306 The Geophysical Structure of the Sea 



In general, the isobaric surfaces and the surfaces of equal dynamic depth (level sur- 

 faces) intersect. These lines of intersection are termed dynamic isobaths and are 

 usually plotted at 5 dyn.mm intervals. In this way the topography of the pressure 

 surface is obtained. On the other hand, the lines of intersection of the pressure surfaces 

 with a level surface are denoted as isobars or lines of equal pressure. These give a 

 chart of the pressure distribution at a given level. In oceanography it is more cus- 

 tomary to represent the pressure field by charts of the dynamic topography of espe- 

 cially selected isobaric surfaces. 



It should be emphasized that for a representation of the pressure distribution in the 

 ocean only the actual water or sea pressure is used without taking the air pressure into 

 account. If the total pressure is required the sea-level pressure of the atmosphere 

 which on a crude average is about 10 decibars must be added. Furthermore, it should 

 also be remembered that dynamic topographies are referred to the physical sea-level 

 from which the measurements are made and not to the ideal sea level (the geoid) 

 which is defined as the surface of zero gravitational potential (dynamic depth zero). 

 The topography of the physical sea-level is unknown, so that in practice these 

 topographies are always represented only as relative topographies, i.e., relative to the 

 unknown topography of the physical sea-level. Expressed in another way, they 

 are dynamic topographies relative to a physical sea-level assumed as "plane" (plane 

 in a geodetic sense). In order to obtain the absolute dynamic topography, the 

 absolute dynamic topography of the physical sea-level would have to be known, 

 and for this a determination of the dynamic depth of the pressure values would have 

 to be carried out starting from the physical sea-level. 



A convenient and practical representation of the mass distribution is obtained by 

 use of dynamic sections — or to be more specific, vertical sections — of pressure sur- 

 faces and the isosteric surfaces. Both of these sets of surfaces vary only slightly from 

 the horizontal, and the vertical scale must be considerably exaggerated in order to 

 obtain visible gradients. Usually, however, the inclination of the isobaric curves as 

 compared with that of the isosteric ones is so slight that horizontal lines in the co- 

 ordinate system can be taken as isobars. The specific volume anomaly is usually 

 used instead of the specific volume itself and the mass field is therefore represented by 

 lines of equal anomaly. 



The two sets of curves (the isobars and the isosteres) divide the vertical surface into 

 a number of parallelograms formed by wavy lines; they are the cross-sections of tubes 

 formed by the intersection of (invariably) two isobaric surfaces and two isosteric 

 surfaces. These differently-shaped parallelepipeds were denoted isobaric-isosteric 

 tubes by Bjerknes (1900); they are denoted as unit tubes or solenoids if areas of units 

 in pressure and specific volume are drawn on vertical sections. 



The terminology "solenoid" is also used when the sets of curves are drawn at inter- 

 vals of several units. If the mass field is given by Hnes of equal anomaly S at unit inter- 

 vals of a (in the CGS system: 10~^), and the pressure field by isobars at intervals of 

 1 db (in the CGS system: 10^ dyn. cm^^), then a parallelogram formed by intersection 

 of two isosteres and two isobars will enclose one solenoid of the CGS system. In 

 practice, isosteres are usually drawn for every 20 of these units so that a surface ele- 

 ment of the isobaric-isosteric tube contains 400 CGS solenoids. The solenoid is 

 assigned a positive or negative sign depending on whether, on rotation in a positive 



