Browsing by Subject "Geophysics of the Hydrosphere"

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  • Erkkilä, Anttoni (Helsingin yliopisto, 2021)
    Gulf of Bothnia was simulated with NEMO sea model and LIM3 sea ice model. The results were used to count ice area, ice thickness, ice season length and the dates for freezing ang thawing. Results contained years from 1975 to 2059 but only years from 2006 were mostly used. Before 2006 the model was forced with a statistical history run. It was compared to thickness observations and the ice model was considered reliable enough. Atmospheric forcings became from three earth system models and two emission scenarios. With smaller emissions results about ice conditions and its trends vary between model runs with different focings a lot more than with higher emissions. In many case a propability distribution was used on results unlike on earlier similar researches which have mostly used only medians of ice parameters. Propability distribution for ice area was made using cumulative Gumbel probability distribution which enabled counting time periods longer than the time series. It also increased the reliability of extreme results. According to the results the northen part of the Bay of Bothnia freezes in every winter at 2050s. Ice thickness is 80cm at the most and median is 50cm. In almost every result southern part of the Bothnian Sea has over 50% chance for ice to occur and the median of thickness is more than 10cm. Length of the ice season close to 2050s can exeed 150 days often at the northen Bay of Bothnia but elsewhere it is closer to 100 days and varies a lot. The counted propability distributions for ice area let us suspect that a part of the Bay of Bothnia would remain unfreezed at least once every 30 years or more propably every 20 years in years from 2006 to 2059. In contradiction, not much can be said about large areas, because results vary a lot about wheter the whole Gulf of Bothnia can be expected to freeze close to 2050s.
  • Kuittinen, Salla (Helsingin yliopisto, 2021)
    Auringon säteilyn voimakkuus vaihtelee vuodenaikojen mukaan Suomessa. Tämän seurauksena järvien kerrostuneisuudessa tapahtuu muutoksia. Värriön luonnonpuistossa järvien tutkimus on ollut vähäistä, ja lämpötilaseurantaa ei aikaisemmin alueen järvissä ole tehty ympärivuotisesti. Aineiston avulla selvitettiin Kuutsjärven ja Tippakurulammen lämpötilan muutoksia, eroavaisuuksia, päivittäisiä muutoksia sekä lumen vaikutusta kevään lämpenemiseen tarkastelujakson aikana. Tutkimusaineisto on kerätty Värriön luonnonpuiston Kuutsjärvestä ( 67° 44'N ja 29° 36'E) sekä Tippakurulammesta (67° 46'N ja 29° 37'E) vuosina 2009–2012. Molemmissa järvissä oli yksi termistoriketju järven syvimmässä kohdassa. Termistoreita ketjussa oli viisi kappaletta. Tulosten tarkastelussa apuna käytettiin Ilmatieteen laitoksen sää- ja lumihavaintoja. Auringon säteilyn voimakkuus on merkittävin Kuutsjärven ja Tippakurulammen lämpötiloihin vaikuttava tekijä. Päivittäisissä muutoksissa ei ole eroavaisuuksia järvien välillä. Tippakurulampi lämpenee tummempana ja matalampana järvenä pohjaa myöten kesän aikana. Kesän aikana sedimenttiin varastoitunut lämpö purkautuu talvella veteen. Kevään täyskierto on molemmissa järvissä lyhyt ja tapahtuu toukokuun loppupuolella. Syksyn täyskierto on Tippakurulammessa Kuutsjärveä pidempi, ja Tippakurulampi jäähtyy sekoittuneessa tilassa. Eroavaisuudet järvien välillä johtuvat koko- ja värieroista.
  • Ala-Könni, Joonatan (Helsingin yliopisto, 2019)
    Mixing processes under a seasonal ice cover in boreal lakes have received little attention from the physical limnological community. Even though the water is calm under the ice cover, many different phenomena are still able to cause mixing in the water column, which in turn affects the gas fluxes as well as physical and chemical properties of the water. Lakes in the boreal zone are very numerous. Understanding their behaviour helps us predict the effects of climate change in the boreal zone. In my thesis I present the various mechanisms that reign under the ice cover, and attempt to see these mechanisms in action in lake Kuivajärvi. Emphasis was placed on internal waves and the various components of the energy balance that can induce mixing. Data was collected with thermistor chains and a measurement raft between 24.1. – 3.5.2017. Two types of internal waves were observed during the ice-on season of 2016 – 2017. Short period barotropic seiches were observed during the whole ice-on season and transient long period baroclinic seiches were observed on two occasions. Other mixing processes seen in the lake were sediment heating in the dead of winter, penetrative convection caused by short wave radiation in the spring and diurnal stratification and mixing during spring caused by the daily heating and nightly cooling. Some mixing under the ice cover was found to depend on the meteorological conditions prevailing over the lake during the previous summer and just before the ice-on in late autumn, while others were more predictable. Long period internal waves and sediment heating are set in motion by meteorological conditions, while the spring mixing and overturn are more stable, due them being more a function of the orbital mechanics of our planet than the prevailing weather. Varying surface conditions of the lake ice cover make the measurement of especially the surface temperature complicated. Snow and ice are under a continuous metamorphosis due to the weather. This makes surface emissivity difficult to estimate, causing significant errors in the measurement of the outgoing longwave radiation. This in turn causes problems in defining the surface temperature from it. Also, the precipitation heat flux is difficult to estimate due to the lack of knowledge on the surface temperature.
