Kjell Arne Mork

Reversal of the 1960s to 1990s freshening trend in the northeast North Atlantic and Nordic Seas

Hydrographic time series in the northeast North Atlantic and Nordic Seas show that the freshening trend of the 1960s–1990s has completely reversed in the upper ocean. Since the 1990s temperature and salinity have rapidly increased in the Atlantic Inflow from the eastern subpolar gyre to the Fram Strait. In 2003–2006 salinity values reached the previous maximum last observed around 1960, and temperature values exceeded records. The mean properties of the Atlantic Inflow decrease northwards, but variations seen in the eastern subpolar gyre at 57°N persist with the same amplitude and pattern along the pathways to Fram Strait. Time series correlations and extreme events suggest a time lag of 3–4 years over that distance. This estimate allows predictions to be made; the temperature of Atlantic water in the Fram Strait may start to decline in 2007 or 2008, salinity a year later, but both will remain high at least until 2010.

Volume and Heat Transports to the Arctic Ocean Via the Norwegian and Barents Seas

The main aim of this paper has been to present a holistic view of the Atlantic water flow along the Norwegian Coast and into the Barents Sea. It has focused on the period starting in the mid-1990s, with simultaneous arrays of moored current meters in the Svinoy section and the Barents Sea Opening. These detailed measurements have provided the bases for improved estimates of means and variations in fluxes, and their forcing mechanisms. Mean volume and heat fluxes associated with Atlantic water in the Norwegian Atlantic Slope Current (NwASC) are 4.3 Sv and 126 TW respectively for the Svinoy section, showing no significant trends, and 1.8 Sv and 48 TW for the Barents Sea Opening, where positive trends have been found in both measures. These estimates are probably higher than the long-tem mean, since hydrographic data along the Norwegian Coast show that the periods of direct current measurements are the prolongations of a period that started in the late 1970s, since when Atlantic water has become warmer and saltier. The close resemblance, throughout the record, between temperature variations in the Kola section and the AMO-index back to the early 20 century illustrates the importance of large-scale longterm variations in the Barents Sea system. Although the magnitudes of these variations are relatively small in comparison with inter-annual variations, other studies have shown them to be of major importance for ecosystem changes (ACIA, 2005). 2 The different forcing effects of the NwASC and the Atlantic inflow to the Barents Sea to similar atmospheric systems are noted. The results strongly suggest that the relative distribution of the Norwegian Atlantic Current entering the Barents Sea and passing through the Fram Strait is very sensitive to storm tracks. Thus, changes in the predominant storm tracks may trigger major changes, including feedback mechanisms, for the Barents Sea climate and the heat budget of the Arctic Ocean.

The Norwegian Atlantic Current in the Lofoten basin inferred from hydrological and tracer data (129I) and its interaction with the Norwegian Coastal Current

[1]xa0From three hydrological sections taken across the Lofoten Basin in May 2000, we estimated geostrophic transports of 7.2 Sv (Sverdrup = 106 m3 s−1) for the Norwegian Atlantic Current (NAC) and its division between northern (4.8 Sv) and eastern (2.4 Sv) branches. From 129I/127I concentration ratio measurements and transport estimations, we calculated 129I mass flux across the three sections. It appears that in the Lofoten Basin (a) 129I tracer-laden Norwegian Coastal Current (NCC) is transporting northward 55 kg/y of 129I assuming a volume flux of 0.7 Sv, (b) the estimated mass fluxes of 129I by the NAC and the NCC are comparable, (c) the total mass flux of 129I by the NAC and NCC, accounts only for about 1/3 of the 129I annual discharge (350 kg/y) from two reprocessing plants based in France and UK. If these measurements are representative of annual mean, it suggests an important transfer of 129I outside the NAC/NCC system.

Carbon export by small particles in the Norwegian Sea

Despite its fundamental role in controlling the Earths climate, present estimates of global organic carbon export to the deep sea are affected by relatively large uncertainties. These uncertainties are due to lack of observations as well as disagreement among methods and assumptions used to estimate carbon export. Complementary observations are thus needed to reduce these uncertainties. Here we show that optical backscattering measured by Bio-Argo floats can detect a seasonal carbon export flux in the Norwegian Sea. This export was most likely due to small particles (i.e., 0.2–20 μm), was comparable to published export values, and contributed to long-term carbon sequestration. Our findings highlight the importance of small particles and of physical mixing in the biological carbon pump and support the use of autonomous platforms as tools to improve our mechanistic understanding of the ocean carbon cycle.

