How fresh water forcing affects the NCC
-an exploratory study

The NCC can be described as an agent for draining runoff from most of Northern Europe into the salty water masses that make up the bulk of the world ocean. We shall briefly present some preliminary results on how the NCC, and the Skagerrak/North Sea in general, are affected by fresh water forcing. First, we provide relevant background information, based on simulation results from the present project. Second, we examine the relative role of the various fresh water sources, and then we present results from validation of various representation of fresh water and Baltic fluxes. Finally, results from a coupled bio/physics model are considered, based on different formulations of how the main source (the Baltic) is represented in the circulation model. An in-depth study of this topic is presently being planned, but for the time being, further activities are pending new funding.

The results that are presented here, are a fragmented collection of experiences from a variety of activities at Norwegian Meteorological Institute (met.no). An in-depth study of this topic is presently being planned, but for the time being, further activities are pending new funding.

In the present project, the Norwegian Coastal Current (NCC) was found to be too wide and lacking in bouyancy. These results are discussed in Section 3 in Melsom (2005), and the salinity bias in the NCC is also presented here. The problem of salt biases in the NCC is not only found in results from HYCOM. Here, we present results from an exploratory study, using MI-POM, met.no's version of the Princeton Ocean Model. This is also the model that is presently used for operational purposes at met.no.

Results from the investigation of the relative role of the various fresh water sources are presented here. Generally, we find that removing run-off has a local impact. The main exception is the Baltic flux of brackish waters into the Kattegat, which (when removed) gives rise to substantial differences downstream, from the Kattegat, through the Skagerrak and into the North Sea. In this context, it should be mentioned that the brackish Baltic inflow is a result of drainage in the Baltic, i.e., it constitutes an integrated response upon entering the presently considered domain.

Validation results for various representations of fresh water fluxes and Baltic fluxes in 1998 are presented here. We find that errors are reduced somewhat when climatolical values for river run-off are replaced by observational run-off data. However, the inclusion of volume fluxes through the Belts and Öresund from a storm surge model that includes the Baltic Sea has the largest impact on the model errors. Nevertheless, the model vs. transect data validation reveal the same problem as the HYCOM results: The NCC is too salty in the model, and the front between the NCC and the Atlantic water is too weak. A possible remedy might be to reduce the specified salinity of the Baltic outflow, but if the Baltic Sea was included in the simulation domain (rather than indirectly from a separate volume flux model), we would also model the temporal variability of salinity in the Belts and Öresund.

Results from the coupled model system for concentrations of diatoms are presented here. Note that the only difference between the results that are presented, is the representation of Baltic outflow in the circulation model. The dominating impact of the Baltic outflow on the results from the phytoplancton model is evident. It is more difficult to find similarities than differences in these results, possibly with the exception of the local maxima in the central Skagerrak that are seen in both simulations. Note also the mesoscale structures of the concentrations in this regions, particularly in the bottom depiction.

Summary

Evaluation of fresh water contributions:
• The relative role of the various fresh water sources

The Baltic outflow may be represented in the model in several ways:
1. by a relaxation of water mass properties in the southern Kattegat towards prescribed (climatological) values
2. by prescribed fluxes of water across an open boundary in the southern Kattegat
3. by merging in the southern Kattegat, using results from a model that includes the Baltic Sea in its domain
4. by specifying or nesting results at an open boundary using results from a model that includes the Baltic Sea in its domain
5. by expanding the model domain to include the Baltic Sea

This is our experience with some of these options:
• Transport of salt anomalies in the HYCOM results (item 1 above)
• Validation for various representations of fresh water fluxes (item 2 in Control expt. vs. 4 in Expt. 3)
• Baltic outflow and the concentration of diatoms (item 2 vs. 3)

This document was written by Arne Melsom, Jon Albretsen, and Lars Petter Røed.