PERU CURRENT 



a first reference to the countercurrent. Augustine de Zarate, who was 

 sent to Peru by the King of Spain soon after Pizarro's expeditions, 

 gave a more complete report on conditions without however under- 

 standing their cause. Later accounts by early English voyagers were 

 mostly concerned with piratical adventures. Frezier, an experienced 

 observer, described a passage from Callao to Valparaiso (1717) as 

 follows : 



"The Regularity of the Winds at E. S. E. and S. E. and the 

 Breezes at S.W. along the coast of Peru, made the Navigation 

 so tedious, before the Method was found of running out to sea, 

 that Ships were six or seven months of sailing from Lima to La 

 Conception, because they only advanced by the Help of some 

 small Northern Blasts and the Land-Breezes, during the night, 

 and some part of the morning. This shows that the Want of 

 understanding Natural Philosophy among Sailors is a greater 

 Evil than is imagined." 

 Alexander von Humboldt was the first (1802) to take the temper- 

 ature of the coastal current and to make strictly scientific observa- 

 tions. He demonstrated that the temperature decreased approaching 

 the coast from seaward and concluded that this represented a flow of 

 cold water from high latitudes. 



De Tessan (1844) partially solved the problem with his suggestion 

 of an upwelling of the lower layers as an explanation of the low tem- 

 perature. Many scientists have since contributed to the description 

 and store of facts concerning the phenomenon now named the Peru 

 Current, in accordance with modern usage and a more geographical 

 designation. 



Sailing directions and other manuals of navigation describe the 

 Peru Current as part of the counterclockwise system of currents in 

 the South Pacific. At a mean latitude of about 50° S. the surface 

 water, encouraged by the west-wind drift of sub-antarctic latitudes 

 moves eastward and later sets in an oblique northeast direction toward 

 the west coast of South America and thence as a cold current north- 

 ward along the coast to the Galapagos Islands. The current follows 

 the trend of the coast in a general north and north-northeast direction, 

 varying slightly under the influence of the wind. The width of this 

 stream is about 120 miles at the latitude of Valparaiso. Temperatures 

 in the current range from 39° F. in latitude 47° S. increasing to 52° F. 

 off Valparaiso (Lat. 33° S.), 57° F. off Coquimbo (Lat. 30°S.), and 

 64° F. near Arica (Lat. 18°30'S.). The average velocity is about 15 

 miles per day. The drift along the Peruvian Coast is about one knot 

 or less between Arica and Pisco (Lat. 13°30'S.), with a westerly set 

 felt at times. The greatest strength is attained between Paita (Lat. 

 5° S.) and the Galapagos Islands where vessels have occasionally been 

 set 50 miles to the west-northwest in 24 hours. Seasonally under the 

 influence of a northerly wind, a southerly set is experienced inshore 

 in certain areas. This same northerly wind can bring torrential rains 

 to an otherwise arid coast and can cause the inflow of warmer equa- 

 torial water as far south as Salaverry (Lat. 8°S.) with an accompa- 

 nying widespread mortality of fish and birds. Curving westward 

 near Cape Blanco the stream passes the Galapagos Islands and joins 

 the South Equatorial Current. 



The surface movements of sea water are due primarily to mete- 

 orological causes. Ocean currents are produced mostly by the agency 

 of the wind. The center of the semi-permanent anticyclone in the 

 South Pacific is located off the north coast of Chile near Lat. 30° S., 

 Long. 90° W. Its position is relatively stable. On the southern side is 

 the zone of westerlies, northward that of the Southeast Trade Winds; 

 on the east and west are the coastwise winds, influenced by the usual 

 offshore and onshore daily alternations. This ocean wind system tends 

 to blow counterclockwise around a high pressure area of prevailing 

 tranquility in which deep transparent blue water and a clear horizon 

 are a marked characteristic. This is the "Great South Sea" named 

 Pacific by Magellan because of the steady gentle winds that drove his 

 fleet across the wide waters. 



On the southern edge of this South Pacific expanse, the west wind 

 drift along the edge of the sub-antarctic convergence, and the result- 

 ing effect of these cool waters on the greater part of the west coast of 

 South America, determine the kind of climate that will prevail. The 

 drift also determines what fish will exist in the waters and the kind of 

 birds in the air above. It has also determined the economic circum- 

 stances of life ashore. 



