C. Mauritzen

Production of dense overflow waters feeding the North Atlantic across the Greenland-Scotland Ridge. Part 1: Evidence for a revised circulation scheme

Abstract The circulation in the Norwegian and Greenland Seas and the production of dense overflow waters feeding the North Atlantic across the Greenland-Scotland Ridge is reconsidered in light of a more complete set of hydrographic and tracer data. In contrast to previous theories of dense-water formation by deep convection in the Iceland and Greenland Seas, an alternative circulation scheme is presented in which the Atlantic Water in the northward flowing Norwegian Atlantic Current becomes gradually more dense by heat loss to the atmosphere and in which the dense water is transported to the outflow regions along the boundary currents surrounding the Iceland and Greenland Seas at shallow and intermediate depths. The overall circulation in the Nordic Seas that is consistent with these data is described. The cyclonic flow, fed by the Norwegian Atlantic Current, consists of three distinct branches: flow westward in the Fram Strait; flow northward through the Fram Strait and then through the Arctic Ocean; and flow passing through the Barents Sea and then through the Arctic Ocean. The branches, with modified hydrographic properties, return southwards as distinct layers of the East Greenland Current, the former two to supply the Denmark Strait overflow, the latter to supply the Iceland-Scotland overflow. The principal water mass product of the Iceland and Greenland Seas is an intermediate water mass that supplies the Iceland-Scotland overflow.

Arctic Ocean Warming Contributes to Reduced Polar Ice Cap

Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150–900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local 1°C maximum was observed in the mid-1990s, followed by an intervening minimum and an additional warming that culminated in 2007 with temperatures higher than in the 1990s by 0.24°C. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 1°C and higher were observed in the Eurasian and Makarov Basins. The AW warming was associated with a substantial (up to 75–90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised upward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice–ocean model in turn suggest a loss of 28–35 cm of ice thickness after 50 yr in response to the 0.5 W m−2 increase in AW ocean heat flux suggested by the 1D model. This amount of thinning is comparable to the 29 cm of ice thickness loss due to local atmospheric thermodynamic forcing estimated from observations of fast-ice thickness decline. The implication is that AW warming helped precondition the polar ice cap for the extreme ice loss observed in recent years.

On the origin of the warm inflow to the Nordic Seas

Abstract Warm and saline waters enter the Nordic Seas from the south as part of the warm-to-cold water transformation of the thermohaline circulation of the northern North Atlantic. One explanation for the origin of the Nordic Seas Inflow is a “shallow source hypothesis” under which the Inflow waters are a modification of upper ocean subtropical waters. Warm waters from the subtropical gyre are carried to the eastern North Atlantic by the North Atlantic Current and branch northwards, joined by poleward upper thermocline flow along the upper continental slope, to provide the Nordic Seas Inflow. Along this pathway the upper water column is progressively cooled and freshened by winter convection, the subpolar mode water transformation process, and this sets the Inflow characteristics. A “deep source hypothesis” provides an alternative explanation for the characteristics of the Nordic Seas Inflow and the pathway delivering the waters to the Inflow. Under this hypothesis Inflow waters originate from the core of the Mediterranean Overflow Waters in the Gulf of Cadiz carried northward at mid-depth by the eastern boundary undercurrent in the subtropics, continuing into the subpolar gyre along the eastern boundary, and rising from depths near 1200m in the Rockall Trough to less than 600m to cross the Wyville-Thomson Ridge into the Faroe-Shetland Channel and thence to the Nordic Seas. The deep source hypothesis focus is on lower thermocline source waters beneath the sill depth for the Nordic Seas Inflow, in contrast to the shallow source hypothesis focus of transformation of upper thermocline waters above the sill depth. On the basis of regional water mass distributions, geostrophic shear, and direct current measurements, we reject the deep source hypothesis in favor of the shallow source hypothesis. Rather than a flow of deep Mediterranean Overflow Water along the eastern boundary rising from depth to feed the Nordic Seas, the Inflow is supplied directly by transformed North Atlantic Current waters from the same depth range as the Inflow. Mediterranean Overflow Water is a constituent of the Inflow, but only through its contribution to defining the temperature-salinity relationship of the interior of the subtropical gyre from which the North Atlantic Current draws its water, rather than by direct northward advection of Mediterranean Outflow Water via the eastern boundary undercurrent crossing from the subtropics into the subpolar domain. Instead, the eastern boundary undercurrent wholly expels its transport of Mediterranean Overflow Water into the eastern edge of the subtropical gyre, creating the mid-depth westward extending subtropical salinity plume. For the lower thermocline isopycnal range of the core of the Mediterranean Overflow Water, measurements show that at the Wyville-Thomson Ridge the flow is southward and descending (rather than northward and rising), representing cold spillover from the Faroese Channels entraining warm upper ocean waters as the spillover plume descends and warms into the northern Rockall Trough. No evidence is found in Rockall Trough northward of 54°N (Porcupine Bank) for direct advection of Mediterranean Overflow Waters via an eastern boundary undercurrent extension, and the very dilute Mediterranean Overflow Waters in the Trough reflect the interaction of the Wyville-Thomson Ridge plume descending into the Trough from the North, and lower thermocline waters delivered to the Trough from the south.

