The 2023 marine heatwave in the North Atlantic tropical ocean

Abstract

Abstract. In the context of climate change, marine heatwaves (MHWs) are becoming more intense and frequent and/or lasting longer. During the year 2023 and based on the Copernicus Marine forecasting system, the Mercator Ocean International MHW bulletin (https://www.mercator-ocean.eu/ocean-intelligence/ocean-bulletins-and-insights/marine-heatwaves-archive/, last access: 14 May 2025) highlighted week after week a MHW event occurring in the North Atlantic (NA) tropical ocean. In this paper, we propose a 4D characterisation of this event using the Copernicus Marine global reanalyses. We demonstrate how this 2023 MHW event in the NA tropical ocean is extraordinary compared to previous years. All indices commonly used for characterising MHWs (intensity, duration, total activity and area) reached values not observed before at the surface but also in the subsurface. The timing of the event and its vertical structure differ across the basin, with the MHW developing first in the north-east, with peaks of intensity in May and progressively moving south-westward across the basin. A characterisation of MHWs at all vertical levels reveals that the vertical structure differs across subregions with different processes at play: in the eastern and subtropical centre of the gyre heat propagates from the surface to the subsurface, spanning beyond the mixed layer depth, whereas in the Caribbean region, abnormally warm waters at depth are transported from remote equatorial regions by eddies traversing the area.