RESUMEN

The ocean absorbs >90% of anthropogenic heat in the Earth system, moderating global atmospheric warming. However, it remains unclear how this heat uptake is distributed by basin and across water masses. Here we analyze historical and recent observations to show that ocean heat uptake has accelerated dramatically since the 1990s, nearly doubling during 2010-2020 relative to 1990-2000. Of the total ocean heat uptake over the Argo era 2005-2020, about 89% can be found in global mode and intermediate water layers, spanning both hemispheres and both subtropical and subpolar mode waters. Due to anthropogenic warming, there are significant changes in the volume of these water-mass layers as they warm and freshen. After factoring out volumetric changes, the combined warming of these layers accounts for ~76% of global ocean warming. We further decompose these water-mass layers into regional water masses over the subtropical Pacific and Atlantic Oceans and in the Southern Ocean. This shows that regional mode and intermediate waters are responsible for a disproportionate fraction of total heat uptake compared to their volume, with important implications for understanding ongoing ocean warming, sea-level rise, and climate impacts.

RESUMEN

The Atlantic Meridional Overturning Circulation (AMOC) regulates the heat distribution and climate of Earth. Here we identify a new feature of the circulation within the North Atlantic Subtropical Gyre that is associated with the northward flowing component of the AMOC. We find that 70% of the water that flows northwards as part of the AMOC circulates the Gyre at least once before it can continue northwards. These circuits are needed to achieve an increase of density and depth through a combination of air-sea interaction and interior mixing processes, before water can escape the latitudes of the Gyre and join the northern upper branch of the AMOC. This points towards an important role of the Gyre circulation in determining the strength and variability of the AMOC and the northward heat transport. Understanding this newly identified role of the North Atlantic Subtropical Gyre is needed to properly represent future changes of the AMOC.

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RESUMEN

Mesoscale eddies stir along the neutral plane, and the resulting neutral diffusion is a fundamental aspect of subgrid-scale tracer transport in ocean models. Calculating neutral diffusion traditionally involves calculating neutral slopes and three-dimensional tracer gradients. The calculation of the neutral slope traditionally occurs by computing the ratio of the horizontal to vertical locally referenced potential density derivative. However, this approach is problematic in regions of weak vertical stratification, prompting the use of a variety of ad hoc regularization methods that can lead to rather nonphysical dependencies for the resulting neutral tracer gradients. Here we use a VErtical Non-local Method "VENM," a search algorithm that requires no ad hoc regularization and significantly improves the numerical accuracy of calculating neutral slopes, neutral tracer gradients, and associated neutral diffusive fluxes. We compare and contrast VENM against a more traditional method, using an independent objective neutrality condition combined with estimates of spurious diffusion, heat transport, and water mass transformation rates. VENM is more accurate, both physically and numerically, and should form the basis for future efforts involving neutral diffusion calculations from observations and possibly numerical model simulations.

RESUMEN

The water mass transformation (WMT) framework weaves together circulation, thermodynamics, and biogeochemistry into a description of the ocean that complements traditional Eulerian and Lagrangian methods. In so doing, a WMT analysis renders novel insights and predictive capabilities for studies of ocean physics and biogeochemistry. In this review, we describe fundamentals of the WMT framework and illustrate its practical analysis capabilities. We show how it provides a robust methodology to characterize and quantify the impact of physical processes on buoyancy and other thermodynamic fields. We also detail how to extend WMT to insightful analysis of biogeochemical cycles.

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