Production of dense overflow waters feeding the North Atlantic across the Greenland-Scotland Ridge. Part 2: An inverse model

In Part 1 of this paper (Deep-Sea Research I, 43, 769–806) an alternative large-scale circulation scheme for the Nordic Seas and the Arctic Ocean was developed, the motivation being to explain the formation and subsequent circulation of the waters that feed the dense overflows across the Greenland-Scotland Ridge into the North Atlantic. The new scheme emphasizes the water changes following the direct advective paths of the warm Atlantic inflow to dense overflows at shallow and intermediate depths. In this paper the new circulation scheme is evaluated quantitatively in terms of an inverse box model. For each water mass, conservation statements are written for mass, heat and salt, and an optimal solution is sought that deviates the least from the observed currents and air-sea fluxes, and yet conserves mass, heat and salt. It is found that the proposed circulation scheme is consistent with the conservation statements, and with the water masses, currents and air-sea fluxes in the region. Estimates of volume transport are in this way assigned to the various branches of the proposed circulation scheme. The inverse model is also used to test various hypotheses, and in particular, it is found that the major site of dense-water formation is the Norwegian Atlantic Current east of the Greenland and Iceland Seas. It is estimated that the intensity of this direct pathway exceeds that of the open-ocean gyres 6:1. Lastly, it is found that the dense water mass formation process within the Norwegian Atlantic Current does not induce a significant seasonal signal in the volume transport. This is consistent with observations of the flow field across the Greenland-Scotland Ridge, which so far have not shown indications that the dense overflows are associated with a seasonal signal.

Evidence in hydrography and density fine structure for enhanced vertical mixing over the Mid‐Atlantic Ridge in the western Atlantic

Received 24 August 2001; revised 22 February 2002; accepted 25 March 2002; published 15 October 2002. [1] Anomalous conditions exist in the salinity, oxygen, and nutrient fields over the western flank of the Northern Hemisphere Mid-Atlantic Ridge. We examine possible advective sources for this anomaly, but determine that vertical mixing is the most likely cause. We proceed to use knowledge gained from the Brazil Basin Tracer Release Experiment in the South Atlantic (where microstructure and fine structure were obtained to explore the intensity, spatial distribution, and mechanisms of mixing in the deep ocean) to interpret density fine structure from common conductivity-temperature-depth data in the North Atlantic. These data support the hypothesis that the anomalous hydrographic conditions are associated with enhanced levels of vertical mixing. The inferred levels of vertical diffusivity over the Northern Hemisphere Mid-Atlantic Ridge are as high as in the South Atlantic: 1–10 � 10 � 4 m 2 /s. INDEX TERMS: 4532 Oceanography: Physical: General circulation; 4536 Oceanography: Physical: Hydrography; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: diapycnal mixing, fine structure, recirculation, Mid-Atlantic Ridge (MAR), hydrography, silicate

Atlantic Climate Variability and Predictability: A CLIVAR Perspective

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

Observational program tracks Arctic Ocean transition to a warmer state

Over the past several decades, the Arctic Ocean has undergone substantial change. Enhanced transport of warmer air from lower latitudes has led to increased Arctic surface air temperature. Concurrent reductions in Arctic ice extent and thickness have been documented. The first evidence of warming in the intermediate Atlantic Water (AW, water depth between 150 and 900 meters) of the Arctic Ocean was found in 1990. Another anomaly, found in 2004, suggests that the Arctic Ocean is in transition toward a new, warmer state [Polyakov et al., 2005, and references therein].

