Biennial Variability of ENSO

El Niño-Southern Oscillation (ENSO) has profound effects on global and regional climate scales, and is key for making accurate seasonal forecasts and more realistic long-term climate simulations. However, for such as an important cycle, we do not fully understand it yet, so increasing our knowledge of it can help our climate and seasonal predictions dramatically. Although ENSO is often thought of as being on a 3-7 year cycle, quasi-biennial frequencies, on the order of every 18 to 30 months, also play an important role. While approximately biennial frequencies often have a low magnitude compared to other spectra, this is mainly because biennial frequencies often exhibit their highest amplitude in certain spikes that often coincide with ENSO events. Fast Fourier transform filters are used to isolate bands of frequencies of surface temperature data over land and sea from the 20th Century Reanalysis. These frequencies play an important role in ENSO, being responsible for much of the variance not just in a thin band of the eastern equatorial Pacific, but in tropical waters across all ocean basins, and beyond.