Storms and other extreme weather events aren't divided equally between the two halves of our globe. The Southern Hemisphere is roughly 24 percent stormier than the Northern, in fact, for reasons that until now haven't been all that clear.
A new study looking in detail at global storm patterns identifies the heights of mountain ranges and the circulation of energy around the oceans as two primary factors in determining how storms brew above and below the equator.
To reach their conclusions, a team of researchers from the University of Chicago and the University of Washington ran a series of modified climate models, looking at how changing variables such as topography and ocean currents had an impact on the number of storms that were created.
"You can't put the Earth in a jar, so instead, we use climate models built on the laws of physics and run experiments to test our hypotheses," says University of Chicago climate scientist Tiffany Shaw.
The researchers changed one part of their climate models at a time. When they flattened the land masses of the Northern and Southern Hemispheres, half of the difference in storminess disappeared.
They then put the brakes on a major global 'conveyor belt' of ocean current, produced when warm water cools and sinks in the Arctic, flows south, and rises with wind-driven upwellings in the Antarctic. Added to the flattening of the world's mountains, this loss in energy transfer brought storm levels in the two hemispheres into line with each other.
What's more, satellite observations indicated the frequency of storms has been increasing in the Southern Hemisphere since the 1980s, while it's remained largely unchanged in the Northern Hemisphere.
That's likely down to changes in currents brought on by shifts in the atmosphere and ocean temperatures. These shifts are happening worldwide, but in the north, they're balanced out by the loss of sea ice and snow and increased sunlight absorption.
"The Southern Hemisphere is projected to become stormier, whereas Northern Hemisphere storminess changes are muted due to a tug of war between tropical and polar climate changes," write the researchers in their published paper.
It was only after World War II that scientists took on the challenge of tracking the weather and climate around the world – although sailors had, of course, known the different conditions in each hemisphere for a long time.
With the arrival of large-scale, real-time satellite imagery at the beginning of the 1980s, researchers could acquire a wealth of new information on weather conditions, allowing them to map out and monitor how they were changing over the years.
These new findings will be fed back into climate change models to help us better understand how the warming planet might affect storms and weather patterns differently in the two hemispheres – and which variables will be most important.
"By laying this foundation of understanding, we increase confidence in climate change projections and thereby help society better prepare for the impacts of climate change," says Shaw.
"One of the major threads in my research is to understand if models are giving us good information now so that we can trust what they say about the future. The stakes are high, and it's important to get the right answer for the right reason."
The research has been published in PNAS.