Holocene oscillations in temperature and salinity of the surface subpolar North Atlantic

The Atlantic meridional overturning circulation (AMOC) transports warm salty surface waters to high latitudes, where they cool, sink and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm northwestern European climate, and acts as a control on the global climate. Past climate fluctuations during the Holocene epoch (~11,700 years ago to the present) have been linked with changes in North Atlantic Ocean circulation. The behaviour of the surface flowing salty water that helped drive overturning during past climatic changes is, however, not well known. Here we investigate the temperature and salinity changes of a substantial surface inflow to a region of deep-water formation throughout the Holocene. We find that the inflow has undergone millennial-scale variations in temperature and salinity (~3.5 °C and ~1.5 practical salinity units, respectively) most probably controlled by subpolar gyre dynamics. The temperature and salinity variations correlate with previously reported periods of rapid climate change. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic. Model studies predict a weakening of AMOC in response to enhanced Arctic freshwater fluxes, although the inflow can compensate on decadal timescales by becoming more saline. Our data suggest that such a negative feedback mechanism may have operated during past intervals of climate change

Toward Improved Estimation of the Dynamic Topography and Ocean Circulation in the High Latitude and Arctic Ocean: The Importance of GOCE

The Norwegian Sea circulation plays a key role in maintaining the mild climate of the northwestern Europe via the transport of warm Atlantic Water pole-ward. The first paper addresses the advective currents connecting the two branches of the Norwegian Atlantic Current and shows the general spin up of the Norwegian Sea circulation during winter with the exception of the flow over the Mohn Ridge. The variability in the surface velocities in the Norwegian Sea is found to be deep reaching, which supports the use of altimetry to monitor the variability of the poleward transport of Atlantic Water. A strengthening and weakening of the Atlantic inflow east of the Faroe Islands has a consistent response along the entire slope current. However, a stronger western inflow, observed north of the Faroe Islands, is associated with more flow of Atlantic Water into the slope current. This finding suggest that a substantial fraction of Atlantic Water that eventually enters the Barents Sea or the Arctic through the Fram Strait, may originate from the western inflowing branch of Atlantic Water to the Nordic Seas, and that the two branches of northward flowing Atlantic Water cannot be considered as separate flows. Paper 2 examines the influence of the surface circulation, eddy activity and local heat loss on the spatial distribution and temporal evolution of dense water formation in the Lofoten Basin. Evidence of intrusion of Atlantic Water into the central Lofoten Basin due to buoyant waters in the eastern part of the basin is found. With the support of hydrographic and satellite datasets, the concept of separate western and eastern regions of the Lofoten Basin is introduced and a link between the western Lofoten Basin and Faroe Shetland overflow waters is identified. Paper 3 addresses an anomalous anticyclonic vortex in the Nordic Seas, which is situated in the western Lofoten Basin. The vortex’ surface and vertical characteristics on seasonal, inter-annual, and climatological time-scales are quantified, relevant forcing mechanisms are addressed, and its uniqueness in the Nordic Seas is documented. In the final paper, a new mean dynamic topography (MDT) is estimated for the North Atlantic and the Arctic from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite gravity anomaly data. The new GOCE-based MDT is assessed and compared to independent steric height observations, other state-of-the-art MDTs and three coupled sea-ice-ocean models, showing its usefulness in studies of high latitude ocean circulation

The inflow of Atlantic water, heat, and salt to the Nordic Seas across the Greenland-Scotland Ridge

The flow of warm, saline water from the Atlantic Ocean (the Atlantic inflow or just inflow) across the Greenland-Scotland Ridge into the Nordic Seas and the Arctic Ocean (collectively termed the Arctic Mediterranean) is of major importance, both for the regional climate and for the global thermohaline circulation. Through its heat transport, it keeps large areas north of the Ridge much warmer, than they would otherwise have been, and free of ice (Seager et al. 2002). At the same time, the Atlantic inflow carries salt northwards, which helps maintaining high densities in the upper layers; a precondition for thermohaline ventilation. The Atlantic inflow is carried by three separate branches, which here are termed: the Iceland branch (the North Icelandic Irminger Current), the Faroe branch (the Faroe Current), and the Shetland branch (Fig. 1.1). These are all characterized by being warmer and more saline than the waters that they meet after crossing the Ridge, although both temperature and salinity decrease as we go from the Shetland branch, through the Faroe branch, to the Iceland branch. All these branches therefore carry, not only water, but also heat and salt across the Ridge.</p

Atlantic origin of observed and modelled freshwater anomalies in the Nordic Seas

Between 1965 and 1990, the waters of the Nordic Seas and the subpolar basins of the North Atlantic Ocean freshened substantially1. The Arctic Ocean also became less saline over this time, as a consequence of increasing runoff1, 2, 3, 4, but it is not clear whether flow from the Arctic Ocean was the main source of the Nordic Seas salinity anomaly. As a region of deep-water formation, the Nordic Seas are central to the Atlantic meridional overturning circulation, but this process is inhibited if the surface salinity is too low2. Here we use the instrumental record of Nordic Seas hydrography, along with a global ocean–sea-ice model hindcast simulation, to identify the sources and magnitude of freshwater that has accumulated in the Nordic Seas since 1950. We find that the freshwater anomalies within the Nordic Seas can mostly be explained by less salt entering the southern part of the basin with the relatively saline Atlantic inflow, with seemingly little contribution from the Arctic Ocean. We conclude that hydrographic changes in the Nordic Seas are primarily related to changes in the Atlantic Ocean. We infer that if the Atlantic inflow and Nordic Seas both freshen similarly, this would render the Atlantic meridional overturning circulation relatively insensitive to Nordic Seas freshwater content