A strong flow, a weak link
The lower limb of the Atlantic Meridional Overturning Circulation carries dense water south from the subpolar North Atlantic. Jia-Jia Chen and M. Susan Lozier combined climate simulations with observations from the Overturning in the Subpolar North Atlantic Program and followed that transport across density surfaces. The strongest variability appeared on surfaces well below the density of maximum overturning.[1]
From monthly to multidecadal timescales, the large swings at selected densities reflected redistribution within the lower limb and showed no strong link to local or downstream overturning variability. A busy meter in one density layer can be recording water shifting between layers rather than a strengthening of the circulation as a whole.[1]
The basin counts both directions
Strong variability in dense water leaving the Labrador Sea was tightly linked to variability entering the basin. Because inflow and outflow run in opposite directions, they largely canceled in basin-wide diagnostics. In this accounting, the Labrador Sea behaves like a conduit carrying two-way changes rather than a source that amplifies the signal on its own.[1]
Transport changes originating in the Irminger and Iceland basins exerted the strongest downstream influence. Earlier parts of the route can therefore carry more information than one large swing at the Labrador exit. The circulation's meaning depends on the basin the water came through and the density at which it traveled, as well as where it leaves.[1]
Reading the meter
What a strong subpolar transport signal means for the circulation as a whole remains unresolved. The contribution is narrower and useful: before giving that signal climatic meaning, the accounting has to include compensation across density layers and cancellation between opposing flows. Local magnitude and system-wide change are different measures.[1]
The useful signal to watch next is whether a change remains coherent in the basin total and in downstream overturning, rather than how large the jump looks at one boundary current. When observing systems keep both flow directions and shifts between densities in the same ledger, it becomes easier to separate a noisy pipe from a change in the whole plumbing system.[1]