Marine heat waves are classified as a period of unusually high temperatures in the ocean. During the summer of 2019, an extreme marine heat wave devastated regional ecosystems and caused concern among fishery and wildlife managers along the west coast of North America. These events can cause catastrophic effects to economic, societal, and biological sectors. Thus, understanding the process and mechanisms involved is critical.
The top layer of the ocean, known as the “mixed layer”, is a layer of water in which the temperature remains consistent with depth. This layer is typically tens of meters deep and acts as a shield to protect the water below from becoming warmer. In a new study, partially funded by MAPP, new research could explain recent extreme marine heatwaves, and points to a future with more frequent and extreme heat waves as global temperatures continue to rise.
In a new Bulletin of the American Meteorological Society, authors Dillon Amaya, Michael Alexander, Antonietta Capotondi, Clara Deser, Kristopher Karnauskas, Arthur Miller, and Nathan Mantua, used a combination of ocean observations and models to estimate the depth of the northeast Pacific Ocean mixed layer back to 1980, and also project out into the future. It was determined that the mixed layer has thinned by nearly 9 feet in some regions of the North Pacific and by 2100, the mixed layer will be 12 feet thinner than what it is today - a 30% decrease of where it is today. In support of previous findings, pronounced changes in the mean mixed layer may have significant implications for the frequency and strength of marine heat waves in the future.
Figure (b): Probability distributions of JJA ensemble mean CESM1-LE mixed layer depth anomalies averaged in NEPac during the “present”(blue; 2005–34) and “future” (orange; 2070–99). Vertical black lines mark JJA 2019 mixed layer depth anomalies values from GODAS (solid) and Argo (dashed) data averaged in the same region. Figures (e) and (g): Simulated JJA mixed layer depth trend (m decade–1 ; shading) from 1980 to 2015, and 2016-2099 in ensemble means of CESM1-LE.
An excerpt from a CIRES press release explains the motivation and findings of the study: "Marine heatwaves will be more intense and happen more often in the future," said Dillon Amaya, a CIRES Visiting Fellow and lead author on the study out this week in the Bulletin of the American Meteorological Society's Explaining Extreme Events. "And we are now understanding the mechanics of why. When the mixed layer is thin, it takes less heat to warm the ocean more."
Researchers note that anthropogenic contribution to mixed layer depths is unknown and that more research is needed to look into mechanisms that may offset the projected effects of a thinner mixed layer with future mare heat waves.
Read the study here.
Materials provided by https://cires.colorado.edu/news/marine-heatwaves-becoming-more-intense-more-frequent
The Modeling, Analysis, Predictions, and Projections (MAPP) Program is a competitive research program in NOAA Research's Climate Program Office. MAPP's mission is to enhance the Nation's and NOAA's capability to understand, predict, and project variability and long-term changes in Earth's system and mitigate human and economic impacts. To achieve its mission, MAPP supports foundational research, transition of research to applications, and engagement across other parts of NOAA, among partner agencies, and with the external research community. MAPP plays a crucial role in enabling national preparedness for extreme events like drought and longer-term climate changes. For more information, please visit www.cpo.noaa.gov/MAPP.