Abstract
During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are thus associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean. We simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere–ocean model. In our model, dynamic-thermodynamic icebergs influence the climate through two direct effects. First, melting of the icebergs causes freshening of the upper ocean, and second, the latent heat used in the phase-transition of ice to water results in cooling of the iceberg surroundings. This cooling effect of icebergs is generally neglected in models. We investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off. At the peak of the simulated Heinrich event, icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects. The cause of this intriguing result must be sought in the involvement of a secondary mechanism: facilitation of sea-ice formation, which can disturb deep water production at key convection sites, with consequences for the thermohaline circulation. We performed additional sensitivity experiments, designed to explore the effect of the more plausible distribution of the dynamic icebergs’ melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid-latitude North Atlantic (NA) Ocean. The early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments, which must be a distribution-effect: the dynamically distributed icebergs quickly affect western NADW formation, which synergizes with direct sea-ice facilitation, causing an earlier sea-ice expansion and climatic response. Furthermore, compared to dynamic-thermodynamic icebergs, a homogeneous hosing overestimates the fresh water flux in the Eastern Ruddiman belt, causing a fresh anomaly in the Eastern North Atlantic, leading to a delayed recovery of the circulation after the event.
Highlights
Paleoceanographic records provide ample evidence that ‘‘great armadas of icebergs’’ (Heinrich 1988) were a prominent feature of the last glacial climate
During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean
The strong cooling in the North Atlantic due to the icebergs is hypothesized to originate from a disturbed ocean circulation, which is weakened by the meltwater associated with the iceberg armadas (e.g. Broecker et al 1993; McManus et al 2004; Gherardi et al 2005)
Summary
Paleoceanographic records provide ample evidence that ‘‘great armadas of icebergs’’ (Heinrich 1988) were a prominent feature of the last glacial climate (reviews: Hemming 2004; Andrews 1998). G. Hewitt et al 2006) In these so-called hosing studies, the freshening effect of the melting icebergs is typically simplified to homogeneous and instantaneous dumping of freshwater on a designated ocean area that corresponds to the Ruddiman belt. This hosing-approach neglects the dynamic nature of icebergs, floating under influence of local forces such as the currents and the winds. In a series of sensitivity experiments we unravel the cooling and the freshwater effects, paying special attention to the role of (facilitated) sea-ice and the consequences for the North Atlantic deep water (NADW) formation
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