We present observations of temperature, salinity and phytoplankton from late winter of 1999 obtained along a meridional transect running from 52 to 72°N through the northern North Atlantic and Norwegian Sea. The field work was conducted with the intention to find evidence for a postulated relationship between convection and primary production in the open ocean in winter. In the investigated area, convection ventilates water masses in a deep mixed layer which may cover several hundreds of meters, depending on atmospheric forcing and stratification. Inspired by the results of a coupled phytoplankton convection model, we postulated that sinking phytoplankton from the seasonal thermocline in autumn is dispersed within the mixed layer by vertical orbital motions and turbulence induced by convection. Orbital motions allow intermittent visits of plankton to the euphotic layer from much deeper depths. Hence, convection distributes the chance of plankton receiving light over the entire mixed layer. This supports production and allows compensation of losses due to sinking. We hypothesised that a winter stock of living phytoplankton in the open ocean would always coincide with the deep mixed layer formed by convection, irrespective of its depth. Indeed, along the entire meridional transect, living phytoplankton was found within the layer, whereas only marginal amounts or virtually no phytoplankton existed beneath it. The layer extent varied between 300 and 800 m. In the northern North Atlantic, average concentrations of chlorophyll a (as a measure of phytoplankton biomass) in the mixed layer were about 0.3 mg m -3, and 0.6 mg m -3 in the Norwegian Sea. We concluded that this winter stock forms the inoculum for a spring production in the open ocean. KEY WORDS: Primary production · Winter · Convection · Vertical orbital motionsResale or republication not permitted without written consent of the publisher

No abstract

Abstract. The three-dimensional flow, temperature and salinity fields of the North Atlantic, including the Arctic Ocean, covering the time period 1992 to 2006 are simulated with the numerical ocean model CODE. The simulation reveals several new insights and previously unknown structures which help us to clarify open questions on the regional oceanography of Icelandic waters. These relate to the structure and geographical distribution of the coastal current, the primary forcing of the North Icelandic Irminger Current (NIIC) and the path of the Atlantic Water south-east of Iceland. The model's adaptively refined computational mesh has a maximum resolution of 1 km horizontal and 2.5 m vertical in Icelandic waters. CTD profiles from this region and the river discharge of 46 Icelandic watersheds, computed by the hydrological model WaSiM, are assimilated into the simulation. The model realistically reproduces the established elements of the circulation around Iceland. However, analysis of the simulated mean flow field also provides further insights. It suggests a distinct freshwater-induced coastal current that only exists along the south-west and west coasts, which is accompanied by a counter-directed undercurrent. The simulated transport of Atlantic Water over the Icelandic shelf takes place in a symmetrical system of two currents, with the established NIIC over the north-western and northern shelf, and a hitherto unnamed current over the southern and southeastern shelf, which is simulated to be an upstream precursor of the Faroe Current (FC). Both currents are driven by barotropic pressure gradients induced by a sea level slope across the Greenland-Scotland Ridge. The recently discovered North Icelandic Jet (NIJ) also features in the model predictions and is found to be forced by the baroclinic pressure field of the Arctic Front, to originate east of the Kolbeinsey Ridge and to have a volume transport of around 1.5 Sv within northern Denmark Strait. The simulated multi-annual mean Atlantic Water transport of the NIIC increased by 85 % during 1992 to 2006, whereas the corresponding NIJ transport decreased by 27 %. Based on our model results we propose a new and further differentiated circulation scheme of Icelandic waters whose details may inspire future observational oceanography studies.

The global tide is simulated with the global ocean general circulation model ICON-O using a newly developed tidal module, which computes the full tidal potential. The simulated coastal M2 amplitudes, derived by a discrete Fourier transformation of the output sea level time series, are compared with the according values derived from satellite altimetry (TPXO-8 atlas). The experiments are repeated with four uniform and sixteen irregular triangular grids. The results show that the quality of the coastal tide simulation depends primarily on the coastal resolution and that the ocean interior can be resolved up to twenty times lower without causing considerable reductions in quality. The mesh transition zones between areas of different resolutions are formed by cell bisection and subsequent local spring optimisation tolerating a triangular cell’s maximum angle up to 84°. Numerical problems with these high-grade non-equiangular cells were not encountered. The results emphasise the numerical feasibility and potential efficiency of highly irregular computational meshes used by ICON-O.

The three-dimensional flow, temperature and salinity fields of the North Atlantic including the Arctic Ocean covering the time period 1992 to 2006 are simulated with the numerical ocean model CODE. The model reveals several new insights and previously unknown structures which help us to clarify open questions on the regional oceanography of Icelandic waters. These relate to the structure and geographical distribution of the coastal current, the primary forcing of the North Icelandic Irminger Current (NIIC), the path of the Atlantic Water south-east of Iceland and the structure of the North Icelandic Jet (NIJ). The model's adaptively refined computational mesh has a maximum resolution of 1 km horizontal and 2.5 m vertical in Icelandic waters. CTD profiles from this region and the river discharge of 46 Icelandic watersheds, computed by the hydrological model WaSiM, are assimilated into the simulation. The model realistically reproduces the established elements of the circulation around Iceland. However, analysis of the simulated mean flow field also provides further insights. It suggests a distinct freshwater-induced coastal current that only exists along the south-west and west coasts which is accompanied by a counter-directed undercurrent. The simulated transport of Atlantic Water over the Icelandic shelf takes place in a symmetrical system of two currents, with the established NIIC over the north-western and northern shelf, and a current over the southern and south-eastern shelf herein called the South Icelandic Current (SIC). Both currents are driven by topographically induced distortions of the Arctic Front's barotropic pressure field. The SIC is simulated to be an upstream precursor of the Faroe Current (FC). The recently discovered North Icelandic Jet (NIJ) also features in the model predictions and is found to be forced by the baroclinic pressure field of the Arctic Front, to originate east of the Kolbeinsey Ridge and to have a volume transport of around 1.5 Sv within northern Denmark Strait. The simulated multi-annual mean Atlantic Water transport of the NIIC increased by 85% during 1992 to 2006, whereas the corresponding NIJ transport decreased by 27%. Based on our model results we propose a new and further differentiated circulation scheme of Icelandic waters whose details may inspire future observational oceanography studies