Presentation Information
[2ASPR-24]Lightning Talks by ECRs No.7
○Pierre Simiganoschi (Tohoku University)
Marine heatwaves (MHWs) are prolonged periods of extreme ocean warming and can affect how carbon dioxide (CO₂) is exchanged between the ocean and atmosphere, but exactly MHWs affect this change is still unclear—especially over short periods of days to weeks. The impact of short-term events is important to understand because they are increasing in frequency and can have a greater impact on FCO₂ compared to longer events. Most studies rely on monthly-averaged data, which removes short-term signals that can hold valuable information about CO₂ transfer. Using daily data from 1982–2023 across the global ocean, we separated the CO₂ flux response into its underlying drivers: wind, the difference in CO₂ between ocean and air, and temperature-driven (thermal) and biological/mixing-driven (non-thermal) effects.
We find that weaker winds during MHWs are a dominant driver for reduced CO₂ intake in ocean sink regions. Weaker wind slows the rate at which the ocean and atmosphere exchange gases. During shorter MHWs (about 10–30 days), this wind effect is the main driver of changes in CO₂ exchange. In longer MHWs lasting more than a month, warming builds up in a stratified layer and becomes the dominant driver responsible for less CO₂ intake, while biological activity and mixing partly counteract it. The wind effect is also seasonal: it matters most in spring and autumn, when the drop in wind is large relative to normal conditions, while in summer competing effects tend to cancel out. Together, these results show that a marine heatwave is best understood as a "compound event": warm water and weak winds occur at the same time, both driven by the same weather patterns, and together they reshape how the ocean takes up or releases CO₂.
Importantly, the same calm, stratified conditions that alter CO₂ exchange also affect ocean life. Wind and mixing supply nutrients to the sunlit surface waters where phytoplankton grow. When MHWs suppress this mixing, fewer nutrients reach the surface, which can limit the biological growth that normally helps draw CO₂ out of the atmosphere—linking these physical changes directly to the productivity that sustains marine food webs. Understanding how warming, winds, carbon, and biology are connected is therefore key to predicting how extreme ocean events ripple through marine ecosystems. As a next step, we plan to combine these observations with climate-model projections of MHW intensity and frequency, to explore how these shifts in ocean CO₂ exchange may evolve and what they could mean for ocean productivity in a warming world.
We find that weaker winds during MHWs are a dominant driver for reduced CO₂ intake in ocean sink regions. Weaker wind slows the rate at which the ocean and atmosphere exchange gases. During shorter MHWs (about 10–30 days), this wind effect is the main driver of changes in CO₂ exchange. In longer MHWs lasting more than a month, warming builds up in a stratified layer and becomes the dominant driver responsible for less CO₂ intake, while biological activity and mixing partly counteract it. The wind effect is also seasonal: it matters most in spring and autumn, when the drop in wind is large relative to normal conditions, while in summer competing effects tend to cancel out. Together, these results show that a marine heatwave is best understood as a "compound event": warm water and weak winds occur at the same time, both driven by the same weather patterns, and together they reshape how the ocean takes up or releases CO₂.
Importantly, the same calm, stratified conditions that alter CO₂ exchange also affect ocean life. Wind and mixing supply nutrients to the sunlit surface waters where phytoplankton grow. When MHWs suppress this mixing, fewer nutrients reach the surface, which can limit the biological growth that normally helps draw CO₂ out of the atmosphere—linking these physical changes directly to the productivity that sustains marine food webs. Understanding how warming, winds, carbon, and biology are connected is therefore key to predicting how extreme ocean events ripple through marine ecosystems. As a next step, we plan to combine these observations with climate-model projections of MHW intensity and frequency, to explore how these shifts in ocean CO₂ exchange may evolve and what they could mean for ocean productivity in a warming world.
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