English: Schematic of ventilation in an idealized high-latitude ocean basin. The basin extends from a high-latitude landmass (or, equivalently, the North Pole) to an arbitrary mid-latitude zonal boundary of length L. The basin contains n sloping isopycnal layers of (spatially and temporally variable) thickness h and meridional velocity v that outcrop into the upper-ocean mixed layer (of depth H) within the region. Like the real-world high-latitude oceans, the basin hosts a double-celled overturning circulation, with inflow in intermediate layers and outflow in surface and deep layers. Ventilation is quantified with a surface-sourced tracer C, which crosses the mixed layer base with vertical velocity w and at an integrated rate FC. The tracer has interior sources and sinks SC within the basin, and is exported through the mid-latitude boundary at a rate TC.