No abstract

The El Niño-Southern Oscillation (ENSO) cycle of alternating warm El Niño and cold La Niña events is the dominant year-to-year climate signal on Earth. ENSO originates in the tropical Pacific through interactions between the ocean and the atmosphere, but its environmental and socioeconomic impacts are felt worldwide. Spurred on by the powerful 1997-1998 El Niño, efforts to understand the causes and consequences of ENSO have greatly expanded in the past few years. These efforts reveal the breadth of ENSO's influence on the Earth system and the potential to exploit its predictability for societal benefit. However, many intertwined issues regarding ENSO dynamics, impacts, forecasting, and applications remain unresolved. Research to address these issues will not only lead to progress across a broad range of scientific disciplines but also provide an opportunity to educate the public and policy makers about the importance of climate variability and change in the modern world.

No abstract

Abstract. Beginning from the hypothesis by Bjerknes [1969] that oceanatmosphere interaction was essential to the E1 Nifio-Southern Oscillation (ENSO) phenomenon, the Tropical Ocean-Global Atmosphere (TOGA) decade has not only confirmed this but has supplied detailed theory for mechanisms setting the underlying period and possible mechanisms responsible for the irregularity of ENSO. Essentials of the theory of ocean dynamical adjustment are reviewed from an ENSO perspective. Approaches to simple atmospheric modeling greatly aided development of theory for ENSO atmospheric feedbacks but are critically reviewed for current stumbling blocks for applications beyond ENSO. ENSO theory has benefitted from an unusually complete hierarchy of coupled models of various levels of complexity. Most of the progress during the ENSO decade came from models of intermediate complexity, which are sufficiently detailed to compare to observations and to use in prediction but are less complex than coupled general circulation models. ENSO theory in simple models lagged behind ENSO simulation in intermediate models but has provided a useful role in uniting seemingly diverse viewpoints. The process of boiling ENSO theory down to a single consensus model of all aspects of the phenomenon is still a rapidly progressing area, and theoretical limits to ENSO predictability are still in debate, but a thorough foundation for the discussion has been established in the TOGA decade.

No abstract

We investigate the response of El Niño to natural radiative forcing changes over the past 1000 years based on numerical experiments employing the Zebiak-Cane (Zebiak and Cane, 1987) model

Forecasts of El Niño climate events are routinely provided and distributed, but the limits of El Niño predictability are still the subject of debate. Some recent studies suggest that the predictability is largely limited by the effects of high-frequency atmospheric 'noise', whereas others emphasize limitations arising from the growth of initial errors in model simulations. Here we present retrospective forecasts of the interannual climate fluctuations in the tropical Pacific Ocean for the period 1857 to 2003, using a coupled ocean-atmosphere model. The model successfully predicts all prominent El Niño events within this period at lead times of up to two years. Our analysis suggests that the evolution of El Niño is controlled to a larger degree by self-sustaining internal dynamics than by stochastic forcing. Model-based prediction of El Niño therefore depends more on the initial conditions than on unpredictable atmospheric noise. We conclude that throughout the past century, El Niño has been more predictable than previously envisaged.

An analysis of historical sea surface temperatures provides evidence for global warming since 1900, in line with land-based analyses of global temperature trends, and also shows that over the same period, the eastern equatorial Pacific cooled and the zonal sea surface temperature gradient strengthened. Recent theoretical studies have predicted such a pattern as a response of the coupled ocean-atmosphere system to an exogenous heating of the tropical atmosphere. This pattern, however, is not reproduced by the complex ocean-atmosphere circulation models currently used to simulate the climatic response to increased greenhouse gases. Its presence is likely to lessen the mean 20th-century global temperature change in model simulations.