ORA-20C is a 10-member ensemble of ocean reanalyses covering the 20th century using atmospheric forcing from ERA-20C. Its main purpose is to provide initial conditions for the ocean component of the CERA-20C streams. One of the main uncertainties when conducting an experiment like ORA-20C is on the state of the ocean in the 1900s. Preliminary experiments were conducted to characterize the ocean model drift and the model response to time varying forcing and changes in the observing system. These experiments were used to provide an ensemble of ocean initial states for ORA-20C. ORA-20C shows a large spread in the first part of the century before the assimilation strongly constrains all the members toward an ocean state that is comparable with other reanalysis products. The ORA-20C reanalysis highlights the issues that need to be solved in order to provide a consistent 20th century ocean record. Dealing with the transition between the poorly observed early 20th century and the well-sampled later decades is a major challenge to take up before a product such as ORA-20C can be used for the detection of robust climate signals in the ocean subsurface. Treatment of model error is key to deal with changes of the observing system. A better bias correction algorithm may help constraining the solution in the first half of the century and stand as a concrete lead to explore.
}, year = {2016}, journal = {ERA Report Series}, number = {24}, month = {2016}, publisher = {ECMWF}, language = {eng}, }