1,720,975 research outputs found
A Comparison of Parameterized, Simulated and Measured Turbulent Mixing in the Gulf of Finland, the Baltic Sea
No full textThree time series of shear microstructure measurements (duration 13, 24 and 14 h respectively) have been performed in 3 different wind forcing regimes as well as in 3 different background density stratification and current velocity shear situations at the entrance to the Gulf of Finland, in July 1998. Vertical shear of current velocity was enhanced by near-inertial waves during the first (A1) and the third (A3) time series, the background stratification weakened continuously from A1 toward A3. We compared eddy diffusivities based on the Richardson number parameterization and eddy diffusivities simulated using the two equation k-e turbulence closure (General Ocean Turbulence Model, GOTM) with the �measured� ones. For two out of the three time series the eddy diffusivities calculated via the Richardson number parameterization and via the k-e model simulation agreed well with the experimental data. However, summing up the discrepancy of all three time series both methods resulted in a remarkable and consistent bias against the measured eddy diffusivity. On the contrary, the calculations with a new parameterization scheme, which considers the internal wave kinetic energy fitted well for all three time series. Similarly the modified k-e simulations which considered the internal wave energy level matched better the measured profiles.JRC.DDG.H.3 - Global environement monitorin
An alternative parameterization of eddy diffusivity in the Gulf of Finland based on kinetic energy of high frequency internal wave band
No full textIn July 1998, Three time series of shear microstructure measurements (duration 13, 24 and 14 h respectively) were performed in 3 different wind forcing regimes as well as in 3 different background density stratification and current velocity shear situations at the entrance to the Gulf of Finland. We compared the �measured� eddy diffusivities with the diffusivities parameterized using the Richardson number, the Brunt-Väisäla frequency and with the diffusivities simulated using the two equation k-e turbulence closure. Summing up the discrepancy of all three time series the methods applied resulted in a remarkable and consistent bias against the measured eddy diffusivity. Contrary to this result, the calculations with the new suggested parameterization scheme, which accounts for the internal wave kinetic energy of the super-inertial frequency band, fitted well for all three time series. Similarly, the modified k-e simulations accounting for the super-inertial internal wave energy matched better the measured profiles.JRC.DDG.H.3 - Global environement monitorin
Yet Another Assessment of Climate Change in the Baltic Sea Area: Breakpoints in Climate Time Serie
No full textThe aim of the present study is to assess changes in the Baltic Sea climate based on different available meteorological data sources (ERA40 and ERA-INTERIM) and various published Baltic Sea climate indices. This regional assessment will be presented in relation to global climate change and assessments available from the literature.
The climate of the Baltic Sea which is located between 50N and 70N is mainly influenced by the competition of westerly humid air flow and easterly continental type air masses and is therefore highly variable. We are investigating air temperature, wind speed, cloud cover, solar radiation and precipitation. Comparisons to climate indices of general relevance as the Baltic ice cover will be conducted.
Using regression analysis we could confirm the following basic trends, increase in air temperature, increase in precipitation, increase in cloudiness. The increase in air temperature in the Baltic Sea area (0.02K/year) is much more rapid then the warming trend for the global air temperature (0.005K/year). The increase in cloudiness has resulted in an effective reduction of incoming solar radiation therefore the accelerated warming is not a result of increased solar radiation, but likely due to an increased net long wave radiation input. Further it has to be mentioned that not all available data sets confirmed the trend in cloudiness, ERA40 data show a nonsignificant decrease instead. No clear trend in the wind velocities could be detected, but wind velocities from ERA40 reanalysis project show an insignificant increase in wind speeds.
Results from model runs with the GETM model (General Estuarine Transport Model, http://getm.eu) show sea surface warming consistent with the increase in heat flux forcing and with satellite observations. The warmer sea surface without an adequate warming in the deeper parts results in a much stronger vertical density stratification and consequently to reduced vertical mixing. A more thorough inspection of the available regional and global data provides some reasonable doubt concerning the application of least square regression analysis to the available time series. Indeed it can be shown by a test based on the F statistics that most of the analyzed time series cannot be considered as stationary and therefore drawing simple regression lines trough these datasets is statistically incorrect. Testing for structural breakpoints in these time series reveals for many investigated parameters and also for many tested climate indices the existence of such breakpoints in the 70-80ties of the last century. Therefore it has to be concluded that the simple trend estimation for many climate parameters is statistically incorrect. Instead for statistical investigations it has to be assumed that there exist either 2 different climate states with either 2 different means or alternatively with 2 different trends which have to be estimated separately.JRC.DDG.H.3 - Global environement monitorin
The Regime Shift in the Baltic Sea area – caused by the change of the NAO sign?
No full textIn the Baltic Sea area during the late 1980s air and sea surface temperature increased. A longer growing season and increases in phytoplankton biomass as well as changes in the zooplankton and fish communities accompanied this. These changes are supposed to represent a regime shift in the ecology of the central Baltic Sea that could have been caused by a related sign change in the North Atlantic Oscillation (NAO) index. But the NAO has changed sign more frequently and most of the time no corresponding regime shift did occur therefore we should check this hypothesis more thorough.
We investigated additional to the NAO a broad range of variables including air temperature, sea surface temperature, phytoplankton and zooplankton biomass, fish abundance and oxygen. The time series data are analyzed with respect to autocorrelation, linear trends and the occurrence of breakpoints whereby always tests for statistical significance are conducted.
Tests for structural breakpoints in these time series reveal for some of the investigated variables the existence of such breakpoints in the 70-80ties of the last century. But surprisingly in many physical and biological variables and in the most common climate indices no clear breakpoints can be identified. Specifically the change of the NAO sign around 1987, which is proposed to be the reason for an ecological regime shift in the Baltic Sea is not statistically significant. The coincidence of the sign change and an ecological regime shift could just be pure random, but even the evidence for a clear ecological regime shift is missing.
