Abstract
The El Nino and Southern Oscillation (ENSO) ‘diversity’ has been considered as a major factor limiting its predictability, a critical need for disaster mitigation associated with the trademark climatic swings of the ENSO. Improving climate models for ENSO forecasts relies on deeper understanding of the ENSO diversity but currently at a nascent stage. Here, we show that the ENSO diversity thought previously as ‘complex,’ arises largely as varied contributions from three leading modes of the ENSO to a given event. The ENSO ‘slow manifold’ can be fully described by three leading predictable modes, a quasi-quadrennial mode (QQD), a quasi-biennial (QB) mode and a decadal modulation of the quasi-biennial (DQB). The modal description of ENSO provides a framework for understanding the predictability of and global teleconnections with the ENSO. We further demonstrate it to be a useful framework for understanding biases of climate models in simulating and predicting the ENSO. Therefore, skillful prediction of all shades of ENSO depends critically on the coupled models’ ability to simulate the three modes with fidelity, providing basis for optimism for future of ENSO forecasts.
Highlights
The largest signal of climate variability on inter-annual time scale arising from ocean-atmosphere interaction in the tropics[1,2], the El Nino and Southern Oscillation (ENSO) has deep and wide ranging impacts across the globe[3,4]
The first Extended EOF (EEOF) represents a quasi-quadrennial (QQ) mode (Fig. 1c), the second EEOF a quasi-biennial (QB) mode (Fig. 1f) while the third EEOF representing a decadal modulation of the QB (DQB) mode (Fig. 1i)
It has been known that some EL Ninos do evolve simultaneously over most of equatorial Pacific basin, while some others starts evolving from the east, here, we show that they are two different modes of ENSO
Summary
The largest signal of climate variability on inter-annual time scale arising from ocean-atmosphere interaction in the tropics[1,2], the El Nino and Southern Oscillation (ENSO) has deep and wide ranging impacts across the globe[3,4]. We try to separate the temporal modes with different spatial patterns associated with them for which the evolutionary history would be distinct This may be achieved using an Extended EOF (EEOF)[28] analysis with a suitable number of lagged copies of the fields. Based on monthly data of SST and sea level pressure (SLP) over the tropical Pacific between 1854 and 2004, we carry out an EEOF analysis with lagged copies of the fields up to 18 months This analysis reveals three distinct ENSO ‘modes’, a quasi-quadrennial mode, a quasi-biennial mode and a decadal modulation of the quasi-biennial mode with distinct spatial patterns as well as temporal evolution of the spatial patterns. The identified modes are from de-trended data and represent the leading modes of natural ENSO variability
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