The surface waters in the Peru Current are relatively low in 

 temperature close to shore, with progressively rising temperature off- 

 shore. There is a remarkable uniformity of low temperature extend- 

 ing northward through the greater length of the current, which is not 



ordinarily affected by latitude or season. This condition is made 

 possible mostly by an upwelling from cooler intermediate layers of 

 water and not to the northerly flow of surface waters. Otherwise 

 these surface waters would gradually warm as they flowed northward. 

 The inevitable result of the meteorological circumstances along the 

 coast accelerates a flow of surface water offshore which results in a 

 vertical circulation described as upwelling. One process in upwelling 

 has been attributed to deviation, in which divergence of surface water 

 offshore is a result of the earth's rotation. Another process would be 

 the effect of offshore winds and ocean drift. 



Upwelling along the coast is the cause of the varied marine life 

 of this area. It brings to the well-illuminated upper layers the inex- 

 haustible store of nutrient salts from the ocean depths, upon which 

 the plant elements depend for life. This rich life gives to the coastal 

 water its typical greenish color in contrast to the indigo or blue of the 

 open ocean. 



Recent current and temperature studies reveal the upwelling as 

 practically limited to the upper 100 to 150 fathoms. Surface temper- 

 atures close to shore along the coast range between 58° F. and 64° F. 

 and average about 18° F. lower than latitude alone would indicate. 

 The atmospheric temperatures as a result are maintained at a similar 

 unchanging low average all the way up the coast from about latitude 

 38° S. to the Gulf of Guayaquil. 



The drift north, strong inshore and diminishing offshore, is the 

 most evident feature of the Peru Current. The decrease in northerly 

 set offshore is accompanied by an increasing westerly set. Upwelling 

 off both Chile and Peru is the final evidence of the movement offshore 

 of surface layers. Frequent observations of countercurrents close 

 inshore are associated with the usual coastal eddies due to contours 

 of the coast and variations with the season. 



The Peru Current, off Punta Aguja (Lat. 6° S.), normally swings 

 to the west and converges with the Equatorial Countercurrent run- 

 ning east. The line of convergence marked by a tide rip may run in 

 an irregular line from Punta Aguja to the Galapagos Islands. The 

 Equatorial Countercurrent normally turns northward along the coasts 

 of Ecuador, Colombia, and Central America but seasonally it swings 

 to the south during January to March and as a result a countercurrent 

 of warm water moves down the coast of Peru, displacing the ordinarily 

 cold water of the Peru Current. This influx of warm water named 

 "El Nino" may reach as far south as Salaverry (Lat. 8° 13' S.). This 

 is a drastic temperature reversal and is associated with widespread 

 mortality in littoral invertebrates, fish, and birds. A common result 

 of this disturbance to the planktonic life is observed in the extensive 

 discoloration of the water. The phenomenon is always accompanied 

 by marked changes in the fauna of the sea, including the disappear- 

 ance of the typical cool water birds, and the invasion of the region by 

 equatorial species. 



A similar current change occurring farther south in latitudes 8°S. 

 to 13° S. during the months of April through June is called "Aguaje" 

 and is caused by an inshore movement of outlying warmer water. 

 After the occurrence of the "Aguaje", penetration and warming in 

 shallow water by the sun, hastens decomposition of the plankton 

 in such a manner as to discolor the ocean and produce hydrogen sul- 

 phide in a quantity sufficient to poison fish. This is but one of the 

 several causes contributing to the widespread mortality. 



Along the northwest extension of the current (Lat. 3° S.), towards 

 the Galapagos Islands, maximum velocity is reached. The cool green- 

 ish water mixes with the warmer blue waters of the South Equatorial 

 Current, often forming distinct bands or stripes. These mixing zones 

 often prove very rich in sea life. Conditions are also favorable for sea 

 birds. This is in marked contrast to the barren ocean areas offshore 

 and northward. 



The climate of the coastal region of Peru is arid with prevailing 

 winds of small force from the southeast quadrant. Here, as on the 

 northern coast of Chile, the cool waters of the Peru Current produce 

 a stable condition of the atmosphere with considerable cloudiness but 

 no rainfall of consequence. On the coast there are no storms and no 

 sudden changes of temperature although the sky is frequently cloudy 

 and threatening. The immediate coast of Peru has a regular alter- 

 nating breeze locally called virazon which blows toward the land in 

 the daytime, and terral which blows toward the sea at night. Farther 

 offshore, the southeast trades always prevail. 



Air temperature over the current is affected by the cool water 

 from the upwelling. Conditioned by the hydrography of the sub- 

 marine areas, the sharpest lowering of temperature corresponds in posi- 

 tion with the ocean deeps close inshore or where the continental shelf 



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