Unraveling the mechanisms and implications of a stronger mid-Pliocene Atlantic Meridional Overturning Circulation (AMOC) in PlioMIP2

Abstract

Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period in which the atmospheric CO2 concentration was approximately equal to the concentration we measure today (ca. 400 ppm). Sea surface temperature (SST) proxies indicate above-average warming over the North Atlantic in the mid-Pliocene with respect to the pre-industrial period, which may be linked to an intensified Atlantic Meridional Overturning Circulation (AMOC). Earlier results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the ensemble simulates a stronger AMOC in the mid-Pliocene than in the pre-industrial. However, no consistent relationship between the stronger mid-Pliocene AMOC and either the Atlantic northward ocean heat transport (OHT) or average North Atlantic SSTs has been found. In this study, we look further into the drivers and consequences of a stronger AMOC in mid-Pliocene compared to pre-industrial simulations in PlioMIP2. We find that all model simulations with a closed Bering Strait and Canadian Archipelago show reduced freshwater transport from the Arctic Ocean into the North Atlantic. This contributes to an increase in salinity in the subpolar North Atlantic and Labrador Sea that can be linked to the stronger AMOC in the mid-Pliocene. To investigate the dynamics behind the ensemble's variable response of the total Atlantic OHT to the stronger AMOC, we separate the Atlantic OHT into two components associated with either the overturning circulation or the wind-driven gyre circulation. While the ensemble mean of the overturning component is increased significantly in magnitude in the mid-Pliocene, it is partly compensated by a reduction in the gyre component in the northern subtropical gyre region. This indicates that the lack of relationship between the total OHT and AMOC is due to changes in OHT by the subtropical gyre. The overturning and gyre components should therefore be considered separately to gain a more complete understanding of the OHT response to a stronger mid-Pliocene AMOC. In addition, we show that the AMOC exerts a stronger influence on North Atlantic SSTs in the mid-Pliocene than in the pre-industrial, providing a possible explanation for the improved agreement of the PlioMIP2 ensemble mean SSTs with reconstructions in the North Atlantic.

Past warm climate conditions show a shift in Northern Hemisphere winter variability towards a dominant North Pacific Oscillation

“…However, we find that the response in winter SLP variability is almost opposite to both forcings, especially over the North Pacific. This is in a way 'consistent' with studies on the mid-Pliocene ENSO (Pontes et al, 2022) and AMOC (Weiffenbach et al, 2023), where the changes are opposite to end-of-the century projections. Feng et al (2022) argue that the mid-Pliocene changes in ice sheets and vegetation are in some way a long-term Earth system feedback to elevated atmospheric CO 2 .…”

“…They show that CCSM4-Utr exhibits the largest northward shift of the ITCZ from October to February. Zhang et al (2020) and Weiffenbach et al (2023) show an increased AMOC strength in the PlioMIP2 ensemble. Weiffenbach et al (2023) furthermore explain that models with a closed Bering strait and Canadian Arctic Archipelago (including CCSM4-Utr) show reduced freshwater transport from the Arctic Ocean into the North Atlantic, causing an increase in subpolar North Atlantic salinity, which drives the stronger AMOC.…”

“…Zhang et al (2020) and Weiffenbach et al (2023) show an increased AMOC strength in the PlioMIP2 ensemble. Weiffenbach et al (2023) furthermore explain that models with a closed Bering strait and Canadian Arctic Archipelago (including CCSM4-Utr) show reduced freshwater transport from the Arctic Ocean into the North Atlantic, causing an increase in subpolar North Atlantic salinity, which drives the stronger AMOC. In this case, CCSM4-Utr simulates a stronger AMOC in the mid-Pliocene with values close to the ensemble mean (22 Sv over 19 Sv in the pre-industrial).…”

“…These modes represent an oscillation of mass between the subpolar latitudes and subtropics in the Northern Hemisphere, and are the leading modes of sea-level pressure (SLP) variability in the northern extratropics (Ambaum et al, 2001;Hurrell and Deser, 2010). The NAO plays a significant role in North Atlantic and European climate, for example example a stronger AMOC (Weiffenbach et al, 2023) compared to a weakening of the AMOC in the projected future, as well as strongly reduced ENSO variability (Oldeman et al, 2021), which is not projected for near-future climate.…”

Causes of the weak emergent constraint on climate sensitivity at the Last Glacial Maximum

“…The structural issues of the AMOC and the Southern Ocean might also arises from their preindustrial representations (Sherriff-Tadano and Klockmann, 2021), implying a state dependency in the noise. The state dependency of the AMOC has been studied for the Pliocene, where models show consistent global SST warming and strengthening of the AMOC compared to the pre-industrial period (Weiffenbach et al, 2023). Pliocene paleogeography may drive changes in AMOC rather than SSTs (Burton et al, 2022); however, the warm state of Pliocene North Atlantic SSTs enhance oceanic heat transport by the subtropical gyre, which may be responsible for regional SSTs changes (Weiffenbach et al, 2023).…”