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| 2. |
- Meier, H. E. M., et al.
(author)
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Ventilation of the Baltic Sea deep water: A brief review of present knowledge from observations and models
- 2006
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In: Oceanologia. - 0078-3234. ; 48, s. 133-164
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Journal article (peer-reviewed)abstract
- The ventilation of the Baltic Sea deep water is driven by either gale-forced barotropic or baroclinic salt water inflows. During the past two decades, the frequency of large barotropic inflows (mainly in winter) has decreased and the frequency of medium-intensity baroclinic inflows (observed in summer) has increased. As a result of entrainment of ambient oxygen-rich water, summer inflows are also important for the deep water ventilation. Recent process studies of salt water plumes suggest that the entrainment rates are generally smaller than those predicted by earlier entrainment models. In addition to the entrance area, the Slupsk Sill and the Slupsk Furrow are important locations for the transformation of water masses. Passing the Slupsk Furrow, both gravity-driven dense bottom flows and sub-surface cyclonic eddies, which are eroded laterally by thermohaline intrusions, ventilate the deep water of the eastern Gotland Basin. A recent study of the energy transfer from barotropic to baroclinic wave motion using a two-dimensional shallow water model suggests that about 30% of the energy needed below the halocline for deep water mixing is explained by the breaking of internal waves. In the deep water decade-long stagnation periods with decreasing oxygen and increasing hydrogen sulphide concentrations might be caused by anomalously large freshwater inflows and anomalously high mean zonal wind speeds. In different studies the typical response time scale of average salinity was estimated to be between approximately 20 and 30 years. The review summarizes recent research results and ends with a list of open questions and recommendations.
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| 3. |
- Nohr, Christian, 1966, et al.
(author)
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A dynamic sea ice model based on the formation direction of leads
- 2009
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In: Cold Regions Science and Technology. - : Elsevier BV. - 0165-232X .- 1872-7441. ; 58:02-jan, s. 36-46
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Journal article (peer-reviewed)abstract
- A dynamic ice model is presented using a novel approach where the memory of weak directions in the ice cover is stored. The model computes ice motion, ice deformation and the associated dynamic ice production without the need of a full two dimensional computation. The ice dynamics is based on a viscous-plastic approach in a marginal ice zone with both compressive and shear stresses. The method is very computational efficient but is likely restricted to basin scales of semi size with relatively coherent wind forcing over the basin. The model is applied to the Bothnian Bay of the Baltic Sea and coupled with an ocean model. The results show good agreement when compared against measurements of ice velocity from an upward looking ADCP deployed in the center of the Bothnian Bay and observations from ice charts. As a model application, the dynamic ice production that occurs in addition to the pure thermodynamic growth in a deforming ice cover has been computed over the period 1991-2004. The results show that the dynamic ice production typically increases the ice volume with 80% over the simulation period.
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| 4. |
- Nohr, Christian, 1966, et al.
(author)
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Computation of energy for diapycnal mixing in the Baltic Sea due to internal wave drag acting on wind-driven barotropic currents
- 2009
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In: Oceanologia. - 0078-3234. ; 51:4, s. 461-494
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Journal article (peer-reviewed)abstract
- The pathways of energy supply for mixing the deep waters of the Baltic Sea is largely unknown. In this paper, a parameterization of the internal wave drag forces on barotropic motion is developed and implemented into a two-dimensional shallow water model of the Baltic Sea. The model is validated against; observed sea levels. The dissipation of barotropic motion by internal wave drag that is quantified from the model results show that breaking internal waves generated by wind forced barotropic motions can contribute significantly to diapycnal mixing in the deep water of the Baltic Sea.
