The climatology of polar lows over the Nordic Seas has been investigated using infrared satellite images for the period between 2000 and 2009. The same region was studied in the 1980s using traditional weather charts for the period between 1972 and 1982. One motivation for the present study was to revisit this climatology, but using a different decade and taking advantage of the vastly improved coverage and dissemination of infrared satellite images since the 1980s. The fact that forecasters at the Norwegian Meteorological Institute had introduced a routine to register polar-low events systematically from 2000 and onward also provided a unique opportunity for extending the existing repository of subjectively identified polarlow observations. On average we found 12 polar-low events per year in the region of study. This is more than the earlier investigation, but we believe that this can be explained by the fact that the previous study relied almost uniquely on weather charts with very little information from ocean areas in the Nordic Seas. The largest numbers were found in January with an average of 2.8 polar-low events per year. The study reconfirms the February minimum found in previous studies, but on the basis of our data we could not show that this minimum is statistically significant. It is suggested that this may be explained as a manifestation of the coldest winter month, when a surface-pressure high over the Scandinavian mainland is common and the large-scale atmospheric flow is less favourable to polar-low formation. This hypothesis was tested by calculating the mean sea-level pressure (MSLP) anomaly for January, February and March from an atmospheric reanalysis. This revealed a positive anomaly over Scandinavia and northwest Russia not found in the pressure distributions for January and March.

The northward flow of warm and saline Atlantic Water through the eastern Nordic Seas sustains a springbloom ecosystem that hosts some of the world's largest commercial fish stocks. Abrupt climatic changes, or changes beyond species-specific thresholds, may have severe effects on species abundance and distribution. Here, we utilize a numerical ocean model hindcast to explore the similarities and differences between large-scale anomalies, such as great salinity anomalies, and along-shelf hydrographic anomalies of regional origin, which represent abrupt changes at subannual time scales. The large-scale anomalies enter the Nordic Seas to the south and propagate northward at a speed one order of magnitude less than the Atlantic Water current speed. On the contrary, wind-generated along-shelf anomalies appear simultaneously along the Norwegian continental shelf and propagate northward at speeds associated with topographically trapped Kelvin waves. This process involves changes in the vertical extent of the Atlantic Water along the continental slope. Such a dynamic oceanic response both affects thermal habitats and has the potential to ventilate shelf waters by modifying the cross-shelf transport of nutrients and key prey items for early stages of fish.

High and variable mortality during the egg and larval stages is thought to be an important source of interannual variability in stock size in many marine fish. However, quantitative information about the mortality during these life stages, especially interannual variability, is sparse. Here, we used a time-series covering 35 years (1959–1993) of survey data to estimate mortality during the egg stages of northeast Arctic cod (Gadus morhua) and Northeast Arctic haddock (Melanogrammus aeglefinus). Using a regression approach based on the numbers of eggs in different developmental stages, we calculated the mean instantaneous mortality rate of cod eggs to be 0.17 d−1 (95% CI: 0.15–0.19), which is significantly higher than that for haddock, 0.09 d−1 (95% CI: 0.07–0.12). Interannual variability in egg mortality ranges from ∼0.12 to ∼0.22 d−1 for cod and from ∼0.04 to ∼0.12 d−1 for haddock. The accuracy of these estimates was evaluated by the analysis of synthetic data constructed from a coupled physical–biological model, suggesting that mean mortality and the magnitude of interannual variability were estimated reliably, but not mortality for any given year.

Biological processes and physical oceanography are often integrated in numerical modelling of marine fish larvae, but rarely in statistical analyses of spatio-temporal observation data. Here, we examine the relative contribution of inter-annual variability in spawner distribution, advection by ocean currents, hydrography and climate in modifying observed distribution patterns of cod larvae in the Lofoten -Barents Sea. By integrating predictions from a particle-tracking model into a spatially explicit statistical analysis, the effects of advection and the timing and locations of spawning are accounted for. The analysis also includes other environmental factors: temperature, salinity, a convergence index and a climate threshold determined by the North Atlantic Oscillation (NAO). We found that the spatial pattern of larvae changed over the two climate periods, being more upstream in low NAO years. We also demonstrate that spawning distribution and ocean circulation are the main factors shaping this distribution, while temperature effects are different between climate periods, probably due to a different spatial overlap of the fish larvae and their prey, and the consequent effect on the spatial pattern of larval survival. Our new methodological approach combines numerical and statistical modelling to draw robust inferences from observed distributions and will be of general interest for studies of many marine fish species.

The impact of intense atmospheric mesocyclones—polar lows—on the ocean circulation at high latitudes, as well as the role of ocean feedbacks on the evolution of these atmospheric systems themselves, is under debate. Here, the upper ocean response to atmospheric forcing before, during, and after polar lows events over the Nordic Seas is studied. A set of 96 unique polar low tracks from 2002 to 2010 are collocated with satellite‐based sea surface temperature and altimeter observations, and with surface drifter observations. The satellite data show systematic temperature and sea level drops and enhanced geostrophic kinetic energies over the days leading up to polar low events. These data however reveal little information about the ocean response to the polar lows themselves. The drifter observations largely agree with the satellite data on the response to synoptic conditions, but they also give indication of enhanced upper‐ocean kinetic energies immediately following the passage of polar lows.