Weather of the Baltic Sea

The weather of the Baltic Sea is mainly determined by a westerly air flow at the surface and in the upper air. This west-wind jet stream is particularly marked in the upper air over central and northern Europe and thus over the Baltic Sea. The jet results from the general circulation of the atmosphere over the Northern Hemisphere. 

Its center lies at an altitude of about 5 km, that is, in the center of the troposphere, and is frequently located over the Baltic Sea. The location, the strength, and the cyclonic or an ticy clonic character of this j et stream determines the weather of the Baltic Sea. Two global action centers over the North Atlantic—the Icelandic Low and the Azores High—regulate its dynamics. North Atlantic lows with their fronts and wedges at intermediate altitudes are very often embedded in this westerly wind system. Usually, they move rapidly from west to east across the Baltic Sea and its coasts. 

Over the European continent, the North-Atlantic cyclones weaken during their eastward propagation, but often they cause the weather to be quite variable. That means, a very diverse but usually heavy cloud cover with occasional precipitation prevails over the Baltic Sea. It is generally moist, cool, and rather windy. The global west-wind jet carries moist and cool air masses from the North Atlantic and the North Sea to central and northern Europe (Nehring et al., 1990). This leads to a predominantly maritime influence on the Baltic Sea region. This 

State and Evolution of the Baltic Sea, 1952-2005, edited by Rainer Feistel, Giinther Nausch, and Norbert Wasmund Copyright 2008 John Wiley Sons. Inc. 

Different weather conditions of the Baltic Sea region result from the large geographical extension of the Baltic from southwest to northeast (Hupfer, 1977 Scharnow et al., 1990 Tiesel, 1995). 

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Very diverse and specific weather processes develop in the prevailing transition climate of the Baltic Sea. The following chapter deals with this special Baltic weather in more detail. 

During this large scale, dangerous northeast weather situation, which also led to the formation of instationary B altic Sea cyclones, a polar high sea level was located over Northern Europe and a corresponding lower one over south of the Baltic Sea (Figs. 4.17 and 4.18). 

FIGURE 4.17 Hand-drawn surface weather map of the Baltic Sea on January 29,1954,0700 GET. Of a family of instationary heat cyclones, the first one is emerging. Color figure on CD, Chapter 20.4. 

One of the most important weather processes of the Baltic Sea, which emerge locally under high pressure weather conditions with low winds and intensive insolation, is the land and sea breeze circulation. 

On an average, the land and sea breeze system of the Baltic Sea is up to 200 m high, extends about 300 m toward the Baltic Sea and about 500 m into the hinterland. During the high pressure weather periods in May and June, when the Baltic Sea water is still relatively cold, the sea breeze system of the Baltic Sea is formed most intensively. If in such cases onshore gradient wind exists under certain weather conditions, it is significantly... 

As a rule, the land breeze of this local wind system of the Baltic Sea develops only in the late summer and in autunm at calm high pressure weather conditions and in particular at night and in the morning. Then, the temperature contrast between the still warm Baltic Sea water and the coastal seam, cooling down already more strongly, are strongest. However, the land breeze is—because of the stronger friction—only weakly pronounced in comparison to the Seabreeze. 

In the coming decades it is to be expected that global warming in northern and central Europe—and thus in the Baltic Sea area—will even accelerate rather than just steadily continue. With the warming of air, the surface water of the Baltic Sea is warming up, too (Chapters 5 and 11). The warmer Baltic Sea water may transfer more free energy to the atmosphere, therefore causing more intense weather processes, as they are presently known... 

About 35 years ago, a long-term average state of the Baltic Sea climate was extensively reviewed by Defant (1972). Fundamentals of the associated water balance were resumed by Jacobsen (1980). In the following paragraphs we report some typical seasonal cycles of wind, temperature, precipitation, and closely related quantities for the Baltic Sea area. In some cases we report computed results for a period of almost 60 years in comparison to the standard reference period 1961-1990 used by the German Weather Service (DWD). Most records refer to Warnemunde or Arkona (Riigen Island) some refer to locations in the Baltic Proper or to the entire Baltic Sea. 

On the basis of its marine weather data archive, the Deutscher Wetterdienst recently developed a sea state climatology for 20 subareas of the Baltic Sea (DWD, ). This climatology is subdivided according to wind sea and swell, and it includes... 

This chapter summarizes investigations on satellite-derived sea surface temperature (SST) of the Baltic Sea for the period 1990-2005. The sea surface temperature derived from the Advanced Very High Resolution Radiometer (AVHRR) of the National Oceanic and Atmospheric Administration (NOAA) weather satellites were used in the Baltic Sea research for the investigation of the temperature development, for detailed process studies, and monitoring purposes. 

As described in Section 19.2.3.8, regional models, such as a Baltic Sea model, can get the boundary values for the calculation of surface fluxes from simulations with atmosphere models, which have been carried out previously. This is possible, because the influence of the Baltic Sea on the Northern Hemisphere weather system is only important for local phenomena, and inaccuracy in the feedback from the Baltic Sea to the atmosphere is of minor importance, Schrumm and Backhaus (1999). Widely used datasets, such as the ERA-40 reanalysis data, are improved by assimilation of observations. If surface variables calculated by the ocean model tend to drift away, this is compensated to a large extent by the calculated surface fluxes. For this reason numerical simulations with standalone ocean-ice models can be successful. 

Such a situation appears nearly exclusively only under strong cyclonic northeast weather conditions, whereby the cyclonic curvature of the sea-level isobars is supporting the development of the Baltic cyclones. 

The instationary Baltic Sea lows repeatedly appear as clusters, too. Then they can affect the weather of the whole Baltic Sea area over the period of an entire day (Fig. 4.19). 

As a result of their sudden development, mostly over night, the Baltic cyclones in principal remain undetected by weather models in the central Baltic Sea area, often at pronounced high pressure weather conditions. Therefore, in particular the instationary Baltic cyclones are almost always leading to completely wrong weather predictions for the Baltic Sea area. 

At these high pressure weather conditions, the sea breeze cloudiness is a familiar picture only directly at the coast. It is typical, too, that these particular cumulus clouds dissolve quickly toward the open Baltic Sea due to the prevailing down-welling of the air over the water. Because of often only weak thermal lift above the narrow and wooded coastal and island areas, however, this cloudiness is usually only weakly pronounced or does not appear at all. Also for this reason, these particular coastal regions have a longer duration of sunshine and thus a higher irradiation than the coastal hinterland. Usually in the late afternoon, the sea breeze system and concomitantly the whole seabreeze cloudiness breaks down. If hereafter a warm offshore breeze sets in, the daily maximum temperatures at the Baltic Sea coast occur only in the evening. 

Therefore, the Vb-weather type belongs to the best-known and formidable large-scale weather situations of Europe and the Baltic Sea. 

So the Omega situation is without a doubt one of the large-scale weather situations that influences the weather of the entire Baltic Sea region very positively. 

Thus, Vb-and Omega-weather types are not only important for the direct meteorological conditions in the Baltic Sea region, but also indirect for the basic hydrographic conditions of the whole Baltic Sea. 

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