Reconstructing El Nino-southern oscillation

El Niño-Southern Oscillation (ENSO) is the most important coupled ocean-atmosphericphenomenon to cause global climate variability on interannual time scales. Efforts tounderstand recent, apparently anomalous ENSO behaviour are hampered by the lack of long,high-quality climate records. While instrumental data generally covers the past 150 years,record length is insufficient for the assessment of past changes in the frequency, magnitude,and duration of ENSO. Here, multiproxy networks of high-resolution tree-ring, coral, iceand documentary records derived from eastern and western Pacific ENSO ‘centres of action’are analysed (A.D. 1525-2002). Considerable improvements in ENSO reconstruction areachieved from expanding the use of records from the western Pacific. In particular, ~500years of a continuous 3,722 year ENSO sensitive tree-ring record from New Zealand isintroduced.Although extreme ENSO events are seen throughout a 478-year discrete event analysis, 43%of extreme, 20% of very strong and 28% of all protracted ENSO events occur within the 20thcentury. Principal component analysis was used to extend instrumental records of theSouthern Oscillation Index (SOI) Niño 3.4 Sea Surface Temperature (Niño 3.4 SST) and anewly developed coupled ocean-atmospheric ENSO index (CEI) by 347 years. Significantly,of the three indices reconstructed here, CEI reconstructions were largely found to be the bestpredictors of ENSO. The results suggest that ENSO may be more effectively characterisedusing a coupled ocean-atmosphere index, particularly for December-May periods.Compared to the pre-instrumental period, the late 19th and early 20th centuries indicate aclear trend toward increased ENSO variability over the past 150 years. Significantly, spectralanalysis of reconstructed indices reveals a marked change in the frequency and intensity ofENSO beginning ~A.D. 1850, coinciding with the end of the Little Ice Age and the boomin global industrialisation. This suggests that ENSO may operate differently under natural(pre-industrial) and anthropogenically influenced background states. This study asserts thatrecent ENSO variability appears anomalous in the context of the past five centuries. Giventhe considerable socio-economic impacts of ENSO events, future investigation into theimplications an increasingly anthropogenically-warmed world may have on ENSO is vital