Evaluation and Diagnosis of the Atlantic Meridional Overturning Circulation 3D Structure in Climate Models

“The Atlantic Meridional Overturning Circulation (AMOC), with its large heat and freshwater transports and interaction with the atmosphere and sea-ice, plays a fundamental role in establishing the mean state and the temporal variability of the Earth’s climate system. Existing studies have shown a wide spread of AMOC state in the current state-of-art climate models, CIMIP5, and hence a better understanding of the driving mechanisms is urgently needed. The current analysis, however, typically quantifies the AMOC as one maximum volume transport stream function at certain latitudes. This overly simplified AMOC index, while important, is insufficient for formulating a comprehensive picture of the AMOC structure across the entire Atlantic and characterizing its fundamental role in the climate system, such as transport of heat/freshwater and water mass transformation.

Recognizing this problem, here we propose a collaborative effort to conduct more comprehensive analyses on the structure of the AMOC in climate models. The analyze are built on observational results and high-resolution ocean simulations, and include a) AMOC transport on temperature-salinity plane and density spaces across trans-Atlantic sections at different latitudes, b) water mass transportation due to surface buoyancy forcing as well as diapycnal/isoptcnal mixing in the ocean interior, and c) diapycnal velocity. The overall goals are 1) to derive a better and more comprehensive diagnosis for evaluating the AMOC representation, including time mean structure and temporal variability, in current climate models, and 2) to identify and understand the key physical process or mechanisms that lead to the wide spread of the AMOC state among the CMIP5 models as well as the AMOC variability in individual models.

This project is a direct contribution to the “Process-oriented evaluation of climate and earth system models and derived projections” in the area A: metric for climate and earth model development. The proposed analysis will help a) evaluate the AMOC structure in current earth system models, compared to observations and high-resolution models; and b) isolate the model biases on AMOC structure due to contributions from air-sea interaction and/or oceanic advection and mixing process. Botha re critical for the ongoing efforts to improve our scientific understanding of the changing climate system and its impact, and to enhance our capability to predict climate variability and changes in climate and earth system models.”