THE ANTARCTIC CONVERGENCE AND THE 

 DISTRIBUTION OF SURFACE TEMPERATURES 



IN ANTARCTIC WATERS 



By N. A. Mackintosh, D.Sc. 

 (Plates I-XIV; Text-figs, i-ii) 



PART I. THE ANTARCTIC CONVERGENCE 



INTRODUCTION 

 r-p HE Antarctic convergence, which constitutes the northern hmit of the Antarctic surface water, 

 i was first observed by Meinardus (1923), and it is a boundary of far-reaching importance in the 

 Southern Ocean. It may be regarded as the fine at the surface along which the Antarctic surface water 

 sinks below the less dense sub-Antarctic water, and it is distinguished by a more or less sharp change 

 of temperature at the surface. The change of temperature is much less clearly defined in some longi- 

 tudes than in others, but it is probably correct to say that the convergence is continuous around the 

 Southern Ocean, even though there are regions where it can sometimes hardly be traced. It is essen- 

 tially a feature of the surface, but where it is ill-defined, or when surface temperature records are 

 insufficient, it can usually be assumed to lie in the latitude at which the coldest part of the Antarctic 

 surface layer sinks below 200 m. For further particulars reference should be made to Deacon (1933, 

 pp. 190-3, and 1937, pp. 20-4), and to Bohnecke (1938, p. 201, etc.). 



The importance of the convergence does not lie only in the fact that it is the boundary between the 

 two principal water masses at the surface of the Southern Ocean. Its position is also related to the 

 distribution and movements of the deeper water masses, and there are reasons for believing that it 

 has a connexion with meteorological conditions in the Southern Ocean. Furthermore, it has a special 

 biological significance, for the Antarctic and sub-Antarctic zones, between which it forms the boundary, 

 have in certain respects, a distinct fauna and flora. That is to say, the convergence marks a limit 

 (though not always a very rigid one) to the range of certain species in the plankton (see Hart, 1934, 

 X937 1942; Mackintosh, 1934, i937; John, 1936), the fishes (Norman 1938) and the benthos 

 (Hastings 1943) The distribution of diatom ooze and diatomaceous mud as shown on Admiralty 

 charts, and by Neaverson (1934, plate xvii) indicates a relation also between the convergence and the 



bottom deposits. , r 1 • *• *• ^ 



A number of oceanic species seem to be unaffected by the convergence, but further investigations 

 will probably increase the list of those whose distribution is influenced by it. It is probable that it 

 separates certain species which characterize different water masses or different systems of circulation, 

 but the simple effect of temperature also needs consideration, for the distribution of some species 

 whose upper or lower limit of tolerance approximates to temperatures at the convergence, may be 

 sharply bounded by the abrupt change of temperature. • j u .u w. ^. 



Deacon (1937 p. 23) concludes that the latitude of the convergence is determmed by the latitude 

 reached by the Antarctic bottom water. He drew the line of the convergence on a circumpolar chart 

 fiQ.7 fi/4) and this was based on a number of positions fixed in the Falkland sector, and rather 

 more scattered positions in other sectors. Since then a very considerable number of additional positions 

 have been fixed. These show that Deacon's original line must lie very near the actual mean position, 



