Abstract
The Earth has seen El Niño-Southern Oscillation (ENSO)—the leading mode of interannual climate variability—for at least millennia and likely over millions of years. This paper reviews previous studies from perspectives of both paleoclimate proxy data (from traditional sediment records to the latest high-resolution oxygen isotope records) and model simulations (including earlier intermediate models to the latest isotope-enabled coupled models). It summarizes current understanding of ENSO’s past evolution during both interglacial and glacial periods and its response to external climatic forcings such as volcanic, orbital, ice-sheet and greenhouse gas forcings. Due to the intrinsic irregularity of ENSO and its complicated relationship with other climate phenomena, reconstructions and model simulations of ENSO variability are subject to inherent difficulties in interpretations and biases. Resolving these challenges through new data syntheses, new statistical methods, more complex climate model simulations as well as direct model-data comparisons can potentially better constrain uncertainty regarding ENSO’s response to future global warming.
Highlights
El Niño-Southern Oscillation (ENSO) is the leading mode of natural climate variability that fluctuates irregularly on interannual time scales
Significant advances have been made in the paleoclimate ENSO studies in the last few decades
Numerical experiments using a newer generation of CGCMs with more complete physics have been widely coordinated and analyzed; continuous simulations of past climate for tens of thousands of years in a CGCM is a possibility; isotope-enabled climate models have offered new opportunities to make direct model-data comparison and improve interpretations of proxy records
Summary
El Niño-Southern Oscillation (ENSO) is the leading mode of natural climate variability that fluctuates irregularly on interannual time scales. Atmosphere 2018, 9, 130 still facing significant challenges, for example, in spring 2014 a major El Niño was forecasted but it never materialized later that year [10] It remains highly uncertain how ENSO variability will respond to future global warming, as reviewed in the projections of the current generation of climate models [11,12,13,14,15,16]. Observations of ENSO prior to instrumental records, or paleo ENSO reconstructions, provide an alternative for reconstructing and studying the response of ENSO to various climate forcings They may help to constrain climate models and reduce the uncertainties in simulating the ENSO response to external forcings [19,20].
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