General Notes

The top plot shows the ENSO time series on which the ENSO classification is based. The bottom plot shows the IOD time series on which the IOD classification is based. These time series have a complex definition that can be found in the original research paper (Meyers et al. 2007).

The dark blue lines are the final time series after approximate climate shifts have been removed (see Meyers et al. 2007). The light blue lines (where visible) are the unshifted time series. Where the light blue line is not visible, it is underneath the dark blue line, and hence the climate shift is zero. Climate shifts help to compensate for trends in SST caused by global warming, and also for an apparent climate anomaly from 1954-1960 in the Indian Ocean. The latter anomaly is not well known or understood, and more work is needed. Reducing the influence of climate trends and shifts in this simple way is not ideal, and a better method needs to be found.

The red lines are the normalized Nino 3 index (top plot) and the negative of the Indian Ocean Dipole eastern box SST (so negative means the eastern Indian Ocean is warm). They are shown for comparison with the more complex time series used by this method.

All time series have been normalized with respect to the seasonal long-term mean. The magenta dashed lines indicate one standard deviation, and the dark blue time series symbols become open circles when it exceeds one standard deviation (either positive or negative).

ENSO

A year is counted as being an El Niño year if the dark blue time series in the top plot is larger than 1 for two or more consecutive months between and including June and February of the following year. A La Niña year is defined if the time series is less than -1 for two or more consecutive months in the June to February period.

IOD

A year is counted as being a (+)ve IOD year if the dark blue time series in the bottom plot exceeds 1 for two or more consecutive months between and including June and December. A (-)ve IOD year is defined when the time series is less than -1 for two or more consecutive months in the June to December period. Note that this time series is similar to the negative of the SST anomaly in the Indian Ocean Dipole eastern region.