Transient tracer fingerprints of Atlantic Meridional Overturning Circulation in Observations and Models

Transient tracers offer a unique and important window into the Atlantic Meridional Overturning Circulation (AMOC). While they have been used to estimate the total rate of formation of NADW and to trace the pathways by which watermasses spread, less attention has been paid to the ways in which tracers can tell us about how the overturning is changing. Increasing amounts of observational data and model simulations with transient tracers offer new opportunities to understand the relationship between transient tracers and the large-scale ocean circulation. This proposal has three parts:
1. Analysis of coupled climate models: As demonstrated in this proposal, transient tracers in climate models can be better correlated with long-period variability in the overturning than spot measurements of the overturning itself. However, the fingerprint of overturning variability in tracers such as ideal age and oxygen has a complex three-dimensional spatial structure, with different responses at different latitudes. We propose to conduct a cross-model comparison of different models developed at NOAA GFDL as well as Earth System Models that are part of the IPCC AR5 model intercomparison. We will examine how robust the correlations with overturning are across models and evaluate how much data is required to extract them from other modes of variability. Anand Gnanadesikan will lead this part of the project.
2. Observational data analysis: We will analyze repeat hydrographic sections (Line W between Cape Cod and Bermuda, and elsewhere) to examine whether the patterns of changes in ventilation age and oxygen seen in the models also show up in the observations. We will also explore whether transient tracers in the observations are linked to changes in the stratification, as they appear to be in the models. If found, robust relationships between oxygen, stratification and age would enable a much broader mapping of changes in ventilation pathways within the North Atlantic. Darryn Waugh will take the lead in this part of the project.
3. High-resolution modeling. In order to better understanding the sources of the tracer fingerprints we see in the coupled models, we propose to conduct some high-resolution (1/10 degree) regional simulations of the North Atlantic in which different forcings are applied to change the overturning circulation and the resulting fingerprints on the tracer fields are computed. Insofar as we see the same results as the lower-resolution simulations, these results will help establish the robustness of tracer signatures of overturning change. If they show very different results, this will point to the importance of eddy processes in setting up tracer anomalies. Thomas Haine will lead this part of the project.
Key products that will emerge from this work are maps of mean age change over the North Atlantic over time and hopefully, identification of fingerprints of AMOC variability on the age and oxygen fields. This could allow extension of our estimates of AMOC variability back in time, which would also help to constrain mechanisms for AMOC variability.