The global mode and intermediate waters are key players in the cycling of anthropogenic and natural carbon, and ventilate the subtropical gyres, thus carrying heat, nutrients, and biogeochemical tracers into the ocean interior. However these water masses can be poorly represented within a number of the leading coupled climate models. This proposal aims to identify and quantify the key mechanisms controlling the formation processes and fate of the global mode and intermediate water masses. Further, we aim to evaluate the biogeochemical tracers associated with mode and intermediate water pathways, and attribute biases in these tracers to biases either in the water mass formation processes or biogeochemical cycling. We will implement water mass transformation diagnostics, similar to those applied in the Southern Ocean by Iudicone et al. (2008), allowing explicit online diagnosis of the specific diapycnal mixing processes (e.g. diffusion, cabbeling, mixed-layer processes) exchanging and forming mode and intermediate waters. We will run these diagnostics in the National Center for Atmospheric Research (NCAR)-CESM and National Oceanic and Atmospheric Administration (NOAA)/Geophysical Fluid Dynamics Laboratory (GFDL)-ESM2M depth coordinate models, and compare the results with the GFDL-ESM2G isopycnal coordinate model. We will diagnose the mean and projected mode and intermediate water mass formation with our main objectives being to:

1. Quantify the relative contributions of different processes in forming mode and intermediate waters, so as to better understand the differences between the two models.
2. Evaluate whether biases in biogeochemical tracers arise from biases in the physical or biogeochemical solutions.
3. Elucidate how changes in winds, surface forcing, and eddy processes affect both mode and intermediate water pathways and biogeochemical cycling.

The proposed objectives will be undertaken through collaboration between Princeton University, Johns Hopkins University and NOAA/GFDL. Implementing the novel transformation diagnostics tool into the widely used NCAR and GFDL Earth System Models will provide a more thorough analysis method for the ocean physical and biological circulation, which observationalists commonly diagnose through major water masses. In addition, given that mode and intermediate waters are important in the global ocean uptake and storage of anthropogenic carbon, as well as the outgassing of natural carbon, quantifying the projected change in carbon distribution through these water masses and the processes responsible for such changes will be of major interest to the scientific community and general public.