Numerical models and long term monitoring - How can numerical models be used to support in situ sampling and survey design for long term hydrographic monitoring in standard sections?

Abstract

As a part of regular monitoring of the marine environment, IMR conducts 10 fixed transects on a multiannual basis during which hydrographical, chemical and plankton data are collected at the same positions several times a year. The transect data sets, which in some cases span up to seven decades, have been vital to the understanding of long-term variability and trends in environmental and climate conditions. As an alternative approach to assemble physical data, numerical circulation models are widely used. There is a large variety of model data archives available, both internally at the IMR and from publicly open data portals, but it is difficult to consider the precision of the different models as they have different properties, resolution, coverage area etc. This report assesses how well existing model products developed and/or intensively used by the Oceanography and Climate Research Group can be utilised to assess and support the shipboard monitoring on the transects. Main focus is on TOPAZ, which is the only fully operational model with a model domain covering all the transects considered here. The results show that TOPAZ reproduces interannual variability and multiannual trends well. However, temperature, salinity and current velocity values, as well as seasonal variability and extreme conditions are less well represented. The operational (internal) Norkyst model show the best skill reproducing current velocities, but do not cover all transects. Using TOPAZ to assess how well the present sampling strategy captures the spatial variability in hydrographic variables suggests that the current sampling strategy is well designed in terms of the horizontal spacing of the fixed transects, although the sections in the northern Norwegian Sea and southern Barents Sea show a significant co-variability of Atlantic Water towards the Arctic Ocean. Assessing the impact of sampling frequency on long-term monitoring efforts in one of the transects, suggests a minimum sampling frequency of 3-4 transects per year to prevent loss of information relating to interannual variability and trends. We note, however, that a full assessment of the impact of sampling frequency on the transects must include also chemical and plankton observations and models, as well as the need of capturing short-term variations like the seasonal cycle.

Numerical models and long term monitoring - How can numerical models be used to support in situ sampling and survey design for long term hydrographic monitoring in standard sections?