An ensemble of eddy-permitting global ocean reanalyses from the MyOcean project

Abstract

com level, transports across pre-defined sections, and sea ice parameters. The eddy-permitting nature of the global reanalyses allows also to estimate eddy kinetic energy. The results show that in general there is a good consistency between the different reanalyses. An intercomparison against experiments without data assimilation was done during the MyOcean project and we conclude that data assimilation is crucial for correctly simulating some quantities such as regional trends of sea level as well as the eddy kinetic energy. A second objective is to show that the ensemble mean of reanalyses can be evaluated as one single system regarding its reliability in reproducing the climate signals, where both variability and uncertainties are assessed through the ensemble spread and signal-to-noise ratio. The main advantage of having access to several reanalyses differing in the way data assimilation is performed is that it becomes possible to assess part of the total uncertainty. Given the fact that we use very similar ocean models and atmospheric forcing, we can conclude that the spread of the ensemble of reanalyses is mainly representative of our ability to gauge uncertainty in the assimilation methods. This uncertainty changes a lot from one ocean parameter to another, especially in global indices. However, despite several caveats in the design of the multi-system ensemble, the main conclusion from this study is that an eddypermitting multi-system ensemble approach has become mature and our results provide a first step towards a systematic comparison of eddy-permitting global ocean reanalyses aimed at providing robust conclusions on the recent evolution of the oceanic state.

South Atlantic meridional transports from NEMO-based simulations and reanalyses

“…The assimilation of observations in eddy-permitting models introduces variability that would otherwise only appear with higher resolution, as in ORCA0083. According to Masina et al (2015), this higher variability in the ORAs is in better agreement with the eddy kinetic energy estimates from the ocean surface current velocities (OSCAR) product than that of the FRMs. Although some of the ORAs have more transport variability than others throughout the basin, the western boundary variability remains a dominant feature, particularly northward of 25 • S. In Fig.…”

“…The different levels of variability in the Agulhas leakage between ORCA025 and ORAs may be attributed to the impacts of the SLA assimilation (Backeberg et al, 2014). However, even among ORAs these Agulhas patterns differ; for example, the weaker contributions in ORAP5 may be due to smoothing from the superobservation method applied to the altimeter data (Mogensen et al, 2012), as also noted by Masina et al (2015). Figure 13a shows the monthly time series of both ψ max and the maximum southward flow in the basin interior (as in Fig.…”

Landfast ice thickness in the Canadian Arctic Archipelago from observations and models

“…See the appendix for a complete description of the method. We also investigate ice thickness values from a selection of the highest-resolution models (Storto et al, 2011;Forget et al, 2015;Haines et al, 2014;Masina et al, 2015), from the Ocean Reanalysis Intercomparison (ORA-IP) Chevallier et al, 2016) (Table 2) and from the Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS) (Zhang and Rothrock, 2003). Supporting 2 m temperature data were obtained from ERA-Interim (Dee et al, 2011).…”

Thermodynamic and Dynamic Ice Thickness Changes in the Canadian Arctic Archipelago in NEMO-LIM2 Numerical Simulations

“…as well as two dimensional (2D) sea surface height (SSH) and sea ice fields (concentration and thickness) are taken from from the Global Ocean Reanalysis and Simulations (GLORYS2v3) produced by Mercator Ocean (Masina et al, 2015). Open 25 boundary conditions (temperature, salinity and horizontal ocean velocities) are derived from the monthly GLORYS2v3 product as well.…”