Geologic map of Australia

Reto Stöckl/NASA Goddard Space Flight Center

Australian continent shifts with the seasons

Every summer, the "land down under" does a little dance: It slowly shifts 1 millimeter to the northwest and dips down 2 to 3 millimeters on its northwestern side. When winter comes, it shuffles back by the same amount, while its southeastern coast does the downward dipping. What triggers these seasonal undulations? A new study, the first to reveal the shift, has an answer: a large-scale movement of water between the Southern and Northern hemispheres. One researcher tracked the continental migration based on nearly a decade of GPS data. Data from another set of satellites that monitors subtle changes in Earth’s gravitational field tracked the movement of water, showing that during the Australian summer (winter in the Northern Hemisphere) large amounts evaporate south of the equator. That loss of weight—equivalent to a depth of between 20 and 30 millimeters of water across a broad swath of the globe centered on the South Pacific—causes Earth’s crust to spring upward in the region. That, in turn, drives Australia slightly toward the northwest. And because the upward spring is greater closer to the South Pacific, the motion also tips the northwestern part of the continent downward, relative to the southeastern part. Six months later, when all that water returns from the snowfields of the Northern Hemisphere, Australia migrates back toward the southeast and settles into its wintertime position, the researcher reports online in the Journal of Geophysical Research: Solid Earth. The newly discovered motions could help track seasonal shifts in the world’s water, as well as what such shifts do to our planet’s center of mass.