Wind-Driven Variability of the Large-Scale Recirculating Flow in the Nordic Seas and Arctic Ocean

The varying depth-integrated currents in the Nordic seas and Arctic Ocean are modeled using an integral equation derived from the shallow-water equations. This equation assumes that mass divergence in the surface Ekman layer is balanced by convergence in the bottom Ekman layer. The primary flow component follows contours of f /H. The model employs observed winds and realistic bottom topography and has one free parameter, the coefficient of the (linear) bottom drag. The data used for comparison are derived from in situ current meters, satellite altimetry, and a primitive equation model. The current-meter data come from a 4-yr record at 75 8 Ni n the Greenland Sea. The currents here are primarily barotropic, and the model does well at simulating the variability. The ‘‘best’’ bottom friction parameter corresponds to a spindown time of 30‐60 days. A further comparison with bottom currents from a mooring on the Norwegian continental slope, deployed over one winter period, also shows reasonable correspondence. The principal empirical orthogonal function obtained from satellite altimetry data in the Nordic seas has a spatial structure that closely resembles f /H. A direct comparison of this mode’s fluctuations with those predicted by the theoretical model yields linear correlation coefficients in the range 0.75‐0.85. The primitive equation model is a coupled ocean‐ice version of the Princeton Ocean Model for the North Atlantic and Arctic. Monthly mean depth-averaged velocities are calculated from a 42-yr integration and then compared with velocities predicted from an idealized model driven by the same reanalyzed atmospheric winds. In the largely ice-free Norwegian Sea, the coherences between the primitive equation and idealized model velocities are as high as 0.9 on timescales of a few months to a few years. They are lower in the remaining partially or fully ice-covered basins of the Greenland Sea and the Arctic Ocean, presumably because ice alters the momentum transferred to the ocean by the wind. The coherence in the Canadian Basin of the Arctic can be increased substantially by forcing the idealized model with ice velocities rather than the wind. Estimates of the depth-integrated vorticity budget in the primitive equation model suggest that bottom friction is important but that lateral diffusion is of equal or greater importance in compensating surface Ekman pumping.

Variability of the Norwegian Atlantic Current and associated eddy field from surface drifters

The Norwegian Atlantic Current (NwAC) and its eddy field are examined using data from surface drifters. The data set used spans nearly 20 years, from June 1991 to December 2009. The results are largely consistent with previous estimates, which were based on data from the first decade only. With our new data set, statistical analysis of the mean fields can be calculated with larger confidence. The two branches of the NwAC, one over the continental slope and a second further offshore, are clearly captured. The Norwegian Coastal Current is also resolved. In addition, we observe a semipermanent anticylonic eddy in the Lofoten Basin, a feature seen previously in hydrography and in models. The eddy kinetic energy (EKE) is intensified along the path of the NwAC, with the largest values occurring in the Lofoten Basin. The strongest currents, exceeding 100 cm s−1, occur west of Lofoten. Lateral diffusivities were computed in five domains and ranged from 1–5 × 107 cm2 s−1. The Lagrangian integral time and space scales are 1–2 days and 7–23 km, respectively. The data set allows studies of seasonal and interannual variations as well. The strongest seasonal signal is in the NwAC itself, as the mean flow strengthens by approximately 20% in winter. The EKE and diffusivities on the other hand do not exhibit consistent seasonality in the sampled regions. There are no consistent indications of changes in either the mean or fluctuating surface velocities between the 1990s and 2000s.

The Arctic and Subarctic Oceans/Seas

Abstract Observations made during the 1990s and 2000s indicate that the Arctic physical environment and associated ecosystem are undergoing remarkable changes. The observed reduction in Arctic sea ice extent is arguably the strongest, most powerful visual symbol of climate change. The Arctic Ocean is changing as well. Here we discuss, from an observational point of view, the present understanding of the circulation, water masses, and stratification of the Arctic Ocean, highlighting the changes that have taken place during the past few decades. Many of these ocean signals evolve rapidly, making it essential that the state of the Arctic/Subarctic is observed continually, using all the recent advances that have been made in high-latitude monitoring and in Earth System understanding. Taking recent investigations of the Arctic atmosphere and sea ice as guides, we deem it likely that signatures of anthropogenic climate change in the Arctic/Subarctic Seas will begin to emerge above the high level of natural variability within the next decade.