Long-term ocean temperature trend and marine heatwaves

Marine heatwaves (MHWs) can cause irreversible damage to marine ecosystems and livelihoods. Appropriate MHW characterization remains difficult, because the choice of a sea surface temperature (SST) temporal baseline strongly influences MHW identification. Following a recent work suggesting that there should be a communicating baseline for long-term ocean temperature trends (LTT) and MHWs, we provided an effective and quantitative solution to calculate LTT and MHWs simultaneously by using the ensemble empirical mode decomposition (EEMD) method. The long-term nonlinear trend of SST obtained by EEMD shows superiority over the traditional linear trend in that the data extension does not alter prior results. The MHWs identified from the detrended SST data exhibited low sensitivity to the baseline choice, demonstrating the robustness of our method. We also derived the total heat exposure (THE) by combining LTT and MHWs. The THE was sensitive to the fixed-period baseline choice, with a response to increasing SST that depended on the onset time of a perpetual MHW state (identified MHW days equal to the year length). Subtropical areas, the Indian Ocean, and part of the Southern Ocean were most sensitive to the long-term global warming trend.