684 The Stratospheric Circulation 



rise, of course, to large differences in the development of the deep currents. The closest 

 approach to the ideal case pictured in Fig. 323 is found in the Pacific. The meridionally 

 oriented sections, although they are based on insufficient data and often do not reach 

 right to the bottom show the approximately symmetrical arrangement of the 

 subpolar intermediate currents about the equator. Warm water sinks in the convergence 

 region of both these currents as it is required in Fig. 323 ; such downwards motions are 

 indeed indicated by a downward bulging of the isothermal layers in the meridional 

 temperature sections. At greater depths the deep layers of the Pacific are almost uniform 

 and there is no special differentiation to indicate any particular motion (Wust, 1929, 

 \9Z0b). This is supported also by the absence of any temperature inversions which are 

 very characteristic of the Atlantic and the Indian Ocean. 



The marked asymmetry of the polar components in the Atlantic Ocean due to the 

 almost complete absence of the Arctic current branches gives rise to a strong develop- 

 ment of the southward directed North Atlantic Deep Current. This provides the only 

 compensation here for the Antarctic water carried north by the intermediate and 

 bottom current. Disregarding at the moment the water layers from about 1000 to 

 1500 m between 50° N and 20° N. (particularly on the eastern side) the oceanic spaces 

 underneath are filled with relatively salinity- and oxygen-rich waters. The structure of 

 these waters indicate by their vertical structure a sub-Arctic origin. Its principal 

 characteristic is the oxygen content of the core layer and the distribution of this shows 

 clearly its origin from the area east and south-east of Greenland and from the boundary 

 zone between the East Greenland Current and the Irminger Current south-west of 

 Iceland as well as from regions in the north of the Labrador Sea. These are the same 

 regions that form the source of the sub-Arctic bottom water (p. 680). WiJST termed 

 this sub-Arctic bottom water as the "Lower North Atlantic Deep Waters" as opposed 

 to the "Middle Deep Water" occurring above. In these regions mentioned above the 

 almost homogeneous structure of the sea during autumn and winter allows the surface 

 waters to sink to great depths forming there the source of the more or less horizontal 

 southward water transport between 1 500 and 2500 m depth. 



In the "Meteor" cruise made in late winter 1935 the kind of conditions were found 

 along a profile south of Greenland which are required to allow the autumn and winter 

 convection to proceed to great depths. The oxygen distribution along this profile 

 (Fig. 324) clearly shov/s this downwards tendency of the surface layers (Wattenberg, 

 1938). Below 1000 m the source for the middle North Atlantic Deep Water is formed 

 here. When this water moves further to the south the transport obviously keeps 

 closely to the western side of the ocean due to the influence of the Coriolis force, but 

 even after crossing the equator it still prefers the western side and the effect of the 

 Middle Atlantic Ridge is clearly noticeable. The upper layers of this southward water 

 movement show the effect of mixing with Mediterranean water (see later) since the 

 [TS]-relationship for the core layer at 35° N. shows a definite reversal point (see Fig. 

 325); apart from this the curve as far as 50° S. is almost a straight line and indicates 

 gradual mixing with the water types above and below. Beyond 30° S. these waters 

 enter the deep-reaching circumpolar flow of the very southern latitudes and under the 

 influence of this are deflected to the east. The pressure conditions in the core layer of 

 the North Atlantic Deep Water are best indicated by the topography of the 2000- 

 decibar surface. Figure 326 shows immediately that the main course of the isobaths is in 



