An Assessment of ENSO-Scale Secular Variability

Abstract

Secular changes in the spatial and temporal structures of the El Niño/Southern Oscillation (ENSO) cycle using four different versions of global sea surface temperature (SST) analysis are examined. The assessments were made for both multidecadal climate means and multidecadal measures of variability by separating the SST variations into low-frequency (periods longer than 30 years) and high-frequency components. The reliability of these estimates is also addressed. This study substantiates a conceptual framework that views the multidecadal, low-frequency variations as a varying climate ?base state? upon which ENSO-scale variability is superposed. The secular changes of the climate base state were quantified both in space and in time. The analysis suggests that multidecadal SST variability has been concentrated in the South Atlantic and Indian Ocean Basins. The Pacific is dominated by the ENSO-scale variability. The analyses reveal that variations in the climate base state and ENSO-scale variability were positively correlated; that is, ENSO-scale variability is higher (lower) when the climate mean SST is relatively warmer (colder). However, the quantification of secular changes of the ENSO-scale variability was found to be sensitive to the particular SST analysis used. Therefore, the conclusions from this study are subject to further verification by using more variables and longer records.