i82 DISCOVERY REPORTS 



The additions to the layer are made therefore to a water mass which is large enough, 

 and lasting enough, to show some resilience to changes. If Antarctic water flowed only 

 towards the north much greater seasonal changes would be expected. 



MOVEMENTS OF THE ANTARCTIC SURFACE WATER IN 

 THE FALKLAxND SECTOR 



Present knowledge of the surface currents in the Falkland Sector is based principally 

 on evidence obtained from charts showing the distribution of temperature and salinity, 

 and from charts showing the topography of the sea surface relative to a deep isobaric 

 surface, which because of the lesser movement at great depths has been assumed to be 

 horizontal. The general conclusions from these charts are shown as current arrows in 

 Fig. 8. The length of the arrows is intended to give an approximate idea of the relative 

 speed of the surface currents, but more precise information and topographical charts 

 will be given in a later report. 



There are three principal factors which cause movement in the Antarctic surface 

 layer, and they can be summarized as follows : 



(i) A thermohaline influence, which produces a vertical circulation in the ocean as 

 a result of density diff'erences maintained in different latitudes largely by diff"erent 

 climatic conditions. 



(2) The efl^ect of the prevailing wind systems and pressure difl^erences. 



(3) An eff'ect due to the excess of precipitation over evaporation, and to the liberation 

 of fresh water when ice and snow melt in summer. 



The low air temperature and the small heating effect of the sun in the Antarctic 

 regions give rise to heavy water at the surface : this water, although it has a low salinity, 

 is still heavier than the more saline but warmer water in the tropical and sub-tropical 

 regions. The presence of this heavy surface water in the south is shown by the slope 

 of the isobaric surfaces in the sea downwards from south to north. The slopes of the 

 o, 300, 600, and 1000 decibar surfaces relative to the 3000 decibar surface are shown in 

 Fig. 9 (p. 184). Since the 3000 decibar surface can be regarded as almost horizontal 

 they may be taken to represent the actual slopes. The vertical scale is in "dynamic 

 centimetres," each of which equals 1-02 cm., and the figure shows the difference be- 

 tween the depths at which the isobaric surfaces were found and the depths of the hori- 

 zontal positions they would occupy in a uniform and motionless sea of 0° C. and 35 7 00 

 salinity. Because the water at the southern end of the section is heavier than that in 

 the north the same pressure is reached at lesser depths. The presence of heavier surface 

 water in the south is partly due to the presence of a movement towards the east, south 

 of 40° S, which is maintained by an external factor, the wind. In this movement the 

 layers of equal density will attain a more or less constant slope downwards to the north ; 

 but there is ample evidence to show that the slope is also the result of a continuous 

 meridional circulation. 