In summary we strongly advocate to apply sound statistical procedures for detecting regime shifts instead of eye fitting and qualitative descriptions. A strong hypothesis like the postulation of a regime shift 1987 caused by the change in the NAO sign does require strong evidence, but it seems we might not have that.JRC.H.1 - Water Resource
Regime Shifts and Trends in the Baltic Sea area: a statistical approach
No full textDuring the late 1980s air and sea surface temperature started to increase in the Baltic Sea area. Sea ice broke up earlier and overall ice coverage declined. This resulted in a longer growing season and in increases in phytoplankton biomass as well as changes in the zooplankton and fish communities. These changes are often considered to represent a regime shift in the ecology of the central Baltic Sea, which according to some authors could be caused by a related sign change in the North Atlantic Oscillation (NAO).
The aim of this investigation is to inspect relevant physical and ecosystem variables for trends and structural breakpoints in the concerned time series. We use sound statistical methods that include confidence tests at the 5% error probability level.
For this purpose we investigated a broad range of physical variables including air temperature, wind speed, sea surface temperature, ice cover, precipitation, oxygen as well as ecosystem variables such as phytoplankton biomass, zooplankton and several fish species from the Baltic Sea region.
Most of these time series do exhibit a statistical significant linear trend. But tests for structural breakpoints in these time series reveal only for some investigated variables the existence of a breakpoint in the 70-80ties of the last century. In contradiction to the seemingly well established “regime shift” hypothesis in the Baltic Sea no clear breakpoint can be identified in many physical variables and also not in most ecosystem variables including fish. Finally also the proposed reason for the supposed ecological regime shift in the Baltic Sea, the change of the NAO sign at around 1987, is not statistically significant.
Therefore we conclude that the Baltic Sea regime shift does remain a hypothesis and most physical and ecosystem time series data from the Baltic Sea region are statistically best described by a linear trend.JRC.H.1 - Water Resource
Global climate change: Did we pass a tipping point?
No full textSystems that have an internal feedback mechanism can principally behave non-linear. The climate on earth is such a complex system with internal feedback and a non-linear characteristic should therefore be expected. Nevertheless it is common practice to publish linear trends from many global data sets without considering their relevance. The classical example is the often cited (IPCC) global warming trend of 0.05°C/decade for air temperature during the last 150 years. The inadequacy of this linear trend model over such a long period is obvious when looking at the difference of the observed 2009 mean temperature to that estimated from the calculated linear trend. The trend calculation gives an underestimation of about 48% (~0.36°C to low, compared to a total change of 0.75°C). A similar inspection of several global atmospheric and oceanographic data time series provides reasonable doubt concerning the validity of the linear model for century long time series. Using a test for breakpoints we can show that most of the analyzed global climate time series contain statistical significant structural changes (breaks in the mean or in the slope of a linear regression). We will reveal the existence of breakpoints for most investigated parameters at the end of the 70ties of the last century. Breakpoints detected at a comparable time in many different regional and global climate variables are a strong indication for a corresponding regime shift in the state of the global climate. The enormous difference between trends calculated with and without consideration of breakpoints invalidates predictions based only on overall linear trends. Despite these statistical indications for the existence of a tipping point we cannot infer the underlying process dynamics from the statistics alone. The final confirmation has to come from an accurate and complete geophysical description of the climate system, a difficult and complex task still to be done.JRC.H.1 - Water Resource
The Variability of Parameters Controlling the Cyanobacteria Bloom Biomass in the Baltic Sea
No full textThe formation of wintertime excess of dissolved inorganic phosphorus concentration (eDIP) in the surface layer and its influence to the late-summer diazotrophic cyanobacteria development have been evaluated on the base of 30 years nutrients and river runoff monitoring data in different Baltic Sea sub-basins, relatively short satellite chlorophyll a (Chl a) and modelled upper mixed layer depth (UMLD) data. The processes responsible for the formation of the surface and bottom phosphate concentration pools in winter, the exchange processes between the pools and the amount of phosphate transported from the bottom pool to the surface are not fully understood. We did not find any significant correlation between the wintertime bottom layer (BL) DIP and upper mixed layer (UML) DIP for the deeper Baltic Sea sub-basins. Instead, we found a significant correlation between UML DIP and UMLD in winter. With the present considerable trend of decrease of the dissolved inorganic nitrogen concentration (DIN) in the Baltic Proper mainly the DIP variability determines the excess phosphate, which is regarded as the main phosphate source for the late summer cyanobacterial blooms in theJRC.H.3 - Global environement monitorin
Observation, Parameterization and Simulation of Turbulent Mixing in the Gulf of Finland, the Baltic Sea
No full textAbstract-Three time series of shear microstructure measurements (duration 13, 24 and 14 h respectively) were performed in 3 different wind forcing regimes as well as in 3 different background density stratification and current velocity shear situations at the entrance to the Gulf of Finland, in July 1998. Vertical shear of current velocity was enhanced by near-inertial waves during the first (A1) and third (A3) time series. We compared a Richardson number based parameterization and an estimation using the two equation k-e turbulence closure (General Ocean Turbulence Model, GOTM) with �measured� eddy diffusivities. For two out of the three time series eddy diffusivities calculated via a Richardson number parameterization and via simulation using the k-e model agreed well with the experimental data. However, in the case of relatively high current velocity shear and weak background density stratification (time series A3) both applied methods resulted in a remarkable discrepancy against the measured eddy diffusivity. On the contrary, calculations with a new parameterization scheme, which considers the internal wave kinetic energy fitted well for all three time series. Similarly the modified k-e simulations which considered the internal wave energy level better matched the measured profiles.JRC.H.3 - Global environement monitorin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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