ALBERT

Description: We present a robust method for diagnosing total diapycnal diffusivities, i.e. explicitly applied plus numerically induced diffusivities, from tracer release experiments in numerical z-level models. To this extent, numerical experiments differing only in the advection scheme used (CTRD using 2nd order centred differences, UPWIND using the upwind/upstream advection scheme, QUICK using the quicker advection scheme after Farrow and Stevens (1995) and FCT after Gerdes et al. (1991)) are analysed and compared. To obtain regionally resolved estimates of diapycnal diffusivities, individual inert dye tracers are released in dynamically different regions of a North Atlantic model, namely (i) in the interior of the subtropical gyre and (ii) in the western boundary current. Diagnosed diffusivities are robust with respect to changes in temporal and spatial sampling of the simulated dye tracer for both advection schemes and for both regions. The numerically induced diffusivity is generally positive, but can become negative for centred differences advection numerics after several months of simulated tracer dispersion.

Description: Diapycnal diffusion is a key process in the ocean, responsible for water mass transformation and the conversion of kinetic energy into potential energy. Despite its widely assumed importance in controlling ocean dynamics, diapycnal diffusion is difficult to quantify both in the real ocean and in ocean models. Here we focus on z-level models, arguably the most common vertical grid scheme of current ocean general circulation models. We examine different methods to diagnose diapycnal diffusivities in z-level models. Different scenarios are investigated, including the impact of advection and vertical convergence or divergence of isopycnals. In all cases we find that the transformation from z-space to density space has to be performed very carefully in order to obtain reliable and robust estimates of diapycnal diffusivities (and the associated diapycnal fluxes). A method involving the tracer flux taken from the work of Griffies et al. (2000) seems to be most appropriate in this respect and is suggested as our method of choice for subsequent applications to 3-dimensional ocean circulation models

Description: Highlights: • Modelled fish biomass was affected by interannual variability in the plankton food. • The effects were small compared with the high variability in observations. • Fish were highly affected by changes in the larval mortality of anchovy. Abstract: The Northern Humboldt Current System is the most productive eastern boundary upwelling system, generating about 10 % of the global fish production, mainly coming from small pelagic fish. It is bottom-up and top-down affected by environmental and anthropogenic variability, such as El-Niño Southern Oscillation and fishing pressure, respectively. The high variability of small pelagic fish in this system, as well as their economic importance, call for a careful management aided by the use of end-to-end models. This type of models represent the ecosystem as a whole, from the physics, through plankton up to fish dynamics. In this study, we utilised an end-to-end model consisting of a physical–biogeochemical model (CROCO-BioEBUS) coupled one-way with an individual-based fish model (OSMOSE). We investigated how time-variability in plankton food production affects fish populations in OSMOSE and contrasted it against the sensitivity of the model to two parameters with high uncertainty: the plankton accessibility to fish and fish larval mortality. Relative interannual variability in the modelled fish is similar to plankton variability. It is, however, small compared with the high variability seen in fish observations in this productive ecosystem. In contrast, changes in larval mortality have a strong effect on anchovies. In OSMOSE, it is a common practice to scale plankton food for fish, accounting for processes that may make part of the total plankton in the water column unavailable. We suggest that this scaling should be done constant across all plankton groups when previous knowledge on the different availabilities is lacking. In addition, end-to-end modelling systems should consider environmental impacts on other biological processes such as larval mortality in order to better capture the interactions between environmental processes, plankton and fish.