Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing

Nicolas Brown,Eric D Galbraith

doi:10.5194/cp-12-1663-2016

Abstract

Abstract. It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through “hosing” experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean–atmosphere model when integrated under a particular background climate state. We compare these unhosed oscillations to hosed oscillations under a range of background climate states in order to examine how the global imprint of AMOC variations depends on whether or not they result from external freshwater input. Our comparison includes surface air temperature, precipitation, dissolved oxygen concentrations in the intermediate-depth ocean, and marine export production. The results show that the background climate state has a significant impact on the character of the freshwater-forced AMOC interruptions in this model, with particularly marked variations in tropical precipitation and in the North Pacific circulation. Despite these differences, the first-order patterns of response to AMOC interruptions are quite consistent among all simulations, implying that the ocean–sea ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. Nonetheless, freshwater addition leads to a more complete shutdown of the AMOC than occurs in the unhosed oscillations, with amplified global impacts, evocative of Heinrich stadials. In addition, freshwater inputs can directly impact the strength of other polar haloclines, particularly that of the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.

Highlights

“Abrupt” climate changes were initially identified as decadal–centennial temperature changes in Greenland ice deposited during the last ice age (Dansgaard et al, 1984), and subsequently recognized as globally coherent climate shifts (Voelker, 2002; Alley et al, 2003)
The long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through “hosing” experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean–atmosphere model when integrated under a particular background climate state
The results show that the background climate state has a significant impact on the character of the freshwater-forced AMOC interruptions in this model, with marked variations in tropical precipitation and in the North Pacific circulation

Summary

Introduction

“Abrupt” climate changes were initially identified as decadal–centennial temperature changes in Greenland ice deposited during the last ice age (Dansgaard et al, 1984), and subsequently recognized as globally coherent climate shifts (Voelker, 2002; Alley et al, 2003). As evocatively described by Broecker (1994), armadas of icebergs, periodically discharged from the northern ice sheets to melt across the North Atlantic (MacAyeal, 1993), would have spread a freshwater cap that impeded convection and through the Stommel (1961) feedback, would have thrown a wrench in the overturning Inspired by this idea, generations of numerical models have been subjected to freshwater “hosing” experiments, whereby the sensitivity of the AMOC to varying degrees of freshwater input has been tested, and the responses have been shown to vary as a function of background climate state and experimental design (Fanning and Weaver, 1997; Ganopolski and Rahmstorf, 2001; Schmittner et al, 2002; Timmermann et al, 2003; Rahmstorf et al, 2005; Stouffer et al, 2006; Krebs and Timmermann, 2007; Hu et al, 2008; Otto-Bliesner and Brady, 2010; Kageyama et al, 2013; Gong et al, 2013; Roberts et al, 2014). We take advantage of the fact that the same model exhibits previously undescribed spontaneous AMOC interruptions and resumptions, which appear very similar to stadial-interstadial variability, to reveal what aspects of the abrupt changes are a result of the hosing itself rather than consequences of the changing AMOC

Methods

Results

Conclusion