Svein Østerhus - Independent Researcher (original) (raw)
Papers by Svein Østerhus
Earth System Science Data Discussions, 2016
Measuring temperature and salinity profiles in the world's oceans is crucial to understand oc... more Measuring temperature and salinity profiles in the world's oceans is crucial to understand ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker POLARSTERN has been the platform of numerous CTD deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD (conductivity, temperature, depth) data. In total 131 datasets (one dataset per cruise leg) containing data from 10063 CTD casts are now freely available at <a href="https://doi.pangaea.de/10.1594/PANGAEA.860066" target="_blank">doi:10.1594/PANGAEA.860066</a>. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation etc.) are described in detail. This compilation is special not only with regard to the qua...
The Faroe Bank Channel is more than 200 m deeper than any other passage across the Greenland-Scot... more The Faroe Bank Channel is more than 200 m deeper than any other passage across the Greenland-Scotland Ridge. It is therefore the main outlet for the densest overflow of cold water from the Arctic Mediterranean (Nordic Seas + Arctic Ocean) to the North Atlantic and is estimated to carry about one third of the total overflow flux across the ridge. Since 1988 the Faroese Fisheries Laboratory has monitored the hydrography of the channel with regular CTD cruises along a standard section southeast of the sill and since 1995 the Nordic WOCE and later the VEINS programmes have maintained a 75 kHz ADCP (Acoustic Doppler Current Profiler) mooring in the channel, located on the sill. Using the CTD data and the almost three year long ADCP time series, average fluxes have been calculated to 2.5 Sv (106 m3/s) for the total volume flux below 450 m depth, 1.9 Sv for ISOW, and 1.5 Sv of water colder than +0.5°C (NSDW + NWSAIW).
Is the oceanic heat transport with Atlantic water towards the Arctic changing?
The flow of Atlantic water (Atlantic inflow) across the Greenland-Scotland Ridge (GSR) is critica... more The flow of Atlantic water (Atlantic inflow) across the Greenland-Scotland Ridge (GSR) is critical for conditions in the Nordic Seas and Arctic Ocean by importing heat and salt. All three branches crossing the GSR have been monitored since the mid-1990ies and the transports of water and heat have been estimated. The Atlantic inflow, that forms the surface part of the thermohaline circulation in the North Atlantic, is affected by wind forcing and freshwater input but the most important driving appears to be the cooling of the ocean by the atmosphere in the subarctic seas and increasing of salinity in the Arctic Ocean through freezing of seawater. This results in the sinking of the surface waters that subsequently flow out of the area close to the bottom over the GSR. This removal of water from the Arctic region by the overflow generates sea level slopes that drive a northward transport of water and heat. With global climate change, the Arctic atmosphere is expected to warm and freshw...
The Nordic WOCE travel-resistant ADCP system
Proceedings of the IEEE Fifth Working Conference on Current Measurement, 1995
ABSTRACT
Oceans '97. MTS/IEEE Conference Proceedings, 1997
1 2 3
The Norwegian Sea one hundred years after
IOP Conference Series: Earth and Environmental Science, 2009
... Equipped with improved instruments and methods, and the new specially designed research vesse... more ... Equipped with improved instruments and methods, and the new specially designed research vessel Michael Sars, an extensive investigation of the Nordic ... the science of the Nordic Seas on an astonishingly modern footing (Mork, 1989) and as Blindheim & Østerhus (2005) put ...
Journal of Atmospheric and Oceanic Technology, 1999
The Ocean Ambient Sound Instrument System (OASIS) consists of a conventional RD Instruments Acous... more The Ocean Ambient Sound Instrument System (OASIS) consists of a conventional RD Instruments Acoustic Doppler Current Profilers (ADCPs), modified to allow the recording of high-quality ambient sound in the frequency range from 1 to 75 kHz. In addition to the usual capabilities of an ADCP, this combination of acoustic instrumentation allows wind direction and speed to be inferred from a subsurface platform. The method of wind speed determination from ambient sound levels is explained identifying some of the techniques and limitations. Wind direction is inferred from surface drift velocities using the ADCP data. The design of both the hardware and software required to make combined ADCP and ambient sound recordings is discussed. The capabilities of the system are demonstrated using observations made in the Norwegian Sea at Ocean Weather Station Mike. Using previously published algorithms (and calibration constants) ambient sound-based wind speeds are found that closely match direct wind observations. The typical standard deviations for hourly wind speed estimates is 1.5 m s Ϫ1 when using acoustic frequencies less than 10 kHz. Higher acoustic frequencies show greater variance in wind speed estimates. OASIS estimates of the 12-h average wind directions have an error standard deviation of 25Њ with no mean bias.
