6 2 DISCOVERY REPORTS 



ocean— roughly a channel with parallel sides — will cause the water movement to be 

 directed almost due east, and the lateral movements to be more restricted than they are 

 where an extensive ocean lies to the north. 



The convergence also appears to be ill-defined in the Tasman Sea west of New 

 Zealand. The water at St. 923 in section 13 (Plates XXVIII-XXX) belongs to the sub- 

 Antarctic current which flows eastwards, south of Stewart Island, but that at St. 924, 

 about 80 miles from the west coast of South Island in 44 18' S, belongs to the current 

 which sweeps towards the north along the west coast, and contains a large proportion 

 of subtropical water. Between Sts. 924 and 925 the section lies too nearly in the direc- 

 tion of the current to give a clear indication of the changes of temperature and salinity 

 at right angles to the coast, but they both appear to increase gradually away from the land. 

 West of South Island the water more than 100 miles offshore has the temperature 

 and salinity of subtropical water and is no doubt derived from the East Australian cur- 

 rent and from the easterly movement south of Australia. Nearer the coast the water 

 has a lower temperature and salinity, and is partly sub-Antarctic, some of it probably 

 having upwelled in the coastal region from the poorly saline antarctic intermediate 

 current. Surface current observations show that part of the northward current turns 

 towards the east round the northern end of South Island into the Cook Strait (New 

 Zealand Pilot, 1930, p. 29), and this branch of the current probably contains the bulk 

 of the sub-Antarctic water. Off the west coast of North Island the water is much warmer 

 and more saline. As far as the scanty data allow it to be determined, the principal 

 boundary between the waters of sub-Antarctic and subtropical origins lies between 

 Sts. 924 and 925 in section 13, and it approaches the land in the neighbourhood of 

 Cape Egmont. It follows approximately the 12 C. isotherm and the 34-9 °/ 00 isohaline. 

 There are very few observations to show the position of the subtropical convergence 

 east of New Zealand. The 'Dana' crossed a well-marked boundary in 42 S, 177 E, 

 about 100 miles from the eastern end of the Cook Strait (Schott, 1934, p. 241). The 

 temperature and salinity measurements along section 14, and others made farther east 

 (Appendix I) which have been used in the construction of Figs. 8 and 11 (pp. 29, 45) 

 show that the isotherms and isohalines bend towards the south in the deep water east 

 of the Chatham Islands. There appears to be a southward movement of warm highly 

 saline water, with some resemblance to the Brazil and Agulhas currents (see pp. 54, 55), 

 and the subtropical convergence probably bends towards the south. 



Farther east the isotherms, isohalines and convergence recede towards the north. 

 According to the calculations of salinity based by Wiist (1929, pp. 59-60) on the old 

 density measurements of the * Challenger ' the surface water at St. 287 (Murray, 1895, 

 p. 1092) in 36 32' S, 132 52' W had a salinity of 34-94 °/ 00 , while the surface salinity 

 at St. 289 in 39 41' S, 131 23' W was only 34-25 °/ 00 . These data suggest that there 

 is a sharp increase of salinity from south to north in this region and indicate that there 

 may be a sharp convergence, possibly in 36-37 S. The temperature difference between 

 the Challenger stations was, however, only i-8° C, and no sudden temperature changes 

 were recorded anywhere in the neighbourhood (Tizard and others, 1885, p. 802). 



