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21.
Laura Tuomi Olga Vähä-Piikkiö Pekka Alenius Jan-Victor Björkqvist Kimmo K. Kahma 《Ocean Dynamics》2018,68(1):17-33
The Stokes drift is an important component in the surface drift. We used the wave model WAM to evaluate the mean values and exceedance probabilities of the surface Stokes drift in the Baltic Sea. As there is no direct way to verify the accuracy of the modelled Stokes drift, we compared the bulk parameters calculated by the wave model against buoy measurements to ensure the quality of the wave hindcast. Furthermore, we evaluated the surface Stokes drift from measured wave spectra to assess the accuracy of the modelled surface Stokes drift. The importance of the Stokes drift as a component of the total surface drift was evaluated by calculating the hindcast mean values and percentiles of the surface Stokes drift. The mean values were between 0.08 and 0.10 ms?1 in the open sea areas, thus being of the same order of magnitude as the mean wind shear currents. The highest values of the surface Stokes drift were slightly larger than 0.6 ms?1. The comparison of modelled Stokes drift values to estimates obtained from measured spectra suggests that the mean values are well represented by the model. However, the higher modelled values are most likely slightly too large because the wave energy was overestimated during high wind situations in some of the sub-basins, such as the Gulf of Finland. A comparison to a drifter experiment showed that use of the Stokes drift improves the estimate of both the drift speed and the direction in the Gulf of Finland. Parameterised methods to evaluate the Stokes drift that are used, e.g. in currently available Baltic Sea drift models, overestimate the smaller values (under 0.3 ms?1) and underestimate the larger values of the Stokes drift compared to the values calculated by the wave model. The modelled surface Stokes drift direction mostly followed the forcing wind direction. This was the case even in the Gulf of Finland, where the direction of the wind and the waves can differ considerably. 相似文献
22.
Kirsti Jylhä Stefan Fronzek Heikki Tuomenvirta Timothy R. Carter Kimmo Ruosteenoja 《Climatic change》2008,86(3-4):441-462
Changes in indices related to frost and snow in Europe by the end of the twenty-first century were analyzed based on experiments
performed with seven regional climate models (RCMs). All the RCMs regionalized information from the same general circulation
model (GCM), applying the IPCC-SRES A2 radiative forcing scenario. In addition, some simulations used SRES B2 radiative forcing
and/or boundary conditions provided by an alternative GCM. Ice cover over the Baltic Sea was examined using a statistical
model that related the annual maximum extent of ice to wintertime coastal temperatures. Fewer days with frost and snow, shorter
frost seasons, a smaller liquid water equivalent of snow, and milder sea ice conditions were produced by all model simulations,
irrespective of the forcing scenario and the driving GCM. The projected changes have implications across a diverse range of
human activities. Details of the projections were subject to differences in RCM design, deviations between the boundary conditions
of the driving GCMs, uncertainties in future emissions and random effects due to internal climate variability. A larger number
of GCMs as drivers of the RCMs would most likely have resulted in somewhat wider ranges in the frost, snow and sea ice estimates
than those presented in this paper. 相似文献
23.
The annual nature of organic laminations in the sediment from a small lake, Lampellonjärvi (61°04'N; 25°04'E), was determined. The core was obtained using an in situ freezing method and the laminations were checked by a detailed microscopical analysis of adhesive tape preparations. A series of six radiocarbon dates were obtained for levels in the core which had previously been dated by means of varve counts to ages between A.D. 182 and AD. 1513. The radiocarbon dates were between 547 and 1525 years older. Two additional 14 C dates from the lower part of the same profile (ca. 1880 B.C. and ca. 3100 B.C.), however, gave expected results. Erosion of old organic terrestrial material due to agricultural activity in the surroundings of the lake was assumed to have been the cause of abnormally old radiocarbon ages. Dates from the pre-agricultural period had a deviation from varve years similar to the difference between tree rings, historical documents and radiocarbon dating recorded in other studies. 相似文献