The cross-equatorial pressure gradient and summer monsoon rainfall in northern and central Australia in January 1974
The occurrence of the Australian monsoon is examined in relation to the cross-equatorial gradient which stretched across the equator between the winter-hemisphere subtropical anticyclone and the summer-hemisphere monsoon low pressure system situated over the Australian continent in January 1974. The pressure gradient is calculated in two parts, both parts of the same sign directed from north to south. The first part is expressed as the difference in pressure between 25 ºN and the equator, and the second part as the difference in pressure between the equator and 25 ºS. The day by day sequence of these gradients is compared with daily rainfall values recorded at selected stations i n north and central Australia i n January 1974. The diagnostic results support the conclusions obtained from numerical trajectory calculations, that a pronounced cross-equatorial gradient is favourable to rainfall in the summer hemisphere. In particular, a strong pressure gradient in the northern winter hemisphere directed southwards appears to be associated with substantial rainfall in the northern part of the area studied, between 11 ºS and 18 ºS, whereas when a strong gradient extends further southwards into the interior of the continent, heavy rainfall occurs between 18 ºS and 25 ºS, and it becomes less rainy north of 18 ºS.