The Lifecycle of Tracer Variance in the North Atlantic

Oceanic tracer distributions are shaped by turbulent mixing, which may be understood via a traver variance budget. There is a tracer variance lifecycle: variance production by turbulent flows stirring large-scale gradients, redistribution by currents, and variance dissipation by molecular diffusion. A comprehensive tracer variance budget, including variance transport, is diagnosed from North Atlantic hydrographic observations and reveals three different regimes. In the subtropical thermocline, there is a local balance between diapycnal production by microscale turbulence and molecular dissipation. In intermediate waters and the subpolar gyre, the local balance is between isopycnal production by mesoscale stirring and dissipation. Near gyre boundaries, a three-way balance emerges between isopycnal production by mesoscale stirring, variance transport by currents, and dissipation downstream. Transitions between mixing regimes are determined by water-mass contrasts and circulation strength. Our expectation is that these different mixing regimes apply to the global ocean and affect how the ocean sequesters and redistributes anthropogenic heat.