Jüri Elken

A view of the Canary Basin thermocline circulation in winter

During January and February 1989 the recirculation of the subtropical gyre in the eastern North Atlantic was surveyed with a three-ship experiment. The analysis of hydrographic measurements and velocity data from a shipboard acoustic Doppler current profiler reveals the synoptic-scale circulation patterns and water mass distributions in the Canary Basin. The geostrophic transport stream function estimated with a horizontally varying reference level of no motion highlights the major currents in three layers representing the vertical structure of the horizontal circulation. The classical circulation scheme is shown by the stream function in the upper 200 m: the Azores, Canary, and North Equatorial currents. Unlike the deep-penetrating Azores Current, the Canary Current and the North Equatorial Current are restricted to the upper 200 m. Both carry North Atlantic Central Water along the water mass boundary with South Atlantic Central Water. South Atlantic Central Water flows through the passage between the Cape Verde archipelago and Africa via narrow currents into the area north of 14.5°N. At the southern edge of the subtropical gyre we identify an eastward flow of Antarctic Intermediate Water between 700 and 1200 m.

Estuarine transport versus vertical movement and mixing of water masses in the Gulf of Finland (Baltic Sea)

Weekly mapping of vertical temperature and salinity fields was carried out across the Gulf of Finland in summer 2006 and spring 2007. Using successive cross-gulf vertical sections of salinity and wind data from the region the variations of estuarine and transverse circulation are described. Changes of deep layer phosphate-phosphorus concentrations are found to be related to the described variations in circulation patterns in a season with strong vertical stratification of the water column. Cumulative volume transport estimates were obtained using the results of a 3D baroclinic circulation model (HIROMB). We suggest that the north-easterly winds, which intensify the estuarine circulation and lead to the upwelling events along the southern coast of the Gulf of Finland, could have a major impact to the Gulfpsilas ecosystem by importing more saline and phosphorus rich waters. These events with many-fold more intense upward movement and mixing of deep waters (upward diapycnal transport) could contribute significantly to the ventilation of deep layers of the northern Baltic proper.

Recent Change—Marine Circulation and Stratification

This chapter describes recent change in the circulation and stratification of the Baltic Sea. A recent warming trend in sea-surface waters has been clearly demonstrated by in situ measurements, remote sensing data and numerical models. Trends in sea-surface temperature (SST) for the past three to four decades based on remote sensing data generally agree with trends determined from in situ observations. Models suggest the current warming within the Baltic Sea lies within the range experienced during the past 500 years. The salinity and stratification of the deep waters are strongly linked to the major inflows of North Sea water that occur sporadically and bring high-saline water into the deep layers of the Baltic Sea. The major inflows normally occur during winter and spring and bring cold oxygen-rich waters into the deep basins. Since 1996, large inflows have also occurred during summer, bringing in warm low-oxygen water.

Direct Estimates of the Lateral Eddy Diffusivity in the Gulf of Finland of the Baltic Sea (Based on the Results of Numerical Experiments with an Eddy Resolving Model)

Numerical modeling was applied to study the generation of transversal jet filaments observed in the summer of 1999 after an upwelling event off the northern coast of the Gulf of Finland. An eddy resolving model well reproduces the mesoscale coherent structures observed. It was shown that they represent manifestations of instability of alongshore baroclinic jet currents of an upwelling-downwelling origin. An estimate of the effective lateral eddy diffusivity in the mesoscale coherent structures equal to 500 m2/s was obtained as a result of statistical processing of pseudorandom model fields of the temperature and current velocity.

Sub-regional observing and forecast system for the NE Baltic: Needs and first results

Following the operational observing system development under BOOS and application of local stand-alone coastal circulation models, acquisition of HIROMB forecasts started regularly in 2005. The forecasts, working on-line with the operational observations (automatic coastal stations and recently, also FerryBox), proved very useful in numerous cases, especially during storm surges and broadcasting relevant warnings well in advance of critical floodings. For the intermediate and downstream services that require resolving of detailed patterns of mesoscale and coastal circulation, installation and application of 0.5-mile resolution HIROMB started in 2007. The system is integrated with the Baltic-wide HIROMB and is driven by sub-regional HIRLAM weather data with enhanced resolution, forecasted and delivered by the Estonian Meteorological and Hydrological Institute.

