Protracted’ ENSO episodes and their impacts in the Indian Ocean region

Abstract

Abstract Evidence for a quasi-decadal (9–13 yr) signal interacting with the quasi-biennial (QB) (2–2.5 yr) and low-frequency (LF) (2.5–8 yr) components of the El Nino Southern Oscillation (ENSO) phenomenon to produce both ‘protracted’ El Nino and La Nina episodes is examined using historical patterns of climatic variables over the Indian Ocean basin in the global context. Across the Indian Ocean basin, seasonal rainfall correlations on LF ENSO and quasi-decadal timescales show that the lower frequency signal displays ‘ENSO-like’ characteristics. Thus, there is evidence that the quasi-decadal signal alone can provide an important modulation of Indian Ocean climatic patterns. However, examinations of the Indian Ocean situation in the wider global context provide strong evidence that ‘protracted’ El Nino and La Nina episodes result from interactions between QB, LF and quasi-decadal signals in the climate system. The seasonal tropospheric velocity potential correlations show that QB, LF and quasi-decadal changes in mass overturning and convective regimes are of global extent. These mass overturnings are most coherent on LF ENSO timescales, but are also clearly evident in seasonal QB and quasi-decadal signal sequences. The situation in October–December (OND) also suggests that a propensity exists in the climate system for drought or flooding extremes to occur over central-eastern Africa to central Asia on QB to decadal timescales, and that this may be particularly manifest during ‘protracted’ El Nino and La Nina episodes. An investigation of individual ‘protracted’ El Nino and La Nina episodes reveals that the magnitude and phasing of the QB, LF and quasi-decadal signals can lead to periods during ‘protracted’ El Nino (La Nina) episodes, which start to take on ‘La Nina-like’ (‘El Nino-like’) characteristics, before reverting back again. This phasing nature involves opposing combinations of the above signals, especially on QB and LF ENSO timescales. Ultimately, it can lead to modulations of climatic variables that are different from those usually observed over ‘classical’ ENSO-sensitive regions. Such characteristics may have major consequences for global teleconnection patterns, and thus have the potential to impact on both regional and remote climatic signatures.