Contribution of Ekman Transport to the ENSO Periodicity Estimated With an Extended Wyrtki Index

Off‐equatorial wind stress curl anomalies that mainly drive geostrophic transport are essential for the phase transition of the El Niño‐Southern Oscillation (ENSO) induced by a recharge‐discharge process. The ENSO‐induced zonal wind stress anomaly also drives surface Ekman currents, which may counteract the geostrophic transport, but its effect on ENSO periodicity remains unclear in the recharge oscillator theory. Here, we extended an ENSO diagnostic framework, called the Bjerknes‐Wyrtki‐Jin index, to evaluate Ekman transport. The extended method applied to reanalysis data sets shows that the ENSO period lengthens by approximately 30%, reconciling the discrepancy between the observed ENSO frequency and estimates from the conventional method. This result demonstrates that the self‐inhibitory process by Ekman transport is an inherent feature of the observed ENSO periodicity. In addition, Ekman transport efficiently works with a wide meridional structure of ENSO, suggesting that the meridional width of individual ENSO events explains the difference in their duration. Plain Language Summary The dominant interannual climate variability, El Niño‐Southern Oscillation (ENSO), is an oscillatory phenomenon accompanying anomalous sea surface temperature in the eastern equatorial Pacific. ENSO drives extreme weather all over the world; therefore, understanding its periodic nature is important. Previous studies have developed a powerful tool, called the Wyrtki period index, to diagnose ENSO periodicity based on the known ENSO dynamics; however, it has a systematic bias that leads to a shorter ENSO period than those observed. Here, we extended the Wyrtki period index to include the contribution of the surface Ekman transport explicitly, which retards the ENSO phase reversal. Our diagnostics show that the ENSO period is approximately 30% longer than that of the conventional method and reproduces the observed ENSO spectrum well. Key Points Equatorial wind stress pattern associated with El Niño‐Southern Oscillation (ENSO) drives the Ekman transport, which acts to slow down the ENSO recharge‐discharge process We extended a theoretical ENSO period index to demonstrate the role of meridional Ekman transport that lengthens the ENSO periodicity Ekman transport contributes to lengthening the ENSO period by 30% relative to the estimation by geostrophic transport alone