*Review* **Instability of Atlantic Meridional Overturning Circulation: Observations, Modelling and Relevance to Present and Future**

**Zhengyu Liu 1,2**

<sup>1</sup> Department of Geography, The Ohio State University, Columbus, OH 43210, USA; liu.7022@osu.edu

<sup>2</sup> School of Geography Science, Nanjing Normal University, Nanjing 210024, China

**Abstract:** The Atlantic Meridional Overturning Circulation (AMOC) has changed dramatically during the glacial–interglacial cycle. One leading hypothesis for these abrupt changes is thermohaline instability. Here, I review recent progress towards understanding thermohaline instability in both observations and modelling. Proxy records available seem to favor thermohaline instability as the cause of the abrupt climate changes during the glacial–deglacial period because the deep North Atlantic water mass and AMOC seemed to have changed before the North Atlantic climate. However, most fully Coupled General Circulation Models (CGCMs) so far seem to exhibit monostable AMOC, because (1) these models have failed to simulate abrupt AMOC changes unless they are forced by an abrupt change of external forcing and, (2) these models have shown opposite freshwater convergence from the current observations. This potential model bias in the AMOC stability leaves the model projection of the future AMOC change uncertain.

**Keywords:** thermohaline instability; model bias; abrupt changes
