480 Tides of the Oceans 



The main pictures of the M 2 - and K x - co-tidal lines are, with a few ex- 

 ceptions, that of rotary waves (amphidromies contra solem), in both hemi- 

 spheres. In most cases, their development is unsymmetrical; often there is 

 a more or less pronounced crowding of the co-tidal lines and, in extreme 

 cases they degenerate into real nodal lines with opposite phases on both sides. 

 Mostly there is no discontinuity, but we have narrow bands in which the 

 phase changes rapidly from one locality to another. For instance, the am- 

 phidromies of the semi-diurnal and diurnal tides of the North Atlantic Ocean 

 are symmetrical, nearly pure nodal lines are found for the semi-diurnal tides 

 in front of the Gulf of Bengal and between Japan and New Guinea. Some- 

 times a compact group of co-tidal lines starts out from the centre of the 

 amphidromy running into only one direction; then it is difficult to judge 

 in how far it is better to represent the observations by a nodal line or by 

 an amphidromy. 



Theoretically the amplitude becomes zero in every centre of an amphidromy 

 and on every nodal line. In practice one finds a minimum of the amplitudes 

 in the vicinity of the amphidromy and the range increases with the distance 

 from the centre of the amphidromy and the same holds true for the nodal 

 line. The maximum values of the tidal ranges are found on the coasts, even 

 if one allows for the increase in amplitude caused by the shelf. But this is 

 not the case on the coasts of islands, inasmuch as these are usually closer 

 to the centre of the amphidromy. Thus, according to Dietrich, in the im- 

 mediate viscinicy of the pseudo-nodal line between Japan and New Guinea 

 the smallest mean spring tide range of the semi-diurnal tide is only 16 cm; 

 west of this point it reaches with increasing distance from the nodal line 

 to 44, 100, 120 and 150 cm. Towards the east there is an increase from 16 

 to 60, 89, 116, 136, 152, and up to 176 cm. The distribution of the amplitudes 

 is also quite characteristic for amphidromies if there are sufficient data of 

 localities available. Thus, in the centre Solomon Islands the amphidromy 

 of M 2 lies near Nusosonga (8°25'S., 157°14'E. lat.) which has an amplitude 

 of cm. The small spring tide range of 12 cm must be attributed to the S 2 

 tide. The tidal ranges increase rapidly with the distance from this centre. 

 The spring tide range of the eastern Solomon Islands is 121 cm; to the south- 

 west in the Louisiade Archipelago it is 75 cm, on the border of the Great 

 Barrier Reef of Australia it is 172 cm and on the Australian continent in 

 Queensland, under the influence of the shallow coral coast, it increases to 

 476 cm. There is always a close correlation between the co-tidal lines and the 

 amplitudes; one-sided crowding of the co-tidal lines extending to the coast 

 is reflected there in a decrease in the range of the tide, so that such minima 

 in range can always be regarded as a characteristic sign of a rapid variation 

 in the phase. 



In judging the amplitudes along the coast one should consider the increase 

 in the amplitude through the co-oscillation of the shelf sea with the tides 



