NSFGEO-NERC: How well can we constrain past changes in ocean heat and freshwater content?

About This Grant

The primary goal of this project is to develop new monthly estimates of changes in ocean heat and freshwater content. The ocean absorbs more than 90% of the excess heat associated with the Earth’s energy imbalance, and spatial patterns of sea level changes are regulated by both heat and freshwater content, emphasizing the. importance of this work. While observations of ocean temperature and salinity needed to determine heat and freshwater content are sparse, improvements can be made by systematically testing mapping methods, employed to obtain global fields from the observations, in ocean models and by including additional sources of data that need bias adjustments before usage. The project is designed to perform these types of method testing and bias corrections through collaborations with other groups on a national and international level. The outcomes of the project will help advance our understanding of past and future changes in ocean heat and freshwater content which is important for the health of the planet, economies, and communities. Sharing of data products and software and dissemination of the results will make the outcomes of the project openly available to the community. This is a project jointly funded by the National Science Foundation’s Directorate for Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Opportunity. This Opportunity allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award recommendation, each Agency funds the proportion of the budget that supports scientists at institutions in their respective countries. This project will substantially advance the scientific knowledge of historical ocean changes through improved observation-based reconstructions of ocean heat and freshwater content. A critical aspect of the work concerns improved and more holistic knowledge of the uncertainties associated with these reconstructions which will help identify to what extent global and regional changes can be constrained. The main novel aspects of the project are: (1) the use of state-of-the-art modeling systems as “test beds” of statistical mapping methods where known truth fields provide an objective assessment of mapping method strengths and limitations, and (2) the development of ensemble bias corrections for the large amount of available Expendable BathyThermograph (XBT) ocean temperature data based on plausible probe and manufacturer assignments. Biases in the XBT data have been the source of an unquantified aspect of uncertainty in estimates of global and regional ocean heat content change in the past. Combining (1) and (2), the project will develop new observation-based estimates that represent a major step forward in the quantitative understanding of ocean environmental change. This new information will be used to identify leading dynamical mechanisms for observed changes in heat and freshwater content and to provide new observationally constrained projections of global surface temperature and sea-level rise. Improved quantification of ocean heat and freshwater content changes and their uncertainties will also aid the evaluation and development of new Earth system models and associated forecast systems and help characterize near-surface and subsurface heat and freshwater extremes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.