Meridional heat transport on a part of the northern hemisphere in November 1947, July 1949, and February 1950

Summary Horizontal flux of enthalpy and latent heat has been computed from synoptic maps over a large part of the northern hemisphere for several isobaric levels during 3 months. The flux of enthalpy and also the flux of latent heat is lowest in July and 3 to 4 times larger in winter. The flux is largest at 850 mb. The large scale eddy conductivity for the enthalpy increases with the horizontal meridional temperature gradient showing that the atmospheric energy exchange processes become more effective when the gradients are strong. From the convergence of the flux values the mean heating of an air column has been computed. This heating has a winter maximum of about 1° C/24 hrs at latitude 60°–70° N. The values for the polar region are considerably lower, indicating a considerable mean descending motion in the region. The correlation between flux or convergence of flux and circulation indices is found to be rather small. There is even a negative correlation between convergence of enthalpy and the zonal index.

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Zusammenfassung Der horizontale Transport von Enthalpie und latenter Wärme wurde auf Grund von Wetterkarten für verschiedene Druckniveaus über einem großen Teil der nördlichen Hemisphäre für drei verschiedene Monate berechnet. Sowohl der Enthalpie- wie der latente Wärmestrom ist am geringsten im Juli und drei- bis viermal größer im Winter; am größten ist er bei 850 mb. Der Großaustauschkoeffizient der Enthalpie nimmt mit dem horizontalen meridionalen Temperaturgradienten zu, was sich durch die Steigerung der atmosphärischen Energieaustauschprozesse bei Verstärkung des Gradienten erklärt. Aus der Strömungskonvergenz wurde die durchschnittliche Erwärmung einer Luftsäule berechnet. Diese Erwärmung zeigt ein Maximum von zirka 1° C/24 Studen im Winter in einer Breite von 60 bis 70° N. Die Werte für das Polargebiet sind beträchtlich niedriger und lassen eine merkliche absinkende Strömung erkennen. Die Korrelation zwischen Strömung oder Strömungskonvergenz und Zirkulationsgrößen hat sich als eher niedrig erwiesen. Die Korrelation zwischen der Konvergenz der Enthalpie und dem Zonalindex ist sogar negativ.

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Résumé Le flux horizontal d'enthalpie et de chaleur latente a été calculé à partir de cartes synoptiques couvrant une grande partie de l'hémisphère nord et cela pendant 3 mois pour plusieurs niveaux isobariques. Le flux d'enthalpie ainsi que celui de chaleur latente est à son minimum en juillet et est 3 à 4 fois plus grand en hiver. Le flux le plus important se trouve au niveau de 850 mb. Le coefficient de la conductivité turbulente de l'enthalpie à grande échelle croît avec le gradient horizontal méridional de température, et montre ainsi que les processus de l'échange d'énergie atmosphérique augmentent avec la force du gradient. L'auteur a calculé le réchauffement moyen d'une colonne d'air en partant de la convergence des valeurs du flux. Ce réchauffement atteint un maximum hivernal d'environ 1° C/24 h à 60°–70° de latitude N. Les valeurs des régions polaires sont notablement plus petites et laissent reconnaître un mouvement descendant considérable dans cette région. La corrélation entre le courant ou la convergence de flux et les valeurs de la circulation s'est avérée plutôt petite. Il y a même une corrélation négative entre la convergence de l'enthalpie et l'index zonal.

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With 6 Figures.     

Spatial variability of soil water content in the covered catchment at Gårdsjön,Sweden

The spatial variability of soil water content was investigated for a 6300 m² covered catchment on the Swedish west coast. The catchment podzol soil is developed in a sandy—silty till with a mean depth of 43 cm and the dominant vegetation is Norway spruce. The acid precipitation is removed by a plastic roof and replaced with lake water irrigated under the tree canopies. On two occasions, in April and May 1993, TDR measurements were made at 57–73 points in the catchment using 15 and 30 cm long vertically installed probes. The water content pattern at the two dates, which occurred during a relatively dry period, were similar. The range of water content was large, from 5 to 60%. In May 1993 measurements also were made in areas of 10 × 10 m, 1 × 1 m and 0·2 × 0·2 m. The range and standard deviation for the 10 × 10 m area, which apart from a small-scale variability in soil hydraulic properties and fine root distribution also had a heterogeneous micro- and macro-topography, was similar to the range and standard deviation for the catchment. The 1 × 1 m and 0·2 × 0·2 m areas had considerably lower variability. Semi-variogram models for the water content had a range of influence of about 20 m. If data were paired in the east-–west direction the semi-variance reflected the topography of the central valley and had a maximum for data pairs with internal distances of 20–40 m. The correlation between soil water content and topographic index, especially when averaged for the eight topographically homogeneous subareas, indicated the macro-topography as the cause of a large part of the water content variability.     

