Mass transport induced by internal Kelvin waves

Abstract A two-layer system with a deep lower layer in a semi-infinite ocean is investigated theoretically. Tidal forcing is applied, and expressions for an internal Kelvin wave are derived. Terms of second order in wave amplitude are retained, to get the wave averaged drift in the flow. The along-shore wave is damped due to friction. Accordingly, the damping in wave drift is compensated by a second order mean motion normal to the direction of wave propagation. Furthermore, the second order momentum equations are shown to relate to the divergence of wave energy flux, in the same manner as barotropic waves. The derived internal Kelvin wave theory is applied to a semi-enclosed basin. This basin has a constriction at the basin entrance in order to enhance internal wave generation. On the northern hemisphere, the wave propagates anti-clockwise around the basin. A numerical model is applied to examine this wave motion. It is shown that the waves obtained may induce a net circulation in the basin. An attempt is made to connect the results from the semi-enclosed basin to the Van Mijen fjord in Svalbard. This fjord is ice-covered a large part of the year, preventing energy input from wind and enhancing internal wave motion. Numerical model results demonstrate an internal wave as predicted. However, the model assumptions do not fully resolve the complexity of the system. Further investigation is needed to understand the motion in this particular fjord

Evaluation of a sub-kilometre NWP system in an Arctic fjord-valley system in winter

Terrain challenges the prediction of near-surface atmospheric conditions, even in kilometre-scale numerical weather prediction (NWP) models. In this study, the ALADIN-HIRLAM NWP system with 0.5 km horizontal grid spacing and an increased number of vertical levels is compared to the 2.5-km model system similar to the currently operational NWP system at the Norwegian Meteorological Institute. The impact of the increased resolution on the forecasts’ ability to represent boundary-layer processes is investigated for the period from 12 to 16 February 2018 in an Arctic fjord-valley system in the Svalbard archipelago. Model simulations are compared to a wide range of observations conducted during a field campaign. The model configuration with sub-kilometre grid spacing improves both the spatial structure and overall verification scores for the near-surface temperature and wind forecasts compared to the 2.5-km experiment. The subkilometre experiment successfully captures the wind channelling through the valley and the temperature field associated with it. In a situation of a cold-air pool development, the sub-kilometre experiment has a particularly high near-surface temperature bias at low elevations. The use of measurement campaign data, however, reveals some encouraging results, e.g. the sub-kilometre system has a more realistic vertical profile of temperature and wind speed, and the surface temperature sensitivity to the net surface energy is closer to the observations. This work demonstrates the potential of sub-kilometre NWP systems for forecasting weather in complex Arctic terrain, and also suggests that the increase in resolution needs to be accompanied with further development of other parts of the model system

Currents and mean circulation induced by trapped internal waves

Bølgemekanismer i havet slik som ”Stokes drift” bidrar til å systematisk forflytte store mengder vannpartikler i en bestemt retning over store avstander. Stokes drift har blitt studert grundig for overflatebølger for eksempelvis knyttet til drift av oljeflak. I dette PhD-arbeidet er fokus bølger i arktiske farvann der vannet er lagdelt og der vi finner indre bølger som drives av tidevann og vind. Disse bølgene driver bølgedrift på tilsvarende måte som i overflaten. Vi begrenser oss til indre bølger fanget nær kyst eller skrånende havbunn. Slike bølger forflytter seg i en bestemt retning, slik at den tilhørende bølgedriften vil kunne transportere biologisk materiale, forurensende partikler eller andre oppløste substanser systematisk over store avstander over tid. I delvis lukkede bassenger, som fjorder, vil dessuten bølgedriften kunne drive en sirkulasjon i hele fjorden. Ved bruk av realistiske verdier viser vi at den reelle bølgetransporten kan være signifikant, noe som ikke er dokumentert tidligere for disse bølgene. Vi ser på to typer fangede bølger. Den første og viktigste er den indre Kelvinbølgen, som er fanget av jordrotasjonen, og vil forflytte seg nær kyst med kysten på sin høyre side på den nordlige halvkule. Ved bruk av idealiserte teoretiske modeller beregner vi hvordan driften i denne bølgen blir, når lagdelingen og drivkraften bak er kjent. For typiske verdier i Barentshavet eller Baffin Bay finner vi at den maksimale driften kan være omkring 2-3 cm/s, når bølgen er under is. Uten isdekke blir bølgedriften mindre, siden isens friksjon faktisk bidrar til å dytte middelstrømmen fremover. For å studere den indre Kelvinbølgen nærmere foretar vi målinger og numeriske modellkjøringer i en fjord på Svalbard. Resultatene viser at vi har et tydelig bølgesignal som forflytter seg rundt fjorden. Bølgen er drevet av tidevannet, som strømmer inn og ut av fjorden jevnlig. Bølgedriften har dermed effekt over lang tid, hvilket kan ha store klimatologiske konsekvenser i denne og lignende fjorder. En annen bølgetype vi ser på er fanget av skrånende havbunn. Denne bølgen heter på fint "the Stokes interfacial edge wave", og ligner på overflatevarianten som tegner fine mønstre på sandstranden. Mekanismen som fanger denne bølgen henger sammen med at bølgehastigheten varierer med dypet. Bølgen bøyer av mot grunnere vann, reflekteres og fortsetter slik bortover. Vi ser på tilfellet der vi har et tynt vannlag nær havbunnen, med et veldig tykt lag over. Dette kan sees i Danmarkstredet mellom Island og Grønland, der tungt bunnvann siger sørover langs vestsiden nær kontinentalskråningen. Bølgene vil her kunne gå på skilleflaten mellom vannlagene, nær skråningen. De vil være vanskelig å observere, og estimat av bølgedriften vil ikke bli like nøyaktige som for Kelvinbølgen. Likevel, våre teoretiske estimater gir en signifikant drift med nær samme størrelsesorden som den indre Kelvinbølgen

