A role for icebergs in the 8.2 ka climate event

Abstract

We investigate the potential role of icebergs in the 8.2 ka climate event, using a coupled climate model equipped with an iceberg component. First, we evaluate the effect of a large iceberg discharge originating from the decaying Laurentide ice sheet on ocean circulation, compared to a release of an identical volume of freshwater alone. Our results show that, on top of the freshwater effect, a large iceberg discharge facilitates sea-ice growth as a result of lower sea-surface temperatures induced by latent heat of melting. This causes an 8% increased sea-ice cover, 5% stronger reduction in North Atlantic Deep Water production and 1°C lower temperature in Greenland. Second, we use the model to investigate the effect of a hypothetical two-stage lake drainage, which is suggested by several investigators to have triggered the 8.2 ka climate event. To account for the final collapse of the ice-dam holding the Laurentide Lakes we accompany the secondary freshwater pulse in one scenario with a fast 5-year iceberg discharge and in a second scenario with a slow 100-year iceberg discharge. Our experiments show that a two-stage lake drainage accompanied by the collapsing ice-dam could explain the anomalies observed around the 8.2 ka climate event in various climate records. In addition, they advocate a potential role for icebergs in the 8.2 ka climate event and illustrate the importance of latent heat of melting in the simulation of climate events that involve icebergs. Our two-stage lake drainage experiments provide a framework in the discussion of two-stage lake drainage and ice sheet collapse.

Relative sea-level data from southwest Scotland constrain meltwater-driven sea-level jumps prior to the 8.2 kyr BP event

“…Carlson and Clark, 2012), with a relatively modest 767 freshwater perturbation at ~8270 yrs BP triggering its eventual reorganization. Thereafter, it 768 is possible that sea-ice-albedo feedbacks (Wiersma and Jongma, 2010;Otto-Bliesner and Brady, 2010) and freshwater fluxes were important in forcing the 8.2 event (Teller et al,770 2002; Törnqvist and Hijma, 2012 hypotheses that lie at the heart of this study. Timing of events are expressed as age probability density functions (pdfs) (3σ) and horizontal bars (2σ probability ranges).…”

Simulating Heinrich event 1 with interactive icebergs

“…The Jongma et al (2009) iceberg module was refined, similarly to (Wiersma and Jongma 2010), by piping the freshwater flux and the latent heat flux associated with the icebergs' basal and lateral melt to the local ocean layer as opposed to the surface-layer. More precisely, for a given iceberg of height H, we calculate a freshwater flux for the melting that is given to all model layers between the surface and H, weighted by each model layer thickness.…”

“…Adding freshwater to a series of vertical layers in the ocean tends to dampen the impact of freshwater on ocean feedbacks (as described in details below). This improvement was added by Wiersma and Jongma (2010) since the effect of freshwater induced halocline on the ocean was found to be too extreme and thus unrealistic. Furthermore, as in (Wiersma and Jongma 2010), there is a linear dependence of the wave erosion on the sea-ice fraction to mimic a first-order dampening effect of sea-ice on the (wind-dependent) waveheight In addition, icebergs that are about to get grounded are weakly repulsed (orthogonal repulsion of 0.003 m/s) instead of fixated.…”

“…This improvement was added by Wiersma and Jongma (2010) since the effect of freshwater induced halocline on the ocean was found to be too extreme and thus unrealistic. Furthermore, as in (Wiersma and Jongma 2010), there is a linear dependence of the wave erosion on the sea-ice fraction to mimic a first-order dampening effect of sea-ice on the (wind-dependent) waveheight In addition, icebergs that are about to get grounded are weakly repulsed (orthogonal repulsion of 0.003 m/s) instead of fixated. This almost immobilizes the grounding icebergs, while allowing changing winds and currents the opportunity to move the icebergs to deeper waters.…”

“…Traditionally, cooling of the surface waters is regarded as a process that enhances deep water formation, as it results in principle in a higher density of the upper water mass. However, the surface cooling associated with melting icebergs, in conjunction with the freshening due to the melting ice, can facilitate the formation of sea-ice (Jongma et al 2009;Wiersma and Jongma 2010). This can have Fig.…”