Global Marine Heatwaves and Cold-Spells in Present Climate to Future Projections

Marine heatwave (MHW) characteristics, such as total days, average duration, and mean intensity, have increased in most global oceans. In contrast, marine cold-spell (MCS) characteristics have a decreasing trend over most regions. However, the difference between MHW and MCS in the mean and trend in the present and future climate is unclear. Here, we provide a comparative global assessment of these changes based on satellite sea surface temperature for 1982-2020 and climate model projections for 2021-2070. Globally, there are clear differences in the mean and trend between MHW and MCS metrics. On seasonal timescales, MHWs mainly occur in summer and autumn, while MCSs generally occur in winter and spring. The occurrence area of MHWs shows an increasing trend during 1982-2020, while the occurrence area of MCSs shows an opposite trend. Furthermore, the increasing trend in MHW average duration led to a near-permanent MHW status in the future projections. Upper-ocean warming due to anthropogenic forcing is closely related to increase in MHWs and decrease in MCSs. In addition, the shallower ocean mixed layer depth in the future is favorable for the occurrence of MHWs. In contrast, the projected weakening of the Atlantic meridional overturning circulation facilitates the occurrence of MCSs in the subpolar North Atlantic. These results provide a global baseline for the difference and relationship between MHW and MCS under present climate and future projections.