Development of our understanding of the ENSO Cycle: Impact on Southern Hemisphere hydrology (Invited Presentation)

In a landmark paper published in 1969, Jacob Bjerknes synthesized diverse paths of oceanic and atmospheric research to reveal an elegant system of coherent ocean/atmosphere interactions over the equatorial Pacific that subsequently became known as ENSO. The seven decades of research which laid the groundwork for this fundamental advance in our understanding of climate variability is briefly reviewed, with emphasis on associated southern hemisphere hydrologic anomalies.

Observational studies during the past three decades have more clearly revealed the hemispheric pattern of "typical " ENSO cycle circulation and hydrologic anomalies. This research will be reviewed in the context of the NCEP/NCAR reanalysis, the Xie-Arkin global precipitation data set, and analyses of streamflow data.

The Bjerknes synthesis has been the dominant paradigm for ENSO studies for the past three decades. It has fully fulfilled the prophesy of Jule Charney, that Bjerknes "has uncovered for us a class of long-period ocean-atmosphere interactions which promises to supply the physical basis on which dynamical models can be built to account for extra-seasonal climatic fluctuations." However, recent observational studies have revealed pronounced regime-like interdecadal variability in the character of ENSO, as well as what some have interpreted from the relatively short time series to be interdecadal "modes" of variability in both hemispheres. One of these has been described by some authors as "ENSO-like" variability on an interdecadal time scale . While significant for prediction and long term water resource planning, the interrelationships, if any, between interannual ENSO cycle variability and the various manifestations of interdecadal "modes", are at present murky. We speculate on the near-term prospects for a new synthesis, which will bring together what are at present poorly understood aspects of interdecadal variability, in a coherent framework comparable to what Bjerknes developed for interannual variability thirty years ago.