Fingerprinting of the Atlantic meridional overturning circulation in climate models to aid in the design of proxy investigations

It is desirable to design proxy investigations that target regions where properties reconstructed from calibrated parameters potentially carry high-fidelity information concerning changes in large-scale climate systems. Numerical climate models can play an important role in this task, producing simulations that can be analyzed to produce spatial "fingerprints" of the expected response of various properties under a variety of different scenarios. We will introduce a new method of fingerprinting the Atlantic meridional overturning circulation (AMOC) that not only provides information concerning the sensitivity of the response at a given location to changes in the large-scale system, but also quantifies the linearity, monotonicity and symmetry of the response. In this way, locations that show high sensitivities to changes in the AMOC, but that exhibit, for example, strongly nonlinear behavior can be avoided during proxy investigations. To demonstrate the proposed approach we will use the example of the response of seawater temperatures to changes in the strength of the AMOC. We present results from an earth-system climate model which has been perturbed with an idealized freshwater forcing scenario in order to reduce the strength of the AMOC in a systematic manner. The seawater temperature anomalies that result from the freshwater forcing are quantified in terms of their sensitivity to the AMOC strength in addition to the linearity and monotonicity of their response. A first-order reversal curve (FORC) approach is employed to investigate and quantify the irreversibility of the temperature response to a slowing and recovering AMOC. Thus, FORCs allow the identification of areas that are unsuitable for proxy reconstructions because their temperature versus AMOC relationship lacks symmetry.