1 92 Density of Water Masses in Ocean, Vertical and Horizontal Density Distribution 



rule that in the Northern Hemisphere the higher density values are found to the left 

 of the direction of flow, while in the Southern Hemisphere they are found to the 

 right (see Fig. 89). 



Horizontal charts of the density for 400 and 1 500 m in the Atlantic Ocean have 

 been given in Plate 7 to supplement the above brief remarks. The first chart shows the 

 Gulf Stream system very clearly by the strong concentration of the isopycnals into a 

 narrow belt running from the Gulf of Mexico through the Florida Straits to the New- 

 foundland Banks and beyond to the north-east. Compared with this very large hori- 

 zontal density gradient, that connected with the equatorial currents is only very 

 small. In the Gulf Stream region the 400 m chart indicates another phenomenon that 

 is characteristic of stronger gradient currents and is apparently missing in pictures 

 of the surface current. On the right-hand side of the Gulf Stream some isopycnals 

 deviate outward and turn into a south or south-west direction, opposite to the direction 

 of the narrow band surrounding a strong longish density maximum at the right-hand 

 side of the current core. These backward-turning isopycnals indicate the presence of a 

 countercurrent to the right (to the east) of the Gulf Stream which is of considerable 

 importance for the dynamics of this ocean current near the American coast. 



In the Southern Hemisphere the isopycnals are strongly concentrated in the regions 

 of the Agulhas Current, the Brazil Current and the Falkland Current. In addition, a 

 steady rise of density exists in the Southern Hemisphere extending around the entire 

 southern ocean which is associated with the broad circumpolar West Wind Drift of 

 the higher southern latitudes. All density charts down to 800 m show very much the 

 same picture, though the density gradient becomes gradually smaller and the density 

 maxima of the subtropics are thereby somewhat displaced towards the poles. At first, 

 a different distribution begins to appear below 1000 m, which dominates in the 

 1 500 m chart. This is the density gradient from high northern latitudes to the mini- 

 mum zone between 35° and 40° S. This north to south density gradient becomes less 

 and less pronounced with increasing depth and below 4000 m the horizontal density 

 differences become already very small. 



4. Potential Density and Isentropic Analysis 



In earlier times potential density was considered a significant property on which to 

 form an opinion about the state of vertical equilibrium of oceanic stratification. As 

 already stated (see p. 1 88) potential density is calculated from the in situ salinity and 

 the potential temperature. Since the latter differs only at great depths from the in 

 situ temperature and then by only a few tenths of a degree centigrade, the difference 

 between o-^ and a^ remains very small and is almost insignificant as shown in Table 79. 

 It thus makes little difference whether the vertical density distribution is judged by 

 means of the customary a^ or of the more correct oq. The potential density has recently 

 become of greater interest due to the introduction of the method of isentropic analysis. 

 In meteorology, the investigation of the distribution of individual meteorological 

 elements on surfaces of equal entropy has been modernized and this has led to ap- 

 preciable success. Parr (1938, 1938^) has studied the spreading of oceanic water types 

 in a similar way by following the changes in salinity and temperature on surfaces of 

 equal density ct<. 



