Papers by Michael Karcher
The radionuclide 99 Tc had been discharged from the nuclear reprocessing facility in Sellafield (... more The radionuclide 99 Tc had been discharged from the nuclear reprocessing facility in Sellafield (UK) into the Irish Sea in increased amounts in the 1990s. We compare the simulated dispersion of 99 Tc in surface water as calculated by a hydrodynamic model and an assessment box model with field-observations from 1996 to 1999 to study concentrations, pathways and travel times. The model results are consistent with the observations and show the typical pathway of dissolved radionuclides from the Irish Sea via the North Sea along the Norwegian Coast. Subsequently the contaminated water separates into three branches of which the two Arctic branches bear the potential for future monitoring of the signal in the next decades. The results of the hydrodynamic model indicate a large variability of surface concentrations in the West Spitsbergen Current which has implications for future monitoring strategies. According to the observed and simulated distributions we suggest an improved box model structure to better capture the pattern for concentrations at the surface. #
Journal of Geophysical Research: Oceans, 2014
With a numerical model we test the sensitivity of the Arctic Ocean cir-4 culation at mid-depth (2... more With a numerical model we test the sensitivity of the Arctic Ocean cir-4 culation at mid-depth (212-1200 m) to the change in the sea ice rheology pa-5 rameter P * that controls the sea ice compressive strength. We show that the 6 reduction of the sea ice strength via P * within commonly used envelope re-7 duces the sea ice extent and consequently enhances the ocean surface heat 8 loss in the marginal ice zone. This leads to cooling of the Atlantic water in-9 flow into the Arctic Ocean. As a result Eurasian Basin and Amerasian Basin 10 temperatures are in average cooled by 0.1 • C and 0.05 • C, respectively. An 11 increased sea ice drift speed in the central Arctic leads to an enhanced cir-12 culation of the anticyclonic Beaufort Gyre of the Amerasian Basin, which 13 in turn weakens the cyclonic Atlantic water circulation below and enhances 14 the recirculation of the Atlantic water in the Eurasian Basin. Consequently 15 the balance of the volume fluxes through the Arctic gateways changes. Fram 16 Strait net outflow increases by 0.46 Sv, Barents Sea Opening net inflow in-17 creases by 0.19 Sv and Davis Strait net outflow decreases by 0.28 Sv. This 18 may spread the effects of the sea ice strength change beyond the limits of 19 the Arctic Ocean and into the deep water convection zones in the North At-20 lantic. These substantial effects should be considered also in the model op-21 timization efforts where P * is commonly used as one of the tuning param-22 eters to achieve better sea ice simulations, whereas the effects on the ocean 23 circulation are rarely taken into account.
Eos, Transactions American Geophysical Union, 2001
Journal of environmental radioactivity, 2004
The radionuclide (99)Tc had been discharged from the nuclear reprocessing facility in Sellafield ... more The radionuclide (99)Tc had been discharged from the nuclear reprocessing facility in Sellafield (UK) into the Irish Sea in increased amounts in the 1990s. We compare the simulated dispersion of (99)Tc in surface water as calculated by a hydrodynamic model and an assessment box model with field-observations from 1996 to 1999 to study concentrations, pathways and travel times. The model results are consistent with the observations and show the typical pathway of dissolved radionuclides from the Irish Sea via the North Sea along the Norwegian Coast. Subsequently the contaminated water separates into three branches of which the two Arctic branches bear the potential for future monitoring of the signal in the next decades. The results of the hydrodynamic model indicate a large variability of surface concentrations in the West Spitsbergen Current which has implications for future monitoring strategies. According to the observed and simulated distributions we suggest an improved box model...
Global and Planetary Change, 2015
ABSTRACT Large freshwater anomalies clearly exist in the Arctic Ocean. For example, liquid freshw... more ABSTRACT Large freshwater anomalies clearly exist in the Arctic Ocean. For example, liquid freshwater has accumulated in the Beaufort Gyre in the decade of the 2000s compared to 1980-2000, with an extra ≈ 5000 km3 — about 25% — being stored. The sources of freshwater to the Arctic from precipitation and runoff have increased between these periods (most of the evidence comes from models). Despite flux increases from 2001 to 2011, it is uncertain if the marine freshwater source through Bering Strait for the 2000s has changed, as observations in the 1980s and 1990s are incomplete. The marine freshwater fluxes draining the Arctic through Fram and Davis straits are also insignificantly different. In this way, the balance of sources and sinks of freshwater to the Arctic, Canadian Arctic Archipelago (CAA), and Baffin Bay shifted to about 1200 ± 730 km3 yr− 1 freshening the region, on average, during the 2000s. The observed accumulation of liquid freshwater is consistent with this increased supply and the loss of freshwater from sea ice. Coupled climate models project continued freshening of the Arctic during the 21st century, with a total gain of about 50,000 km3 for the Arctic, CAA, and Baffin Bay (an increase of about 50%) by 2100. Understanding of the mechanisms controlling freshwater emphasizes the importance of Arctic surface winds, in addition to the sources of freshwater. The wind can modify the storage, release, and pathways of freshwater on timescales of O(1-10) months. Discharges of excess freshwater through Fram or Davis straits appear possible, triggered by changes in the wind, but are hard to predict. Continued measurement of the fluxes and storage of freshwater is needed to observe changes such as these.
