Sailors have known for centuries that the most fearsome storms are found in the southern oceans. Now scientists using satellite data, have put a number to it – the Southern Hemisphere (SH) is, on average, about 24% stormier than the Northern Hemisphere (NH).
Until now, there hasn’t been an explanation for why this phenomenon exists. But in a new study in PNAS, US climate scientists have identified the two major drivers: ocean circulation and the large mountain ranges in the North.
“The Southern Hemisphere has a stronger jet stream and more extreme weather events than the Northern Hemisphere,” the authors write.
“We show that the stormier Southern Hemisphere is induced by nearly equal contributions from topography and the ocean circulation, which moves energy from the Southern to Northern Hemisphere.”
Ocean circulation and topography are to blame.
To investigate the variation, the team used a combination of evidence from satellite observations, theory, and physics-based computer simulations of Earth’s climate.
“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,” explains lead author Dr Tiffany A. Shaw, Associate Professor in the Department of the Geophysical Sciences at the University of Chicago.
They used an atmospheric model of Earth’s climate to simulate real-life observed storminess asymmetry and then removed different variables one at a time to measure the impact on it.
The first variable was topography; when every mountain on Earth was flattened in the simulation storminess asymmetry was reduced by about half to 12%. This is because large mountain ranges disrupt air flow in a way that reduces storms, and there are more of them in the NH.
Do you care about the oceans? Are you interested in scientific developments that affect them? Then our new email newsletter Ultramarine, launching soon, is for you. Click here to become an inaugural subscriber.
The other half of the storminess asymmetry can be attributed to ocean circulation.
Globally, energy is transported through sea water sinking in the Arctic, travelling along the bottom of the ocean, rising near Antarctica, and then flowing up near the surface.
“The ocean circulation contributes to a stormier Southern Hemisphere by transporting energy from the Southern to the Northern Hemisphere thereby creating a larger equator-to-pole surface energy flux imbalance in the Southern Hemisphere,” the authors write.
This causes an energy difference between the two hemispheres that, when eliminated from the model, resulted in the removal of the other half of the difference in storminess.
Will it continue to get even stormier in the Southern Hemisphere?
By examining how storminess has changed since the advent of satellite-based observations in the 1980s the researchers found that, while the average change in storminess in the NH has been negligible, the SH is getting even stormier.
This is because changes in the SH’s ocean means energy transport toward the equator is stronger, an effect canceled out in the NH due to sunlight absorption from the loss of sea ice and snow.
The authors say this trend is consistent with other climate model projections of the climate response to increased CO2.
“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.”