Multidecadal changes in the North Atlantic bloom phenology driven by temperature extremes

The North Atlantic bloom has an important role in the ocean carbon cycle and carbon sequestration. Observational studies have highlighted significant changes in bloom magnitude and phenology over the past decades, while model projections have predicted the bloom to decline or even collapse during the 21st century. Here we use a variety of satellite and in situ observations to study how temperature extremes drive multidecadal changes (1998-2024) in the North Atlantic bloom phenology. On average, spring MHWs trigger higher net primary production (NPP) across the whole North Atlantic. Summer MHWs, on the other hand, trigger a dipolar zonal response, with reduced NPP at lower latitudes and the opposite effect at higher latitudes. Such sub-regional differences in the impact of temperature extremes on the North Atlantic bloom also result in distinct changes in the bloom phenology over the past decades. These include net shifts in the bloom peak, such as a shift from a spring to a summer peak in NPP in the Iceland Basin and an intensification of spring NPP in the Irminger Sea. We further investigate the link between temperature extremes and changes in the bloom by studying the time-delay response of the bloom to long-lasting MHWs (e.g. the 2023 MHW) and the contrasting impact of opposite temperature extremes such as marine cold spells that have been observed in the region.