Geophysical Monograph Series, 2000
Hydrographical evidence for an isopycnal intrusion corroborated by acoustic Doppler current profiling
Journal of Geophysical Research, 1989
ABSTRACT
Upper layer cooling and freshening in the Norwegian Sea in relation to atmospheric forcing
Deep Sea Research Part I: Oceanographic Research Papers, 2000
Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and sali... more Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and salinity since the 1960s. Time series from Weather Station “M”, from Russian surveys in the Norwegian Sea, from Icelandic standard sections, and from Scottish and Faroese observations in the ...
The Bottom Pressure Field In The Arctic Mediterranean
The f/H field in the Arctic Mediterranean is dominated by topography, and the re- gion is dominat... more The f/H field in the Arctic Mediterranean is dominated by topography, and the re- gion is dominated by many closed f/H contours. Simple analytical expressions for the bottom geostrophic velocities are derived by integrating the vortivity equation over areas enclosed by closed f/H contours. When the f/H contours are not closed within the Arctic Mediterranean we use current measurements as boundary conditions. Due to the strong topographic gradients the bottom currents will mainly follow topogra- phy, and the bottom pressure may be approximated as a function of f/H. We show that this approximation is valid even on very long contours enclosing most of the Arctic Mediterranean. Thus, given the wind stress and density fields, and boundary condi- tions on open f/H contours, we present a method to estimate the bottom pressure field in the Arctic Mediterranean. To validate this method we compare the solutions from the analytical expressions to current mesurements near bottom in several locations within the Nordic Seas and to the near bottom pressure field from a numerical model.
The Nuka Arctica Line
In order to develop an accurate understanding of the mean circulation and its variability in the ... more In order to develop an accurate understanding of the mean circulation and its variability in the northeast Atlantic over a wide range of timescales, the Nuka Arctica project was established to measure currents directly using a hull-mounted ADCP onboard the container vessel `Nuka Arctica' that operates between Denmark and Greenland on a three-week schedule. Since late 1999, the `Nuka Arctica' has collected velocity data from nearly 50 transects. The circulation around the Reykjanes Ridge stands out very clearly with a southward flow on its eastern flank and a well-defined northward flow along its western slope. We also find a well-defined flow northward through the Iceland Basin. The East Greenland Current as it passes around Cape Farewell stands out clearly. These mean flow patterns appear to be robust thanks to the many sections available. In contrast, except for the East Greenland Current, these currents cannot be discerned in any single section. There are two reasons for ...
Processes beneath Filchner-Ronne Ice Shelf contribute to the production of deep and bottom waters... more Processes beneath Filchner-Ronne Ice Shelf contribute to the production of deep and bottom waters of the Weddell Sea. As part of a programme aimed at determining how those processes are affected by climatic forcing we instrumented two sites beneath the ice shelf in early 1999. One site has yielded an 18-month long record of current and temperatures, while the second site is still active and has provided records three years in length. Unlike long term records from beneath the western Ronne Ice Shelf the new records show marked interannual variability. Variability in oceanographic conditions beneath the ice shelf is a response to variability in conditions north of the ice front. In particular, the thermohaline forcing on the sub-ice shelf cavity is a function of the amount of brine release from sea-ice formation over the open continental shelf. The primary control on the sea-ice formation rate is the amount of open water available for the formation of fresh ice. In the Austral summer ...
Direct observatons of inflow to the Nordic Seas through the Faroe Shetland Channel 1994-1998
Hydrographic variability during the decade of the 1990s in the Northeast Atlantic and southern Norwegian Sea. ICES Mar. Sci. Symp. 219: 111-120
Deepwater formation in the Southern Weddell Sea, processes, pathways and fluxes
Ice Shelf Water (ISW) is the final product of the melting process underneath the floating ice she... more Ice Shelf Water (ISW) is the final product of the melting process underneath the floating ice shelves in the Antarctica. Recent drastic break-ups of the Larsen ice-shelf in the Weddell Sea has vitalised the question if this melting has increased. The most efficient production of ISW takes place under the immense Ronne - Filchner Ice shelves in the Southern Weddell Sea. The corresponding ISW flow out of the region was located in 1977, and a key location for long term monitoring was identified. Since then we have succeeded in occupying this station with instrumented moorings for about 5 years, which have given us valuable information of variability of the sub-ice shelf and continental shelf circulation. However, in addition to serve as an indicator for Antarctic ice-cap melting, it turns out that the ISW flow contribute to the formation of the Antarctic bottom water. Therefore it is driving the thermohaline circulation, which has a great impact on the global climate. There are several...