Absolute flow field estimation for the Nordic seas from combined gravimetric, altimetric, and in situ data

[1]xa0In the Nordic seas, we combine a computation of absolute surface current flow derived from geodetic data with in situ historical hydrographic data to estimate the absolute volume, heat, and salt transports as a function of depth. Our mean dynamic topography (MDT) is calculated from marine, airborne and satellite gravimetry, combined with satellite altimetry, using a new algorithm called the iterative combination method (ICM). Residual noise in the gravimetric geoid is the limit on MDT resolution and is suppressed using a Gaussian filter with a width at half-peak amplitude of 59 km. Detailed and coherent flow paths for surface geostrophic currents are clearly identified. ICM MDT was used as fixed boundary condition to transform historical hydrography into absolute estimates of volume, heat, and salt transport, replacing the assumption of an isobaric surface at a predetermined depth. For the inflow of Atlantic Water (potential temperature Θ > 6°C) through the Faroe-Shetland Channel into the Nordic seas, we obtain time-averaged fluxes between 1993 and 1996 of 3.5 Sv (volume), 121 TW (heat), and 124 × 106 kg s−1 (salt), very close to reported observations from acoustic Doppler current profiler moorings and conductivity-temperature-depth data. For the Svinoy section, we obtain a northward transport of Atlantic Water (S > 35.0, T > 5.0°C) of 3.9 Sv in the eastern branch of the Norwegian Atlantic Current comparable with reported measurements of 4.2 Sv. Similarly good agreement is found for the Hornbanki and Iceland-Faroe Ridge sections and for monitoring Atlantic Water outflow across the Barents Sea Opening to the Arctic shelf.

Synchrony in marine growth among Atlantic salmon (Salmo salar) populations

More synchronous growth was observed between close, than more distantly separated populations of Atlantic salmon (Salmo salar), during both the first and the second year at sea. The marine growth of seven Norwegian popula- tions, located between 608N and 708N, were correlated with sea surface temperatures (SSTs) in the Barents Sea, the Nor- wegian Sea, and the North Sea, and it was found that growth correlated best with the water temperatures in the area located closest to their home river. Growth was also compared with three broad-scale climate indices (North Atlantic Os- cillation (NAO), Atlantic Multidecadal Oscillation, and subpolar gyre), with the strongest relationship occurring with the NAO index. However, SSTs explained more of the variability than the climatic indices did. Growth increment for the first year, but not the second year, was higher for southern than northern populations, mainly because of later smolt migration to sea in the north, and hence, a shorter growth season. For multi-sea-winter fish, all populations except one had a negative trend in growth with years for both the first and the second year at sea. For the second year at sea, this was most pro- nounced after the beginning of the 1980s. This is in accordance with the negative trend in pre-fishery abundance of adult salmon during the same period. Resume´ : On observe plus de croissance synchronisee entre les populations adjacentes que plus eloignees chez le saumon atlantique (Salmo salar), tant durant la premiere que la seconde annee en mer. Nous avons mis en correlation la croissance en mer de sept populations norvegiennes, etablies entre 608 Ne t 708N, et les temperatures de surface de la mer (SST) dans la mer de Barents, la mer de Norvege et la mer du Nord; la croissance est le mieux correlee avec les temperatures des si- tes situes le plus pres de leur riviere natale. Nous avons aussi comparela croissance avec trois indices climatiques a grande echelle (loscillation nord-atlantique (NAO), loscillation atlantique multidecennale et le tourbillon subpolaire) et la relation la plus forte setablit avec lindice NAO. Cependant, les SST expliquent une plus grande partie de la variabilite ´ que les indices climatiques. Lincrement de la croissance pour la premiere annee, mais non pour la seconde, est plus eleve ´ pour les populations du sud que pour celles du nord, surtout a cause de la migration plus tardive des saumoneaux vers la mer dans le nord et ainsi, de la periode de croissance plus courte. Chez les poissons qui passent plusieurs hivers en mer, toutes les populations, sauf une, affichent une tendance negative de la croissance en fonction des annees, tant pour la pre- miere que pour la deuxieme annee en mer. Pour la deuxieme annee en mer, la tendance est plus prononcee apresl e debut des annees 1980. Cela concorde avec la tendance negative dans labondance des saumons adultes pendant la meme pe ´- riode. (Traduit par la Redaction)

