Understanding the freshwater budget of the Atlantic Ocean: Controls, Responses, and the Role of the AMOC

The Atlantic Meridional Overturning Circulation (AMOC) is an interactive player in the Atlantic  Ocean freshwater budget. In model simulations, the AMOC responds to surface freshwater flux  (precipitation – evaporation + river runoff + ice melt; “P-E+R+M”) perturbations in the subpolar  North Atlantic; it is also influenced by P-E+R+M over the broader Atlantic through salt/freshwater  advection and inter-basin exchanges (e.g., Agulhas Leakage). In turn, the AMOC drives changes in  salt transport across 35oS and affects P-E+R+M through its influence on Atlantic sea surface  temperature, sea ice extent, and other processes. Yet, the intrinsic time scales and mechanisms driving and responding to Atlantic Ocean freshwater budget variability are not known. Moreover, changes in the global hydrological cycle, melting of the Greenland Ice Sheet, and retreat of Arctic  sea ice are among the most robust features of climate projections. We propose to investigate the  interconnections between P-E+R+M and oceanic transport of heat and freshwater/salt; and how  they affect, and are influenced by, AMOC variability on decadal to multidecadal timescales. We will perform targeted analyses of representative Coupled Model Inter-comparison Project Phase 5  (CMIP5) models; the new AMOC ensemble of NSF-DOE CESM; an eddy-permitting simulation  with the Accelerated Climate Modeling for Energy (ACME) v0 model; and output from ACME v1,  when it becomes available. In addition, we will perform perturbation experiments using the standard-resolution CESM. The objectives are: 
1. Investigate the spatio-temporal patterns of P-E+R+M associated with the AMOC  variability in the selected coupled simulations; examine how they project onto the total surface  freshwater flux variability, and how they differ among the models; 
2. Analyze the freshwater budget of the Atlantic Ocean in selected coupled climate  simulations, focusing on the interplay between P-E+R+M, storage, and interocean exchanges  due to the AMOC, the wind-driven circulation, and interocean exchange; identify drivers and  response terms, the time scales on which they operate, and their controls; 
3. Investigate the role of Agulhas Leakage in the freshwater budget of the Atlantic, by tracing  the pathway of Agulhas Leakage water through the Atlantic, and assessing its impact on the  Atlantic stratification and the AMOC; 
4. Elucidate the physical mechanisms and feedbacks that connect P-E+R+M forcing, oceanic  freshwater transport adjustment and AMOC variability through targeted experimentation using  the CESM. 
This research is responding to “CVP – AMOC-Climate Linkages in the North and/or  South Atlantic” competition. The proposed model evaluation will utilize existing and emerging  observations; the sensitivity experiments and tracer simulations are designed to understand flow  pathways of the AMOC, and how they respond to surface and inter-basin forcing changes. Both of  these aspects are listed priorities of the CVP solicitation. We anticipate that our results will improve  comparison between climate model simulations and measurements. This research also addresses an  objective of NOAA’s long-term climate goals outlined in NOAA’s Next-Generation Strategic Plan,  namely, improved scientific understanding of the changing climate system and its impacts.