Meridional Overturning Exchange with the Nordic Seas
The flow of Atlantic water towards the Arctic crosses the Greenland-Scotland Ridge in three curre... more The flow of Atlantic water towards the Arctic crosses the Greenland-Scotland Ridge in three current branches. By the heat that it carries along, it keeps the subarctic regions abnormally warm and by its import of salt, it helps maintain a high salinity and hence high density in the surface waters as a precondition for thermohaline ventilation. In mid 1990's an extensive monitoring program for all three branches was lunched as a Nordic contribution to WOCE and is still going on. The western branch, the Irminger Current, has been monitored by means of traditional current meters moorings on a section crossing the current northwest of Iceland. A number of ADCPs have been moored on a section going north from the Faroes, crossing the Faroes Current. The eastern branch, the Continental Slope Current, is monitored by ADCPs moorings across the Faroe-Shetland Channel. CTD observations from research vessels along all the current meter sections are obtained on seasonal basis. Here we presen...
The interaction between the AMOC and the Nordic Seas through the Iceland-Scotland Gap
The exchange of water, heat, and salt across the Greenland-Scotland Ridge is a two-way coupling b... more The exchange of water, heat, and salt across the Greenland-Scotland Ridge is a two-way coupling between the Nordic Seas and the Atlantic Ocean that is essential for both the Atlantic Meridional Overturning Circulation (AMOC) and the conditions in the Nordic Seas and the Arctic. Climate models do not, however, give any clear indication, how this coupling will be affected by climate change (IPCC, 2007). Most of the exchange occurs east of Iceland, through the Iceland-Scotland Gap. This includes about half of the total deep overflow of water from the Nordic Seas and 90% of the Atlantic inflow to the Nordic Seas. In this region, systematic measurements have been carried out for more than a decade and time series of flow properties (temperature and salinity) and transports have been obtained for all the main exchange branches. A complementary set of time series is provided by ocean models, constrained by atmospheric input from reanalysis data sets. For some exchange branches, there are c...
Earth System Science Data Discussions, 2016
Measuring temperature and salinity profiles in the world's oceans is crucial to understand oc... more Measuring temperature and salinity profiles in the world's oceans is crucial to understand ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker POLARSTERN has been the platform of numerous CTD deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD (conductivity, temperature, depth) data. In total 131 datasets (one dataset per cruise leg) containing data from 10063 CTD casts are now freely available at <a href="https://doi.pangaea.de/10.1594/PANGAEA.860066" target="_blank">doi:10.1594/PANGAEA.860066</a>. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation etc.) are described in detail. This compilation is special not only with regard to the qua...
The Faroe Bank Channel is more than 200 m deeper than any other passage across the Greenland-Scot... more The Faroe Bank Channel is more than 200 m deeper than any other passage across the Greenland-Scotland Ridge. It is therefore the main outlet for the densest overflow of cold water from the Arctic Mediterranean (Nordic Seas + Arctic Ocean) to the North Atlantic and is estimated to carry about one third of the total overflow flux across the ridge. Since 1988 the Faroese Fisheries Laboratory has monitored the hydrography of the channel with regular CTD cruises along a standard section southeast of the sill and since 1995 the Nordic WOCE and later the VEINS programmes have maintained a 75 kHz ADCP (Acoustic Doppler Current Profiler) mooring in the channel, located on the sill. Using the CTD data and the almost three year long ADCP time series, average fluxes have been calculated to 2.5 Sv (106 m3/s) for the total volume flux below 450 m depth, 1.9 Sv for ISOW, and 1.5 Sv of water colder than +0.5°C (NSDW + NWSAIW).
Is the oceanic heat transport with Atlantic water towards the Arctic changing?
The flow of Atlantic water (Atlantic inflow) across the Greenland-Scotland Ridge (GSR) is critica... more The flow of Atlantic water (Atlantic inflow) across the Greenland-Scotland Ridge (GSR) is critical for conditions in the Nordic Seas and Arctic Ocean by importing heat and salt. All three branches crossing the GSR have been monitored since the mid-1990ies and the transports of water and heat have been estimated. The Atlantic inflow, that forms the surface part of the thermohaline circulation in the North Atlantic, is affected by wind forcing and freshwater input but the most important driving appears to be the cooling of the ocean by the atmosphere in the subarctic seas and increasing of salinity in the Arctic Ocean through freezing of seawater. This results in the sinking of the surface waters that subsequently flow out of the area close to the bottom over the GSR. This removal of water from the Arctic region by the overflow generates sea level slopes that drive a northward transport of water and heat. With global climate change, the Arctic atmosphere is expected to warm and freshw...
