Chapter XX 



The Stratospheric Circulation 



1. Introduction 



Beyond the polar convergence (oceanic polar front) towards the poles the oceanic 

 stratosphere reaches upward to the sea surface and is here subject to the full influences 

 of the atmosphere (radiation, evaporation, precipitation, freezing processes and 

 others). The water types continually formed by the climatic conditions here are 

 heavier, due to their low temperature and in spite of their low salinity, than the waters 

 of the adjacent convergence regions of the oceanic troposphere. Thus, in relation to 

 these latter water types they tend to sink, intruding beneath the oceanic troposphere, 

 until they reach a depth corresponding to their density. The sinking, strongly favoured 

 by the thermo-haline structure, reaches down to great depths. After sinking, the 

 almost horizontal spread of the water underneath of the troposphere causes a layered 

 leaf-like structure in the oceanic stratosphere. When this structure is sufficiently well 

 developed it is therefore possible to tell from it something about the path of spread 

 of the water masses and gain thereby an insight into the stratospheric circulation. This 

 is the method that has been used up to the present time in the study of the water 

 movements inside the stratosphere. In the absence of sufficient direct current measure- 

 ments, however, the results of such investigations were largely only of a qualitative 

 nature. Preparation of the observational data according to dynamic methods can 

 provide further insight into the nature of the stratospheric oceanic flow, but at the 

 present time only a few investigations of this type have been made. All these methods, 

 of course, give mean conditions only. Over large parts of the ocean, however, especially 

 for the deeper layers the basic prerequisite of stationary movements will be satisfied. 

 But aperiodic disturbances of shorter or longer duration and of greater intensity un- 

 doubtedly occur. By means of the observations available at present, and also due to 

 the manner in which they have been gained, it seems hardly possible to draw any 

 conclusions about the nature of these disturbances. 



The surface layers of the oceanic stratosphere poleward (the polar fronts) are, of 

 course, subject to wind influence, so that also in the polar and subpolar seas wind- 

 driven ocean currents are generated. The complicated orographic configuration of the 

 continents in the Northern Hemisphere affects the nature of these surface currents 

 and exerts strong influence during their transformation into gradient currents. In 

 this way, piling up (Stau) phenomena play the principal role, and meridionally oriented 

 coasts in higher latitudes form excellent guiding channels for southward outbreaks 

 of the cold polar water masses. The zonal polar circulation obtains in that way 

 meridional components, so that on the eastern sides of polar land mass water flows 

 south, while on the western sides mainly water of subtropical origin flows north. 



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