Annual Sea Surface Height Variability in the Nordic Seas

Altimeter data from May 1995 to February 2002, together with wind, heat, and fresh water fluxes and hydrographic data, are used to study the annual variability in sea surface height (SSH) in the Nordic Seas. The main contributions to the SSH are quantified with error estimates. The amplitude of the annual variation of the SSH ranges from 4 to 8 cm with largest variability in the deeper basins (Lofoten, Norwegian, and Greenland Basins) and along the Norwegian coast. The seasonal steric height can explain 1.5-4 cm of the SSH variation and arises from changes in the air-sea heat flux. However, the associated seasonal changes in the surface currents that arrive from the steric height and the heat flux are weak (a few mm/s). This is because their contributions to the SSH are nearly constant over large areas. From altimetry and hydrography, we calculated seasonal changes in bottom pressure and currents. In the Greenland and Norwegian Seas, the seasonal anomalous large-scale circulation is cyclonic during spring and anticyclonic during autumn. It is most apparent in the Norwegian Sea with a seasonal variation in speed of 1-2 cm/s. This variation can be explained by changes in the averaged wind-stress curl over the Norwegian Sea. The annual cycle of the Norwegian Atlantic slope current is also revealed. It is strongest in winter and weakest in summer with an annual amplitude in speed of 2-4 cm/s.

Advective and atmospheric forced changes in heat and fresh water content in the Norwegian Sea, 1951–2010

Climate variability in the Norwegian Sea was investigated in terms of ocean heat and fresh water contents of Atlantic water above a reference surface, using hydrographic data during spring 1951–2010. The main processes acting on this variability were examined and then quantified. The area-averaged water mass cooled and freshened, but a deepening of the reference surface resulted in a positive trend in the heat content of 0.3 W m−2. Air-sea heat fluxes explained about half of the interannual variability in heat content. The effect of the advection of Atlantic and Arctic waters on the variability varied with time, apparently due to large-scale changes in the ocean circulation. The data are consistent with the explanation that changing wind patterns caused buffering and then release of Arctic water in the Iceland Sea during the late 1960s to early 1970s, and this caused large hydrographic changes in the Norwegian Sea.

Steric height variability in the Nordic Seas

[1]xa0The variability of steric height in the Nordic Seas is analyzed on seasonal, interannual, and decadal timescales using a comprehensive data set of temperature and salinity observations for the second half of the twentieth century. Results from a regional Ocean General Circulation Model (OGCM) are used to assess the reliability of the averaging and the temporal interpolation of the inhomogeneous distributed observation data. The annual cycle explains only a minor part of the monthly variability for most of the region. The analysis on interannual to decadal timescales confined to the Norwegian Sea displays a clear rising trend starting at the end of the 1960s with particularly strong changes along the Barents Sea opening (6 to 7 cm). Moreover, a general freshening is found for the entire Norwegian Sea. In addition, a north-south dipole of the thermal component of the steric height variability is identified. This dipole elevates the general rising trend along the Barents Sea opening and reduces it in the southern Norwegian Sea. The bulk of the interannual variability in steric height is governed by variations in the local meridional wind stress that determine the relative distribution of Atlantic and Arctic waters in the Norwegian Sea. In addition, reduced heat loss to the atmosphere strongly correlates with the North Atlantic Oscillation winter index. This, in turn, may in particular explain the large steric height increase found at the Barents Sea opening.

Stability of the Thermohaline Circulation to Noisy Surface Buoyancy Forcing for the Present and a Warm Climate in an Ocean General Circulation Model

Abstract The stabilities of two different circulation regimes in the North Atlantic, 1) the present thermohaline circulation and 2) a weaker thermohaline circulation, are compared using the Hamburg Large Scale Geostrophic (LSG) ocean circulation model. The latter circulation regime is obtained by restoring the LSG model toward an on average 4°C warmer air surface temperature corresponding to a doubled atmospheric content of CO2. The stabilities of these stationary states are investigated by imposing various amounts of stochastic noise on the surface freshwater flux. The simulations show more variability on secular timescales for the present than for the warm climate. Since the modeled static stabilities for the two climates are relatively similar, the different rates of variability are probably connected to other mechanisms. In the present climate at high latitudes the two buoyancy fluxes due to heat and freshwater are of similar magnitudes but with opposite signs; thus switches between convective and non...