Comparison of current simulations by the state-of-the-art operational models in the Gulf of Finland with ADCP measurements

Current modeling results from two operational models (Baltic-wide HIROMB-BS01 with 1 nautical mile resolution and subregional HIROMB-EST with 0.5 mile resolution) were compared with ADCP measurements at four sites of different topographic and hydrographic background in the Gulf of Finland. Both models predict a reasonable match with the observed subsurface currents in most of the cases, although there are no data to carry out the assimilation of mesoscale oceanographic features. The correlations of the modeled currents with the observed counterparts yield quite often the values above 0.5, both in the low-frequency and high-frequency range. In general, the models tend to generate less current variability (smaller standard deviations) than observed. There is also some site-specific bias of mean currents. Near-bottom and coastal currents are highly constrained by the topography. High current speeds (above 20 cm/s) were measured in the deep layers of the rough-bottom area around a small island. Such currents were in general well simulated by the 0.5 nm model, only slightly biased by the dominating direction. The modeled near-bottom currents are simply transformable into more realistic results if the correct topography is taken into account. Both the observations and the subregional model results revealed similar halocline-intensified EOF modes of vertical current structure, the amplitudes of which were correlated quite well.

BOOS/HIROMB-based marine forecasts in Estonia: Problems, experiences and challenges

In January 2005, a severe storm hit the Baltic Sea region. Estonian towns Pärnu and Haapsalu were flooded with the storm surge waters. Considerable damages and economic loss occurred. Due to the BOOS cooperation, numerical forecasts became available and they were convincingly much more accurate than the forecasts made by the traditional methods. Advantages of modern operational oceanography were clearly demonstrated and Estonian Center of Environmental Investments launched a project to establish HIROMB-based marine forecasts in Estonia. In winter 2006, two severe oil pollution events occurred in the Gulf of Finland. Hind- and forecasts of oil drift, done with the HIROMB/SeaTrackWeb were of great importance in the practical management of those critical situations. The presentation gives an overview of problems, experiences and challenges on the implementation of BOOS/HIROMB-based marine forecasts in Estonia. We consider also validation of forecasts and the need to apply local VHR forecast models that are linked to the Baltic-wide operational system.

Portable coastal operational oceanographic system to monitor the harbor-related environmental impacts in Estonia

A portable coastal operational oceanographic system is presented. It has been developed and applied to monitor the harbor-related environmental impacts (primarily spreading of suspended particulate matter and oil spills) in the Estonian coastal sea. Besides the traditional in situ monitoring methods, the system incorporates operational modeling and remote sensing. Application and validation of the system is demonstrated in two particular coastal areas - Muuga Bay and Pakri Bay.

Pathways of suspended particles released in the bottom boundary layer of the Bornholm Deep, Baltic Sea (numerical simulations)

A model system consisting of a circulation model and a random-walk model is developed to simulate suspended particulate matter transport in the bottom boundary layer (BBL) of the southern Baltic Sea. The circulation model is based on POM, the Princeton Ocean Model, in which the vertical grid size is refined towards the bottom in order to resolve BBL properly. 3D fields of velocity, vertical and lateral apparent diffusivities generated by the circulation model are used as an input for the random walk model to simulate transport and dispersion of particles with prescribed settling velocity. The random-walk scheme allows for non-uniform vertical profiles of the vertical apparent diffusivity, and test runs have been done to make sure that the model does not display unrealistic removal of particles from highly turbulent BBL and further accumulation in low-diffusivity above-lying layers. A number of numerical experiments have been performed to study pathways of suspended particles released in the BBL in the centre of the Bornholm Deep at different wind conditions. At northerly and easterly winds the particles initially move westward and then get involved into either northern or southern detours around the Deep. The particles from the northern detour are finally absorbed into the Slupsk Furrow while those of the southern detour do not enter the Furrow keeping on cyclonic rotation within the Bornholm Basin. To the contrast, for the westerly and southerly wind conditions the particles move to the northeast for some 20 km and then get involved into the cyclonic rotation. The cyclonic rotation implies the convergence of currents in the BBL due the Ekman transport and, in view of continuity, the upwelling, so that the particles will remain trapped within the Bornholm Deep if the settling velocity is large enough to overcome the upwelling. Since the westerly wind conditions dominate in the climatic sense, the trapping effect may be considered as an important factor that controls dispersion of chemical warfare agents dumped in the Bornholm Deep after the World War II.