http://hdl.handle.net/1956/926 Search the Channel search http://bora.uib.no/simple-search http://hdl.handle.net/1956/2393 Title: On the Nordic Seas’ role in the Atlantic Meridional Overturning Circulation <br/> <br/>Authors: Iovino, Doroteaciro <br/> <br/>Abstract: In the present climate, the North Atlantic thermohaline circulation (THC) plays a fundamental role in the global transport of heat at high latitudes. The response of the North Atlantic Ocean-Nordic Seas THC to surface forcing and basin geometries in an idealized one-hemisphere basin is analyzed to better understand the processes that are fundamental to the modeled circulation. Focusing on the dynamics of the Nordic Seas, analytical and numerical modeling highlight the relevance of a sill (Greenland-Scotland Ridge) in setting the properties of the water masses formed in and exported from a marginal sea. Finally, the influence of the convective activity in the Greenland Sea for the overflow, and thus the overturning, is assessed using hydrographic data (from 1950 to present), a regional ocean model, and a unique tracer release experiment. Greenland Sea Gyre water is estimated to contribute less than 1 Sv, and there is no evidence for causality between changes in the Greenland Sea and the overflow. http://hdl.handle.net/1956/1307 Title: Aspects on interactions between mid- to high lattitude atmospheric circulation and some surface processes <br/> <br/>Authors: Byrkjedal, Øyvind <br/> <br/>Abstract: The Arctic is a hot topic in Climate Research. A large number of signs of a warming Arctic Climate have been identified the latest years. This is of major concern in light of the increasing atmospheric content of greenhouse gases. The climate research community projects future warming of the climate in the high latitudes as a response to increased amounts of anthropogenic release of greenhouse gases since the pre-industrial era. The overall objectives of this work has been to study the mid- and high latitude climate and climate variability, and to evaluate how well some climate processes that contribute to determine the Arctic climate and variability are represented and simulated in climate models. A new data set of storm tracks trajectories and statistics over the Northern Hemisphere for the period 1948-2002 has been developed. The variability of the cyclones extending to the Nordic Seas is studied in particular, and it is found that both the number of storms and their intensity exhibits a strong decadal and interannual variability. The ocean volume transports into and out of the Nordic Seas shows a relatively close relation to the wintertime cyclone intensity and cyclone count. To have confidence in future projections of climate, it is necessary to evaluate how the model behaves in a climate regime different from modern day. To do this two model simulations of the last glacial maximum (LGM) was performed. The reconstructions of sea surface temperatures in the Nordic Seas in LGM differ from perennial sea ice cover to having open ocean during the summer. The large scale atmospheric circulation patterns of the two different climate reconstructions are studied. It is found that the perennial sea ice cover produces a circulation pattern which may be too zonal to support the existence of the large north Eurasian ice sheets. In the case with seasonally open ocean the air masses carries larger amounts of heat and moisture towards the ice sheets and represents a larger degree of meridional circulation. The current general circulation models, including several of those used by the IPCC, show considerable disagreement in simulating present day high latitude climate. This is of major concern and reduces the confidence in future model projections of high latitude climate. To investigate how turbulent vertical exchange processes in the Arctic boundary layer is represented by the climate models a simulation with high vertical resolution in the lower part of the atmosphere is performed. This reveals that the coarse vertical resolution commonly employed in the climate models are unable to reproduce important exchange processes in the Arctic boundary layer. In the case of our model this results in a warm bias over the Arctic Ocean. By increasing the vertical resolution we achieve a better representation of vertical turbulent exchange processes with the result of reproducing more realistic surface fluxes and surface air temperatures. <br/> <br/>Description: This thesis is based on the following papers: Paper I Sorteberg A., Kvamstø N. G. and Byrkjedal Ø. (2005). Wintertime Nordic Seas Cyclone Variability and its Impact on Oceanic Volume Transports Into the Nordic Seas. In: The Nordic Seas, An Integrated Perspective. H. Drange, T. Dokken, T. Furevik, R. Gerdes and W. Berger. Washington DC, American Geophysical Union. 