Freshwater routing in eddy-permitting simulations of the last deglacial: the impact of realistic freshwater discharge

Abstract

Abstract. Freshwater, in the form of glacial runoff, is hypothesized to play a critical role in centennial- to millennial-scale climate variability, such as the Younger Dryas and Dansgaard–Oeschger events, but this relationship is not straightforward. Large-scale glacial runoff events, such as Meltwater Pulse 1a (MWP1a), are not always temporally proximal to subsequent large-scale cooling. Moreover, the typical design of hosing experiments that support this relationship tends to artificially amplify the climate response. This study explores the impact that limitations in the representation of runoff in conventional “hosing” simulations has on our understanding of this relationship by examining where coastally released freshwater is transported when it reaches the ocean. We particularly focus on the impact of (1) the injection of freshwater directly over sites of deep-water formation (DWF) rather than at runoff locations (i.e. hosing), (2) excessive freshwater injection volumes (often by a factor of 5), and (3) the use of present-day (rather than palaeo) ocean gateways. We track the routing of glaciologically constrained freshwater volumes from four different inferred injection locations in a suite of eddy-permitting glacial ocean simulations using the Massachusetts Institute of Technology General Circulation Model (MITgcm) under both open and closed Bering Strait conditions. Restricting freshwater forcing values to realistic ranges results in less spreading of freshwater across the North Atlantic and indicates that the freshwater anomalies over DWF sites depend strongly on the geographical location of meltwater input. In particular, freshwater released into the Gulf of Mexico generates a very weak freshwater signal over DWF regions as a result of entrainment by the turbulent Gulf Stream. In contrast, freshwater released into the Arctic with an open Bering Strait or from the Eurasian ice sheet is found to generate the largest salinity anomalies over DWF regions in the North Atlantic and GIN (Greenland–Iceland–Norwegian) seas region respectively. Experiments show that when the Bering Strait is open, the Mackenzie River source exhibits more than twice as much freshening of the North Atlantic deep-water formation regions as when the Bering Strait is closed. Our results illustrate that applying freshwater hosing directly into the North Atlantic with even “realistic” freshwater amounts still overestimates the amount of terrestrial runoff reaching DWF regions. Given the simulated salinity anomaly distributions and the lack of reconstructed impact on deep-water formation during the Bølling–Allerød, our results support that the majority of the North American contribution to MWP1a was not routed through the Mackenzie River.

Towards spatio-temporal comparison of simulated and reconstructed sea surface temperatures for the last deglaciation

“…Finally, the reconstructed meltwater peaks could be too high or the models' responses to them too strong, leading to a spatially too homogeneous SST response (He and Clark, 2022). Insights into this potential explanation could be gained from coupled atmosphere-ocean-ice sheet simulations (Ziemen et al, 2019) or replacing local meltwater input with freshwater fingerprints obtained from eddy-resolving ocean models (Love et al, 2021).…”

Towards spatio-temporal comparison of transient simulations and temperature reconstructions for the last deglaciation

“…Finally, the reconstructed meltwater peaks could be too high or the models' responses to them too strong, leading to a spatially too homogeneous SST response (He and Clark, 2022). Insights on this potential explanation could be gained from coupled atmosphere-ocean-ice sheet simulations (Ziemen et al, 2019) or replacing local meltwater input by freshwater fingerprints obtained from eddy-resolving ocean models (Love et al, 2021). to the timing of fluctuations, could be combined with a more insightful measure for temporal patterns.…”

A New Eulerian Iceberg Module for Climate Studies

“…The meltwater from the icebergs reduces surface salinity and reduces or suppresses the formation of deep water and weakens the AMOC and the North Atlantic heat transport, leading to a substantial cooling. For the understanding of the role of HEs on climate, melting icebergs become particularly important, as the sensitivity of the AMOC to freshwater input depends on the location of the input (Condron & Winsor, 2012; Love et al., 2021; Maier‐Reimer & Mikolajewicz, 1989; Schiller et al., 1997; Swingedouw et al., 2022). Releasing icebergs from an ice sheet has a different effect on climate than releasing liquid melt water at the same location.…”