Comparing global trends in marine cold spells and marine heatwaves using reprocessed satellite data

Abstract

Abstract. Climate change is causing extreme climate events to become more frequent and more severe. Marine heatwaves (MHWs) and marine cold spells (MCSs) are prolonged, discrete periods of anomalously high or low ocean temperatures with wide-ranging impacts from dramatic shifts in biodiversity to changes in fishery yields. Previous research has found that MHWs are increasing in frequency and intensity, but MCSs remain less well understood. We used sea surface temperature (SST) data to compare the global observed MCS and MHW intensities and trends in their frequency over the period 1982–2021. These events were also assigned a category from I (moderate) to IV (extreme). Our findings show that in large areas of the ocean it can be said with 99 % confidence that MCSs have become less frequent and MHWs have become more frequent. In those regions, the occurrence of MCSs has typically reduced by one event every 5 years, while there is one extra MHW every 5 to 10 years. However, parts of the Southern Ocean go against these trends with MHWs becoming slightly less frequent and MCSs becoming more frequent. The trend of increasing numbers of MHWs and decreasing numbers of MCSs is mostly due to increases (and decreases) in category I and II events. Category III and IV MHWs are less common than the milder events, and a trend analysis demonstrated that across most of the ocean they are occurring at a slightly increased rate. However, the spatial extent of the ocean affected by these events is increasing at a faster rate. The occurrences of category III and IV MCSs are infrequent, and a trend analysis also found that their rates of occurrence, as well as the spatial extent of the ocean affected, remain near constant. Many of our results are in agreement with previous studies; what is significant about this study is the fact that it uses a different, higher-resolution input SST dataset. The similarity of the results between the different research efforts strengthens the argument that these events are not just a feature of their input dataset.