The Time-Dependent Response of a Two-Basin Ocean to a Sudden Surface Temperature Change

Abstract

Building on previous work using single-basin models, we here explore the time-dependent response of the Atlantic Meridional Overturning Circulation (AMOC) to a sudden global temperature change in a two-basin ocean-ice model. We find that the previously identified mechanisms remain qualitatively useful to explain the transient and the long term time-mean responses of the AMOC in our simulations. Specifically, we find an initial weakening of the AMOC in response to warming (and vice versa for cooling), controlled by the mid-depth meridional temperature contrast across the Atlantic basin. The long-term mean response instead is controlled primarily by changes in the abyssal stratification within the basin. In contrast to previous studies we find that for small-amplitude surface temperature changes, the equilibrium AMOC is almost unchanged, as the abyssal stratification remains similar due to a substantial compensation between the effects of salinity and temperature changes. The temperature-driven stratification change results from the differential warming/cooling between North Atlantic Deep Water and Antarctic Bottom Water, while the salinity change is driven by changes in Antarctic sea ice formation. Another distinct feature of our simulations is the emergence of AMOC variability in the much colder and much warmer climates. We discuss how this variability is related to variations in deep-ocean heat content, surface salinity, and sea ice in the deep convective regions, both in the North Atlantic and Southern Ocean, and its potential relevance to past and future climates.

Overturning Pathways Control AMOC Weakening in CMIP6 Models

“…In cooler, glacial climates, the equilibrium AMOC probably shoaled, decoupling the upper and lower cells of the MOC and thus isolating the AMOC from the Indo-Pacific Ocean (Baker et al, 2020(Baker et al, , 2021Ferrari et al, 2014;Nadeau & Jansen, 2020). The AMOC may also shoal in response to GHG forcing with its transient response investigated in idealized models (e.g., Bonan et al, 2022;Chang & Jansen, 2022;Sun et al, 2020). The Indo-Pacific overturning circulation responds rapidly to GHG forcing, weakening to compensate changes in the AMOC through communication between the North Atlantic and Indo-Pacific Oceans via wave processes (Sun et al, 2020).…”