Upper ocean,conditions in the Greenland Sea are investigated for detecting preconditioning of the... more Upper ocean,conditions in the Greenland Sea are investigated for detecting preconditioning of the water column,for deep convection and how,these conditions are simulated,in the models. Observations indicate that periods of intense deep-water formation were also associated with dense upper ocean water masses, hence we focus on the upper ocean hydrography. We chose to analyze properties at 200m which,reside below summer,mixed,layer
Eos, Transactions American Geophysical Union, 2005
Geophysical Monograph Series, 2000
The complexity of the state-of-the-art Ocean General Circulation Models (OGCMs) has increased and... more The complexity of the state-of-the-art Ocean General Circulation Models (OGCMs) has increased and the quality of the model systems has improved considerably over the last decades. The improvement is caused by a variety of factors ranging from improved representation of key physical and dynamical processes, parallel development of at least three classes of OGCM systems, accurate and cost-effective numerical schemes, an unprecedented increase in computational resources, and the availability of synoptic, multi-decadal atmospheric forcing fields. The implications of these improvements are that the current generation of OGCMs can, for the first time, complement available ocean observations and be used to guide forthcoming ocean observation strategies. OGCMs are also extensively used as laboratories for assessing cause-relationships for observed changes in the marine climate system, and to assess how the ocean system may change in response to, for instance, anomalous air-sea fluxes of heat, fresh water and momentum. The Nordic Seas are a particularly challenging region for OGCMs. The challenge is caused by characteristic length scales of only a few to about ten km, a variety of complex and interrelated ocean processes, and extreme air-sea fluxes. In the paper, an overview of the status of the prognostic modelling of the Nordic Seas marine climate system is given. To exemplify the status, output from two widely different, state-of-the-art OGCM systems are used. The paper also addresses processes that are still inadequately described in the current generation of OGCMs, providing guidelines for the future development of model systems particularly tailored for the Nordic Seas region.
As the world warms, the expectation is that the freshwater outflows from the Arctic Ocean to the ... more As the world warms, the expectation is that the freshwater outflows from the Arctic Ocean to the North Atlantic will strengthen and may act to suppress the rate of the climatically-important Atlantic meridional overturning circulation. Hitherto, however, we have lacked the system of measurements required to estimate the totality of the freshwater flux through subarctic seas. Though observations remain patchy and rudimentary in places, we piece-together the results from recent large-scale observational programmes together with associated modelling, to establish preliminary maps of the rates and pathways of freshwater flux through subarctic seas. These fluxes are calculated according to two reference salinities, S = 34.8 to conform with the majority of estimates reported in the literature, and S = 35.2, the salinity of the inflowing Atlantic water, to calculate the freshwater balance of the 'Arctic Mediterranean'. We find that 148 mSv of freshwater enters the Nordic Seas across its northern boundary. There it is supplemented by around 54 mSv of freshwater from Baltic runoff, Norwegian runoff, P À E and Greenland ice melt, so that the total freshwater contribution to the Nordic Seas from all sources is 202 mSv. Of this, around 51 mSv of freshwater is estimated to pass south to the deep Atlantic in the dense water overflows leaving an assumed balance of 151 mSv to leave the Nordic Seas in the upper water export through Denmark Strait. The corresponding estimate for the freshwater outflow west of Greenland is 103 mSv relative to 35.2 so that the total freshwater flux reaching the North Atlantic through subarctic seas is around 300 mSv.
Ocean Science, 2009
We present the late summer distribution and transports of freshwater components in the upper west... more We present the late summer distribution and transports of freshwater components in the upper western part of the Fram Strait during 1998Strait during , 2004Strait during and 2005. Hydrographic data and and water δ 18 O values are analyzed to distinguish Atlantic Water, ice-melt (IMW) and freshwater removal from ice formation (IFW), and Meteoric Water (precipi-5 tation and riverine sources; MW). Concentrations of these water masses are combined with volume transport estimates from an inverse model. The average liquid freshwater transport relative to a reference salinity of 34.92, was 2500 km 3 /yr or 80 mSv southward, which is at the upper end of values reported in the literature. Our results indicate that not only the region of the continental slope but also parts of the East Greenland 10 Shelf are important for freshwater transports. The average transports of MW and IFW were 160 mSv (5000 km 3 /yr) and 90 mSv (2800 km 3 /yr) southward, respectively. The southward transport of MW was higher in 2005 than in 1998, but was compensated by a higher IFW transport. These differences in transports were associated with stronger southward velocities and the absence of 15 northward velocities over the continental slope and the eastern East Greenland Shelf in 2005. A simulation using the North Atlantic-Arctic Ocean Sea Ice Model (NAOSIM) shows that the high transport of MW in the Fram Strait in 2005 is in agreement with the temporary storage of river water on the Siberian shelf in the mid-1990s, which reached the north of Greenland in 2003. Our results indicate that IFW follows the 20 same pathways as MW before reaching the Fram Strait.