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| 5. |
- Nohr, Christian, 1966
(author)
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Oceanographic studies of the Baltic Sea with emphasis on sea ice and mixing processes
- 2009
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Doctoral thesis (other academic/artistic)abstract
- This thesis comprises of model estimates of the water and heat budgets, re-estimating of the budget of deep-water mixing energy, model studies of generation mechanisms for internal waves, sea ice dynamermics and finally sea ice monitoring. Common site for all the studies are the Baltic Sea. A Baltic Sea model was used as a tool for synthesizing available data to be able to analyze the Baltic Sea water and heat balances. The accuracy in the long-term water and heat balances was quantified, while the accuracy of the individual terms is still unknown. The study illustrates the possibility of negative precipitation minus evaporation rates. The calculated inter-annual variability of the heat loss between atmosphere and Baltic Sea indicates large variations (+/- 10 Wm^{-2}). Despite an atmospheric warming no trend was seen in the annual mean water temperature. Computations suggest that breaking internal waves, generated by wind forced barotropic motions, contribute significantly to the diapycnal mixing in the deep water of the Baltic Sea. Similar computations have previously been performed for tides in the World Ocean. However, the primary driver of barotropic motions in the Baltic Sea is the local weather. This causes the generated internal waves to have periods well above the inertial period. The stochastic forcing and the complex topography imply that the described energy transfer can be quite efficient even though the waves have super inertial periods. The diffusivity due to the dissipation of the barotropic motion conforms to earlier estimates and this mechanism also explains observed seasonal and spatial variations in vertical diffusivity. Ice motion and ice thickness in the center of the Bothnian Bay was monitored with a bottom mounted ADCP for an entire winter season. The ice motion was primarily driven by the wind but with a clear influence of internal ice stresses and ice thickness. A rough force balance computation gave compressive ice strength 4 times larger than normally used in numerical ice models. The ridges made up 30-50% of the total ice volume showing that dynamical processes are important for the total ice production. The development of a dynamic ice model includes a novel viscous-plastic approach where a memory of weak directions in the ice cover were stored. The model computes the ice motion, the ice deformation and the associated dynamic ice production and the results shows good agreement when compared with measurements of ice velocity from the ADCP. The results also show that the dynamic ice production typically increases the ice volume with 80% over the simulation period.
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| 6. |
- Omstedt, Anders, 1949, et al.
(author)
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Calculating the water and heat balances of the Baltic Sea using ocean modelling and available meteorological, hydrological and ocean data
- 2004
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In: Tellus Series a-Dynamic Meteorology and Oceanography. - : Stockholm University Press. - 0280-6495. ; 56:4, s. 400-414
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Journal article (peer-reviewed)abstract
- In this paper we aim to analyse Baltic Sea water and heat balances for the BALTEX/BRIDGE study period and to put these into a climatic perspective. The study period-the three years starting October 1999-was a time of enhanced observational and modelling activities in the Baltic Sea region and of the major field activity of BALTEX Phase I programme. The present study follows the example of earlier work, where Baltic Sea modelling was used as a tool for synthesizing available data and closing the water and heat balances. The modelling approach was validated with independent data sets of observations from salinity and temperature. The model simulation was also compared with the coupled atmosphere-Baltic Sea model system, HIRLAM-BALTEX, which was run in a delayed data assimilation mode. The results indicate that accurate long-term net water and net heat balances (mean errors over decadal time scales are about 600 m(3) s(-1) and 2 W m(-2) respectively) can be calculated using current Baltic Sea modelling and meteorological and hydrological data available from the BALTEX data centres. The accuracy of the individual terms in the water and heat balances is, however, still unknown. The study illustrated that negative net precipitation rates are possible, with the year 2002 standing out from the rest of the 30-yr study period. The calculated inter-annual variability of the net heat loss between atmosphere and Baltic Sea during the BALTEX/BRIDGE period indicated large variations ( 10 W m-2). It has also been shown that the Baltic Sea annual mean temperature has not increased during the studied period despite an atmospheric warming of I degreesC. The reason has been explained by the heat balance that indicated no trend in the Baltic Sea net heat loss.
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| 8. |
- Rudels, B., et al.
