Oceanic hindcast simulations at high resolution suggest that the Atlantic MOC is bistable

All climate models predict a freshening of the North Atlantic at high latitude that may induce an abrupt change of the Atlantic Meridional Overturning Circulation (hereafter AMOC) if it resides in the bistable regime, where both a strong and a weak state coexist. The latter remains uncertain as there is no consensus among observations and ocean reanalyses, where the AMOC is bistable, versus most climate models that reproduce a mono-stable strong AMOC. A series of four hindcast simulations of the global ocean at 1/12° resolution, which is presently unique, are used to diagnose freshwater transport by the AMOC in the South Atlantic, an indicator of AMOC bistability. In all simulations, the AMOC resides in the bistable regime: it exports freshwater southward in the South Atlantic, implying a positive salt advection feedback that would act to amplify a decreasing trend in subarctic deep water formation as projected in climate scenarios

Copernicus Marine Service Ocean State Report

This is the final version. Available from Taylor & Francis via the DOI in this record

Couplage d'un océan NEMO avec deux modèles de glace hémisphériques Arctique et Antarctique

Two new NEMO-SAS (stand alone surface module) configurations are derived from a global ORCA grid. A FORTRAN tool is developed to transform the ORCA input files and define the OASIS weights & addresses to map fluxes and surface variables between one global and two hemispheric grids. A tri-component coupled system is set up, including an ocean on the ORCA2 global grid and two hemispheric NEMO-SAS with SI3 sea-ice. Removing North Pole folding communications in ice model helps to improve computing performance, since our ORCA2 coupled configuration is twice faster and two times cheaper than its single executable counterpart. The North Pole folding free hemispheric grid discretisation simplifies the coding of the future neXtSIM model, while relying on the same grid points than the ocean, thus avoiding inaccurate interpolation between ice and ocean grid

A high-resolution ocean and sea-ice modelling system for the Arctic and North Atlantic oceans

As part of the CONCEPTS (Canadian Operational Network of Coupled Environmental PredicTion Systems) initiative, a high-resolution (1/12°) ice–ocean regional model is developed covering the North Atlantic and the Arctic oceans. The long-term objective is to provide Canada with short-term ice–ocean predictions and hazard warnings in ice-infested regions. To evaluate the modelling component (as opposed to the analysis – or data-assimilation – component, which is not covered in this contribution), a series of hindcasts for the period 2003–2009 is carried out, forced at the surface by the Canadian GDPS reforecasts (Smith et al., 2014). These hindcasts test how the model represents upper ocean characteristics and ice cover. Each hindcast implements a new aspect of the modelling or the ice–ocean coupling. Notably, the coupling to the multi-category ice model CICE is tested. The hindcast solutions are then assessed using a verification package under development, including in situ and satellite ice and ocean observations. The conclusions are as follows: (1) the model reproduces reasonably well the time mean, variance and skewness of sea surface height; (2) the model biases in temperature and salinity show that while the mean properties follow expectations, the Pacific Water signature in the Beaufort Sea is weaker than observed; (3) the modelled freshwater content of the Arctic agrees well with observational estimates; (4) the distribution and volume of the sea ice are shown to be improved in the latest hindcast due to modifications to the drag coefficients and to some degree to the ice thickness distribution available in CICE; (5) nonetheless, the model still overestimates the ice drift and ice thickness in the Beaufort Gyre