Soil frost effects on soil water and runoff dynamics along a boreal transect: 2. Simulations

A physically based SVAT‐model was tested with soil and snow physical measurements, as well as runoff data from an 8600 m² catchment in northern Sweden in order to quantify the influence of soil frost on spring snowmelt runoff in a moderately sloped, boreal forest. The model was run as an array of connected profiles cascading to the brook. For three winter seasons (1995–98) it was able to predict the onset and total accumulation of the runoff with satisfactory accuracy. Surface runoff was identified as only a minor fraction of the total runoff occurring during short periods in connection with ice blocking of the water‐conducting pores. Little surface runoff, though, does not mean that soil frost is unimportant for spring runoff. Simulations without frost routines systematically underestimated the total accumulated runoff. The possibility of major frost effects appearing in response to specific combinations of weather conditions were also tested. Different scenarios of critical initial conditions for the winter, e.g. high water saturation and delayed snow accumulation leading to an increased frost penetration, were tested. These showed that under special circumstances there is potential for increased spring runoff due to soil frost. Copyright © 2001 John Wiley & Sons, Ltd.     

Altered Benthic Prey-Availability Due to Episodic Oxygen Deficiency Caused by Drifting Algal Mats

Abstract. Large quantities of drifting algal mats have become a common phenomenon on shallow sandy bottoms in the northern Baltic Sea. A decomposing algal mat rapidly induces hypoxic or anoxic conditions in the underlying sediment and interferes with the normal living-mode of the benthos, i.e., mobility, feeding, and predator-prey relationships. Field surveys have shown that bivalves, such as Macoma balthica , avoid the unfavourable anoxic conditions by emerging at the sediment surface. Due to low reburrowing rates these bivalves are exposed to epibenthic predators once the drift algae disperse. A series of aquarium experiments were conducted to test for differences in the survival of M. balthica when exposed to continuous predation without prior stress and short-term predation after hypoxic stress induced by algae (= 20% 02-saturation). Sublethally stressed adult M. balthica , that normally are outside the size-range of the isopod predator Saduria entomon , became significantly more susceptible to predation when lying exposed at the sediment surface. The same effects were found with two other predators, the brown-shrimp Crangon crungon and the flounder Platichthysflesus . Drift algal mats induced an escape-reaction in the infauna more rapidly than mere hypoxia. This response is affected by temperature. which further underlines the importance of drift algae influencing prey availability. This paper discusses the role of the drift algae as a regulating factor for the infauna and demonstrates the importance of external structuring factors, such as hypoxic stress and algal mats, on predator-prey interactions.     

The Bothnian Sea ice stream: early Holocene retreat dynamics of the south‐central Fennoscandian Ice Sheet

The Gulf of Bothnia hosted a variety of palaeo‐glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice‐sheet divide to a major corridor of marine‐ and lacustrine‐based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo‐ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high‐resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo‐ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo‐ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south‐central Bothnian Sea. It was activated after initial margin retreat across the Åland sill and into the Bothnian basin, and the exclusive association of the ice‐stream pathway with crevasse squeeze ridges leads us to interpret a short‐lived stream event, under high extension, followed by rapid crevasse‐triggered break‐up. We link this event with a c. 150‐year ice‐rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine‐based ice‐sheet sector.     

Spatial variation of wetlands and flux of dissolved organic carbon in boreal headwater streams

In order to investigate the relation between water chemistry and functional landscape elements, spatial data sets of characteristics for 68 small (0·2–1·5 km²) boreal forest catchments in western central Sweden were analysed in a geographical information system (GIS). The geographic data used were extracted from official topographic maps. Water sampled four times at different flow situations was analysed chemically. This paper focuses on one phenomenon that has an important influence on headwater quality in boreal, coniferous forest streams: generation and export of dissolved organic carbon (DOC). It is known that wetland cover (bogs and fens) in the catchment is a major source of DOC. In this study, a comparison was made between a large number of headwater catchments with varying spatial locations and areas of wetlands. How this variation, together with a number of other spatial variables, influences the DOC flux in the streamwater was analysed by statistical methods. There were significant, but not strong, correlations between the total percentages of wetland area and DOC flux measured at a medium flow situation, but not at high flow. Neither were there any significant correlations between the percentage of wetland area connected to streams, nor the percentage of wetland area within a zone 50 m from the stream and the DOC flux. There were, however, correlations between catchment mean slope and the DOC flux in all but one flow situations. This study showed that, considering geographical data retrieved from official sources, the topography of a catchment better explains the variation in DOC flux than the percentage and locations of distinct wetland areas. This emphasizes the need for high‐resolution elevation models accurate enough to reveal the sources of DOC found in headwater streams. Copyright © 2007 John Wiley & Sons, Ltd.     

Competitive effects of fish in structurally simple habitats: perch,invertebrates, and goldeneye in small boreal lakes

We determined the associations between Eurasian perch Perca fluviatilis, invertebrates, and common goldeneye Bucephala clangula in boreal lakes. Our aim was to test the hypothesis that perch play a controlling role in small, oligotrophic, acidic, and poorly vegetated boreal lakes, affecting both invertebrate numbers and community structure. In addition, we predicted that perch impact lake usage by goldeneye. In the observational part of our study, we first explored the association between perch, invertebrates and goldeneye (pairs and broods per shore km) in 18 boreal, poorly vegetated lakes. Perch densities were associated negatively with invertebrate abundance and lake usage by goldeneye broods. In the experimental part in three fish-free lakes, we found that upon successful perch introductions, the number and biomass of invertebrates, the proportion of large dytiscids, and lake usage by goldeneye broods significantly decreased. We conclude that perch apparently plays a key role as a predator of invertebrates in boreal lakes with few aquatic macrophytes. It is evident that perch can strongly affect their prey populations and communities, and this predation may have an indirect effect on species, e.g. goldeneye, that consume the same prey.