Norway sea, have
a salt content 2-3%, less than the Atlantic regions named.
The distribution of saltness in the depths of the Pacific ocean is not yet sufficiently known, but it seems to correspond in the main with that ascertained for the Atlantic (q.v.).
The surface currents of the Pacific have not been studied in the same detail as those of the Atlantic, and their seasonal variations are little known except in the monsoon regions. Speaking generally, however, it may be said that they are for the most part under the direct control of the prevailing winds. The North Equatorial current is due to the action of the north-east trades. It splits into two parts east of the Philippines, one division flowing northwards as the Kuro Siwo or Black Stream, the analogue of the Gulf Stream, to feed a drift circulation which follows the winds of the north Pacific, and finally forms the Cali fornian current flowing south-eastwards along the American coast. Part of this rejoins the North Equatorial current, and part probably forms the variable Mexican current, which follows the coasts of Mexico and California close to the land. The Equatorial Counter-current flowing eastwards is largely assisted during the latter half of the year by the south-west monsoon, and from July to October the south-west winds prevailing east of 15o° E. further strengthen the current, but later in the year
the easterly winds weaken or even destroy it. Between the Kuro Siwo and the Asiatic coast; i.e., east of the Kuriles and Kam chatka, a band of cold water with a slight movement to the south ward, known as the Oya Siwo, forms an analogue of the "cold wall" of the Atlantic. In the Japan sea, too, on the Siberian coast, as well as in the Yellow sea on the Chinese coast, there flows southward, during the greater part of the year, cold water poor in salt. This current is easily recognized from its green colour in the Straits of Formosa as far as Hong Kong. In the Southern Hemisphere the South Equatorial current is produced by the south-east trades, and is much more vigorous than its northern counterpart. On reaching the western Pacific, part of this cur rent passes southwards, east of New Zealand, and again east of Australia, as the east Australian current parts northwards to join the Equatorial Counter-current. In the higher latitudes of the south Pacific the surface movement forms part of the west wind-drift of the Roaring Forties.
The cold water of this stream, with contributions from the depths, on the coasts of Chile and Peru, gives rise to the scanti ness of the rain in this region as far northward as Callao and Payta, an effect analogous to that of the Benguela current along the coast of South-west Africa. Sometimes it appears that this Peruvian current in the northern part is hemmed in by the warmer water of the "El nirio" current, which spreads southward from the coast of Colombia and Ecuador. This has disastrous con sequences for the animal world : the guano-birds and the fish die. It also has fatal effects on the coast regions, for the rain asso ciated with the warm current entails great economic injury on this erstwhile desert-like region. Such a catastrophe occurred last in Peru between January and March 1925.
See Reports of expeditions of the U.S.S. "Albatross" and "Thetis" (1888-92) ; A. Agassiz, Expedition to the Tropical Pacific (1899-1900, 1904-05) ; H.M.S. "Challenger" (1873-76) ; "Egeria" (1888-89 and 1899) ; "Elisabeth" (1877) ; "Gazelle" (1875-76) ; "Planet" (1906) ; "Penguin" (1891-1903) ; "Tuscarora"
; "Vettor Pisani" (1884) ; "Vitiaz" (1887-88) ; also observations of surveying and cable ships, and special papers in the Annalen der Hydrographie; for dis tribution of temperature and for currents see H. Thorade, p. 17 (1909) ; G. Schott, p. 2 ( 191 0) ; B. Schulz, p. 177 (191I) ; F. Zorck, p. 166 (1928) ; for salinity see G. Schott, p. 148 (1928) and in the Archly der Seewarte (G. Schott, 1891; C. Puls, 1895).