158: 137-156 Paper II Byrkjedal Ø., Kvamstø N. G., Meland M. and Jansen E. (2006). Sensitivity of Last Glacial Maximum Climate to sea-ice condition in the Nordic Seas. Climate Dynamics 26(5): 473-487 Paper III Byrkjedal Ø., Esau I. and Kvamstø N. G. (2006). Sensitivity of simulated wintertime Arctic atmosphere to vertical resolution in the ARPEGE/IFS model. Submitted to Climate Dynamics http://hdl.handle.net/1956/804 Title: The sensitivity of the present day Atlantic meriodinal overturning circulation to freshwater forcing <br/> <br/>Authors: Otterå, Odd Helge; Drange, Helge; Bentsen, Mats; Kvamstø, N. G.; Jiang, Dabang <br/> <br/>Abstract: Mounting evidence indicates that the Atlantic Meridional Overturning Circulation (AMOC) was strongly reduced during cold climate episodes in the past, possible due to freshwater influx from glacial melting. It is also expected that the freshwater input to high northern latitudes will increase as human-induced global warming continues, with potential impacts on the AMOC. Here we present results from a 150 years sensitivity experiment with the Bergen Climate Model (BCM) for the present-day climate, but with enhanced runoff from the Arctic region throughout the integration. The AMOC drops by 30% over the first 50 years, followed by a gradual recovery. The simulated response indicates that the present-day AMOC might be robust to the isolated effect of enhanced, high-latitude freshwater forcing on a centennial time scale, and that the western tropical North Atlantic may provide key information about the long-term variability, and by that monitoring, of the AMOC. http://hdl.handle.net/1956/624 Title: Evaluation of ocean carbon cycle models with data-based metrics <br/> <br/>Authors: Matsumoto, K.; Sarmiento, J. L.; Key, R. M.; Aumont, O.; Bullister, J. L.; Caldeira, Ken; Campin, J. M.; Doney, S. C.; Drange, Helge; Dutay, J. C.; Follows, M.; Gao, Yongqi; Gnanadesikan, A.; Gruber, N.; Ishida, A.; Joos, F.; Lindsay, K.; Maier-Reimer, E.; Marshall, J. C.; Matear, R. J.; Monfray, P.; Mouchet, A.; Najjar, R.; Plattner, G. K.; Schlitzer, R.; Slater, R.; Swathi, P. S.; Totterdell, I. J.; Weirig, M. F.; Yamanaka, Y.; Yool, A.; Orr, J. C. <br/> <br/>Abstract: New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new databased metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model. http://hdl.handle.net/1956/435 Title: Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from denstity overturns <br/> <br/>Authors: Fer, I.; Skogseth, R.; Haugan, Peter Mosby <br/> <br/>Abstract: Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional conductivity-temperature-depth profiles and short-term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1-dbar vertically averaged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, e, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity Kz is estimated using Osborn’s model assuming a constant mixing efficiency. Survey-averaged profiles suggest enhanced mixing near the bottom with values of Kz and e, when averaged within the overflow, equal to 10 x 10-4 m2 s-1 and 3 x 10-8 W kg-1, respectively. Kz is found to decrease with increasing buoyancy frequency as N-1.2 (±0.3), albeit values of N covered only 0.5–8 cph (1 cph = 2p/3600 s-1). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m2, when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than but, considering the errors in estimates of Kz, comparable with 100 W m2, the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream. http://hdl.handle.net/1956/430 Title: Ice shelf water overflow and bottom water formation in the southern Weddell Sea <br/> <br/>Authors: Foldvik, A.; Gammelsrød, T.; Østerhus, Svein; Fahrbach, E.; Rohardt, G.; Schröder, Michael; Nicholls, Keith W.; Padman, L.; Woodgate, R. A. <br/> <br/>Abstract: [1] Cold shelf waters flowing out of the Filchner Depression in the southern Weddell Sea make a significant contribution to the production of Weddell Sea Bottom Water (WSBW), a precursor to Antarctic Bottom Water (AABW). We use all available current meter records from the region to calculate the flux of cold water (<−1.9°C) over the sill at the northern end of the Filchner Depression (1.6 ± 0.5 Sv), and to determine its fate. The estimated fluxes and mixing rates imply a rate of WSBW formation (referenced to −0.8°C) of 4.3 ± 1.4 Sv. We identify three pathways for the cold shelf waters to enter the deep Weddell Sea circulation. One path involves flow constrained to follow the shelf break. The other two paths are down the continental slope, resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36°W and 37°W. Mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m. Probably aided by thermobaricity, the plume water at this depth can flow at a speed approaching 1 m s−1, implying that the flow is occasionally supercritical. We postulate