Computer simulations show that both ocean dynamics, such as the Gulf Stream, and mountain ranges influence the pattern of storm tracks on Earth. This also explains why Earth's storm tracks are so different from those on the gas giant Jupiter.
Storm tracks are regions where swirling eddies are concentrated. On Jupiter, eddies gather into belts at particular latitudes, meaning that the storm tracks circle the planet symmetrically.
But on Earth the storm tracks in the atmosphere, corresponding to regions where weather is highly variable, do not circle the planet uniformly. Although Earth's storm tracks broadly occupy the mid-latitudes, they contain both tilt and along-stream variability in strength. To understand how weather patterns will change in a future climate, we need to understand what mechanisms are responsible for controlling the present-day patterns.
"The main candidates for causing asymmetric storm tracks on Earth are ocean dynamics and mountain orography -- the shape of mountain ranges. Our study, published in the Journal of Climate, examines their relative roles using a special type of computer model, and answers a long-standing debate on the subject," says Dr Chris Wilson of the National Oceanography Centre.
Weather and climate in Europe are affected by small changes to the position of the North Atlantic storm track. Some previous studies suggest that the Gulf Stream has a large warming effect on European climate, since it carries large amounts of warm water from the tropics towards the continent. However, another study suggests that ocean dynamics such as the Gulf Stream are negligible and that the reason for the relative warmth of Europe for its latitude is that mountain orography causes the jet stream to deviate, carrying warm air from the tropics.
Both ocean dynamics and mountain orography provide potential mechanisms to break the symmetry of the storm track, mechanisms which are not present on Jupiter.
"We used an intermediate-complexity coupled ocean-atmosphere climate model to perform a set of highly idealised experiments. First, we made an Earth with a static ocean, which stored and released heat but didn't transport it, and with flat continents without mountains. Then we introduced ocean dynamics and orography in stages, until we got a realistic Earth. This set of four experiments allowed us to study the individual effect of ocean dynamics and orography on the storm track pattern, as well as the effect from their interaction."
Full-complexity climate models require a huge amount of supercomputer resources, so are not suitable to highly idealised experiments. Intermediate-complexity models contain simpler equations describing the physics of the system, but fill a useful niche, allowing testing of ideas before full-complexity simulation. For this study, the intermediate-complexity model FORTE replicates the observed storm tracks to quite a high degree of accuracy in the control simulation, adding confidence to the results from the idealised experiments.
The model experiments show that ocean dynamics and mountain orography play comparable roles in shaping the pattern of Earth's storm tracks. Ocean dynamics act to shift the storm tracks poleward and induce tilt over the western North Atlantic, and mountain orography causes along-stream variability in the storm tracks and tilt over the western Pacific. The interaction between ocean dynamics and orography has a minor local effect on the storm tracks.
"Our study shows that ocean dynamics do influence atmospheric storm tracks and therefore European weather and climate. We do not yet know how sensitive forecasts of European weather and climate will be to the detailed representation of ocean dynamics. However, we have shown that both oceans and mountains influence storm tracks to a similar degree but with different effects," says Wilson.
The researchers are Chris Wilson and Bablu Sinha of the National Oceanography Centre, and Ric Williams of the University of Liverpool.
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