Plankton matter -deep blue oceans spawn fewer tropical storms
17/8/2010 New Scientist By colouring ocean waters, the microscopic plants encourage hurricanes and typhoons.The finding suggests that ocean colour, seen from space, could one day be used to predict changes in the number and intensity of hurricanes and typhoons.Climate change is predicted to decrease the amount of phytoplankton in the ocean, which the study suggests would mean many fewer hurricanes and typhoons hitting American and Asian coastlines.
Clear ocean waters look deep blue, but they become a murkier shade of blue depending on how much phytoplankton – tiny floating plants that are at the base of the ocean food web – they contain.
To quantify the effect this colouring has on tropical cyclones, Anand Gnanadesikan of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, and colleagues modelled the north Pacific subtropical gyre, a massive region of circulating ocean currents over which cyclones form. They found that clearer ocean waters would mean fewer cyclones.
Clear blue gyres
The centres of gyres tend to be clearer than their edges, allowing solar heat to penetrate to nearly 100 metres, while it gets no further than 5 to 10 metres at the edges. This means that areas of ocean that are murky with phytoplankton get warmer nearer the surface, while clearer waters warm up further down.
That deeper heat is carried away from the gyre by its associated deep currents, creating a region that is cooler than the rest of the tropics. There are no such currents to carry away the warmer waters of the murky regions. A cooler area changes patterns of regional air convection and so reduces the number of cyclones over the gyre.
Gnanadesikan and colleagues modelled two cases: one in which the gyre had half its normal chlorophyll levels, and another in which it was completely clear. They found that a clear gyre would reduce the number of tropical cyclone days over the north Pacific by about 70 per cent, whereas reducing the chlorophyll by half led to a 35 per cent drop.
The findings are backed up by records from the 1960s, which show that cyclone activity in the Pacific subtropical gyres during the 1960s was about 20 per cent lower than today, while chlorophyll levels were about half of present-day levels – a trend consistent with the model.
The modelling also showed that the reduction was more pronounced for cyclones whose winds had top speeds greater than 33 metres per second, suggesting that ocean colour can influence the intensity of cyclones as well as their number.
Where’s it headed?
Past observations and models predict that warmer oceans will produce less phytoplankton but local effects complicate matters, making it difficult for now to predict how the colour of the gyres may change in coming decades. For instance, increased industrial activity in China has deposited more iron-rich dust in the Pacific over the past 40 years, but it’s not known whether this results in more nutrients (and hence more colour) at the edges of the gyre or at the centre.
“The potential for change is significant, but our understanding of the underlying dynamics is so rudimentary that it’s not clear that we know which direction the change would be,” says Gnanadesikan.
There’s also the issue of Atlantic hurricanes, which will require more complicated analysis, says Gnanadesikan. The formation of these hurricanes is affected by sea-surface temperatures in the Pacific, and preliminary analysis shows that if the Pacific gyres were completely clear, Atlantic hurricane activity would fall by 12 per cent, he says.
Climate modeller Jimy Dudhia of the National Center for Atmospheric Research in Boulder, Colorado, says that using ocean colour to predict future hurricane activity “is reasonable, given that [the technique] has been validated against past seasons”. And Jorge Sarmiento, who studies ocean biogeochemistry and circulation at Princeton University, says that the work “deserves serious consideration”.
Journal reference: Geophysical Research Letters, DOI: 10.1029/2010gl044514
Go to: http://www.newscientist.com/article/dn19319-deep-blue-oceans-spawn-fewer-tropical-storms.html