Abstract
Evidence from paleo-proxy records suggests that the Atlantic Meridional Overturning Circulation (AMOC) can be in both an AMOC on state, the AMOC as we observe it today, and an AMOC off state, where the AMOC becomes extremely weak or even collapses. The freshwater transport due to the AMOC () at 34°S in the Atlantic has often been used as an indicator for bi-stability, with a positive suggesting a monostable AMOC and a negative suggesting a bi-stable AMOC. Often studies have shown that the sign of the divergence of the might be a good indicator of AMOC bi-stability. In this study we investigate how model bias affects the sign of across all latitudes in the Atlantic basin, through a detailed analysis of the Coupled Model Inter-Comparison Project 5 (CMIP5) model ensemble. , in the CMIP5 models is generally too positive in the southern Atlantic due to a salinity bias, while in the subtropical North Atlantic the values of are influenced by a combination of velocity and salinity biases. We compare these results to observations, reanalysis products and Hadley Centre Global Environmental Model version 3 global configuration version 2, a current generation coupled model which exhibits a stable AMOC off state, and discuss the differences that can lead to the possibility of a bi-stable AMOC as opposed to a monostable AMOC.
Highlights
The Atlantic Meridional Overturning Circulation (AMOC) forms an important part of the Atlantic climate system, transporting heat northward resulting in warmer temperatures in the regions surrounding the subpolar North Atlantic than other regions at a similar latitude
We compare these results to observations, reanalysis products and Hadley Centre Global Environmental Model version 3 global configuration version 2, a current generation coupled model which exhibits a stable AMOC off state, and discuss the differences that can lead to the possibility of a bi-stable AMOC as opposed to a monostable AMOC
The value of Mov at 34°S is often considered an indicator for AMOC bi-stability with a negative value indicating a bistable AMOC and positive values indicating a monostable AMOC
Summary
The Atlantic Meridional Overturning Circulation (AMOC) forms an important part of the Atlantic climate system, transporting heat northward resulting in warmer temperatures in the regions surrounding the subpolar North Atlantic than other regions at a similar latitude. The freshwater transport by the AMOC has been proposed as an indicator of AMOC bi-stability (Rahmstorf, 1996; de Vries and Weber, 2005) and is referred to as Mov (Fov in some studies). In the case of an AMOC collapse an initial positive (negative) value of Mov will cause an anomalous import of salt (freshwater) into the Atlantic destabilising (stabilising) the AMOC off state. The development of RTHC increases the freshwater transport into the Atlantic by the AMOC helping to further stabilise the AMOC off state. This Mov value, is no longer associated with the behaviour of the AMOC on state. The paper is concluded with a discussion on the implications for AMOC bi-stability (Section 5)
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