La Niña’s fingerprint is all over plants in eastern Australia and southern Africa in this pair of vegetation images from the SPOT satellite. In these dry grasslands, plants live and die by the rain. Other places, like forests, store up water in the soil, so a few exceptionally dry or wet months don’t have such a large impact. But in the ecosystems common in Australia and southern Africa, you can almost measure the rain by the health of the vegetation, and vice versa. In February 2008, La Niña brought heavy rain to eastern Australia and southern Africa, and the vegetation responded in kind. Areas of bright green point to strong plant growth in both regions. Though an abundance of growing things may seem good, the excess rainfall was accompanied by devastating floods in both southern Africa and eastern Australia.
How does La Niña—a cooling of the eastern Pacific Ocean—affect plant growth on the other side of the globe in Africa? La Niña occurs when strong trade winds blow across the Pacific Ocean. The winds push sun-warmed surface water west towards Australia. Cool water rises to replace the surface water in the east. As a result, the Pacific Ocean is cooler than normal in the east off South America and warmer in the west off Australia. Warm, moist air rises over the pool of warm water in the western Pacific, where it generates abundant rain in eastern Australia and Indonesia. The rising air travels east in the upper atmosphere, drops as cool, dry air over the eastern Pacific, and then blows west as the strong trade winds that drive La Niña.
This circulation pattern is so large that it influences circulation the world over. Echoing the circulation over the Pacific, an identical pattern drives wind and rain in the Indian Ocean. Warm, moist air rises in the west, while cool, dry air sinks in the east. The effects of the Indian Ocean circulation pattern are evident in these images. The warm, moist air dropped heavy rain over southern Africa, where plants responded with gusto. The cool, dry air sank over central and western Australia, which was drier than normal. Brown indicates that plants were more sparse or less healthy than average in these regions.
NASA image created by Jesse Allen, using data provided by the United State Department of Agriculture Foreign Agriculture Service and processed by Jennifer Small and Assaf Anyamba, NASA GIMMS Group at Goddard Space Flight Center. Caption by Holli Riebeek, caption information courtesy Assaf Anyamba (UMBC/GEST at Goddard Space Flight Center).
