Marine heatwaves and surface chlorophyll-a extremes in the global ocean

Abstract

Scientific understanding of the impacts of marine heatwaves (MHWs), high sea-surface temperature (SST) events, and the processes which drive them has gained considerable momentum in recent decades, following observations of their widespread environmental disruption. However, MHWs can cooccur with low or high chlorophyll-a counts constituting a compound event where more than one physical or biogeochemical stressor coincide, to often have synergistic consequences in the environment. Regional processes conduct the biological response to MHWs and thus impact upon ocean productivity rates temporarily. This study analyses satellite observations and CMIP6 model projections of daily SSTs and chlorophyll-a concentrations to quantify annual frequencies of three extreme oceanic events for 17 open ocean biomes, categorised by distinct biogeochemical regimes (Fay et al., 2014). The percentage of days a year that have MHWs, low compound events (MHW-low chlorophyll-a) and high compound events (MHW-high chlorophyll-a) have been obtained at the ocean biome as well as global scale. Projections to the end of the 21st century follow shared socio-economic pathways (SSPs) associated to 2.6 W/m2 and 8.5 W/m2 of radiative forcing, respectively (O’Neill et al., 2014). MHW hotspots were identified in biomes defined by a degree of seasonal ice coverage and in the eastern equatorial Pacific region, with the latter also experiencing the highest number of low compound events per year whilst high latitudes instead experience more high compound events annually, on average. The knowledge gap surrounding the potential impact to marine species and the ocean carbon budget as a result of compound events should motivate future research into the mechanisms behind the regional variability in chlorophyll-a responses to MHWs that has been quantified here. Especially as this work reassures that conditions defining extreme events in the ocean today increase in frequency, in line with rising global average temperatures, with the potential to become a near-permanent state of the ocean by 2100 (Frölicher et al., 2018; Laufkötter et al., 2020).

Date of Award	24 Jan 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Oliver D Andrews (Supervisor) & Fanny M Monteiro (Supervisor)