A Dynamical Reconstruction of the Global Monthly Mean Oxygen Isotopic Composition of Seawater

We present a dynamically consistent gridded data set of the global, monthly-mean oxygen isotope ratio of seawater (δOsw). The data set was created from an optimized simulation of an ocean general circulation model constrained by global monthly δOsw data collected from 1950 until 2011 and climatological salinity and temperature data collected from 1951 to 1980. The optimization was obtained using the adjoint method for variational data assimilation, which yields a simulation that is consistent with the observational data and the physical laws embedded in the model. Our data set performs equally well as a previous data set in terms of model-data misfit but brings an improvement in terms of the seasonal cycle and physical consistency. As a result the data set does not show any sharp transitions between water masses or in areas where the data coverage is low. The data assimilation method shows high potential for interpolating sparse data sets in a physically meaningful way. Comparatively big errors, however, are found in our data set in the surface levels in the Arctic Ocean mainly because the influence of isotopically highly depleted precipitation is not preserved in the sea-ice model, and the low model resolution of about 285 km horizontally. The data set is publicly available and it is anticipated to be useful for a large range of applications in (paleo-) oceanographic studies. Keypoints: • Physically consistent data set of the global, 3-dimensional oxygen isotopic distribution of the ocean ©2018 American Geophysical Union. All Rights Reserved. • Using the adjoint method to fit an ocean general circulation model to observational data • Data set available for potential applications in (paleo-)oceanography ©2018 American Geophysical Union. All Rights Reserved. Plain Language Summary The ratio of the heavier to the lighter oxygen isotope in seawater varies over different areas and for different water masses in the ocean. In this study we present a global gridded data set of the monthly-mean oxygen isotope ratio of seawater (δOsw). The data set is publicly available and will be useful for a large range of applications. It can, for example, help to reconstruct past ocean states, or be used as lower boundary for an atmospheric model to investigate the isotopic composition of the atmosphere. The data set is taken from a simulation of a numerical ocean model that is in consistency with global δOsw observations collected from 1950 until 2011 and monthly salinity and temperature observations collected from 1951 to 1980. To obtain the model simulation, we used a data assimilation method that brings the model simulation into consistency with the observational data. The ocean model is based on physical laws and therefore, our data set is also in consistency with the ocean physics. Our data set is similarly close to the observations than a previous data set, but brings an improvement because it is based on the ocean physics and because it has a seasonal cycle.

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