The Evaluation of Ocean Reanalyses in Their Determining Trends in Global Ocean Heat Content with a Novel Method

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Detecting and understanding oceanic change and variability has become increasingly important, given the fact that they serve as major evidence for climate change as well as the need to interpret climate variability patterns such as the much-debated recent global surface warming slowdown. Central to this issue is the ability to reliably represent the ocean state globally and over its full depth. Many researchers have turned to describe and understand oceanic change and variability from ocean reanalyses (ORA), which are the combination of ocean models, atmospheric forcings, and ocean observations through various data assimilation methods and potentially provide more accurate information than observation-only or model-only based ocean estimations. Also, ORAs provide more variables and quantities to interpret and understand the oceanic change and variability. At present, estimates from ORAs, especially their temporal evolutions, are still sensitive to the change of the observing system, the errors of the dynamical model, atmospheric forcing, and the data assimilation methods. Many of the uncertainties are often flow dependent and are hard to identify and estimate. This research proposes a novel, yet simple way to discover the possible systematic errors and biases in the temperature estimates from ORAs and then utilizes the most physically consistent ORAs to perform analysis on the changes in global and regional OHC. The proposed study can potentially provide a unified view for the recently much-debated global surface warming hiatus, and will advance understanding of the decadal variability of ocean heat uptake as well as provide useful implications about the imbalance of radiation at the top of the atmosphere. The project will support an early career scientist and a graduate student contributing to building the next generation of scientists. In addition, it will contribute to the development of k-12 educational material explaining the role of the oceans in regulating climate change.In this project, a simple but rarely used method will be used to evaluate a large set of the existing ORA products and determine if they are suitable for examining climatic-scale changes in global and regional ocean heat content (OHC). This evaluation is based on examining the changes of the thermal-steric and halo-steric contributions to the global steric height over time. To first order, the size of the global halo-steric component should be very small, as it is controlled by changes in freshwater input into the global ocean. After evaluating existing ORAs, the ones selected as having the smallest potential errors in long-term OHC will be analyzed with a goal to better understand the processes that determine the observed OHC change and variability. The team will assess whether the decadal-scale slight reduction in surface temperature was real and whether it reflected transport of extra heat to deeper layers of the ocean. Researchers will also carry out the analyses with a focus to separate the contributions of isopycnal movement (heave) and change in water mass properties along isopycnals (spiciness). If available in the selected ORAs, the vertical heat flux and the associated dynamical variables will also be examined.