PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM 



two years' periodicity, especially dur- 

 ing the 1960's; at other times the 

 rhythms were less regular. 



The mechanism of the equatorial 

 air-sea rhythms is illustrated in Fig- 

 ure IV-5, which shows that a six- 

 month, smoothed time-series of 

 atmospheric pressure in Djakarta, 

 Indonesia (6°S. 107 E.), exhibits the 

 same long-period trends as the sea- 

 surface temperatures measured at 

 Canton Island and by ships crossing 

 the equator at 165W. When the 

 barometric pressure in Djakarta is 

 lower than normal, the equatorial 

 easterlies heading for the Indonesian 

 low become stronger than normal; 

 this automatically intensifies the 

 Pacific equatorial upwelling and cools 

 the sea surface. The parallelism of 

 the time-series of Djakarta pressure 

 and Canton Island sea temperature 

 is thereby assured. 



If wind profiles are observed along 

 the equator at two opposite phases 

 of the air-sea rhythm, as exemplified 

 by November 1964, with its cool 

 ocean and aridity, and November 

 1965, with its warm ocean and abun- 

 dant rainfall at Canton Island, it is 



found that in November 1964 the 

 equatorial easterlies swept uninter- 

 ruptedly from South America past 

 Canton Island toward a deeper-than- 

 normal Indonesian low, whereas in 

 November 1965 they stopped short 

 of reaching Canton Island. The equa- 

 torial upwelling — a by-product of 

 the equatorial easterlies — extended 

 almost to Indonesia in November 

 1964, while being confined to a much 

 smaller area east of Canton Island 

 a year later. Concomitantly, the 

 equatorial rainfall was confined to the 

 neighborhood of Indonesia in No- 

 vember 1964; the following year it 

 expanded from the west to beyond 

 Canton Island, while Indonesia suf- 

 fered serious drought. 



The propulsion of the air-sea 

 rhythms resides in the atmospheric 

 thermally driven equatorial circula- 

 tion over the Pacific, which has its 

 heat source (by condensation) in the 

 rising branch, and heat sink (by 

 radiative deficit insufficiently com- 

 pensated by scarce precipitation) in 

 its descending branch near South 

 America. The oceanic counterpart to 

 this atmospheric circulation is, in 

 part, the westward surface drift and 



Figure IV-5 — WALKER'S "SOUTHERN OSCILLATION" 



The diagram shows the similarities in trend ot the time-series of sea temperature 

 and pressure measured at and near the equator in the southern hemisphere. The 

 dotted curve that follows that for Djakarta is based on data from Singapore. The 

 rapid oscillations of the sea-temperature curve measured at the equator in 1958 and 

 1959 result from more frequent ship crossings — and hence a greater density of short- 

 period detail — rather than from any unusual natural activity. 



the subsurface return flow and, addi- 

 tionally, the circulation consisting of 

 an upwelling thrust at the equator 

 and sinking motion to the north and 

 south of the equator. These ocean 

 circulations are wind-driven and in- 

 trinsically energy-consuming, but they 

 exert a powerful feedback upon the 

 atmosphere by slowly varying the 

 areal extent of warm water at the 

 equator and thereby varying the ther- 

 mal input for the global atmospheric 

 circulation. 



In November 1964, when cool up- 

 welling water occupied almost the 

 whole Pacific equatorial belt, the at- 

 mosphere received less heat than in 

 November 1965, when the upwelling 

 had shrunk back into a smaller east- 

 ern area. Consequently, the tropical 

 atmosphere swelled vertically from 

 1964 to 1965. This swelling was 

 most conspicuous over the Pacific 

 at 160 W. longitude. Moreover, the 

 swelling of the tropical atmosphere 

 had spread all around the global 

 tropical belt between 1964 and 1965, 

 a global adjustment that is inevitable, 

 since pressure gradients along the 

 equator must remain moderate. 



North and south of the swelling 

 atmosphere in the tropical belt, the 

 gradient of 200-millibar heights in- 

 creased from November 1964 to 

 November 1°65, which indicated 

 increasing westerly winds in the 

 globe-circling subtropical jet streams. 

 This can best be documented in the 

 longitude sector from the area of 

 Pacific equatorial warming eastward 

 across North America and the At- 

 lantic to the Mediterranean. 



The corresponding change at sea 

 level could be seen most dramatically 

 over Europe, where the moving low- 

 pressure centers abandoned their 

 normal track by way of Iceland to 

 Scandinavia and, instead, in Novem- 

 ber 1965 moved parallel to the 

 strengthened subtropical jet stream 

 and invaded central and southern 

 Europe. 



Other associated rearrangements 

 involved the arctic high-pressure sys- 



86 



