Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere‐iceberg‐ocean model

Abstract

[1] We introduce explicit icebergs from a dynamic and thermodynamic iceberg model into an intermediate complexity climate model, which includes the coupled atmosphere-ocean system. This modeling approach allows iceberg meltwater to be injected into the ocean on the basis of thermodynamical considerations along the iceberg trajectories. Icebergs are seeded from known ice sheets in both hemispheres. Adding icebergs to the present-day climate model has a minimal impact, but during the Last Glacial Maximum (LGM), Atlantic overturning strength is reduced by a third, while producing a model state that is consistent with a steady state climate. We test the sensitivity of the model at the LGM to additional Heinrich event-scale fluxes of icebergs from three possible sources: Hudson Strait, the Gulf of Saint Lawrence, and the Norwegian Channel Ice Stream (NCIS). The sensitivity of the ocean is similar for all locations, with differences dominated by the length of the iceberg meltwater pathways to the main ocean convection region. The NCIS events result in more variability and a distinctly different, more northerly, salinity anomaly. We compare these results to a more typical modeling approach, whereby meltwater is injected directly into the ocean at the iceberg source locations, and find that these floods overestimate the oceanic response compared to the iceberg events. Our results suggest that 0.3-0.4 Sv of additional freshwater flux, either as icebergs or freshwater, is required to shut down the North Atlantic meridional overturning, a larger freshwater flux than sometimes suggested because of the localized nature of the release of the freshwater.Citation: Levine, R. C., and G. R. Bigg (2008), Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere-iceberg-ocean model, Paleoceanography, 23, PA4213,

Simulating Heinrich event 1 with interactive icebergs

“…After the iceberg/freshwater release ends in year 300, MOC recovery shows a delay of up to 5 decades when comparing hosing with the CF-icebergs. This is quite consistent with the idea (Jongma et al 2009;Levine and Bigg 2008) that hosing is too efficient at shutting down convection. The less convection remains active, the larger the build up of fresh surface waters in the Eastern North Atlantic Ocean (Figs.…”

“…• Compared to traditional homogeneous fresh-water hosing, Cool & Fresh-icebergs are almost as efficient as hosing at weakening the meridional overturning circulation, even though hosing can be regarded as an in principle too-efficient manner to suppress convection (Jongma et al 2009;Levine and Bigg 2008).…”

“…1), homogeneous freshwater hosing might be expected to be unrealistically efficient at inhibiting convection, when compared to a more plausible iceberg-melt distribution (Jongma et al 2009). Indeed, a Heinrich event has been simulated recently with dynamic icebergs (Levine and Bigg 2008), suggesting that with a more realistic, localized freshwater flux of icebergs, the ''MOC-shutdown may be harder to induce than previously suggested''. However, neither the classical hosing approach nor the above iceberg study take into account the significant amount of latent heat that is needed to melt the icebergs.…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…Indeed, they showed that, contrary to Levine and Bigg (2008) and Jongma et al (2009), icebergs were much more efficient at suppressing deep convection when the latent heat is taken into account. This counter-intuitive result (cooling the surface ocean should generate denser waters) is explained by a 'sea-ice facilitation effect', whereby the cooling effect of icebergs promotes sea-ice formation.…”

“…As such, recent thermo-dynamical iceberg models have been coupled to climate models. One of the first studies to undertake this approach was Levine and Bigg (2008), who accounted for both the distribution of icebergs and their freshwater release. With icebergs, they found a greater freshwater release is required to have the same effect on the AMOC than the traditional freshwater hosing approach.…”