1.
Stranne, Christian, et al.
(författare)
Acoustic mapping of mixed layer depth
2018
Ingår i: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 14:3, s. 503-514
Tidskriftsartikel (refereegranskat) abstract
The ocean surface mixed layer is a nearly universal feature of the world oceans. Variations in the depth of the mixed layer (MLD) influences the exchange of heat, fresh water (through evaporation), and gases between the atmosphere and the ocean and constitutes one of the major factors controlling ocean primary production as it affects the vertical distribution of biological and chemical components in near-surface waters. Direct observations of the MLD are traditionally made by means of conductivity, temperature, and depth (CTD) casts. However, CTD instrument deployment limits the observation of temporal and spatial variability in the MLD. Here, we present an alternative method in which acoustic mapping of the MLD is done remotely by means of commercially available ship-mounted echo sounders. The method is shown to be highly accurate when the MLD is well defined and biological scattering does not dominate the acoustic returns. These prerequisites are often met in the open ocean and it is shown that the method is successful in 95% of data collected in the central Arctic Ocean. The primary advantages of acoustically mapping the MLD over CTD measurements are (1) considerably higher temporal and horizontal resolutions and (2) potentially larger spatial coverage.
2.
Ödalen, Malin, 1982-
(författare)
Model analysis of ocean carbon storage and transport across climate states
2019
Doktorsavhandling (övrigt vetenskapligt/konstnärligt) abstract
The ocean carbon cycle plays a fundamental role in the Earth’s climate system, on decadal to multi-millennial timescales. Of the carbon held in the ocean, the atmosphere, and the terrestrial biosphere combined, more than 90% resides in the ocean. Carbon enters the surface ocean through air-sea gas exchange and from terrestrial sources. It is transported to the deep ocean with the ocean circulation and through the so-called biological pump, where carbon is taken up in the surface ocean by photosynthetic organisms that fall down and decompose at depth. This thesis contributes to the understanding of the processes involved in ocean carbon storage and transport. It examines how these processes respond to model perturbations, and how this response influences our attempts to simulate glacial-interglacial fluctuations in atmospheric carbon dioxide (CO2).The thesis investigates the response of the simulated ocean carbon storage, and distribution of the isotopic tracer δ13C, to changes in physical and biological parameters. In the included studies, we use observational as well as proxy records of oceanic properties to evaluate our model simulations. In addition, we use a climate model to interpret proxy evidence of glacial-interglacial changes in ocean δ13C. By using a separation framework, we identify the origin of the carbon in the model ocean, and attribute observed changes to the processes involved.The results indicate a strong link between ocean carbon storage and the strength of the global ocean overturning circulation. Stronger circulation leads to less carbon storage through a weakening of the biological pump, and through reduced solubility due to an increase in global ocean average temperature.In simulations of glacial climate, we find that biological adaptability to the surrounding nutrient conditions, through a flexible carbon-to-phosphorus ratio (C/P) in ocean photosynthesis, increases the ocean carbon storage compared to simulations where fixed C/P is applied. The biological flexibility improves the model’s ability to reproduce glacial atmospheric CO2. In line with previous research, we find freshwater input to the North Atlantic to be an important factor for reproducing glacial proxy records. The ensemble of simulations that achieve a good representation of glacial-interglacial δ13C indicates a deglacial whole-ocean change in δ13C of 0.28 ± 0.06‰.The thesis underlines the importance of the initial state, and the choice of model parameterisations, for the outcome of model ensemble, and intercomparison studies. Finally, it proposes a new method for estimation of ocean carbon transport, and attribution of this transport to different water masses and carbon system processes.
3.
