Abstract
Marine heatwaves pose an increasing threat to the ocean’s wellbeing as global warming progresses. Forecasting marine heatwaves is challenging due to the various factors that affect their occurrence, including large variability in the atmospheric state. In this study we demonstrate a causal link between ocean heat content and the area and intensity of marine heatwaves in the Tasman Sea on interannual to decadal time scales. Ocean heat content variations are more persistent than ‘weather-related’ atmospheric drivers (e.g. blocking high pressure systems) for marine heatwaves and thus provide better predictive skill on timescales longer than weeks. Using data from a forced global ocean sea-ice model, we show that ocean heat content fluctuations in the Tasman Sea are predominantly controlled by oceanic meridional heat transport from the subtropics, which in turn, is mainly characterized by the interplay of the East Australian Current and the Tasman Front. Variability in these currents is impacted by wind stress curl anomalies north of this region, following Sverdrup´s and Godfrey’s ‘Island Rule’ theories. Data from models and observations show that periods with positive upper (2000m) ocean heat content anomalies or rapid increases in ocean heat content are characterized by more frequent, larger, longer and more intense marine heatwaves on interannual to decadal timescales. Thus, the oceanic heat content in the Tasman Sea acts as a preconditioner and has a prolonged predictive skill compared to the atmospheric state (e.g. surface heat fluxes), making ocean heat content a useful indicator and measure of the likelihood of marine heatwaves.
Highlights
The Tasman Sea is a region of change, where ocean temperatures (Oliver et al, 2014; Roemmich et al, 2015; Shears and Bowen, 2017) and sea level (Church and White, 2006; Hannah and Bell, 2012; Fang and Zhang, 2015) are rising, exceeding global averages (0.2–0.5◦C/decade and 3–5 mm/decade respectively)
In this manuscript we investigated the connection between ocean heat content, heat transport and the area and intensity of Marine heatwaves (MHWs) in the Tasman Sea, on interannual to decadal timescales, by using a coarse-resolution forced ocean hindcast corroborated by optimally interpolated data from Argo, satellite measurements and in situ high-resolution expendable bathythermograph data (HRXBT) temperature observations
The Tasman Sea is a region of oceanic heat convergence, meaning that average surface heat fluxes are directed toward the atmosphere to balance the oceanic heat convergence
Summary
The Tasman Sea is a region of change, where ocean temperatures (Oliver et al, 2014; Roemmich et al, 2015; Shears and Bowen, 2017) and sea level (Church and White, 2006; Hannah and Bell, 2012; Fang and Zhang, 2015) are rising, exceeding global averages (0.2–0.5◦C/decade and 3–5 mm/decade respectively). This provides an opportunity to establish a near real-time measure for inferring heat content trends in the Tasman Sea and a measure for the likelihood of MHWs. To explain the origin of the variability of the TN heat transport, we computed the correlation of the TN heat transport with the wind stress curl from JRA-55-DO (Figure 8).
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