Leetmaa, A., W. Higgins, D. Anderson, P. Delecluse, and M. Latif, 2001: Application of seasonal to interannual predictions: a Northern Hemisphere perspective. In Observing the Oceans in the 21st Century,  Melbourne, Australia: Uniprint Pty. Ltd., 39-47.

Abstract: Climate variability in many parts of the world can impact a range of social and economic sectors. The dominant source of interannual global climate variability is the El Niño-Southern Oscillation phenomenon (ENSO). Over the past few decades scientists have developed a capability to forecast aspects of ENSO and its impacts several seasons in advance using statistical and dynamical tests. This capability was used to mitigate some of the impacts of the major 1997-98 El Niño in many countries. However, ENSO accounts for only part of the observed variability. Other sources of climate variability are also important on seasonal, interannual, and decadal timescales. These fall into two broad classes. The first, of which ENSO is the best-known member, is related to global tropical rainfall variation on intraseasonal, seasonal, and decadal timescales. A better understanding of all the sources of rainfall variability—on all timescales and throughout the global tropics—will likely lead to improved forecast skill, since this builds on existing understanding and forecast systems developed for ENSO. The second class consists of changes in zonal flows in the midlatitudes; examples of this class are the North Atlantic Oscillation and the Arctic Oscillation. More research is required in order to develop an understanding of the origins of these zonal flow changes and to convert this into skillful forecast methodologies. Research into all of these and other yet unexplained phenomena and the subsequent development of improved seasonal forecasts will require expansions of the existing ocean observing system.