Classification of synchronous oceanic and atmospheric El Niño-Southern Oscillation (ENSO) events for palaeoclimate reconstruction

Abstract

Since the mid-1970s, ENSO has changed in character to a predominance of El Niño conditions, the extreme phase of which appears coincidental with increases in global temperature records. Instrumental time series (<150 years) are too short to adequately address the significance of late twentieth-century ENSO variability, thus, multi-century palaeoclimate reconstructions derived from long proxy records are sought. Despite the global influence exerted by ENSO on society, limited consensus exists within the scientific community as to which index best defines the timing, duration and strength of events. Here we address issues associated with the complexity of ENSO characterisation by comparing the 'event capture' ability of two currently used indices of ENSO. It is suggested that the use of a sole ENSO index is undesirable as a given index is only indicative of one physical aspect of the phenomenon, and as such is unlikely to be representative of the wider interactions experienced in the coupled ocean-atmospheric system. In an attempt to describe more of the nature and evolution of ENSO events, the Coupled ENSO Index (CEI) classification scheme was devised to identify synchronous oceanic (Niño 3.4 SST) and atmospheric (Southern Oscillation Index) anomalies associated with ENSO for the instrumental period . The CEI is of practical relevance to the ENSO community as it provides an amplitude preserving instrumental baseline for the calibration of proxy records to reconstruct both components of the ENSO system. Analysis of the nature of instrumental ENSO events from the CEI suggests that the frequency and intensity of post-1970 ENSO events (when 50% of all extreme events identified occur) appears the most anomalous in the context of at least the past century. It is hoped that the CEI will facilitate palaeo-ENSO research to systematically resolve the long-term context of past ENSO behaviour to assess whether the apparently anomalous nature of late twentieth-century variability is unprecedented within existing palaeoclimate archives.

Atypical influence of the 2007 La Niña on rainfall and temperature in southeastern Australia

“…A La Niña is classified when the mean annual value of an ENSO index is above (below) the 70th (30th) percentile for the CEI and SOI (Niño 3.4) (Figure 2a). This yields a similar number of La Niña years in all indices that correspond with documented moderate and strong La Niña events [Gergis and Fowler, 2005].…”

“…The Niño 3.4 describes ENSO variability using sea surface temperature (SST) anomalies in the central Pacific Ocean in an area bounded by 5°N-5°S, 120°W -170°W [Trenberth, 1997]. The Coupled ENSO Index (CEI) combines the low frequency components of the SOI and the Niño 3.4 using the 11-month and 5-month smoothed means of the respective indices [Gergis and Fowler, 2005].…”

How unusual was late 20th century El Niño-Southern Oscillation (ENSO)? Assessing evidence from tree-ring, coral, ice-core and documentary palaeoarchives, A.D. 1525-2002

“…In an attempt describe more of the nature and evolution of ENSO conditions, Gergis and Fowler (2005a) devised the Coupled ENSO Index (CEI) to register synchronous oceanic (Niño 3.4 SST) and atmospheric (Southern Oscillation Index) anomalies for the instrumental period . Anomalies expressed in either Niño 3.4 SST or SOI indices (and therefore perhaps indicative of decoupled or out of phase behaviour) are maintained in the CEI, while fully coupled ocean-atmospheric anomalies result in an amplification of the index.…”

“…Anomalies expressed in either Niño 3.4 SST or SOI indices (and therefore perhaps indicative of decoupled or out of phase behaviour) are maintained in the CEI, while fully coupled ocean-atmospheric anomalies result in an amplification of the index. Where previous studies have chosen to reconstruct the SOI or Niño region SSTs indices alone, here we use the newly introduced coupled ocean-atmosphere ENSO index as a baseline for the calibration of proxy records (Gergis and Fowler, 2005a). Table 1.…”

A climate reconstruction of Sydney Cove, New South Wales, using weather journal and documentary data, 1788-1791

“…According to a facsimile of Bradley's journal (Bradley 1969, p. 357 Gergis and Fowler (2005) were used to calculate ENSO composites to assess any ENSO forcing on Sydney's local temperature and MSLP variations during El Niño and La Niña conditions.…”