Western Pacific Easterly Wind Extremes Drive Marine Heatwaves off Western Australia with La Niña as the Preconditioning Symptom

Abstract Marine heatwaves (MHWs) off Western Australia (WA), including the extreme example in 2011, can substantially impact local marine ecosystems. Understanding WA MHW predictability is key to improving forecast systems and enabling early warnings. Easterly wind stress anomalies in the western and central equatorial Pacific—termed “easterly wind extremes (EWEs)”—can trigger both WA MHWs and La Niña via ocean waveguides. However, the role of La Niña as either merely a preconditioning symptom in EWE-generated WA MHWs or a direct trigger remains unclear. Here, we identify the WA MHWs and La Niña using the vertically averaged temperature (VAT) from sea surface to about 300-m depth. We investigated the relationships between WA VAT MHWs, EWEs, and VAT La Niña using a linear first baroclinic mode reduced-gravity ocean model, and analyzed pycnocline depth extremes as the proxy for WA VAT MHWs and VAT La Niña. We found that the tropical western and central equatorial Pacific wind stress forcing and related oceanic wave dynamics are dominant contributors to WA pycnocline depth variations compared to other oceanic regions. By applying idealized EWE-forced model experiments, we found both EWEs and VAT La Niña typically precede WA VAT MHWs by about 4 months (2–6 months). Notably, 25% of WA VAT MHWs occurred about 4 months (2–6 months) after EWEs, without a preceding VAT La Niña. Our findings reveal a new mechanism for WA VAT MHW generation, highlighting EWEs as key precursors independent of El Niño–Southern Oscillation. This significantly advances our understanding of WA VAT MHW predictability, offering new insights beyond the established knowledge of VAT–La Niña–associated drivers. Significance Statement This study aims to understand the large-scale oceanic dynamical processes that cause marine heatwaves off Western Australia. Using an intermediate-complexity ocean model, we found that easterly wind extremes over the equatorial Western Pacific are the real trigger of these marine heatwaves, as the easterly wind extremes push warm water from the tropics toward the Western Australian coast. La Niña often, but not always, occurs before these marine heatwaves off Western Australia and serves only as a warning sign rather than the actual cause. Our findings provide insight into the prediction systems for marine heatwaves off Western Australia, helping to better manage and reduce their local impacts on Western Australia marine ecosystems.