The importance of westerly wind bursts for ENSO dynamics demonstrated by “wind shaving” coupled model experiments

Abstract:

Westerly wind bursts (WWBs) – short lived, but strong westerly wind events in the equatorial Pacific – have been shown to play an important role in El Niño Southern Oscillation (ENSO) dynamics by providing stochastic forcing and contributing to the irregularity of El Niño, affecting its magnitude, spatial pattern, and timing. WWBs have a broad spectral tail toward longer frequencies due to its state-dependence on underlying sea surface temperature (SST) and a weak memory linked to the Madden-Julian Oscillation. Previous studies highlighted the importance of WWBs for particular El Niño events. Other studies focused on either low- or high-frequency components of the WWB forcing, but their respective roles are still not fully settled, in part due to different definitions of WWBs. Here, we aim to provide a new perspective on the importance of WWBs in ENSO dynamics. To that end, we designed a suite of experiments, using Community Earth System Model 1.2 and each lasting 200 years, in which we limit the transfer of zonal momentum from the atmosphere to the ocean during the daily atmosphere-to-ocean coupling. We do that by replacing zonal surface wind stress anomalies exceeding a preset threshold by the threshold value. That is, strong wind bursts are “shaved”. Across different experiments we successively reduce the value of the threshold thus “shaving” off more of WWBs. In these experiments the magnitude of ENSO and the frequency of extreme El Niño decrease following the threshold magnitude. The atmospheric zonal wind stress variability in a subseasonal band (5-90 days) retains the same power, but that in an ENSO band (2-7 years) weakens. The reduction of the zonal wind stress power in ENSO band results from the suppressed subseasonal Bjerkness feedback operating between WWBs and eastward warm SST anomalies progression, showing the importance of WWB forcing in shaping the characteristics of ENSO including the occurrence of extreme events.