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Marine heatwaves (MHWs) are extreme ocean events that may have severe and long-lasting impacts on marine life. The level of ecosystem stress and whether organisms are displaced or perish during MHWs depends on species’ thermal thresholds and ability to adapt, but also depends on the characteristics of the events, including their depth structure. However, due to a paucity of in situ measurements below the surface, the vertical structure of MHWs is poorly understood. Here we use one of the longest (27 years) records of daily sub-surface ocean temperature anywhere, spanning the water column in 65 m of water off Sydney, Australia, to explore the sub-surface coherence of MHWs and its controls.We show that MHW events often extend over the whole water column in weakly stratified months and are the longest, driven by the intrusion onto the shelf of warm large-scale currents, while during periods of strong stratification, shallow and deep MHW events are typically uncoupled.Shallow events are mostly driven by anomalous air-sea fluxes, while downwelling favourable winds trigger sub-surface MHWs. The sub-surface events tend to be the most intense, but also the shortest.These results highlight that, when considering coastal areas where maximum biological damage is reported, sea surface temperature extremes such as MHWs, are not representative of the water column below the shallow mixed layer during stratified months., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Marine heatwaves (MHWs) are prolonged extreme oceanic warm water events, that can cause substantial and sometimes devastating impacts on marine ecosystems. Our knowledge of regional and remote drivers, teleconnections, and local-scale processes underpinning large-scale and iconic MHWs has improved significantly over the past decade because of focused research efforts around the world. Dynamical knowledge of MHW drivers, including those from remote sources such as modes of climate variability and their teleconnections, coupled with local MHW amplification or decay processes, is critical to the process-based understanding of MHW predictability. However, the key development needed for effective adaptation and mitigation efforts by marine conservation and fisheries managers is better MHW prediction skill. MHW predictability depends on multiple factors including how we define MHWs, the regions in which they occur – e.g., coastal versus offshore, western boundary versus eastern boundary regions –, the spatial and temporal scales of the events, atmospheric heat flux versus ocean advection-driven events, and driver persistence. This presentation will explore MHW predictability on various timescales, focusing on the physical processes that offer a prediction. Some significant MHW events around Australia will be central to this discussion., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Pacific Equatorial Undercurrent (EUC) transports water originating from a number of distinct source regions, eastward across the Pacific Ocean. It is responsible for supplying nutrients to the productive eastern Equatorial Pacific Ocean. Of particular importance is the transport of iron by the EUC; the limiting nutrient in that region. Although the mean circulation and sources of EUC water are reasonably well understood, it is unclear how the contribution of water from these sources to the EUC vary on seasonal to interannual timescales. Here a Lagrangian analysis is applied to the eddy-resolving OFAM3 ocean simulation in order to identity variability in the makeup of the EUC over an 18 year period (1993-2010). While ENSO has an influence on the variability of source transport contributions to the EUC, the signal of increased (decreased) transport of water from the LLWBCs during El Niño (La Niña) periods does not translate to substantial changes in the makeup of the EUC between 165°E and 140°W. It is hypothesized that this is due to the large spread in travel times of water parcels as they travel from the source regions into the EUC. The consequent erosion of transport anomalies generated at the different western boundary source regions associated with ENSO may help explain why previous studies found little relationship between variability in iron fluxes off Papua New Guinea shelves and the chlorophyll response in the eastern tropical Pacific. Key Points: A Lagrangian framework is used to backtrack EUC water sources Increased LLWBC water during El Niño is smeared out within the EUC Travel times and particle positions from individual sources exhibit large spread