Broomé, Sara, 1989-
(författare)
Atlantic Water in the Nordic Seas : A satellite altimetry perspective on ocean circulation
2020
Doktorsavhandling (övrigt vetenskapligt/konstnärligt) abstract
The Atlantic Water in the Nordic Seas contributes to the mild climate of Northern Europe and is the main oceanic source of heat for the Arctic. The northward bound transport of the warm and saline Atlantic Water is mediated by a topographically constrained cyclonic boundary current along the Norwegian continental slope. The analysis within this thesis is based on satellite observations of dynamic Sea Surface Heights (SSH) from 1993 to the recent present, combined with both hydrographic observations and modelling. It provides some new perspectives and results, as well as corroborates the essential role of bottom topography for the circulation in the Nordic Seas.In the first part of the thesis, the topographic constraint is used in the analysis by examining the satellite-derived SSH along topographic contours. We find stationary along-contour anomalies that indicate deviations from strict topographic steering. However, we show that these deviations are dynamically consistent with, and can be explained by, potential vorticity conservation in an adiabatic steady-state model for flow over a topographic slope. The analysis along topographic contours is further developed to study northward-propagating, low-frequency ocean temperature signals. These signals have an expression in the SSH and their propagation speed is remarkably slow compared to the current speed. We propose a conceptual model of shear dispersion effects, in which the effective advection speed of a tracer is determined not only by the rapid current core, but by a mean velocity taken over the cross-flow extent of Atlantic Water. The model predicts a reduced effective tracer advection velocity, comparable to the one observed.The close connection between anomalies in SSH and heat content is further used to study decadal variability in the Nordic Seas. There is a shift in decadal trends in the mid-2000s, from a period of strong increase in SSH and heat content to a more stagnant period. We find this variability to be forced remotely, rather than by local air-sea heat fluxes. By developing a conceptual model of ocean heat convergence, we are able to explain the broad features of the decadal changes with the temperature variability of the inflowing Atlantic Water from the subpolar North Atlantic.In the final part of the thesis, satellite-derived surface geostrophic velocity fields are used as input to a Lagrangian trajectory model. Based on this, we study the fractionation of the Atlantic Water in the Nordic Seas between the two straits towards the Arctic Ocean: the Barents Sea Opening and the Fram Strait. This Lagrangian approach also provides insights on the origin of the water that reach the straits. We find that it is the frontal current branch, rather than the slope current, that contributes to the variability of the Barents Sea Opening inflow of warm Atlantic Water, and thus potentially to the climate of the Barents Sea and its sea ice cover.
4.
Björk, Göran, 1956, et al.
(författare)
Bathymetry and oceanic flow structure at two deep passages crossing the Lomonosov Ridge
2018
Ingår i: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 14:1, s. 1-13
Tidskriftsartikel (refereegranskat) abstract
The Lomonosov Ridge represents a major topographical feature in the Arctic Ocean which has a large effect on the water circulation and the distribution of water properties. This study presents detailed bathymetric survey data along with hydrographic data at two deep passages across the ridge: a southern passage (80-81°ĝN), where the ridge crest meets the Siberian continental slope, and a northern passage around 84.5°ĝN. The southern channel is characterized by smooth and flat bathymetry around 1600–1700ĝm with a sill depth slightly shallower than 1700ĝm. A hydrographic section across the channel reveals an eastward flow with Amundsen Basin properties in the southern part and a westward flow of Makarov Basin properties in the northern part. The northern passage includes an approximately 72ĝkm long and 33ĝkm wide trough which forms an intra-basin in the Lomonosov Ridge morphology (the Oden Trough). The eastern side of the Oden Trough is enclosed by a narrow and steep ridge rising 500–600ĝm above a generally 1600ĝm deep trough bottom. The deepest passage (the sill) is 1470ĝm deep and located on this ridge. Hydrographic data show irregular temperature and salinity profiles indicating that water exchange occurs as midwater intrusions bringing water properties from each side of the ridge in well-defined but irregular layers. There is also morphological evidence that some rather energetic flows may occur in the vicinity of the sill. A well expressed deepening near the sill may be the result of seabed erosion by bottom currents.
5.
Jakobsson, Martin, et al.
(författare)
Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation
2016
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
Tidskriftsartikel (refereegranskat) abstract
The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions41-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (similar to 140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.
6.
Broomé, Sara, 1989-, et al.
