Quantifying uncertainties on regional sea level change induced by multidecadal intrinsic oceanic variability

A global eddy-permitting (1/4 degrees resolution) ocean general circulation model is shown to spontaneously generate intrinsic oceanic variability (IOV) up to multidecadal timescales (T > 20years) under a repeated seasonal atmospheric forcing. In eddy-active regions, the signature of this multidecadal eddy-driven IOV on sea level is substantial, weakly autocorrelated, and is comparable to (and may clearly exceed) the corresponding signature of internal climate variability (ICV) produced by current coupled climate modelswhose laminar ocean components may strongly underestimate IOV. Deriving sea level trends from finite-length time series in eddy-active regions yields uncertainties induced by this multidecadal IOV, which are of the same order of magnitude as those due to ICV. A white noise model is proposed to approximate the low-frequency tail of the IOV spectra and could be used to update ICV estimates from current climate simulations and projections.