Combining meteorological and oceanic observations for an improved SODA: reconstrucing climate variability during the past century

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The ocean's ability to store and transport heat and freshwater gives it a crucial role in climate variability. The idea that the ocean can sequester excess heat and thus disrupt continental warming trends or shift rainfall patterns is the subject of a rapidly growing research literature. Much of the observational basis for many studies of the hiatus is limited to the years since 2001 when the instrumental coverage of the oceans expanded due to Argo. An effort by meteorologists is refining and extending the surface meteorological record back to the nineteenth century. This proposal is a complementary effort to explore the limits of using data assimilation to combine historical observations, surface meteorology, and ocean dynamics to reconstruct the corresponding record of ocean heat and water storage. By elucidating the historical ocean climate record and the limits to what we can learn about past variability due to the historical data sampling this research provides a key piece of information for efforts to anticipate and predict future climate variability and change. The ocean reanalysis data sets produced will be made freely available to other researchers and the general community, which will foster further research benefits. This work will also form the basis of a dissertation for a University of Maryland graduate student.The ocean and sea ice systems are increasingly being recognized as playing a key role in climate variability and change and so making best use of the limited historical record must be a high priority. The recent hiatus in ocean warming seems to have been confined only to the past decade, but the oceans experienced a much more extended two decade hiatus beginning in 1970 and likely another in the 1940s and 1950s. It has been suggested that the reduced heating in the 1970s was the result of the effects of increasing aerosol loading on net surface heating. If so that should lead to significant differences in heat distribution from the recent hiatus (for which Pacific climate variability is likely important). The relatively cool North Atlantic of the 1940s ended a period of unprecedented warming of the Arctic. Is there evidence to suggest corresponding reductions of northward heat transport into the Arctic? Our understanding of the historical record of salinity is even less certain than temperature, but can the documented trends in upper ocean salinity be shown to alter the ventilation of water masses? The proposal will build on the PI's long term development of the Simple Ocean Data Assimilation (SODA) reanalysis as a tool to explore the ocean's role in climate variability. A key emphasis will be on using observing system simulation experiments and data withholding experiments to explore the accuracy of those reconstructions to better understand the limits of application of such a reconstruction. With the combined use of hydrographic and tide gauge sea level data together in a hybrid ensemble data assimilation system, this proposal is to reanalyze ocean and sea ice variability (including sea level) from 1900 through 2015, and to use observing system experiments to quantify uncertainties. Justification for the work lies in the need to address the US CLIVAR (Climate Variability and Predictability) program's objectives of "describing and understanding the ocean's role in climate variability on seasonal, interannual, and decadal timescales" and the recommendations of the U.S. Global Change Research Plan 2012-2021.