(författare)
Mechanisms of decadal changes in sea surface height and heat content in the eastern Nordic Seas
2020
Ingår i: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 16:3, s. 715-728
Tidskriftsartikel (refereegranskat) abstract
The Nordic Seas constitute the main ocean conveyor of heat between the North Atlantic Ocean and the Arctic Ocean. Although the decadal variability in the subpolar North Atlantic has been given significant attention lately, especially regarding the cooling trend since the mid-2000s, less is known about the potential connection downstream in the northern basins. Using sea surface heights from satellite altimetry over the past 25 years (1993–2017), we find significant variability on multiyear to decadal timescales in the Nordic Seas. In particular, the regional trends in sea surface height show signs of a weakening since the mid-2000s, as compared to the rapid increase in the preceding decade since the early 1990s. This change is most prominent in the Atlantic origin waters in the eastern Nordic Seas and is closely linked, as estimated from hydrography, to heat content. Furthermore, we formulate a simple heat budget for the eastern Nordic Seas to discuss the relative importance of local and remote sources of variability; advection of temperature anomalies in the Atlantic inflow is found to be the main mechanism. A conceptual model of ocean heat convergence, with only upstream temperature measurements at the inflow to the Nordic Seas as input, is able to reproduce key aspects of the decadal variability in the heat content of the Nordic Seas. Based on these results, we argue that there is a strong connection with the upstream subpolar North Atlantic. However, although the shift in trends in the mid-2000s is coincident in the Nordic Seas and the subpolar North Atlantic, the eastern Nordic Seas have not seen a reversal of trends but instead maintain elevated sea surface heights and heat content in the recent decade considered here.
7.
Thompson, Bijoy, et al.
(författare)
A model study of the first ventilated regime of the Arctic Ocean during the early Miocene
2012
Ingår i: Polar Research. - : Norwegian Polar Institute. - 0800-0395 .- 1751-8369. ; 31, s. 10859-
Tidskriftsartikel (refereegranskat) abstract
The tectonic opening of Fram Strait during the Neogene was a significant geological event that transferred the Arctic Ocean from a poorly ventilated enclosed basin, with weak exchange with the North Atlantic, to a fully ventilated ocean stage''. Previous tectonic and physical oceanographic analyses suggest that the early Miocene Fram Strait was likely several times narrower and less than half as deep as the present-day 400 km wide and 2550 m deep strait. Here we use an ocean general circulation model with a passive age tracer included to further address the effect of the Fram Strait opening on the early Miocene Arctic Ocean circulation. The model tracer age exhibits strong spatial gradient between the two major Arctic Ocean deep basins: the Eurasian and Amerasian basins. There is a two-layer stratification and the exchange flow through Fram Strait shows a bi-layer structure with a low salinity outflow from the Arctic confined to a relatively thin upper layer and a saline inflow from the North Atlantic below. Our study suggests that although Fram Strait was significantly narrower and shallower during early Miocene, and the ventilation mechanism quite different in our model, the estimated ventilation rates are comparable to the chemical tracer estimates in the present-day Arctic Ocean. Since we achieved ventilation of the Arctic Ocean with a prescribed Fram Strait width of 100 km and sill depth of 1000 m, ventilation may have preceded the timing of a full ocean depth connection between the Arctic Ocean and North Atlantic established through seafloor spreading and the development of the Lena Trough.
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10.
Brannigan, Liam, et al.
(författare)
Generation of Subsurface Anticyclones at Arctic Surface Fronts due to a Surface Stress
2017
Ingår i: Journal of Physical Oceanography. - 0022-3670 .- 1520-0485. ; 47:11, s. 2653-2671
Tidskriftsartikel (refereegranskat) abstract
Isolated anticyclones are frequently observed below the mixed layer in the Arctic Ocean. Some of these subsurface anticyclones are thought to originate at surface fronts. However, previous idealized simulations with no surface stress show that only cyclone–anticyclone dipoles can propagate away from baroclinically unstable surface fronts. Numerical simulations of fronts subject to a surface stress presented here show that a surface stress in the same direction as the geostrophic flow inhibits dipole propagation away from the front. On the other hand, a surface stress in the opposite direction to the geostrophic flow helps dipoles to propagate away from the front. Regardless of the surface stress at the point of dipole formation, these dipoles can be broken up on a time scale of days when a surface stress is applied in the right direction. The dipole breakup leads to the deeper anticyclonic component becoming an isolated subsurface eddy. The breakup of the dipole occurs because the cyclonic component of the dipole in the mixed layer is subject to an additional advection because of the Ekman flow. When the Ekman transport has a component oriented from the anticyclonic part of the dipole toward the cyclonic part then the cyclone is advected away from the anticyclone and the dipole is broken up. When the Ekman transport is in other directions relative to the dipole axis, it also leads to deviations in the trajectory of the dipole. A scaling is presented for the rate at which the surface cyclone is advected that holds across a range of mixed layer depths and surface stress magnitudes in these simulations. The results may be relevant to other regions of the ocean with similar near-surface stratification profiles.