Tidally induced internal motion in an Arctic fjord

The internal response in a stratified, partially enclosed basin subject to semi-diurnal tidal forcing through a narrow entrance is investigated. The site is located above the critical latitude where linear internal waves of lunar semi-diurnal frequency are not permitted to propagate freely. Generation and propagation of tidally induced internal Kelvin waves are studied, for baroclinically sub- and supercritical conditions at the mouth of the fjord, using a non-linear 3-D numerical model in an idealized basin and in Van Mijenfjorden, Svalbard, using a realistic topography. The model results are compared to observations of hydrography and currents made in August 2010. Results from both the model and measurements indicate the presence of internal Kelvin waves, even when conditions at the fjord entrance are supercritical. The entrance of Van Mijenfjorden is split into two sounds. Sensitivity experiments by closing each sound separately reveal that internal Kelvin waves are generated at both sounds. When the conditions are near supercritical, a wave pulse propagates inward from the fjord entrance at the beginning of each inflow phase of the tidal cycle. The leading crest is followed by a series of smaller amplitude waves characterized as non-linear internal solitons. However, higher model resolution is needed to accurately describe the influence of small-scale mixing and processes near the sill crest in establishing the evolution of the flow and internal response in the fjord

Value of the Copernicus Arctic Regional Reanalysis (CARRA) in representing near-surface temperature and wind speed in the north-east European Arctic

The representation of 2-m air temperature and 10-m wind speed in the high-resolution (with a 2.5-km grid spacing) Copernicus Arctic Regional Reanalysis (CARRA) and the coarser resolution (ca. 31-km grid spacing) global European Center for Medium-range Weather Forecasts fifth-generation reanalysis (ERA5) for Svalbard, northern Norway, Sweden and Finland is evaluated against observations. The largest differences between the two reanalyses are found in regions with complex terrain and coastlines, and over the sea ice for temperature in winter. In most aspects, CARRA outperforms ERA5 in its agreement with the observations, but the value added by CARRA varies with region and season. Furthermore, the added value by CARRA is seen for both parameters but is more pronounced for temperature than wind speed. CARRA is in better agreement with observations in terms of general evaluation metrics like bias and standard deviation of the errors, is more similar to the observed spatial and temporal variability and better captures local extremes. A better representation of high-impact weather like polar lows, vessel icing and warm spells during winter is also demonstrated. Finally, it is shown that a substantial part of the difference between reanalyses and observations is due to representativeness issues, that is, sub-grid variability, which cannot be represented in gridded data. This representativeness error is larger in ERA5 than in CARRA, but the fraction of the total error is estimated to be similar in the two analyses for temperature but larger in ERA5 for wind speed

Polar low variability and future projections for the Nordic and Barents Seas

Polar lows are intense mesoscale cyclones occurring during winter over open sea areas in certain polar sub‐regions. Due to their small size, they are not explicitly represented in present global climate models or Earth system models. In this study 18 members of the CESM Large Ensemble were dynamically downscaled to ∼12 km horizontal mesh width using the quasi‐hydrostatic ALARO model within the HARMONIE script system in climate mode (HCLIM‐ALARO). The domain covers the Nordic and Barents Seas. One historical and two future time‐periods were selected. For validation, the ERA‐Interim reanalysis was also downscaled. A cyclone‐tracking algorithm was used to identify tracks of individual polar lows. Their frequency of occurrence, lifetime, and maximum relative vorticity were estimated. Relative to ERA‐Interim, the historical frequency of occurrence of polar lows was slightly overestimated in the Nordic Seas and underestimated in the Barents Sea, which is likely due to positive biases in sea‐surface temperature and sea‐ice concentration. For future climate projections, the regions of polar low genesis are diagnosed to move northwards in accordance with the sea‐ice retreat. In the Nordic Seas, the number of polar lows decreases at the beginning of the season, while there is an increase in March. In the Barents Sea, a February–April increase in the occurrence of polar lows is seen

