The 2023 marine heatwave in the North Atlantic and the Mediterranean Sea: ocean response to atmospheric circulation

The year 2023 was characterized by record-breaking global surface air and sea surface temperatures (SSTs), the latter reaching a record 21 C in April (excluding the polar regions; Copernicus 2023). As June to October were the warmest on record globally (WMO 2023), extreme and long-lasting marine heatwave (MHW) events were observed, especially in the North Atlantic and the Mediterranean Sea. In general, the occurrence of MHWs in the subtropics and western boundary current regions is predominantly driven by atmospherically induced processes such as the net ocean heat uptake from the atmosphere, associated with a reduction in latent heat loss and increased shortwave radiation (Schlegel et al. 2021; Vogt et al. 2022). The atmospheric circulation with persistent highpressure systems and anomalously weak wind speeds associated with increased insolation is the dominant driver of the above processes. We focus on the state of the atmosphere at the surface and in the mid-troposphere during 2023 and identify specific atmospheric patterns and SST anomaly structures. To detect MHWs and calculate their characteristics we use the daily gridded NOAA OI SST version 2.1 dataset (Huang et al. 2021, updated), derived from the AVHRR satellite, in-situ ship and buoy SST data. For the atmospheric component, we used the mean sea level pressure (SLP), the horizontal wind at 10 m, the geopotential height at 500 hPa (zg500) and the 2 m maximum temperature (Tmax) from the ECMWF ERA5 reanalysis (Hersbach et al. 2020, updated). Atmospheric and ocean datasets are provided globally with a high resolution (0.25). We use daily anomalies with 1983 to 2012 as the reference period (as recommended by Hobday et al. 2018). The evaluation of MHW metrics such as frequency, duration, mean and cumulative intensity in different subregions of the North Atlantic and Mediterranean revealed that the most frequent MHWs were observed in the western Mediterranean (WMED), the longest MHWs in the central northeast Atlantic and the cumulatively most intense MHWs in the northwest Atlantic and central northeast Atlantic. The most intense MHWs are found in the WMED and off Newfoundland. During summer we detect asynchronous, above normal SLP, zg500 and Tmax over the northwest Atlantic, the WMED and the Black Sea, representing a type of blocking condition. A weakened Azores High, associated with reduced wind speed, mixing and upwelling, allows SSTs to rise substantially in the central northeast Atlantic during summer (Copernicus 2023). The first Empirical Orthogonal Function shows an antiphase dipole of SST and zg500 anomalies (explained variances of 43.9 % and 34.3 %, respectively) between the Mediterranean and West of the British Isles as well as monopol SST and zg500 anomalies (explained variances of 57.7 % and 41.9 %, respectively) over the northwestern Atlantic and the Labrador Sea.