E. Linacre and B. Geerts
Surface wind anomalies around 40° S in South America has been found to vary over the last century with an average periodicity of 10 years or more (1). The variation is not sinusoidal, but rather winds change abruptly from one state to the other. For example, the monthly-mean intensity of the westerlies in Southern Chile in July changed abruptly in 1939, 1949, 1967 and 1977. These intensities were deduced from the differences between atmospheric pressures at Punta Galera (41° S) and Punta Arenas (53° S), both on the Pacific coast. Most abrupt changes in this region occurred in March, and it is not clear why.
The same study (1) looked at monthly mean sea-level pressure differences (D p) on the eastern side of the Andes, between a) Buenos Aires, and b) Cordoba (located to the northwest), since 1870. In summer (January-March) there usually is a low near Cordoba, resulting in warm, humid northeasterly winds between the two places. An abrupt D p decrease occurred in 1894 and an increase in 1954, resulting in anomalously wet summers in the northern half of Argentina. In winter (June-August) the typical pressure gradient is reversed as a high-pressure region settles inland, hence the prevailing wind inland from Buenos Aires is southwesterly. A sudden D p decrease occurred in 1899, an increase in 1949 and a decrease1970. A smaller winter pressure gradient leads to warmer winters in northern Argentina.
These occasional jumps either up or down resemble the changes of rainfall at Darwin shown in Fig 10.10 in the textbook. There are also similar suddenly alternating changes of temperature regime in Sydney (2). The switching between El Niño and La Niña events in the Pacific Ocean is also rather discontinuous, especially for strong cases. Rapid climate changes have also been observed at the end of the last Ice Age. And, on a much shorter time scale, we have all experienced sudden weather changes, especially in mid-latitudes, where frontal boundaries dominate the weather charts.
Many scientists have looked for cyclic changes in weather and climate, yet it is important to be aware of the discontinuous nature of the atmosphere, even on long time scales. This non-periodic behaviour must be related to the fundamental properties of fluid flow, in particular, that the atmosphere is chaotic.
(1) Vargas, W.M., J.L. Minetti & A.G. Poblete 1995. Statistical study of climatic jump in the regional zonal circulation over South America. J. Meteor. Soc. Japan 73, 849-56.
(2) Linacre, E.T. 1992. Climate Data & Resources (Routledge) p. 132.