The Role of Internal Waves in Climate Change

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Until recently, scientists had largely felt that most elements impacting climate change have been accounted for. This means we can target key emissions in order to reduce the effects of climate change while also forecasting what projected scenarios may affect the planet based on generally understood global-scale phenomena. However, scientists from the UK and US have recently indicated that underwater waves, known as internal waves, likely play an important role in climate change and these need to be considered in order to understand how we mitigate or face the worst effects of our changing climate. 

Climate models applied in forecasts for future climate change look at global oceanic and atmospheric circulation to derive their estimates. Our understanding of global circulation is that cool waters are generally brought to the tropics and then warm water flows back out towards northern latitudes in a conveyor belt fashion. This is the case in the Atlantic Meridional Overturning Circulation (AMOC), which plays a crucial role in this larger circulation. In fact, researchers had believed that much of the melting in the Arctic and Antarctic is due to this phenomenon, where warmer waters have been circulating in these regions as they come out of the tropics.

A map of the world showing the global ocean current with warmer waters in red and cold currents in blue.
Map of the global meridional overturning circulation (GMOC) of which the Atlantic overturning circulation is a part of. Map: NASA.

However, climate models generally do not adequately consider how underwater waves affects this circulation. Researchers in the UK and US have shown that this is a serious flaw and should be considered in future climate scenarios. Work has shown that underwater waves play a pivotal role in transporting heat and carbon between water layers. In fact, underwater the ocean is complex, made of different water layers that affect water density, heat and nutrient transfer, and other aspects.

Generally, water is colder, denser at the bottom, while it is warmer and lighter at the top. Heat and carbon can move between layers due to turbulence in these layers; micro-scale turbulence in the upper branch of circulation can redistribute how carbon and heat move throughout the layers. This phenomenon is known as cross-density (Diapycnal) mixing of water.

A diagram showing the formation of internal waves underneath the surface of the water.
A simplistic view of the formation of internal waves and the intermixing of ocean layers. Diagram: Caitlin Dempsey using Canva elements.

Research now shows that water moving southward from the North Atlantic to the Southern Ocean could now be moving heat and carbon through different layers, affecting climate model results since this means it is not just lateral circulation between the tropics and poles that affects heat and carbon flux. Such turbulence can significantly affect the amount of heat reaching the Antarctic Ice Sheet, which would affect also the timing of when warmer waters arrive in different seasons.

In fact, because turbulence within ocean layers has not been significantly studied, the researchers have indicated there needs to be better monitoring of oceanic turbulence across the globe. There needs to be turbulence sensors installed at observational levels to get a better sense of how micro-scale turbulence translates into global-scale heating effects.[1] Measurement of heat and carbon transfer in such mixing may need to be more precise across different parts of the globe so that climate models can be better tuned to this phenomenon in order to improve their forecasting capabilities.

We have of course known about cross-density mixing for some time. For instance, we know that cross-density mixing plays an important role in nutrients being distributed across the oceans and this helps life in the oceans to thrive.[2] Phenomenon such as ocean eddies are also affected by vertical water mixing in the oceans.[3] The problem is most studies have been on a small-scale and not focused on climate, which has limited our understanding of how this phenomenon affects the entire global circulation process. 

Climate models have increasingly become critical in mapping government and multinational policies on climate change. However, we may need to re-evaluate some of these models in order to be sure that they are precisely capturing important phenomenon in how the poles, major ice sheets and glaciers are being affected as well as general transfer in heat and carbon across our oceans.

Cross-layer movement within waves and water layers in the deep oceans appears to have a significant effect on heat and carbon transfer and this likely needs more study in order to determine its global-scale significance. While we now have potentially a better theoretical understanding, more empirical data may need to be collected to improve forecasting capabilities for climate models. 

References

[1]    For more on cross-density oceanic mixing affecting heat and carbon transfer in global circulation, including its effect on climate modeling, see:  Cimoli L, Mashayek A, Johnson HL, et al. (2023) Significance of Diapycnal Mixing Within the Atlantic Meridional Overturning Circulation. AGU Advances 4(2): e2022AV000800. DOI: 10.1029/2022AV000800.

[2]    For more on nutrient mixing in how underwater layers mix, see:  Zhu SJ, Zhang J, Matsuno T, et al. (2023) Quantifying the Water Contribution of Subtropical Mode Water and Related Isopycnal/Diapycnal Water Mixing in the Western Pacific Boundary Current Area Using Radiocesium: A Significant Nutrient Contribution From Subtropical Pacific Gyre to the Marginal Region. Journal of Geophysical Research: Oceans 128(4): e2022JC018975. DOI: 10.1029/2022JC018975.

[3]    For more on the role of cross-layer ocean mixing and this effect on eddies, see:  Liu Z and Liao G (2023) Relationship between global ocean mixing and coherent mesoscale eddies. Deep Sea Research Part I: Oceanographic Research Papers 197: 104067. DOI: 10.1016/j.dsr.2023.104067.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.