The Impact of Climate Change on Mixed Layer Depth and its Role in Shaping Past and Future Marine Heatwaves

Abstract:

Climate model projections indicate that upper ocean stratification is expected to increase in response to continued greenhouse gas warming. As a result, the mean mixed layer depth (MLD) is expected to shoal throughout much of the globe. The MLD plays a critical role in the mixed layer temperature budget. In particular, long-term shoaling of the mean MLD would result in a stronger temperature response for the same surface heat flux forcing. This has significant implications for the likelihood and intensity of ocean temperature extremes (i.e., marine heatwaves) and their potential impacts on sensitive marine ecosystems. In this study, we investigate the influence of long-term MLD shoaling on marine heatwaves in the observational record and in future climate model projections, with a particular focus on the Northeast Pacific. We find that recent marine heatwaves, such as the summer 2019 Blob 2.0, and their impacts were likely amplified by multi-decadal shoaling of the mixed layer since 1980. A comparison with a large ensemble of climate models forced with historical radiative forcing reveals that these observed trends are primarily due to internal variability. However, future projections suggest that anthropogenically forced MLD shoaling will become increasingly apparent in the coming decades and will result in more frequent and more intense ocean temperature extremes by 2100. Our analysis takes an important step in determining the influence of climate change on the dynamics that govern marine heatwaves, with implications for future marine ecosystem impacts.