Tapani Stipa

The Structure and Driving Mechanisms of the Baltic Intrusions

Abstract Data from closely spaced CTD profiling performed in the eastern Gotland Basin after the 1993 inflow event are used to study thermohaline intrusions in the Baltic Sea. Two CTD cross sections display abundant intrusive layers in the permanent halocline. Despite the overwhelming dominance of the salinity stratification, diffusive convection is shown to work in the Baltic halocline enhancing diapycnical mixing. To understand the driving mechanisms of observed intrusions, these are divided into different types depending on their structural features. Only two types of observed intrusions are suggested to be strongly influenced by diffusive convection: 1) relatively thin (3–5 m) and long (up to 8 km) intrusions inherent to high-baroclinicity regions and 2) relatively thick (∼10 m) and short (2–5 km) intrusions inherent to low-baroclinicity regions. To verify this hypothesis the linear stability models of 3D and 2D double-diffusive interleaving in approximation of a finite-width front were used. It is sh...

Generation of subsurface cyclonic eddies in the southeast Baltic Sea: Observations and numerical experiments

[1] Closely spaced CTD transects performed in the summertime reveal simultaneous downward/upward bendings of temperature/salinity contours in the seasonal thermocline/ permanent halocline of the Stolpe Channel and the Gulf of Gdansk, which may be interpreted as geostrophically balanced cyclonic eddies in the intermediate layer. To examine processes capable of forming the observed cyclonic eddies, a numerical simulation based on the Princeton Ocean Model (POM) has been initiated. The subsurface cyclones in the Stolpe Channel were satisfactory simulated in model runs under easterly/ northerly wind conditions. Their formation was shown to result from the adjustment of the high potential vorticity (PV) outflow (from the Bornholm Basin via the Stolpe Sill) to low potential vorticity environment by vortex stretching (so-called the PV outflow hypothesis by Spall and Price [1998]). In accordance with the real wind conditions, a cyclonic eddy observed in the intermediate layer of the Gulf of Gdansk was satisfactorily reproduced in a model run with the westerly wind shutdown, which implies westward transport throughout the Stolpe Channel and thereby excludes the PV outflow hypothesis. The subsurface cyclone simulated in the Gulf of Gdansk was traced to form in the course of relaxation of the coastal downwelling baroclinic jet.

Baroclinic adjustment in the Finnish coastal current

Observations from the northern shore of the Gulf of Finland show a quasi-stable brackish coastal current flowing along a sloping bottom. Other examples of similar currents have been found, for example, in the Arctic basin. Generally, baroclinic flows of this type tend to be unstable. To study the stability of such flows, a specially designed numerical experiment has been devised. The simulations show that a current structure resembling that found in the observations results from an internal baroclinic adjustment of the flow to a baroclinically quasi-stable state with monotonic isopycnal vorticity. On the basis of the observations and numerical results, we propose that coastal currents for a certain part of the parameter range have an inherent tendency to evolve towards a quasi-stable state. This baroclinic neutralization is suggested as an explanation of the observed persistence of buoyant coastal currents under favorable conditions.

The dynamics of the N/P ratio and stratification in a large nitrogen-limited estuary as a result of upwelling: a tendency for offshore Nodularia blooms

The effect of upwelling on the preconditions for noxious cyanobacterial blooms in a nitrogen-limited estuary is studied with Nodularia blooms in the Gulf of Finland especially in mind. An idealised physical–chemical model is derived and integrated. The physical development in the model agrees with a classical picture of upwelling for the scales of interest. The chemical component indicates a transient minimum of dissolved inorganic nitrogen to phosphorus ratio in the middle of the gulf, coinciding with the stratification maximum resulting from physical dynamics. A general tendency for offshore cyanobacterial blooms in similar physical and biological conditions is deduced from these results.

Role of eddies and intrusions in the exchange processes in the Baltic halocline

Intrusions observed in the regions of intense mesoscale eddies of the Baltic Sea are studied. The study is based on the data of the closely spaced CTD profilings performed in the Eastern Gotland Basin after the major inflow of 1993. The results obtained supplement the interleaving analysis presented in [6]. Special attention is focused on the description of the intrusions in the intrahalocline cyclonic eddy that was formed during the inflow. A T, S-analysis of the water structure in mesoscale eddies and beyond them revealed the following features: (a) a substantial difference between the T, S indices in cyclonic and anticyclonic eddies, and (b) a gradual decrease in the salinity of the modified North Sea waters is observed during their propagation into the basin. A phenomenological description of the exchange and mixing processes in the Baltic halocline based on the results of numerical simulation of mesoscale eddies formed during the inflow event and an empirical analysis of the interleaving are suggested.