Future changes in marine heatwaves based on high-resolution ensemble projections for the northwestern Pacific Ocean

Abstract

Marine heatwaves (MHWs) are oceanic conditions characterized by extremely high sea surface temperature (SST) anomalies that last for several days to years. Because MHWs have devastating effects on marine ecosystems and significant impacts on fisheries, understanding future MHWs is important for adapting to upcoming climate changes. In this study, we examined future changes in MHWs in the northwestern Pacific Ocean (18–53ºN, 117ºE–170ºW) under two CO₂ emission scenarios using a high-resolution ensemble (four members for each scenario) simulation product using a high-resolution ocean model that satisfactorily resolves the Kuroshio, Kuroshio Extension, and SST fronts. Following global warming, MHWs based on a threshold in the historical period (1981–2005) will increase and intensify (i.e., occur with higher SST anomalies than before). In the historical period, the annual MHW days ranged from 20 to 34 days. Annual MHW days increase to 63–313 days (188 days–all year round) depending on the region under the high CO₂ mitigation (emission) scenario at the end of the twenty-first century of 2076–2100. Furthermore, we investigated the spatial details of future MHWs. Future MHWs reflect the magnitude of SST variability in addition to that of sea surface warming in the twenty-first century; future MHWs are less frequent and more intense in the subtropical–subarctic frontal zone with large SST variability than in other regions.

Appendix

In this section, we discuss why the shape of the SST anomaly histogram at the end of the twenty-first century is wider than that in the historical period (Fig. 5). Several factors affect shape change. First, it is natural to presume that the wider shape of the SST anomaly histogram may result from larger inter-annual variability at the end of the twenty-first century. However, future changes in the SST variability on inter-annual timescales are not large (Figure S6), so this is not the case.

Second, on the ensemble-mean histogram, the shape change can be derived from differences in the degree of future warming among members; the larger the inter-member differences in future warming are, the wider the ensemble-mean SST anomaly histogram would be. To check this point, we remove the mean SST change between the historical period and the end of the twenty-first century from each member and then plot the histograms again (i.e., setting the center of each member’s histogram to zero by translation). The result shows that the shape of the ensemble-mean histograms becomes similar both in the historical period and at the end of the twenty-first century in the KE region and southeast of Hokkaido (Fig. 

Fig. 12

Similar to Fig. 5, but annual-mean SST increases from the historical period at the end of the twenty-first century are removed from future projections. Histogram from MGDSST is not drawn

12a and b). In these regions, the shape change in the ensemble-mean SST anomaly histograms reflects inter-member differences in the degree of future warming. On the other hand, in the south of Japan and the central Japan Sea, the shape of ensemble-mean SST anomaly histograms still differs between the historical period and the end of the twenty-first century, even after this translation (Fig. 12c and d). In these regions, marked histogram shape changes were observed not only in the ensemble-mean but also in each member (Fig. 5c and d).

Third, the acceleration of sea surface warming could be a possible factor that affects the shape change; the SST trend changes with time and becomes larger at the end of the twenty-first century than in the historical period, with some exceptions in the RCP2.6 simulation (Fig. 8). To test the effect of sea surface warming acceleration, we remove the 25-year linear SST trend in the historical period and at the end of the twenty-first century and then plot SST anomaly histograms again. If the histogram shape becomes similar between the historical period and the end of the twenty-first century after the removal of the linear trend in each period, the histogram shape change is linked to accelerated SST changes. However, the results show that the histogram shape is still different between the two periods even after the detrend (Fig. 

Fig. 13

Similar to Fig. 5, but the linear trends of the annual-mean SST in the historical period or at the end of the twenty-first century are removed. Histogram from MGDSST is not drawn

13). Therefore, the acceleration of sea surface warming is not important for the histogram shape change. Other factors are important. For example, differences in sea surface warming among seasons (Fig. 11) influence the change in the SST anomaly histogram by dispersion of the SST anomaly distributions. To understand this point in more detail, further investigation is needed in future studies.