Observational data covering currents and temperature through two contrasting autumn to spring periods (2010/2011 and 2013/2014) at the constricted entrance to the inner basin of the Hornsund fjord, Svalbard, are investigated. This fjord is presently undergoing significant changes, manifested by reduction of the seasonal sea ice cover and retreating tidewater glacier fronts. The data set presented here shows how local wind, tides, and longer period current fluctuations allow warm water of Atlantic origin to reach the innermost basin of the fjord. While the forcing factors of the exchange mechanisms are not seen to vary much between the two studied seasons, the effect of wind events on exchange is assumed to be modified by local stratification and sea ice concentration. The resulting heat transport depends on availability of heat in the central part of the fjord. The observations indicate that available heat in autumn and winter 2013/2014 effectively hindered formation of a stable sea ice cover. While the present data set does not enable direct quantification of the role of oceanic heat in tidewater glacier melting, we find that the net retreat of glacier fronts was 10 times larger in the warm (ocean) period 2013/2014 compared to the colder 2010/2011.

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The duration of current measurements is often short, ranging from a few weeks up to a year. Application of extreme value statistics to derive design levels requires relatively long time series. To mitigate the lack of long-term measurements, the Norwegian standard NS9415 for fish farm design requires the design level of 50-year return period to be derived by multiplication of the current maximum in month-long current measurements by a prescribed conversion factor of 1.85. Here we use twelve data sets of yearlong coastal current measurements to explore the validity of this factor. For each yearlong time series, a design level of 50-year return period is calculated by extreme value statistics and used to calculate estimates of the conversion factor. The mean value of the resulting conversion factor is close to that of NS9415, 1.85 and 1.80 at 5 and 15 m depth, respectively. However, the spread in values is great, both geographically and between months. A conversion factor ranging from 1 to 4 reflects different relative dominance of the driving forces at different coastal regions and different seasons. The absence of a significant seasonal cycle in the conversion factors calculated here, illustrates the difficulty in adjusting for season. The results illustrate and quantify the uncertainty and — often — the lack of conservatism in design levels derived from month long current observations.