Persistent shift of Calanus spp. in the south-western Norwegian Sea since 2003, linked to ocean climate

Abstract The southwestern Norwegian Sea is characterized by an inflow of warm and saline Atlantic water from the southwest and cold and less saline East Icelandic Water (EIW), of Arctic origin, from the northwest. These two water masses meet and form the Iceland-Faroe Front (IFF). In this region, the copepod Calanus finmarchicus plays a key role in the pelagic ecosystem. Time-series of C. finmarchicus and Calanus hyperboreus in May and September, extending back to the early 1990s, were studied in relation to phytoplankton bloom dynamics and hydrography. The main reproductive period of C. finmarchicus started consistently earlier south of the IFF, resulting in different life cycles and stage compositions in the two water masses. In 2003, a sudden shift occurred north of the IFF, resulting in a similar phenology pattern to south of the IFF. Before this, only one generation of C. finmarchicus was produced in the Arctic water, but the earlier reproduction enabled the species to produce two generations after 2003. Simultaneously, C. hyperboreus, an expatriate in the EIW, largely disappeared. Food availability is unlikely the reason for the phenological differences observed across the front, as the typical pattern of the phytoplankton spring bloom showed an earlier onset north of the IFF. Temperature and salinity peaked at record high values in 2003 and 2004, and therefore possible links to oceanography are discussed. The dominant role of Calanus spp. and the potential linkages to water mass exchanges may herald strong effects on the ecosystem and pelagic fish in this subpolar Atlantic region under expected climate change.</jats:p

Seasonal to Decadal Temperature Variations in the Faroe-Shetland Inflow Waters

A 53 years hind-cast simulation with a regional version of the Nansen Center version of the Miami Isopycnic Coordinate Ocean Model (MICOM) has been conducted to explore the nature of the observed seasonal to inter-annual variations in the temperature of the pole-ward flowing Atlantic Water (AW) crossing the Iceland-Scotland Ridge (ISR). It is found that the simulated long-term temperature variations closely resemble observations south of the ridge (Rockall Trough), north of the ridge (Svinøy section) and between (Faroe Shetland Channel, FSC). The simulated temperature on the Faroe Shelf is also compared to daily temperature observations from Mykines, revealing realistic long-term temperature variations, seasonal variations and a realistic seasonal modulation. The simulated time series in the FSC indicates that the phase and amplitude of the annual temperature cycle of the AW have varied by almost one month and 0.15 ºC between the 1960s and 2001, illustrating the difficulty in unambiguously removing the seasonal cycle from the, sparsely sampled, time series. It is argued that the simulated time series can be used to complement the observed time series in periods with sparse sampling. Specifically, the observation-based cold anomaly in the late 1960s and the warming in the early 1980s should be treated with caution. Finally, the analysis indicates that it is not advisable to survey the hydrographic section less than four times a year if reliable decadal scale temperature variations are of interest

The Iceland–Faroe inflow of Atlantic water to the Nordic Seas

The flow of Atlantic water between Iceland and the Faroe Islands is one of three current branches flowing from the Atlantic Ocean into the Nordic Seas across the Greenland–Scotland Ridge. By the heat that it carries along, it keeps the subarctic regions abnormally warm and by its import of salt, it helps maintain a high salinity and hence density in the surface waters as a precondition for thermohaline ventilation. From 1997 to 2001, a number of ADCPs have been moored on a section going north from the Faroes, crossing the inflow. Combining these measurements with decade-long CTD observations from research vessel cruises along this section, we compute the fluxes of water (volume), heat, and salt. For the period June 1997–June 2001, we found the average volume flux of Atlantic water to be 3.5 ± 0.5 Sv (1 Sv = 106 m3·s1). When compared to recent estimates of the other branches, this implies that the Iceland– Faroe inflow is the strongest branch in terms of volume flux, transporting 47% of the total Atlantic inflow to the Arctic Mediterranean (Nordic Seas and Arctic Ocean with shelf areas). If all of the Atlantic inflow were assumed to be cooled to 0 °C, before returning to the Atlantic, the Iceland–Faroe inflow carries a heat flux of 124 ± 15 TW (1 TW = 1012 W), which is about the same as the heat carried by the inflow through the Faroe–Shetland Channel. The Iceland–Faroe Atlantic water volume flux was found to have a negligible seasonal variation and to be remarkably stable with no reversals, even on daily time scales. Out of a total of 1348 daily flux estimates, not one was directed westwards towards the Atlantic