[SLS Pre-Defense] Anthony Meza

“Characteristics of unsteady abyssal circulation inferred from transient tracers”

Many climate models project a weakening of the global overturning circulation under continued anthropogenic forcing. To detect such changes, basinwide mooring arrays have monitored the overturning for up to two decades, yet interannual-to-decadal variability in these records continues to obscure any underlying trends. Here, we test whether dissolved transient tracers (CFC-11, CFC-12, and SF₆) can differentiate steady circulations (piControl) from unsteady ones (Historical/SSP5-8.5) in the GFDL CM4X coupled climate model. To do so, we apply time-invariant and time-varying reduced-order transport models that represent abyssal waters as mixtures of Adélie and Ross Sea bottom waters and preindustrial water. First, we show that the time-invariant model reconstructs piControl tracers within uncertainty bounds. In Historical/SSP5-8.5, however, seafloor SF₆ declines after 2025 even as prescribed atmospheric concentrations continue to rise, and the same model fails to predict it.

In contrast, the time-varying model, which permits the circulation to slow, better fits all simulated tracers and reproduces this decline by reducing the contribution of ventilated bottom waters and increasing the relative fraction of older, unventilated water. Finally, along the S04P hydrographic section, time-invariant models calibrated to pre-2000 data again fail to predict the SF₆ decline, and their misfits grow as abyssal overturning weakens. As long as atmospheric SF₆ continues to rise, our work suggests that a decline in seafloor SF₆ could serve as another indication of slowing abyssal circulation.

Sack Lunch Seminar Series —
Informal seminar series within PAOC (Program in Atmospheres, Oceans, and Climate) that focuses on more specialized topics than the PAOC Colloquium. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.

Contact: sacklunch-committee@mit.edu