The Nordic WOCE travel-resistant ADCP system
Proceedings of the IEEE Fifth Working Conference on Current Measurement, 1995
ABSTRACT
Oceans '97. MTS/IEEE Conference Proceedings, 1997
1 2 3
The Norwegian Sea one hundred years after
IOP Conference Series: Earth and Environmental Science, 2009
... Equipped with improved instruments and methods, and the new specially designed research vesse... more ... Equipped with improved instruments and methods, and the new specially designed research vessel Michael Sars, an extensive investigation of the Nordic ... the science of the Nordic Seas on an astonishingly modern footing (Mork, 1989) and as Blindheim & Østerhus (2005) put ...
Journal of Atmospheric and Oceanic Technology, 1999
The Ocean Ambient Sound Instrument System (OASIS) consists of a conventional RD Instruments Acous... more The Ocean Ambient Sound Instrument System (OASIS) consists of a conventional RD Instruments Acoustic Doppler Current Profilers (ADCPs), modified to allow the recording of high-quality ambient sound in the frequency range from 1 to 75 kHz. In addition to the usual capabilities of an ADCP, this combination of acoustic instrumentation allows wind direction and speed to be inferred from a subsurface platform. The method of wind speed determination from ambient sound levels is explained identifying some of the techniques and limitations. Wind direction is inferred from surface drift velocities using the ADCP data. The design of both the hardware and software required to make combined ADCP and ambient sound recordings is discussed. The capabilities of the system are demonstrated using observations made in the Norwegian Sea at Ocean Weather Station Mike. Using previously published algorithms (and calibration constants) ambient sound-based wind speeds are found that closely match direct wind observations. The typical standard deviations for hourly wind speed estimates is 1.5 m s Ϫ1 when using acoustic frequencies less than 10 kHz. Higher acoustic frequencies show greater variance in wind speed estimates. OASIS estimates of the 12-h average wind directions have an error standard deviation of 25Њ with no mean bias.
Geophysical Monograph Series, 2000
Hydrographical evidence for an isopycnal intrusion corroborated by acoustic Doppler current profiling
Journal of Geophysical Research, 1989
ABSTRACT
Upper layer cooling and freshening in the Norwegian Sea in relation to atmospheric forcing
Deep Sea Research Part I: Oceanographic Research Papers, 2000
Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and sali... more Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and salinity since the 1960s. Time series from Weather Station “M”, from Russian surveys in the Norwegian Sea, from Icelandic standard sections, and from Scottish and Faroese observations in the ...
The Bottom Pressure Field In The Arctic Mediterranean
The f/H field in the Arctic Mediterranean is dominated by topography, and the re- gion is dominat... more The f/H field in the Arctic Mediterranean is dominated by topography, and the re- gion is dominated by many closed f/H contours. Simple analytical expressions for the bottom geostrophic velocities are derived by integrating the vortivity equation over areas enclosed by closed f/H contours. When the f/H contours are not closed within the Arctic Mediterranean we use current measurements as boundary conditions. Due to the strong topographic gradients the bottom currents will mainly follow topogra- phy, and the bottom pressure may be approximated as a function of f/H. We show that this approximation is valid even on very long contours enclosing most of the Arctic Mediterranean. Thus, given the wind stress and density fields, and boundary condi- tions on open f/H contours, we present a method to estimate the bottom pressure field in the Arctic Mediterranean. To validate this method we compare the solutions from the analytical expressions to current mesurements near bottom in several locations within the Nordic Seas and to the near bottom pressure field from a numerical model.
The Nuka Arctica Line
In order to develop an accurate understanding of the mean circulation and its variability in the ... more In order to develop an accurate understanding of the mean circulation and its variability in the northeast Atlantic over a wide range of timescales, the Nuka Arctica project was established to measure currents directly using a hull-mounted ADCP onboard the container vessel `Nuka Arctica' that operates between Denmark and Greenland on a three-week schedule. Since late 1999, the `Nuka Arctica' has collected velocity data from nearly 50 transects. The circulation around the Reykjanes Ridge stands out very clearly with a southward flow on its eastern flank and a well-defined northward flow along its western slope. We also find a well-defined flow northward through the Iceland Basin. The East Greenland Current as it passes around Cape Farewell stands out clearly. These mean flow patterns appear to be robust thanks to the many sections available. In contrast, except for the East Greenland Current, these currents cannot be discerned in any single section. There are two reasons for ...
