Caribbean Marine Heatwaves, Marine Cold Spells, and Co-Occurrence of Bleaching Events

Marine heatwaves (MHWs) and Cold Spells (marine cold spells (MCSs)) are events of extreme positive and negative temperature anomalies, that last 5 days or longer. These events have been diagnosed across oceans from sea surface temperatures (SSTs) and recognized to affect ecosystems' health worldwide. Some studies have focused on the extent, evolution, and drivers of specifically intense and long-lasting regional MHW events. Although extreme temperature anomalies could lead to coral bleaching, mortality, and ecosystems' shift from healthy to barren reefs, no studies have focused on MHWs and MCSs in the Caribbean Sea. This study characterizes them in the Caribbean Sea, explores simultaneous thermal-stress and coral response, and reveals prevailing environmental conditions during their occurrence. We use four decades of SSTs observed daily and a recently published database of worldwide coral bleaching events. We found bleaching events occurring simultaneously with both MCSs and MHWs, often with extreme events of mild intensity and typical duration. We find a positive long-term trend in MHWs' frequency, consistent not only with global warming, but also with the Atlantic Meridional Mode. There is a clear seasonal signal with MHWs occurring more often in summer and MCSs in autumn-winter. Our results suggest MHWs and MCSs development is related to El Nino/La Nina conditions, respectively. Finally, we identify intense extreme temperature events that span large areas as candidates for future studies on their dynamics. Our results are relevant for the development of MHWs and MCSs forecasts applied to conservation strategies and ecosystem-based management of marine resources, particularly in reef systems. Events of extremely warm and cold temperatures that last 5 days or longer in the ocean are denominated Marine Heatwaves (MHWs) and marine cold spells (MCSs), respectively. Analysis of sea surface temperature observed with satellites over the past decades have shown MHWs are occurring more often and have important ecological implications. This study focuses on MHWs and MCSs in the Caribbean Sea over the last four decades to: (a) determine their typical duration, intensity, and frequency; (b) explore if their occurrence coincides with recorded coral bleaching events; (c) determine their temporal variability, as well as correlation with climate indices; and (d) identify the largest and most intense MHW and MCS events. We present evidence of Caribbean MCSs and MHWs linked to widespread coral bleaching. Additionally, we reveal MHWs/MCSs occur more often during summer/winter and El Nino/La Nina conditions and show an increase in MHWs' frequency consistent with long-term warming, and natural climate modes of variability such as the Atlantic Meridional Mode. Our findings contribute to the improvement of tools that anticipate MHWs and MCSs for the management of reefs, which are ecosystems that host large biodiversity, support societal needs, and that are under pressure due to ongoing climate change. Caribbean marine heatwave (MHW) and marine cold spell (MCS) are simultaneous with coral bleaching eventsMHWs/MCSs have a seasonal signal, being more frequent and intense during summer/autumn-winter and in the SE region of the CaribbeanMHWs/MCSs are longer, stronger, and more frequent during the +/- phase of Atlantic Multidecadal Oscillation, Atlantic Meridional Mode and Caribbean, and occur more often during +/- El Nino Southern Oscillation conditions