Towards a unified understanding of AMOC changes under warming and fresh water forcing

There is a long history of global climate model (GCM) studies of the response of the Atlantic Meridional Overturning Circulation to changing greenhouse gases (GHGs). Alongside this is an almost separate branch of the literature studying the AMOC’s response to fresh water input (‘hosing’) with fixed GHGs, focusing on the potential for ‘tipping’ behaviour. Some common model responses are observed among models (e.g. in GHG experiments an initial AMOC weakening associated with warming of the subsurface North Atlantic), but also considerable diversity, especially in the long-term response following stabilisation of GHG concentrations or hosing.

In recent years a few studies have emerged that use in-depth analysis frameworks to give insight into individual model responses, or into the differences between model responses. However the two branches of the literature (GHG and hosing response) have remained largely independent, and there is an increasing recognition that in real-world climate change the ‘smooth’ response to GHGs and potential abrupt ‘tipping’ responses need to be considered together. Given the diversity of model responses it will be valuable to establish whether there is a simple model framework that captures the potential mechanisms of response to GHGs and hosing that have been identified in GCMs. Such a model can then be used to characterise the types of qualitative behaviour that are possible in the more relevant scenario of tipping in a warming climate.   

We present a simple box model of thermally- and haline-driven AMOC change that aims to capture in as simple a form as possible many of the mechanisms of the AMOC responses to GHGs and hosing that have been identified in the literature. To develop this from an earlier model (that captured purely the hosing response), it was found necessary to add both a simple representation of basin-scale energy and water balances, and a simple representation of varying stratification in the sub-polar North Atlantic, increasing the dynamical degrees of freedom of the model.

We show that the model captures a wide range of behaviours seen in GCM experiments, and use it to identify circumstances in which AMOC tipping may be possible without requiring unrealistic additional water input from the Greenland Ice Sheet.