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Published online: 8 February 2005; | doi:10.1038/news050207-6
Typhoons have long-distance contactPhilip BallCyclones may affect each other's trajectories even if thousands of kilometres apart.
| Tropical cyclones can influence each other's behaviour across huge swathes of ocean. © SPL |
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When there's a cyclone around, you want to know where it is headed. But that may be harder to predict than we thought.
Melinda
Peng and Carolyn Reynolds of the Naval Research Laboratory in Monterey,
California, have found that two simultaneous tropical cyclones can
influence one another in ways that are subtle and difficult to
forecast, even if they are nearly 2,000 kilometres apart.
Weather
forecasters currently use computer methods to predict the course of a
cyclone - a fierce storm system that gathers around a region of low
atmospheric pressure. But when there is another cyclone in the
vicinity, Peng and Reynolds warn, small differences in the initial
meteorological conditions used in the model can significantly affect
the prediction of the first cyclone's path and behaviour two days later.
The
distance over which cyclones can affect each other is surprising, the
researchers say. Previous observations of twin cyclones had led experts
to suspect that they could not affect one another if they were more
than 1,500 km apart.
Peng
and Reynolds used a cyclone-prediction model developed by the US Navy
to forecast the trajectories of two real, coexisting cyclones, Katsana
and Parma, that formed in the western Pacific Ocean in late October
2003.
The
predicted courses corresponded fairly closely to those of the real
storms over a two-day period. But a careful look at the results showed
that the forecast for Parma depended on that for Katsana, despite the
fact that the two storms were more than 1,500 km apart for most of
their lives. (Their closest approach was 1,333 km.) Parma did not, in
turn, seem to affect Katsana, which was the stronger of the two
cyclones.
What's
more, for cyclone Katsana, the researchers showed that the predictions
would have been altered significantly if the model's initial conditions
500 kilometres from the cyclone's centre were changed slightly, showing
how surprisingly sensitive these storms can be. "The forecasts can be
very sensitive to the initial state," Peng and Reynolds say. "Although
the forecasts in these cases were good, our tools highlighted the
potential for initial errors to grow."
Two
cyclones together is not uncommon - it happens about three times every
two years in the western North Pacific Ocean, and about once every
three years in the Atlantic.
Despite the potential problems for cyclone forecasting,
Peng and Reynolds say that their results have a positive spin too. The
new technique they used for analysing cyclone interactions, called
singular vector diagnostics, seems to give them a better handle on the
problem than standard methods. "It may tell us how much confidence we
can have in our forecasts," they say.
References
- Peng M. S. & Reynolds C. A. Geophysical Research Letters 32, L02810 (2005). | Article |
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