Global surface ocean temperature anomalies in 2023 and their climate context

Around 3 billion people rely on the ocean for their livelihoods, with around 10% of the world’s population directly relying on fishing. As human-driven climate change causes the world to warm, the ocean and the ecosystems within are increasingly susceptible to heatwave events that can have severe consequences. Such marine heatwaves (MHWs) can last from several days to a year and result in the destruction of ocean habitats and the diminution or relocation of fish species, with knock-on effects for coastal communities. The frequency of MHWs has doubled since 1982 and they are likely to continue to increase in frequency, intensity, and duration. However, the link between MHWs and modes of climate variability remains uncertain. Here, we investigate to what extent maps of temperature anomalies in 2023 can be attributed to large-scale climate modes with centres of action in the Atlantic, North Pacific, and tropical Pacific. Specifically, we regress global sea surface temperatures on to indices of Atlantic Multidecadal Variability (AMV), the 2^nd EOF of North Pacific variability (commonly linked to MHWs), and El Nino/Southern Oscillation (ENSO, which strongly correlates with the 1^st EOF of North Pacific variability). We find that around 30% of the variance in global, annual sea surface temperature anomalies can be explained by a linear combination of these indices. Since 2012, the combination of these indices has been unprecedented, associated with anomalous warming (on top of the global trend) throughout the northern hemisphere. As such, climate variability (which may include a forced component) is currently providing an unusually high baseline for further MHW events. Further work will aim to use decadal prediction models to investigate the predicted evolution of these indices over the coming years.