Currents and eddies stir up ocean life
WOODS HOLE, MASS. — Dennis McGillicuddy says he smiles as he goes to work. He loves mysteries and he's solving a big one - the mystery of what drives the ocean's rich abundance of life.
The question nagged at oceanographers throughout the past century. They must discover this process to understand marine ecology, to better manage fisheries, to and trace what human actions are doing to ocean life.
They need ecological insight to better understand global warming climate change. The growth-decay-renewal cycles of ocean life remove heat-trapping carbon dioxide from the atmosphere. Any disruption of those cycles could exacerbate a warming.
Dr. McGillicuddy explains that the abundance mystery baffled scientists because you can't get at the answer from research ships. It takes a combination of ocean-scanning satellites, instrumented buoys and moorings, plus ships, to get the necessary data. To master the physics, chemistry, and biology involved takes the kind of analytical power he has at the Woods Hole Oceanographic Institution, and the kind of computer power his colleagues have at the Los Alamos National Laboratory in New Mexico and the National Center for Atmospheric Research in Boulder, Colo.
Focus those resources on the productivity question and the long-sought answer hits you in the eye. McGillicuddy explains that they show the interplay of currents, fronts, and massive eddies - the ocean's equivalent of the atmosphere's winds, fronts, and cyclonic storms. That action roils the water stirring up life-sustaining nutrients.
Nitrates, phosphates, and other nutrients concentrate near the sea floor. Upwelling currents carry them into sun-lit surface waters to fertilize microscopic plants called phytoplankton. This is the so-called "grass of the sea" that lies at the base of the ocean's food chains. This process is well known in offshore waters where it supports rich fisheries and in the regular spring-time plankton blooms of the open sea. But such long recognized sources account for only half of the nutrient supply needed to sustain the ocean's full productivity. McGillicuddy says it now seems clear that the ocean's internal "weather" supplies the rest.
Eddies are tens to hundreds of kilometers in extent. They last for weeks to several months. With a research ship, you might visit an area teeming with life only to find it barren when you return sometime later. Such experiences have puzzled oceanographers working mainly from ships. But with data from satellites that track eddies and detect phytoplankton's green chlorophyll, along with a permanent set of instruments to monitor an area where eddies often pass, the mystery disappears. That's what McGillicuddy and colleagues do using an instrumented mooring 50 kilometers south east of Bermuda.
The altimeter on the French/American Topex/Poseidon satellite measures sea surface heights to an accuracy of 4 centimeters. The 20 centimeter or so depression of an eddy shows up as clearly as an atmospheric low on a TV weather map. McGillicuddy says the experimental setup regularly shows nutrients flowing upward when an eddy passes. That's when the phytoplankton bloom and the entire "biomass goes bonkers," he says.
Computer simulations at Los Alamos and the Boulder center mimic the process. McGillicuddy showed several examples during a recent lecture at Woods Hole to illustrate how closely his theory of ocean productivity matches what the real world data show.
Episodes like this make the ocean bloom in areas that otherwise would be barren. Occurring on a global scale, they fertilize an abundance of life that the sea could not otherwise sustain. That abundance is a major climate regulator. Phytoplankton use light and carbon dioxide to make oxygen and carbohydrates. When the tiny plants and animals that feed on them die and sink, they take the carbon with them. If there were less biological activity, more carbon dioxide would remain in the atmosphere to enhance global warming.
McGillicuddy notes that global warming is predicted to be stronger at the poles than at the equator. This already seems to be happening. The temperature difference between equatorial regions and polar latitudes is the main driver of both atmospheric and ocean weather. If that driver weakens, the ocean storms that fertilize the sea may also weaken. It's one more factor to include in computer-based climate simulations.
Meanwhile, McGillicuddy is looking into the way the ocean's internal weather affects sound propagation. He wonders whether whales can locate eddies acoustically and go straight to where the best eats will be found. It's yet another puzzle to investigate. "That's why I have a smile on my face," he says.
(c) Copyright 2001. The Christian Science Monitor