Processes beneath Filchner-Ronne Ice Shelf contribute to the production of deep and bottom waters... more Processes beneath Filchner-Ronne Ice Shelf contribute to the production of deep and bottom waters of the Weddell Sea. As part of a programme aimed at determining how those processes are affected by climatic forcing we instrumented two sites beneath the ice shelf in early 1999. One site has yielded an 18-month long record of current and temperatures, while the second site is still active and has provided records three years in length. Unlike long term records from beneath the western Ronne Ice Shelf the new records show marked interannual variability. Variability in oceanographic conditions beneath the ice shelf is a response to variability in conditions north of the ice front. In particular, the thermohaline forcing on the sub-ice shelf cavity is a function of the amount of brine release from sea-ice formation over the open continental shelf. The primary control on the sea-ice formation rate is the amount of open water available for the formation of fresh ice. In the Austral summer ...
Direct observatons of inflow to the Nordic Seas through the Faroe Shetland Channel 1994-1998
Hydrographic variability during the decade of the 1990s in the Northeast Atlantic and southern Norwegian Sea. ICES Mar. Sci. Symp. 219: 111-120
Deepwater formation in the Southern Weddell Sea, processes, pathways and fluxes
Ice Shelf Water (ISW) is the final product of the melting process underneath the floating ice she... more Ice Shelf Water (ISW) is the final product of the melting process underneath the floating ice shelves in the Antarctica. Recent drastic break-ups of the Larsen ice-shelf in the Weddell Sea has vitalised the question if this melting has increased. The most efficient production of ISW takes place under the immense Ronne - Filchner Ice shelves in the Southern Weddell Sea. The corresponding ISW flow out of the region was located in 1977, and a key location for long term monitoring was identified. Since then we have succeeded in occupying this station with instrumented moorings for about 5 years, which have given us valuable information of variability of the sub-ice shelf and continental shelf circulation. However, in addition to serve as an indicator for Antarctic ice-cap melting, it turns out that the ISW flow contribute to the formation of the Antarctic bottom water. Therefore it is driving the thermohaline circulation, which has a great impact on the global climate. There are several...
Meridional Overturning Exchange with the Nordic Seas
The flow of Atlantic water towards the Arctic crosses the Greenland-Scotland Ridge in three curre... more The flow of Atlantic water towards the Arctic crosses the Greenland-Scotland Ridge in three current branches. By the heat that it carries along, it keeps the subarctic regions abnormally warm and by its import of salt, it helps maintain a high salinity and hence high density in the surface waters as a precondition for thermohaline ventilation. In mid 1990's an extensive monitoring program for all three branches was lunched as a Nordic contribution to WOCE and is still going on. The western branch, the Irminger Current, has been monitored by means of traditional current meters moorings on a section crossing the current northwest of Iceland. A number of ADCPs have been moored on a section going north from the Faroes, crossing the Faroes Current. The eastern branch, the Continental Slope Current, is monitored by ADCPs moorings across the Faroe-Shetland Channel. CTD observations from research vessels along all the current meter sections are obtained on seasonal basis. Here we presen...
The interaction between the AMOC and the Nordic Seas through the Iceland-Scotland Gap
The exchange of water, heat, and salt across the Greenland-Scotland Ridge is a two-way coupling b... more The exchange of water, heat, and salt across the Greenland-Scotland Ridge is a two-way coupling between the Nordic Seas and the Atlantic Ocean that is essential for both the Atlantic Meridional Overturning Circulation (AMOC) and the conditions in the Nordic Seas and the Arctic. Climate models do not, however, give any clear indication, how this coupling will be affected by climate change (IPCC, 2007). Most of the exchange occurs east of Iceland, through the Iceland-Scotland Gap. This includes about half of the total deep overflow of water from the Nordic Seas and 90% of the Atlantic inflow to the Nordic Seas. In this region, systematic measurements have been carried out for more than a decade and time series of flow properties (temperature and salinity) and transports have been obtained for all the main exchange branches. A complementary set of time series is provided by ocean models, constrained by atmospheric input from reanalysis data sets. For some exchange branches, there are c...