Ocean Modelling, 2004
Within the framework of the Arctic Ocean Model Intercomparison Project results from several coupl... more Within the framework of the Arctic Ocean Model Intercomparison Project results from several coupled sea ice-ocean models are compared in order to investigate vertically integrated properties of the Arctic Ocean. Annual means and seasonal ranges of streamfunction, freshwater and heat content are shown. For streamfunction the entire water column is integrated. For heat and freshwater content integration is over the upper 1000 m. The study represents a step toward identifying differences among model approaches and will serve as a base for upcoming studies where all models will be executed with common forcing. In this first stage only readily available outputs are compared, while forcing as well as numerical parameterizations differ.
Deutsche Hydrographische Zeitschrift, 1998
Summary The paper presents the results of a joint project combining numerical model studies and ... more Summary The paper presents the results of a joint project combining numerical model studies and field work in order to investigate the dispersion of anthropogenic radioactivity in the Arctic Ocean. The results obtained underline the essential difference between the dispersion of dissolved contaminants in water and the transport of particulate material in sea ice. Coupled ice-ocean models on different spatial scales
Journal of Geophysical Research, 2011
Journal of Geophysical Research, 2005
The impact of North Atlantic Current (NAC) volume, heat, and salt transport variability onto the ... more The impact of North Atlantic Current (NAC) volume, heat, and salt transport variability onto the Nordic Seas and the Arctic Ocean is investigated using numerical hindcast and sensitivity experiments. The ocean-sea ice model reproduces observed propagation pathways and speeds of SST anomalies. Signals reaching the entrance to the Nordic Seas between Iceland and Scotland originate partly in the lower-latitude North
Journal of Geophysical Research, 2007
We use a subset of models from the coordinated experiment of the Arctic Ocean Model Intercomparis... more We use a subset of models from the coordinated experiment of the Arctic Ocean Model Intercomparison Project (AOMIP) to analyze differences in intensity and sense of rotation of Atlantic Water circulation. We focus on the interpretation of the potential vorticity (PV) balance. Results differ drastically for the Eurasian and the Amerasian Basins of the Arctic Ocean. We find indications that
Journal of Geophysical Research, 2007
1] As a part of the Arctic Ocean Model Intercomparison Project, results from 10 Arctic ocean/ice ... more 1] As a part of the Arctic Ocean Model Intercomparison Project, results from 10 Arctic ocean/ice models are intercompared over the period 1970 through 1999. Models' monthly mean outputs are laterally integrated over two subdomains (Amerasian and Eurasian basins), then examined as functions of depth and time. Differences in such fields as averaged temperature and salinity arise from models' differences in parameterizations and numerical methods and from different domain sizes, with anomalies that develop at lower latitudes carried into the Arctic. A systematic deficiency is seen as AOMIP models tend to produce thermally stratified upper layers rather than the ''cold halocline'', suggesting missing physics perhaps related to vertical mixing or to shelf-basin exchanges. Flow fields pose a challenge for intercomparison. We introduce topostrophy, the vertical component of VÂr r r rD where V is monthly mean velocity and r r r rD is the gradient of total depth, characterizing the tendency to follow topographic slopes. Positive topostrophy expresses a tendency for cyclonic ''rim currents''. Systematic differences of models' circulations are found to depend strongly upon assumed roles of unresolved eddies.
Journal of Geophysical Research, 2007
1] A series of sensitivity experiments using a coupled regional atmosphere-ocean-ice model of the... more 1] A series of sensitivity experiments using a coupled regional atmosphere-ocean-ice model of the Arctic has been conducted in order to identify the requirements needed to reproduce observed sea-ice conditions and to address uncertainties in the description of Arctic processes. The ability of the coupled model to reproduce observed summer ice retreat depends largely on a quasi-realistic ice volume at the beginning of the melting period, determined by the relationship between winter growth and summer decay of ice. While summer ice decay is strongly affected by the parameterization of the sea-ice albedo, winter ice growth depends significantly on the parameterization of lateral freezing. Reciprocal model biases due to uncertainties in the atmospheric energy fluxes can be compensated to a certain extent. However, potential underlying weaknesses of the model cannot be eliminated that way. Since lateral freezing also determines the ice concentration during winter, and thus the heat loss of the ocean and the near-surface air temperature, the model tuning possibilities are limited. A large uncertainty in the model relates to the simulation of long-wave radiation most likely as a result of overestimated cloud cover. The results suggest that uncertainties in the descriptions for Arctic clouds, snow, and sea-ice albedo, and lateral freezing and melting of sea ice, including the treatment of snow, are responsible for large deviations in the simulation of Arctic sea ice in coupled models. Improved descriptions of these processes are needed to reduce model biases and to enhance the credibility of future climate change projections.
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Papers by Michael Karcher