  • Rautiainen, Laura (Helsingin yliopisto, 2020)
    Ocean reanalysis products (ORAs) can provide information on the state of the ocean. Although the different data sources, model configurations, forcing choices and assimilation methods cause the ORAs to deviate from each other, the ensemble approach has been previously found to produce realistic mean states. This raises the question if ORAs could be used for studying temporal and spatial changes in the Arctic Ocean, where measurements are generally sparse. Such study has not been previously published. In this thesis, the changes in the hydrography of the Arctic Ocean are examined over the previous decades based on selected ORAs. Eleven ORAs, TOPAZ4, C-GLORS025v5, ECDA3, GECCO2, GLORYS2v4, GloSea5-GO5, MOVE-G2i, ORAP5, SODA3.3.1, UR025.4 and ORAS5, were chosen for this study due to their overlap over 1993–2010 and the multimodel ensemble (MMM) was formulated based on the products, excluding ECDA3. The data were divided into depth layers and layer-average salinities and temperatures were used to calculate basin-average anomaly time series and trends to study the observed temporal changes. Per-grid trends were also produced to study both spatial and temporal changes in more detail. To assess their reliability, trends from the MMM and individual ORAs were compared to an observational product, EN4.2.0.g10 and the variability in the products and the MMM was assessed using statistical measures. The Eurasian Basin was found to be warming across all layers (up to 0.3 ◦ C decade −1 ) accompanied by salinification, except for localised cooling in the top 100 meters in the western basin, near the Fram Strait (-0.2 ◦ C decade −1 ). This indicates additional heat uptake by the surface 0–100 meters and also increasing heat and salinity content of the AW inflow, while the transport of sea ice out of the AO has increased. The Amerasian Basin, on the other hand, showed a strong freshening trend culminating at the Beaufort Gyre. This is most likely due to the anticyclonic wind forcing and increasing freshwater inflow to the Beaufort Sea. The Amerasian Basin also showed a warming trend in the 300–700 m layers but a cooling trend in the 100–300 m layer north of the Chukchi Sea. The ensemble approach worked well in dampening the extremities of singular ORAs, but some trends observed in the literature were missed due to disagreements between ORAs, especially in the Fram Strait and Beaufort Sea, which suggest that further improvements in both models and measurements are needed in those areas. Furthermore, improvements in deep ocean observations, how models handle the deeper ocean and assimilation methods are needed in order to study trends in the deeper depths in the AO. All in all, as the improvements come, the ORA MMM shows great potential for studies in the AO.
  • Arola, Aleksi (Helsingin yliopisto, 2021)
    Freshwater ecosystems are an important part of the carbon cycle. Boreal lakes are mostly supersaturated with CO2 and act as sources for atmospheric CO2. Dissolved CO2 exhibits considerable temporal variation in boreal lakes. Estimates for CO2 emissions from lakes are often based on surface water pCO2 and modelled gas transfer velocities (k). The aim of this study was to evaluate the use of a water column stratification parameter as proxy for surface water pCO2 in lake Kuivajärvi. Brunt-Väisälä frequency (N) was chosen as the measure of water column stratification due to simple calculation process and encouraging earlier results. The relationship between N and pCO2 was evaluated during 8 consecutive May–October periods between 2013 and 2020. Optimal depth interval for N calculation was obtained by analysing temperature data from 16 different measurement depths. The relationship between N and surface pCO2 was studied by regression analysis and effects of other environmental conditions were also considered. Best results for the full study period were obtained via linear fit and N calculation depth interval spanning from 0.5 m to 12 m. However, considering only June–October periods resulted in improved correlation and the relationship between the variables more closely resembling exponential decay. There was also strong inter-annual variation in the relationship. The proxy often underestimated pCO2 values during the spring peak, but provided better estimates in summer and autumn. Boundary layer method (BLM) was used with the proxy to estimate CO2 flux, and the result was compared to fluxes from both BLM with measured pCO2 and eddy covariance (EC) technique. Both BLM fluxes compared poorly with the EC flux, which was attributed to the parametrisation of k.