scale vagaries in weather that are not predictable. Vulnerability of individuals will also vary according to the diversity of the operation: whether there is irrigation available, whether the farmer has insur- ance, and whether he or she can work off the farm to help out in times of adversity. See also Carbon Dioxide (CO 2 ) Cycle. El Nin  o Southern Oscillation (ENSO) Models. Evaporation and Humidity. Fisheries and Climate. Heat Transport and Climate. Ocean Circulation. Paci\c Ocean Equatorial Currents. Further Reading Glantz MH, Katz RW and Nicholls N (eds) (1991) Tele- connections Linking World-wide Climate Anomalies. Cambridge: Cambridge University Press. Glantz MH (1996) Currents of Change: El Nin J o’s Impact on Climate and Society. Cambridge: Cambridge Uni- versity Press. Suplee C (1999) El Nin  o/La Nin  a. National Geographic, March, 72}95. National Research Council (1996) Learning to Predict Climate Variations Associated with El Nin J o and the Southern Oscillation: Accomplishments and Legacies of the TOGA Program. Washington: National Acad- emy Press. Philander SGH (1990) El Nin J o, La Nin J a, and the South- ern Oscillation. London: Academic Press. Trenberth KE (1997) Short-term climate variations: recent accomplishments and issues for future progress. Bulletin of the American Meteorological Society 78: 1081}1096. Trenberth KE (1997) The deRnition of El Nin  o. Bulletin of the American Meteorological Society 78: 2771} 2777. Trenberth KE (1999) The extreme weather events of 1997 and 1998. Consequences 5(1): 2}15. EL NIN  O SOUTHERN OSCILLATION (ENSO) MODELS S. G. Philander, Princeton University, Princeton, NJ, USA Copyright ^ 2001 Academic Press doi:10.1006/rwos.2001.0401 Introduction The signature of El Nin  o is the interannual appear- ance of unusually warm surface waters in the east- ern tropical PaciRc Ocean. That area is so vast that the effect on the atmosphere is profound. Rainfall patterns are altered throughout the tropics } some regions experience Soods, others droughts } and even weather patterns outside the tropics are affec- ted signiRcantly. From an atmospheric perspective, these various phenomena are attributable to the change in the sea surface temperature pattern of the tropical PaciRc. Why does the pattern change? Whereas sea surface temperatures depend mainly on the incident solar radiation over most of the globe, the tropics are different. There the winds are of primary importance because of the shallowness of the thermocline, the thin layer of large temperature gradients, at a depth of approximately 100 m, that separates warm surface waters from the cold water at depth. The winds, by causing variations in the depth of the thermocline, literally bring the deep, cold water to the surface in regions where the ther- mocline shoals. For example, the trade winds that drive warm surface waters westward along the equator expose cold, deep water to the surface in the eastern equatorial PaciRc. A relaxation of the winds, such as occurs during El Nin  o, permits the warm water to Sow back eastward. The changes in the winds are part of the atmospheric response to the altered sea surface temperatures. This circular argument } the winds are both the cause and conse- quence of sea surface temperature changes } sug- gests that interactions between the ocean and atmosphere are at the heart of the matter. Those interactions give rise to a broad spectrum of natural modes of oscillation. This result has several impor- tant implications. One is that El Nin  o, even though we tend to regard him as an isolated phenomenon that visits sporadically, is part of a continual Suctu- ation, known as the Southern Oscillation. El Nin  o corresponds to one phase of this oscillation, the phase during which sea surface temperatures in the eastern tropical PaciRc are unusually high. La Nin  a is the name for the complementary phase, when temperatures are below normal. (Very seldom are temperatures ‘normal’, as is evident in Figure 1.) To ask why El Nin  o, or La Nin  a, occur, is equivalent to asking why a pendulum spontaneously swings back and forth. Far more interesting questions concern the factors that determine the period and other properties of the oscillation, and the degree to which it is self-sustaining or damped. Only strongly damped oscillations, that disappear at times, require EL NIN  O SOUTHERN OSCILLATION (ENSO) MODELS 827