Amplitude, duration, variability, and seasonal frequency analysis of the El Niño-Southern Oscillation

As the climate is rapidly changing already, it is important to understand how the El Niño-Southern Oscillation (ENSO) has changed in recent decades, and this can shed light on future ENSO characteristics. The ERA5 data were used to analyze the amplitude, duration, variability, and seasonal frequency of El Niño and La Niña, and to investigate whether there have been changes in the amplitude and duration of these events during the period 1959–2021. During this period, variability in the amplitude of El Niño events has been much higher than that of La Niña events. Rainfall anomalies in the Niño 3.4 region also show much higher variability during El Niño compared with La Niña events, while variability in the duration of El Niño events has been much lower than that of La Niña events. The highest frequency of El Niño events is in boreal autumn and winter, while their lowest frequency is in boreal spring and summer. The frequency of La Niña events also varies seasonally, with the highest frequency in boreal autumn, but the lowest frequency in the late boreal spring to mid-summer. Both El Niño and La Niña reach the peak amplitude toward the end of the calendar year, while their minimum amplitudes occur in boreal spring. Significant positive and negative sea surface temperature (SST) trends are identified in the western and eastern tropical South Pacific, respectively, implying the strengthening of the zonal SST gradient in the tropical Pacific during the period 1959–2021. Despite that, the amplitude and duration of El Niño and La Niña events have not changed significantly over the past six decades, implying that these ENSO characteristics have not been largely influenced by a climate change signal.