Evaluation of a sub-kilometre NWP system in an Arctic fjord-valley system in winter

Terrain challenges the prediction of near-surface atmospheric conditions, even in kilometre-scale numerical weather prediction (NWP) models. In this study, the ALADIN-HIRLAM NWP system with 0.5 km horizontal grid spacing and an increased number of vertical levels is compared to the 2.5-km model system similar to the currently operational NWP system at the Norwegian Meteorological Institute. The impact of the increased resolution on the forecasts’ ability to represent boundary-layer processes is investigated for the period from 12 to 16 February 2018 in an Arctic fjord-valley system in the Svalbard archipelago. Model simulations are compared to a wide range of observations conducted during a field campaign. The model configuration with sub-kilometre grid spacing improves both the spatial structure and overall verification scores for the near-surface temperature and wind forecasts compared to the 2.5-km experiment. The subkilometre experiment successfully captures the wind channelling through the valley and the temperature field associated with it. In a situation of a cold-air pool development, the sub-kilometre experiment has a particularly high near-surface temperature bias at low elevations. The use of measurement campaign data, however, reveals some encouraging results, e.g. the sub-kilometre system has a more realistic vertical profile of temperature and wind speed, and the surface temperature sensitivity to the net surface energy is closer to the observations. This work demonstrates the potential of sub-kilometre NWP systems for forecasting weather in complex Arctic terrain, and also suggests that the increase in resolution needs to be accompanied with further development of other parts of the model system

Evaluation of a sub-kilometre NWP system in an Arctic fjord-valley system in winter

Terrain challenges the prediction of near-surface atmospheric conditions, even in kilometre-scale numerical weather prediction (NWP) models. In this study, the ALADIN-HIRLAM NWP system with 0.5 km horizontal grid spacing and an increased number of vertical levels is compared to the 2.5-km model system similar to the currently operational NWP system at the Norwegian Meteorological Institute. The impact of the increased resolution on the forecasts’ ability to represent boundary-layer processes is investigated for the period from 12 to 16 February 2018 in an Arctic fjord-valley system in the Svalbard archipelago. Model simulations are compared to a wide range of observations conducted during a field campaign. The model configuration with sub-kilometre grid spacing improves both the spatial structure and overall verification scores for the near-surface temperature and wind forecasts compared to the 2.5-km experiment. The subkilometre experiment successfully captures the wind channelling through the valley and the temperature field associated with it. In a situation of a cold-air pool development, the sub-kilometre experiment has a particularly high near-surface temperature bias at low elevations. The use of measurement campaign data, however, reveals some encouraging results, e.g. the sub-kilometre system has a more realistic vertical profile of temperature and wind speed, and the surface temperature sensitivity to the net surface energy is closer to the observations. This work demonstrates the potential of sub-kilometre NWP systems for forecasting weather in complex Arctic terrain, and also suggests that the increase in resolution needs to be accompanied with further development of other parts of the model system

Evaluation of a sub-kilometre NWP system in an Arctic fjord-valley system in winter

Terrain challenges the prediction of near-surface atmospheric conditions, even in kilometre-scale numerical weather prediction (NWP) models. In this study, the ALADIN-HIRLAM NWP system with 0.5 km horizontal grid spacing and an increased number of vertical levels is compared to the 2.5-km model system similar to the currently operational NWP system at the Norwegian Meteorological Institute. The impact of the increased resolution on the forecasts’ ability to represent boundary-layer processes is investigated for the period from 12 to 16 February 2018 in an Arctic fjord-valley system in the Svalbard archipelago. Model simulations are compared to a wide range of observations conducted during a field campaign. The model configuration with sub-kilometre grid spacing improves both the spatial structure and overall verification scores for the near-surface temperature and wind forecasts compared to the 2.5-km experiment. The sub-kilometre experiment successfully captures the wind channelling through the valley and the temperature field associated with it. In a situation of a cold-air pool development, the sub-kilometre experiment has a particularly high near-surface temperature bias at low elevations. The use of measurement campaign data, however, reveals some encouraging results, e.g. the sub-kilometre system has a more realistic vertical profile of temperature and wind speed, and the surface temperature sensitivity to the net surface energy is closer to the observations. This work demonstrates the potential of sub-kilometre NWP systems for forecasting weather in complex Arctic terrain, and also suggests that the increase in resolution needs to be accompanied with further development of other parts of the model system