Tropical storms migrate toward poles

Storms are reaching their maximum intensity further north and south every decade

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Tropical storms have been migrating northwards and southwards towards the poles for the past 30 years, a paper in Nature says.

People are becoming less likely to face the worst of a hurricane or typhoon if they live close to the Equator, and more likely if they live on the edges of the tropics.

The storms are reaching their maximum intensity 52-63km further north and south every decade.

The paper, led by the US agency Noaa, underlines a previous findingthat the area defined as the Tropics is clearly expanding.

The researchers believe humans are influencing the changes - with climate change, ozone depletion and aerosols playing a part. They cannot yet be certain which factor is most significant.

The lead author, James Kossin from the National Oceanic and Atmospheric Administration's (Noaa) National Climatic Data Center, says the research is based on observations of the point of maximum intensity of the storm.

He told BBC News: "By the time a serious storm reaches maximum intensity, someone has picked it up and is monitoring it, so we can be pretty confident of this data.

"What we can't be sure of yet is exactly what's causing the trend. There is compelling evidence that the expansion of the tropics is attributable to a combination of human activities, but we don't know which is the primary factor.

"If ozone depletion is mainly to blame, then the situation is likely to stabilise by the middle of the century after ozone-depleting chemicals are phased out. But if climate change is the main factor then there's no end in sight to this phenomenon."

Atmospheric heating

Hurricanes are formed when warm air above warm seas rises upwards to create low pressure, which in turn sucks in more air at the sea surface. The Earth's spin twists the column of air and intensifies the process.

Dr Kossin says the central Tropics are now less "hospitable" to storms, as the ideal conditions for storm creation are shifting slightly north and south. His paper shows that the degree of wind shear in an area an important factor in storm formation, as well as sea surface temperature.

The Tropics dominate world weather by producing a strong updraught of warm moist air which cascades out towards the poles. The area generating the updraught has been expanding since 1979 by 0.5 degrees- 1 degrees latitude per decade.

Warming in the deep tropics causes stronger updraughts that spread further pole-ward, and greenhouse gases are thought to be contributing to this process. The atmosphere is also being heated by tiny particles known as aerosols; some produced by dirty diesel engines and cooking stoves, others by ozone in the troposphere.

Another factor is ozone in the stratosphere. This causes stratospheric cooling, which in turn affects air currents from the tropics.

Noaa says the migration of tropical storms is a global phenomenon with regional variations. In the Southern Hemisphere the Pacific and Indian oceans show a strong pole-ward trend.

The largest shift in the Northern Hemisphere is in the western Pacific. The Atlantic and eastern North Pacific exhibit small movement.

Dr Kossin says the team has tested for the influence of the natural climate system ENSO, but it is unlikely to have played a big part.

Julian Heming, an expert in cyclones at the Met Office, told BBC News that the research did not mean that people very near the Equator would escape severe storms entirely. Typhoon Bopha in 2012 and Typhoon Haiyan both hit land in the Philippines at or below 10 degrees from the Equator.

He said: "It is worth remembering that this paper is not showing any increasing trend in tropical cyclone intensity - just the location of peak intensity."

5:16 PM Share:

Turtle migration driven by hatchling drift experience

The study should inform more effective conserve strategies

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Why do turtles migrate to the places they do? What is it that tells them where to swim as adults?

New research is providing some insight, suggesting the animals' experiences as little hatchlings adrift in ocean currents have a huge influence.

Scientists combined all the available satellite tracking data on adult turtles with models of how the world's sea water moves.

It shows that locations encountered in the earliest years are a powerful draw.

If these foraging sites are favourable and not too distant, the turtles will swim directly back to them as adults, time and time again.

If they are not suitable locations, the adults may simply not undertake migrations and just feed in the open ocean.

"The kind of information we're acquiring is important for designing effective conservation strategies," said Dr Rebecca Scott, who led the study soon to be reported in the journal Ecology.

"We can't protect turtles in their feeding areas if they're going to lots of different locations and we don't really understand why they're going to those places," she told BBC News.

Many animal groups undertake great migrations, and the process of learning where to go on these travels can take several forms.

For some species like the great whales, the mother shows her calf the way. And for some bird species, the young will learn the necessary flight routes by following at the rear of the flock.

But neither of these strategies works for turtles. Once the adult female has laid her eggs on a beach, her involvement in her offspring's development is ended.

And when the hatchlings run down the beach into the water, they are on their own; there is no experienced turtle to follow, and they go where the ocean takes them.

The difficulty of affixing satellite-tracking tags to neonate (baby) turtles means the data on where precisely they venture in their early years is very sparse.

But scientists can get a good idea by looking at the flow patterns of ocean water moving past nesting sites.

Dr Scott and colleagues used this information to build a picture of the likely drift patterns of the hatchlings.

They then compared this view with the ample sat-tracking data that does exist for adult turtles.

The study incorporates ocean current information from drifter buoys (coloured tracks) passing close to turtle nesting sites (stars)

The synthesis strongly suggests that adult sea-turtle migrations and the selection of foraging sites are closely tied to the past experiences of drifting hatchlings.

The big difference of course is that the adults do not drift - they swim more or less direct.

"Hatchlings' swimming abilities are pretty weak, and so they are largely at the mercy of the currents. If they drift to a good site, they seem to imprint on this location, and then later actively go there as an adult; and because they're bigger and stronger they can swim there directly," explained Dr Scott.

"Conversely, if the hatchlings don't drift to sites that are suitable for adult feeding, you see that reflected in the behaviour of the adults, which either do not migrate or they feed [in the open ocean], which is really not the normal strategy for most turtle species."

Dr Scott is currently affiliated to the Geomar Helmholtz Centre for Ocean Research in Kiel, Germany, but was formerly at Swansea University, UK.

Commenting on the study, Prof Brendan Godley from Exeter University told BBC News: "This is a study of great scope using recently developed oceanographic models as well as tracking data from a large number of oceanic drifter buoys to model the routes hatchling turtles might take from a large number of sites around the world.

"The location of adult turtle foraging grounds discovered by satellite tracking correlates with the expected patterns of hatchling movements.

"It is clearly an incremental step forward, highlighting how the combination of high-tech methods can give us real insights into the enigmatic life cycle of marine turtles. In the long term, however, we need to find techniques which allow us to track hatchling turtles from the beach without impediment. Hopefully, this is not too far over the horizon."

6:03 AM Share: