Processes and flow over the Iceland-Faroe Ridge (D2.3)

Summary

Combining the results from the ester Valley Overflow field experiment with data from other in situ current measurements, from satellite tracked drifters, and from satellite altimetry, we find a consistent picture of the passage of Atlantic water across the Iceland-Faroe Ridge, which can be described in terms of two branches.

The Western branch passes through the Western Valley, where it is locked to the steep topography of the Icelandic slope, but part of it re-circulates back onto the ridge before entering the Norwegian Sea. It may have strong surface flow in a narrow current, but it is highly variable and its average volume transport is low. The Eastern branch passes the ridge as a broad flow with a core that shifts back and forth laterally (parallel to the ridge axis). In general, it exhibits weaker surface currents based on observations, but its volume transport is the dominant component and must be fairly stable to account for the stability of transport monitored at a section farther east.

The available data on Mean Dynamic Topography seem to reflect realistic large-scale circulation features, but to smear out the spatial structure of surface currents and transports. The Sea Level Anomaly data from satellite altimetry appear to be well related to surface currents in this region, at least on time scales from weeks to longer. When calibrated with the results from the WOW field experiment, they allow long-term monitoring of volume transport of the Western branch.

By applying a set of commentary modelling approaches, it is confirmed that the combined transport can be assessed and effectively monitored on a section north of the Faroes, disregarding that there is little model consensus on the strength of individual branches. Direct comparisons of model results and observations of the total transport show large discrepancies. Assessment of inherent model uncertainty for this and other inflow branches show that the Atlantic water flow across the Iceland-Faroe Ridge has a higher natural noise level and is less constrained by the applied forcing than could be expected. This new result puts emphasis on model ensemble approaches as the key to understand the climatic sensitivity of the system but also raises a concern about our capability to predict abrupt changes.

Results suggests that adequately tuned ocean models of eddy permitting resolution are sufficient for simulating the main characteristics of the Iceland-Faroe Ridge inflow including horizontal structure, individual characteristics of the two branches as well as seasonality of the net transport.