The theory of statistics of extremes and el niño phenomena : a stochastic approach

The possibility of El Nino being a result of the random superposition of stochastically forced equatorial Kelvin waves is investigated, with the help of the theory of statistics of extremes, which provides us with the tools to evaluate the threshold crossing statistics of the sea level (SL) and sea surface temperature (SST) anomaly fields. A comprehensive review of this theory is given, since it might be relevant to other oceanographic problems. A linear, stratified, equatorial p-plane Kelvin wave ocean model is forced by stochastic zonal winds. The zonal velocity field is used to advect mean zonal temperature gradients to produce SST anomalies. Solutions are obtained in terms of the zonal wavenumber-frequency spectra of SL and SST. These spectra are used to compute the threshold crossing statistics of the respective fields. The time and space scales for the SL and SST excursions above 2 standard deviations are found to be small, when compared with observed El Nino scales. The stochastic assumption is reinterpreted as a possible triggering mechanism, rather than as a full explanation, for El Nino occurrences. Better knowledge of the wind stress forcing spectrum (zonal wavenumber and frequency) is needed in order to test the plausibility of the stochastic argument in a more conclusive way. Thesis Supervisor: Dr. Carl Wunsch Title: Professor of Physical Oceanography

[1]  Allan J. Clarke,et al.  Wind-induced upwelling, coastal currents and sea-level changes , 1974 .

[2]  John M. Wallace,et al.  Planetary-Scale Atmospheric Phenomena Associated with the Southern Oscillation , 1981 .

[3]  K. Wyrtki.  Sea Level During the 1972 El Niño , 1977 .

[4]  D. Luther,et al.  Observations of long period waves in the tropical oceans and atmosphere , 1980 .

[5]  S. Zebiak.  Tropical Atmosphere - Ocean Interaction and the EL Nino/southern Oscillation Phenomenon. , 1984 .

[6]  Mark A. Cane,et al.  Modeling Sea Level During El Niño , 1984 .

[7]  A. E. Gill.  An Estimation of Sea-Level and Surface-Current Anomalies during the 1972 El Niño and Consequent Thermal Effects , 1983 .

[8]  T. Barnett.  Prediction of the El Niño of 1982–83 , 1984 .

[9]  J. O'Brien,et al.  On the Interannual Wind-Driven Response of the Tropical Pacific Ocean , 1983 .

[10]  M. Cane.  Oceanographic Events During El Ni�o , 1983, Science.

[11]  John P. Boyd,et al.  The nonlinear equatorial Kelvin wave , 1980 .

[12]  W. G. Leslie,et al.  Mean annual variation of sea level in the Pacific Ocean , 1980 .

[13]  K. Wyrtki.  The Response of Sea Surface Topography to the 1976 El Niño , 1979 .

[14]  M. Glantz,et al.  Resource management and environmental uncertainty : lessons from coastal upwelling fisheries , 1981 .

[15]  C. Eriksen,et al.  Wind-Generated Equatorial Kelvin Waves Observed Across the Pacific Ocean , 1983 .

[16]  J. O'Brien,et al.  The Seasonal Variability in a Model of the Tropical Pacific , 1980 .