Monthly maps of optimally interpolated in situ hydrography in the North Sea from 1948 to 2013

- Hydrography reconstructions based on Optimal Interpolation
- North Sea monthly hydrography from 1948-2013 with a resolution of 0.2°× 0.2°
- Accurate representation of inter-annual and decadal temperature and salinity changes
- Updated North Sea climatologies with 54 vertical layers
- Accurate reconstruction of the bottom hydrography for ecology and fishery studies

We used a statistical model mainly based on optimal or Gauss-Markov interpolation (OI) to produce monthly maps of in situ hydrography in the North Sea through 66 years (from 1948 to 2013) with a resolution of 0.2° × 0.2°. The in situ hydrography data are approximately 430,000 hydrographic profiles and aggregated thermosalinograph data from various international databases. Duplicates, outliers and vertical density instabilities were removed. Regions with poor OI estimates were replaced with a harmonic reconstruction arising from the most reliable OI estimates. Adjustments for vertical density stability were based on the standards of the World Ocean Atlas. We mapped at 54 depth levels through the water column, focusing on surface and bottom hydrography because this type of map is of particular interest for ecosystem and fisheries research. Average OI temperature and salinity expected errors at the surface are 0.3 °C and 0.1, respectively. OI errors decrease with depth following decrement of signal and noise variances and apparently independent of the data amount (indicating a good data coverage). Alternative error estimates were obtained with the Median Absolute Deviation between our hydrography estimates and time series excluded from the analysis and are on average 0.3 °C and 0.1 salinity units. While our product seems limited for analysis of variability on monthly and seasonal time scales, particularly in the regions of large variability, it is suitable for studies of inter-annual and decadal variability. A comparison with two alternative analyses (KLIWAS and SODA) is discussed. As direct application of our results, we present a new hydrographic climatology of the North Sea at various depths with an improved effective resolution.