(author)
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The interaction between waters from the Arctic Ocean and the Nordic Seas north of Fram Strait and along the East Greenland Current: results from the Arctic Ocean-02 Oden expedition
- 2005
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In: Journal of Marine Systems. - : Elsevier BV. - 0924-7963. ; 55:1-2, s. 1-30
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Journal article (peer-reviewed)abstract
- In spring 2002, the Swedish icebreaker Oden conducted an extensive oceanographic survey of the East Greenland Current from north of Fram Strait to South of Denmark Strait as a part of the Arctic Ocean 2002 programme to study the Nordic Seas, while RV Knorr of Woods Hole worked in the ice-free parts. The Oden survey concentrated on water mass formation in ice covered water and the interactions between the water of the Arctic Ocean and the Nordic Seas. The CTD observations made on Oden are presented by following the Oden cruise track. They describe all major, and many of the more subtle, mixing processes occurring in the Arctic Mediterranean Sea. In Storfjorden, ice formation and brine rejection had created the highest bottom salinities observed in the last 20 years. The Atlantic Water (AW) entering the Arctic Ocean close to Svalbard was cooled and freshened down to 600 m implying that dense, brine enriched shelf water from the northern Svalbard shelf had convected into the Atlantic core. The upper part of the Atlantic Water was cooled by heat loss to the atmosphere and to ice melt. About 30% of the heat went to ice melt leading to a less saline upper layer that eventually forms the embryo of the Arctic Ocean halocline water. North of the Yermak Plateau Atlantic Water as well as Arctic Intermediate Water (AIW) and Nordic Seas Deep Water (NDW) were seen to enter the Arctic Ocean. North of 81 degrees N the Arctic Ocean water masses dominated west of 0 degrees E, while recirculating waters from the south were observed first at 79 degrees N. The properties of the intermediate and deep waters exiting the Arctic Ocean in the East Greenland Current changed considerably from Fram Strait to the Greenland Sea indicating interactions with recirculating waters. The salinity of the Polar Surface Water (PSW) increased from Fram Strait to Denmark Strait but the thickness of the low salinity upper layer also increased and the freshwater content appeared to be conserved. The Denmark Strait overflow plume was stratified and several of the different water masses present at the sill in Denmark Strait at the Oden crossing could contribute to the overflow. Arctic Atlantic Water (AAW) and Recirculating Atlantic Water (RAW) as well as Polar Intermediate Water (PIW) would supply the less dense part, while the ultimate origin of densest component of the overflow, found at the Iceland side of the trench, still is an open question. The low salinity lid found on top of the overflow plume in the Irminger Sea suggests that entrainment of ambient water into the overflow plume was small. (c) 2004 Elsevier B.V. All rights reserved.
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| 9. |
- Wåhlin, Anna, 1970, et al.
(author)
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Inflow of Warm Circumpolar Deep Water in the Central Amundsen Shelf
- 2010
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In: Journal of Physical Oceanography. - 0022-3670. ; 40:6, s. 1427-1434
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Journal article (peer-reviewed)abstract
- The thinning and acceleration of theWestAntarctic Ice Sheet has been attributed to basalmelting induced by intrusions of relatively warm salty water across the continental shelf. A hydrographic section including lowered acoustic Doppler current profiler measurements showing such an inflow in the channel leading to the Getz and Dotson Ice Shelves is presented here. The flow rate was 0.3–0.4 Sv (1 Sv[106 m3 s21), and the subsurface heat loss was estimated to be 1.2–1.6TW.Assuming that the inflow persists throughout the year, it corresponds to an ice melt of 110–130 km3 yr21, which exceeds recent estimates of the net ice glacier ice volume loss in the Amundsen Sea. The results also show a 100–150-m-thick intermediate water mass consisting of Circumpolar DeepWater that has beenmodified (cooled and freshened) by subsurfacemelting of ice shelves and/or icebergs. Thiswatermass has not previously been reported in the region, possibly because of the paucity of historical data.
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