Wind Spatial Structure Triggers ENSO’s Oceanic Warm Water Volume Changes

Abstract

This study demonstrates that the generalization that strong anomalous equatorial Pacific westerly (easterly) winds during El Niño (La Niña) events displays strong adjusted warm water volume (WWV) discharges (recharges) is often incorrect. Using ocean model simulations, we categorize the oceanic adjusted responses to strong anomalous equatorial winds into two categories: (i) transitioning (consistent with the above generalization); and (ii) neutral adjusted responses (with negligible WWV re- and discharge) During the 1980-2016 period only 47% of strong anomalous equatorial winds are followed by transitioning adjusted responses, while the remaining are followed by neutral adjusted responses. Moreover, 55% (only 30%) of the strongest winds lead to transitioning adjusted responses during the pre-2000 (post-2000) period in agreement with the previously reported post-2000 decline of WWV lead time to El Niño-Southern Oscillation (ENSO) events. The prominent neutral adjusted WWV response is shown to be largely excited by anomalous wind stress forcing with a weaker curl (on average consistent with a higher ratio of off-equatorial to equatorial wind events) and weaker Rossby wave projection than the transitioning adjusted response. We also identify a prominent ENSO phase asymmetry where strong anomalous equatorial westerly winds (i.e., El Niño events) are roughly 1.6 times more likely to strongly discharge WWV than strong anomalous equatorial easterly winds (i.e., La Niña events) are to strongly recharge WWV. This ENSO phase asymmetry may be added to the list of mechanisms proposed to explain why El Niño events have a stronger tendency to be followed by La Niña events than vice versa.

Understanding Lead Times of Warm Water Volumes to ENSO Sea Surface Temperature Anomalies

“…The westward shift of the ENSO anomalous wind stress pattern after 2000 is consistent with a shorter ENSO period and WWV lead time (Bunge & Clarke, 2014; Hu et al., 2017; Li et al., 2019). Recent work (Neske et al., 2021) showed that ENSO wind stress curl change contributed to the WWV change after 2000. Therefore, the effect of different wind patterns on ENSO periodicity, which mainly depends on the recharge‐discharge efficiency parameters and (Jin, 1997b; Jin et al., 2020), needs further investigations and will be reported elsewhere.…”

“…The decline of WWV lead time to ENSO SST and shorter ENSO period was argued to be caused by a westward displacement of the ENSO‐associated wind stress anomaly pattern during a La Niña‐like tropical Pacific mean state (An & Wang, 2000; Hu et al., 2017, 2020; Li et al., 2019). By decomposing the WWV response to wind stress into “instantaneous” and “adjusted” components using a linear shallow water ocean model, Neske and McGregor (2018) argued that the decline of WWV lead time to ENSO SST is related to the increasing “instantaneous” contribution prominence after the year 2000, which is possibly related to wind spatial structure changes (Neske et al., 2021). The current diversity in the explanations is a sign that more work is needed to understand the underlying mechanisms controlling WWV lead time to ENSO SST.…”