Trade Winds Weakening
6th July 2006
By GWA Correspondent


In the harbour, in the island, in the Spanish Seas,
Are the tiny white houses and the orange trees,
And day-long, night-long, the cool and pleasant breeze
Of the steady Trade Winds blowing.

There is the red wine, the nutty Spanish ale,
The shuffle of the dancers, the old salt's tale,
The squeaking fiddle, and the soughing in the sail
Of the steady Trade Winds blowing.

And o' nights there's fire-flies and the yellow moon,
And in the ghostly palm-trees the sleepy tune
Of the quiet voice calling me, the long low croon
Of the steady Trade Winds blowing.

John Masefield’s poem from his Sea Ballads was written over a hundred years ago, but the steady reliable trade winds to which he referred are weakening according to a recent study released in the 4th May issue of Nature. The report emanated from Gabriel Vecchi and his team at the University Corporation for Atmospheric Research in New Jersey.

The trade winds blow at low latitudes from an easterly direction at the earth’s surface as a result of the planet’s rotation from west to east. In the Pacific Ocean warm moist air rises near Asia in the Indonesian area and travels east at high level descending in the eastern Pacific near South America. As the poem implies, they blow with consistent reliability and the Pacific conveyor is known as the Walker circulation.

This large scale system is important in that the constancy of direction of the winds also drives the sea currents. Any change has serious implications for marine life, as the trade wind induced currents cause an up welling of the deeper, nutrient rich waters on which so much marine life depends.

Exactly how the conveyor system works is, like so much in atmospheric science, a matter of theory and conjecture. One popular theory goes like this. In order for the total atmosphere to remain energy neutral, water vapour taken into the atmosphere must be balanced by rainfall out. If sea temperatures rise that naturally results in an increase in water vapour the air can hold. However, for complex physical reasons, the atmosphere cannot release the same increased proportion of rain. In order for the atmosphere to release the water vapour as rain, the trade winds must slow down. This enables the energy equation to remain in balance. It’s a theory but leads scientists to conclude that the trade winds will slow down in the future as Man’s activities continue to warm the planet.

Vecchi and his team looked at records of sea level atmospheric pressure going back to the start of the industrial revolution, even including those of the Royal Navy to fill the gaps. They combined these with computer simulations to find that the trade winds have weakened by 3.5% over the past one hundred and fifty years. Further, by extrapolation, they predicted that a further slowing of the order of IO % could be expected by the end of this century.