Characteristics and Causes of the Record-Breaking Marine Heatwaves in subtropical Northeastern Atlantic in 2023 Spring

Marine heatwaves (MHWs) are extreme high temperature events in oceans that have significant impacts on marine environments and ecosystems. Utilizing daily sea surface temperature (SST) data from National Oceanic and Atmospheric Administration"s OISST V2 dataset, along with ERA5 atmospheric reanalysis data from the European Centre for Medium-Range Weather Forecasts and ocean reanalysis data from National Centers for Environmental Prediction, this study analyzed the spatial and temporal features of the record-breaking MHW events in the subtropical Northeastern Atlantic in the spring of 2023. The results indicate that the North Atlantic experienced an unprecedented rise in SST during spring 2023, particularly in the subtropical Northeastern Atlantic (SNA) region, which emerged as a hotspot. The calculation results of the mixed-layer heat budget in the upper ocean layer show that during the entire ocean heatwave event, the net heat flux term plays a dominant role in the energy exchange between the sea and the atmosphere at each stage. The SNA region has endured over 90 days of Marine heatwave conditions, presenting two significant events with clear initiation, peak and decline phases. Their intensity and range changes are regulated by the positive feedback mechanism of wind-evaporation-sea surface temperature: In the initial stage, the abnormal low-pressure system leads to a decrease in sea surface wind speed, seawater evaporation rate and latent heat flux on the sea surface, resulting in an abnormal downward net heat flux. During the peak stage, under the control of the abnormal high-pressure system, with further decreasing wind speed and suppressed evaporation, the SST continues to rise, intensifying the MHW. During the recession stage, increased sea surface wind speeds intensify evaporation, consequently lowering sea surface temperatures and culminating in event termination. In addition, formation and maintenance of these MHW events are closely related to an eastward-propagating Rossby wave from the Pacific Ocean. During the attenuation stage of heatwaves, abnormally warm SST can trigger an eastward-propagating Rossby wave across Eurasia, thereby causing low temperatures in eastern China.