Drought or Deluge?

Ray Schmitt, Woods Hole Oceanographic Institution

If you don’t live near the coast then it’s likely you rarely think of the ocean. But if you ever check the weather forecast or look to the sky for signs of rain, you are indirectly looking for signs of the ocean’s influence on your daily life.

The ocean holds 97 percent of the water on Earth, making it the largest reservoir on the planet and the largest single actor in the global water cycle. Solar heating helps drive evaporation that moves moisture from the ocean to the atmosphere, and air is able to hold more moisture as it warms. That means we can expect the global water cycle to intensify as temperatures rise. But it has been difficult to see how this change plays out on land, in part because of human interventions, like damming rivers, pumping up ground water, and converting native vegetation to crops.

We can, however, turn to the ocean for insight. Because the ocean covers more than two-thirds of Earth’s surface, it follows that most of the rain falls at sea. And when rain falls over the ocean, the sea surface becomes less salty. At the same time, evaporation makes the sea surface saltier by removing fresh water to the atmosphere. This movement of water to and from the sea leaves behind patterns of salinity on the surface of the ocean that give us some insight into what might be in store for us in a warming world. By looking back over 60 years of salinity records, we see signs that salty areas have gotten saltier and the fresh areas have gotten fresher, which means that we should expect dry areas to become dryer and wet areas to get wetter for us on land, as well.

We also recently discovered that the year-to-year variations in salinity in certain parts of the ocean can be used to make accurate predictions of seasonal rainfall on land, often thousand of miles away. The idea is that if some part of the ocean has gotten saltier by evaporation, the atmosphere has taken up this water. We can therefore expect the extra moisture to show up as rainfall somewhere else and, in fact, we do see specific links between places like the eastern North Atlantic and Central Africa; and the western North Atlantic and the U.S. Midwest.

Remarkably, ocean salinity turns out to be a better predictor of rainfall on land than nearly all other variables, and these “teleconnections” between certain regions of the ocean and land can provide accurate rainfall predictions a season or two ahead.

The key challenge becomes to monitor ocean salinity in a way that will tell us whether our future is one of drought or deluge because, as W. H Auden said, “Thousands have lived without love, not one without water.”

Raymond Schmitt is a Senior Scientist at the Woods Hole Oceanographic Institution. He focuses on ocean mixing and microstructure; double-diffusive convection; relationships between small scale mixing processes and large-scale temperature and salinity distributions; the thermohaline circulation of the ocean; and development of microprofiling instrumentation.

Rainfall and evaporation over the ocean is providing insight into changes in Earth’s water cycle and preciptitation patterns on land. (Photo © Woods Hole Oceanographic Insitution)