More rain leads to fewer trees in the African savanna

In 2011, satellite images of the African savannas revealed a mystery: these rolling grasslands, with their heavy rainfalls and spells of drought, were home to significantly fewer trees than researchers had expected. Scientists supposed that the ecosystem's high annual precipitation would result in greater tree growth. Yet a 2011 study found that the more instances of heavy rainfall a savanna received, the fewer trees it had.

The researchers found that under very wet conditions, grasses have an advantage because they can quickly absorb water and support high rates of photosynthesis, the process by which plants convert sunlight into energy. Trees, with their tougher leaves and roots, are able to survive better in dry periods because of their ability to withstand water stress. But this amounts to a disadvantage for trees in periods of intense rainfall, as they are comparatively less effective at utilizing the newly abundant water.

The study highlights the importance of understanding the pattern and intensity of rainfall, not just the total annual precipitation, which is where most research in this area has focused, Xu said. In 50 years, a region may still experience the same overall amount of precipitation. If the intensity changes, however, that will affect the abundance of grasses and trees. This, in turn, will influence the herbivores that subsist on them, and other animals in the biome -- essentially, affecting the entire ecosystem.

Citation:

Princeton University. "More rain leads to fewer trees in the African savanna." ScienceDaily. ScienceDaily, 19 October 2015. <www.sciencedaily.com/releases/2015/10/151019130802.htm>.

Long-range winter weather forecasts could be twice as accurate by taking account of unusual winds miles up in the stratosphere,

Long-range winter weather forecasts could be twice as accurate by taking account of unusual winds miles up in the stratosphere, scientists have found.

High up in the stratosphere is the polar night jet stream, at around 40km (25 miles) above the surface of the Earth -- around the height reached by record-breaking parachutist Felix Baumgartner.

Winds in the polar night jet stream usually blow from the west and have speeds of around 70mph. The research team found that during conditions in which the polar night jet stream wind speeds exceed 90mph, or reverse their direction to flow from the east, forecasts in both the stratosphere and troposphere (the layer of atmosphere closest to the ground), are more skillful.

While it was previously known that a sudden weakening of these winds, and subsequent warming of the stratosphere, was a source of predictability, the researchers found that the opposite was also true when the polar night jet stream strengthened and the polar stratosphere was unusually cold. Such stratospheric conditions occur up to 3-4 times a winter, meaning that around one in five winter 'sub-seasonal' forecasts (those looking 2-4 weeks ahead) might benefit from this effect.

The strength of stratospheric winds can influence the position of the jet stream in the troposphere, having a major influence on weather across the North Atlantic, and allow freezing polar air to travel further south than would normally be expected.

Citation: 

Institute of Physics. "Forecasters look higher for clues to winter weather: Long-range winter weather forecasts could be twice as accurate by taking account of unusual winds miles up in the stratosphere, scientists have found." ScienceDaily. ScienceDaily, 13 October 2015. <www.sciencedaily.com/releases/2015/10/151013102413.htm>.