that such supercriticality acts to limit mixing between the plume and its environment. The transition from supercritical to slower, more uniform flow is associated with very efficient mixing, probably as a result of hydraulic jumps. http://hdl.handle.net/1956/428 Title: The impact of the wind stress curl in the North Atlantic on the Atlantic inflow to the Norwegian Sea toward the Arctic <br/> <br/>Authors: Orvik, Kjell Arild; Skagseth, Øystein <br/> <br/>Abstract: The Norwegian Atlantic Current (NwAC) through the Norwegian Sea serves as a conduit of warm and saline Atlantic water from the North Atlantic to the Arctic Ocean, an important factor for climate and ecology. In this study, we concentrate on how the North Atlantic wind stress curl (NAWSC) affects interannual variability on the major branch of the NwAC—the Norwegian Atlantic Slope Current (NwASC). Based on wind stress data from the NCEP reanalysis and estimated volume transport of the NwASC from current records during 1995–2003 in the Svinøy section (62N), our analysis shows that the volume transport in the NwASC exhibits a maximum correlation of 0.88 with the zonally integrated NAWSC at 55N 15 months earlier. Our findings reveal the NAWSC to be a major forcing for interannual variability of the NwASC in the range of 3.0- to 5.3 Sv (Sv = 106 m3 s1). The 15 month time lag appears to be in accordance with a forced baroclinic Rossby wave in response to the local Ekman pumping changing the baroclinicity and strength of the North Atlantic Current (NAC). After being converted to a nearly barotropic shelf edge current along the Irish-Scottish shelf through interaction with bottom topography, it appears as a barotropic response downstream in the Svinøy section. This result suggests the possibility of predicting conditions influenced by the NwASC more than a year in advance, using the NAWSC as a proxy. http://hdl.handle.net/1956/427 Title: Monitoring the Norwegian Atlantic slope current using a single moored current meter <br/> <br/>Authors: Orvik, Kjell Arild; Skagseth, Øystein <br/> <br/>Abstract: Monitoring the Atlantic inflow (AI) of warm and saline water into the Nordic Seas (Norwegian, Greenland and Iceland Seas) is of great importance becauce of its impact on climate and ecology in Northern Europe and Arctic. In this study, an observation system for establishment of simple, robust and cost effective monitoring of the AI is validated in the Svinøy section, cutting through the AI just to the north of the Faroe-Shetland Channel. We concentrate on the eastern branch of the AI, the Norwegian Atlantic Slope Current (NwASC), an about 40km wide flow along the steep Norwegian slope. The database is an array of 15 current meters on 4 moorings covering the NwASC over a 2-year period 1998–2000. We test the hypothesis that long-term monitoring of the NwASC can be performed by using one single current meter suitable placed in the flow. The volume flux can then be estimated by construction of simple regression models using the single current meter record as the independent variable. For validation of statistical properties as stability, confidence and stationarity, the time series is split into two 1-year segments: a model period and a test period. Gridded correlation fields between currents and volume transport show correlation maxima in the core of the NwASC, ranging from 0.84 on a daily timescale to 0.97 on a monthly timescale. A more comprehensive correlation/ coherence analysis for each current meter record against volume transport on 7-day timescales, enable us to choose the optimal current meter for a linear regression model with (correlation, slope) coefficients of (0.87, 0.13) for the model period and (0.80, 0.13) for the test period. The similarity of the statistical properties for the model and test periods substantiates the stationarity, stability and robustness of the model. A linear regression model underestimates large fluxes and is thus extended to a second degree polynomia. This improves the curve fitting for strong currents with a minor increase in overall correlation, but is more sensistive and less stable. Overall, we find a linear regression model to be more robust and applicable for monitoring the NwASC. The applicability of a linear regression model as an estimator for volume flux of the NwASC is demonstrated using a 2-year time series, and validated against calculated transport. The calculated transport agrees with the statistical analysis and reveals a noisy fit on daily timescale, while the curves coincide well on both 7- and 30-day timescales with correlation coefficients of 0.84 and 0.86, respectively. On all timescales, the calculated and model transport give an overall mean flow of 4.4 Sv and show fluctuations on timescales of days to months, with the seasonal cycle being the most prominent.