Hobart

Seminar Abstract

Wednesday 1 April 2009, 11.30am (Tas time)
CSIRO Auditorium, Hobart

Allan J Clarke
Adrian E Gill Professor of Oceanography
Distinguished Research Professor, Department of Oceanography
Florida State University, USA

A verified estimation of the El Niño index NINO3.4 since 1877

Decadal and longer time scale variability of our best known El Niño Southern Oscillation (ENSO) indices are poorly correlated before 1950 and so our knowledge of interdecadal variability and trend in ENSO indices is dubious, especially before 1950. To address this problem, we constructed and compared physically related monthly ENSO indices. Our base index was El Niño index NINO3.4, the sea surface temperature (SST) anomaly averaged over the equatorial box bounded by 5°N, 5°S, 170°W and 120°W; we also constructed indices based on the Nighttime Marine Air Temperature over the NINO3.4 region (NMAT3.4) and an Equatorial Southern Oscillation Index (ESOI). Our NINO3.4 index used the HadSST2 monthly data set (Rayner et al. 2006), a data set with smaller uncertainty and better geographical coverage than others. In constructing the index, data at each point for a given month were weighted to take into account the typical considerable spatial variation of the SST anomaly over the NINO3.4 box as well as the number of observations at that point for that month. Missing monthly data were interpolated and “noise” was reduced by using the result that NINO3.4 has essentially the same calendar month amplitude structure every year. This 12-point calendar month structure from April to March was obtained by an EOF analysis over the last 58 years, and then was fitted to the entire monthly time series using a least square approach. Equivalent procedures were followed for NMAT3.4 and ESOI. The new ESOI index uses Darwin atmospheric pressure in the west and is based on theory that allows for variations of the atmospheric boundary layer depth across the Pacific.

The new NINO3.4 index was compared with NMAT3.4, the new ESOI and with a record of δ¹⁸O from a coral at Palmyra, an atoll inside the region NINO3.4 (Cobb et al. 2003). Correlation coefficients between NINO3.4 and the three monthly indices mentioned above before 1950 are 0.84, 0.87, 0.73 and 0.93, 0.86, 0.73 for decadal time scales. These relatively high correlation coefficients between physically related but independent monthly time series suggest that we have improved our knowledge of low-frequency variability. All four indices are consistent with a rise in NINO3.4 SST and the weakening of the equatorial Pacific winds since about 1970. These changes appear to be due to anthropogenic forcing.

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Location:
CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
To schedule a seminar, contact:
Clothilde Langlais, (Oceanographic seminars) CSIRO Marine and Atmospheric Research (03) 6232 5399
Natalie Kelly, (Biology/Modelling seminars) CSIRO Marine and Atmospheric Research 0438 452 483
Jillian Enraght-Moony, (seminar administrator) CSIRO Marine and Atmospheric Research (03) 6232 5320
Communications Manager, Antarctic Climate and Ecosystems CRC (03) 6226 2265
Margaret Hazelwood, Institute of Antarctic and Southern Ocean Studies (IASOS) University of Tasmania (03) 6226 2971

Last updated 21/07/09