Why is the Southern Hemisphere getting drier?

In Southern Hemisphere Rainfall Is Decreasing, Leaving Scientists Searching For An Answer the Huffington Post is reporting on a possible reason for decreased rainfall in autumn in southern parts of Australia:

Since the 1970s, southern Australia — and other areas of the Southern Hemisphere —have seen decreased levels of rain between April and May, which is autumn in that part of the Earth. But what's causing this extended drought?

Previous research has pointed the finger at a southward shift in storm tracks and weather systems during the late 20th century.

But a study published today (Oct. 4) in the journal Scientific Reports takes the explanation one step further. Its findings suggest that changing storm patterns and the ensuing droughts are due to a southern shift in the Hadley cell, the large-scale pattern of atmospheric circulation that transports heat from the tropics to the subtropics.

The southward march of this circulation pattern has been greatest in the autumn, and has disproportionately affected southeastern Australia, according to a release describing the study from the Commonwealth Scientific and Industrial Research Organisation (CSIRO).

In Australia, it's led to a shift in the "subtropical dry zone," a region that stretches around the world and receives little rain, by 125 to 250 miles (200 to 400 kilometers) to the south. That's bad news for ecosystems in the area, which rely on fall rain to recharge.

As yet scientists do not know what is cause this phenomenon.