Click here to access a half-hour tutorial I gave on this topic at the 2014 Ocean Sciences Meeting.
The ocean is full of eddies, whorls of water that correspond to the high and low pressure systems in the atmosphere. A major theme of my research has been understanding the impact of these eddies on ocean circulation, biogeochemical cycles, and climate.
In recent years, my group has focussed on the role of eddies in stirring water properties along surfaces of constant density (isopycnals). Exchanging parcels of fluid along such surfaces requires essentially no energy. By contrast, exchanging them across such surface requires pushing light water into dense water and vice-versa, which demands a supply of energy. As a result, the along-isopycnal diffusion coefficient ARedi is up to nine orders of magnitude larger than the cross-isopycnal diffusion coefficient. This means that if you were to inject a dye into the thermocline, over a few months it would spread approximately 10,000 times further in the horizontal than it would in the vertical.
Neither observations nor models show a consensus about the correct value of ARedi. Some models use constant values which range from a few hundred to a few thousand m2s-1. Others use spatial patterns that tend to be high in boundary currents and low in the central gyres. But observations in the gyres show very large values!
In collaboration with Ryan Abernathey at Columbia, my group has been examining the sources and impacts of this uncertainty. Gnanadesikan, Dunne and John (2012) showed that isopycnal mixing was a major source of oxygen to open-ocean oceanic dead zones, where oxygen is so low that fish are unable to live. In Gnanadesikan, Bianchi and Pradal, (2013) demonstrated that changing the ARedi coefficient over the range in the literature can change the volume of the most intensely hypoxic waters (where bacteria stop using oxygen and use nitrate instead for metabolic processes) by an order of magnitude. Pradal and Gnanadesikan (2014) found that changing mixing also changes the temperature of high latitude regions by destabilizing the pycnocline, producing localized warming of up to 7 degrees C! Gnanadesikan, Pradal and Abernathey (2015a) show that the uncertainty in ARedi is associated with a 20% uncertainty in the uptake of anthropogenic carbon dioxide. Gnanadesikan, Pradal and Abernathey (2015b) show that it also affects the distribution of mantle helium and radiocarbon, though not the relationship between them. Gnanadesikan et al. (2017) show that increasing lateral mixing actually increases the amplitude of El Nino. This paradoxical result is explained by the fact that while mixing damps anomalies, it produces a mean state that is more sensitive to perturbations in both the atmosphere and ocean. We are currently working on papers that look at the impact of changing lateral mixing on the biological carbon pump (Gnanadesikan et al., in rev.), on impacts on ocean deoxygenation (Bahl et al. in prep.) and on a review paper (Gnanadesikan et, in rev., Ann. Rev. Mar. Sci.).
Gnanadesikan, Anand, John P. Dunne and Jasmin John, 2012: Understanding why the volume of suboxic waters does not increase over centuries of global warming in an Earth System Model, Biogeosciences, 9, 1159-1172, doi:10.5194/bg-9-1159-2012. Link
Anand Gnanadesikan, Daniele Bianchi and Marie-Aude Pradal, 2013: Critical role for mesoscale eddy diffusion in supplying oxygen to hypoxic ocean waters, Geophysical Research Letters, 40, 5194-5198, doi:10.1002/grl.50998. Link
Pradal, Marie-Aude and Anand Gnanadesikan, 2014: How does the Redi parameter for mesoscale mixing impact global climate in an Earth System Model?, Journal of Advances in Modeling the Earth System, 6,586-601,doi:10.1002/2013MS000273 Link
Gnanadesikan, Anand, Marie-Aude Pradal and Ryan Abernathey, 2015a: Isopycnal mixing by mesoscale eddies significantly impacts oceanic anthropogenic carbon uptake, Geophys. Res. Lett.. 42,4249-4255, doi:10.1002/2015GL064100 Link
Gnanadesikan, Anand, Ryan Abernathey and Marie-Aude Pradal, 2015b: Exploring the isopycnal mixing and helium-heat paradoxes in a suite of Earth System Models, Ocean Science, 11,591-605. Link
Gnanadesikan, Russell, Abernathey and Pradal, Impact of lateral stirring on El Nino in a climate model,in press Journal of Advances in Modeling the Earth System. PDF
Gnanadesikan, Anand, Marie-Aude Pradal and Ryan Abernathey, Changes in the isopycnal mixing coefficient produce significant, but compensated, changes in ocean carbon pumps, in rev. Global Biogeochemical Cycles