Facts of the Matter
Complex systems mesh to create climate
"Climate is what you expect. Weather is what you get."
-- Robert Heinlein
In the past two decades, scientists have been piecing together surprising interactions among various aspects of the global climatic systems.
Until recently it was assumed that the weather and climate in any given place was influenced by its location and not by something that happened on the other side of the world.
Now it is clear that the entire earth is part of a global climatic system where local weather, and hence long-term climate, are affected by previously unsuspected connections.
At the most fundamental level, both weather and climate are manifestations of the laws of thermodynamics: Heat flows from hot to cold, and to maintain a constant temperature any object must lose as much heat as it gains.
The global climate system is complicated by how the Northern Hemisphere contains 85 percent of all land mass on Earth, which affects the heating and cooling of the two hemispheres differently.
At the equator, Earth receives more heat from the sun than it radiates. At the poles, it radiates more than it receives. Since Earth's temperature stays relatively constant, there must be a constant flow of heat from the tropics toward the poles.
The ocean and atmosphere carry heat poleward in both horizontal and vertical motion. But that is only the beginning of the story.
The global circulation of the atmosphere is dominated by regions of semipermanent low and high pressure.
The subtropical high is a semipermanent mound of air located over the oceans near 35 degrees latitude in the Northern and Southern hemispheres.
Each shifts slightly north-south and east-west as the seasons change. They are most intense and closest to the poles in summer.
The polar low is a depression in the atmosphere that forms over high-latitude seas within a polar or arctic air mass. It generally moves eastward, guided by the polar jet stream in the upper atmosphere. As it moves eastward, it closes in on itself and is replaced by a new low near its original location.
We now know that the surface and deep ocean surface, upper and lower atmosphere, and land operate as a heat transfer system that consists of an intertwined complex of coupled systems.
Coupled systems exist between the surface and deep ocean, between ocean and atmosphere, between ocean and land, and between land and atmosphere and between lower and upper atmosphere.
Some of the coupled systems that are being studied are El Nino-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Stratospheric Quasi-bienniel Oscillation (QBO), surface ocean circulation, thermohaline circulation (often called "the great conveyor belt"), upwelling and monsoons, in addition to ongoing studies of weather systems such as hurricanes and frontal storms.
Scientists first became aware of the connections between these systems while studying El Nino, a condition caused when the trade winds weaken and allow a mass of warm water to move eastward across the tropical Pacific Ocean.
We now can correlate with some degree of certainty many changes in weather that occur during El Nino episodes.
Heavy rain in Southern California, an increase in hurricane frequency in the Pacific and decrease in the Atlantic, droughts in Africa and Australia, and decreased monsoon rain in Asia are just a few of the climatic effects thought to be associated with El Nino.
The tropical ocean affects the atmosphere above it by heating the air and adding moisture. The atmosphere also influences the ocean below it to form a couple system, ENSO.
During an El Nino episode, sea level becomes lower in the eastern Pacific as surface pressure increases and higher in the western Pacific where surface pressure decreases. The opposite occurs during the condition called La Nina.
This seesaw in atmospheric pressure and sea level between the eastern and western tropical Pacific is called the Southern Oscillation, often abbreviated as SO. The coupled system is called ENSO and occurs with no particular regularity and with varying severity every three to seven years.
The NAO is the dominant factor affecting the variability of winter climate in the North Atlantic, extending from North America, Europe and much of Northern Asia.
NAO is a large-scale seesaw in atmospheric mass between the subtropical high and the polar low, similar to the SO, that exhibits a tendency to remain in one phase for intervals lasting several years.
A monsoon is the seasonal reversal of prevailing wind direction caused by uneven heating of land and sea. Land heats up quicker than water, so air over large land masses rises as it warms, causing lower pressure. Moist air flows across the surface from the surrounding sea and is uplifted by mountain ranges, causing clouds and rain.
The monsoon is strongest in southern Asia where warm, moist air from the Indian Ocean is lifted over the towering Himalayas, causing torrential downpours.
The monsoon effect occurs to a lesser degree in North America, where summer thunderstorms are common in the southwestern deserts. It even occurs on small islands, being responsible for the familiar and regular afternoon shift in wind that brings rain to quench the thirst of Kona coffee on the southwestern slopes of Mauna Loa.
Surface ocean circulation extends to depths of a few hundred feet and is driven parallel to the global wind patterns that circulate clockwise around the subtropical highs.
Wind blowing toward the equator along the western coastlines causes upwelling as it pushes surface water away from the shore, bringing cold, nutrient-rich water up from the depths.
El Nino interferes with upwelling by deepening the warm surface layer of the eastern Pacific ocean and thus restricting the flow of cold water from the depths.
Circulation in the deep sea comprises most of the mass of ocean water. It is driven by large-scale convection caused by the density differences in sea water, which is determined by temperature and salinity together, so the deep-ocean circulation is also called the thermohaline circulation or "the great conveyor belt."
Warm, salty surface water flows northward in the Gulf Stream, gets chilled and sinks in the extreme North Atlantic to flow south in the middle layer of the Atlantic. There it is cooled further and flows along the sea floor into the Atlantic, Indian and Pacific basins. After warming and rising slowly to the surface, primarily in the Pacific and Indian oceans, the water returns as subsurface flow to the North Atlantic, where it joins the surface flow from the Gulf Stream to repeat the cycle.
Each of these coupled systems can interact with any of the others. If you can picture the myriad possibilities, then you get an idea of the difficulties in making conclusions about the systems and modeling them to make weather forecasts or climate predictions.
Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at firstname.lastname@example.org