19 September 2010

Can tiny marine plants steer some of the world's biggest storms?

Posted by vivienne


An infrared image of Severe Tropical Cyclone Monica, a hurricane with maximum wind speeds of 177mph, which formed in the South Pacific in April 2006 (image from the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies. Facts about Monica from http://earthobservatory.nasa.gov/Features/WorldOfChange/storms.php)

Phytoplankton –  microscopic marine plants – may be small, but they could have a big impact on the routes hurricanes take across the North Pacific. Incredible to think plants less than five millimetres across could change the paths of storms perhaps 500km wide, but the phytoplanktons’ secret is their vast numbers.

Sometimes called the ‘grass of the sea’, phytoplankton are the most abundant marine life form and base of the ocean’s food chain. Like grass, they capture the sun’s energy using a light-absorbing green pigment called chlorophyll and – where they’re blooming en masse – change the ocean’s colour from deep, clear blue to murky turquoise.

A crescent-shaped string of plankton in the North Sea weaving through the Scandinavian region. Norway (left) and Sweden (right), part of the Scandinavian Peninsula, are visible at the top, and Denmark is at bottom right. The image was taken by European Space Agency (ESA) satellite Envisat. The caption and image are taken from the ESA website (http://www.esa.int/esaEO/SEMKVW7Q69G_index_0.html)

Should you have spent this summer in a glass-walled office with no air conditioning, you’ll know transparent materials allow lots of the sun’s heat to get through. Clear ocean water is the same – the sun’s warmth reaches deep into the ocean’s interior. In contrast, plankton-filled water is warm near the ocean’s surface and cold below 100m deep because the plants intercept the lion’s share of the sun’s energy before it can travel far.

This is all theory from computer modelling, of course, according to Anand Gnanadesikan from the Geophysical Fluid Dynamics Laboratory at the US National Oceanic and Atmospheric Administration (NOAA) and colleagues. Isolating the effect of phytoplankton on ocean temperatures in real life is hard because water sloshes around.

But, assuming the theory is correct, Gnanadesikan and team wanted to know if the differences in ocean temperatures between green and blue water could be affecting hurricane formation in the North Pacific. The fuel powering these violent storms is warm ocean water.

The team tested their theory using a computer climate model. I won’t bore you with the details, but they exterminated virtual phytoplankton in the North Pacific subtropical gyre. Phytoplankton blooms verdantly around the world’s gyres – huge, circular ocean currents – because the swirling waters at the gyres’ edges bring nutrients to the surface.

There are five major ocean-wide gyres — the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Each is flanked by a strong and narrow “western boundary current,” and a weak and broad “eastern boundary current” (image and description from NOAA)

The simulated ‘phytocide’ had dramatic effects. With less plankton around, there were 15% fewer hurricanes each year. The remaining hurricanes weren’t steered as far north as before – hurricane activity dropped by 70% in the subtropical Northwest Pacific. Hurricanes tended to meander along the equator instead. More hurricanes wandered through the South China Sea, but hardly any travelled north into the East China Sea and onwards to hit Japan and the South China coast. Virtual inhabitants of these places could breathe a sigh of relief.

Map of the South China Sea. From the United States Department of Energy: http://www.eia.doe.gov/emeu/cabs/schina.html

Map of the East China Sea

But why did removing marine plants confine hurricanes to the equator? The answer is the clearer water strengthened the Hadley Circulation – a huge wind system that moves air between the equator and the subtropics. The Trade Winds – so-named because they helped England’s merchant sailing fleet carry goods across the Atlantic Ocean – are a surface manifestation of this circulation.

Global circulation of Earth's atmosphere displaying Hadley cell, Ferrell cell and polar cell (diagram and caption from sealevel.jpl.nasa.gov)

The Hadley Circulation hoists hot, moist air into the upper atmosphere over the equator causing it to cool and drop the water as rain. The now dry, cold air is spun northwest by the Earth’s rotation and sinks back to the surface over the subtropics.

In the climate model’s plankton-lite Pacific, the sun’s heat penetrated further into the ocean – warming the depths and cooling the surface. This cold surface water chilled the air above it, encouraging more dry, cold air to sink. All this dry, cold sinking air was bad news for violent hurricanes, which need lots of warm, moist air to grow.

The sinking air had to go somewhere – the equator, where it was wafted aloft and carried away in westerly winds in the upper atmosphere. These strong westerlies also scuppered the virtual hurricanes’ chances to form. The complex structure of white, spiralling clouds you see in hurricane pictures need time to set up and the strong winds in the model literally blew them away.

Hurricane Igor snapped from space five days ago showing an eye and a complex structure of spiralling clouds. The storm is due to hit Bermuda tomorrow (Image from http://earthobservatory.nasa.gov/IOTD/view.php?id=45780)

@highlyanne asked me why the warming at depth in plankton-free waters doesn’t encourage hurricanes to form. I’ve answered her question here.

Gnanadesikan, A., Emanuel, K., Vecchi, G., Anderson, W., & Hallberg, R. (2010). How ocean color can steer Pacific tropical cyclones Geophysical Research Letters, 37 (18) DOI: 10.1029/2010GL044514