Breathing Planet

With thee conversing I forget all time,

All seasons, and their change; all please alike.

—John Milton (1867), Paradise Lost, Book IV

circumpolar cyclone

Solar radiation is the principal heat engine powering the Earth system. Across the planet, there are air-sea interactions that are forced by seasonal sunlight, most notably in the high latitudes where the winter-summer contrasts are most extreme. For example, around Antarctica, the absence of solar radiation during the winter cools sea-surface temperatures and causes sea-ice coverage to expand from a summer minimum of 3 million square kilometers to nearly 20 million square kilometers (Fig. 8.1)—across an area nearly equal to that of North America. Eighty-five percent of this sea ice in the unbounded Southern Ocean is formed annually, as opposed to the enclosed Arctic Basin where only 15% of the sea ice is new each year (Fig. 3.1).

Why does Antarctica have circumpolar environmental systems?

As seen already, variable warming or cooling of different regions on the planet influences climate cycles (Figs. 7.2-7.4) as well as the density-driven dynamics of the ocean (Figs. 7.8 and 7.9). Differential heating across the Earth also influences atmospheric circulation patterns and winds which, in turn, drive the surface

Nuclear Blast Radius

FIGURE 8.3 Currents of the Southern Ocean, which is bounded by the Antarctic Continent and the sea floor south of the Subtropical Convergence (Subantarctic Front Zone). The predominant clockwise trajectory of the West Wind Drift in the atmosphere drives the Antarctic Circumpolar Current. The Antarctic Convergence (Antarctic Polar Front Zone) is the northern boundary of the Antarctic marine ecosystem. South of the West Wind Drift is the counter-clockwise East Wind Drift, which drives the Antarctic Coastal Current. Between the East and West Wind Drifts is the circumpolar up-welling zone at the Antarctic Divergence (Fig. 7.8). Modified from Knox (1970) and Berkman (1992).

FIGURE 8.3 Currents of the Southern Ocean, which is bounded by the Antarctic Continent and the sea floor south of the Subtropical Convergence (Subantarctic Front Zone). The predominant clockwise trajectory of the West Wind Drift in the atmosphere drives the Antarctic Circumpolar Current. The Antarctic Convergence (Antarctic Polar Front Zone) is the northern boundary of the Antarctic marine ecosystem. South of the West Wind Drift is the counter-clockwise East Wind Drift, which drives the Antarctic Coastal Current. Between the East and West Wind Drifts is the circumpolar up-welling zone at the Antarctic Divergence (Fig. 7.8). Modified from Knox (1970) and Berkman (1992).

Wind-driven currents around Antarctica (Fig. 8.3) are coupled with the atmosphere, as are thermohaline circulations propagating through the ocean (Figs. 7.8 and 7.9). In both cases, the principal connections involve solar radiation and transferring water between its solid, liquid, and gas phases (Chapter 7: Flowing Planet).

Summer Winter

Summer Winter

FIGURE 8.1 Latitudinal profile of the average Antarctic Surface Water temperatures (from 0 to 10 meters depth) during the austral summer (December through February) and winter (June through August) in the Southern Ocean (Figs. 7.8 and 8.3). Sea ice minimum and maximum coverages are shown as shaded areas during the summer (February 1974) and winter (August 1974), respectively— extending from around 3 million to 20 million square-kilometers each year (Fig. 3.1). Modified from Schwerdtfeger (1970).

FIGURE 8.1 Latitudinal profile of the average Antarctic Surface Water temperatures (from 0 to 10 meters depth) during the austral summer (December through February) and winter (June through August) in the Southern Ocean (Figs. 7.8 and 8.3). Sea ice minimum and maximum coverages are shown as shaded areas during the summer (February 1974) and winter (August 1974), respectively— extending from around 3 million to 20 million square-kilometers each year (Fig. 3.1). Modified from Schwerdtfeger (1970).

currents of the ocean. This coupled movement of the ocean and atmosphere is further regulated by the motion of the planet itself.

On a non-rotating Earth, air would rise where it is warm and sink where is it is cold. These thermodynamic features of the atmosphere basically involve temperature and pressure as defined by Robert Boyle for an ''ideal'' gas (Equation 8.1). For example, in a container with a constant volume, a heated gas will cause pressures to rise—as in a pressure cooker. Higher pressures, which are associated with enhanced molecular motion at higher temperatures, also explain why a balloon (which has a flexible volume) inflates as it is warmed and deflates as it is cooled. Similarly, when a heated gas occupies a larger volume it will have a lower density of molecules—which is why hot air rises over cold air. In relation to the Earth's atmosphere, Eq. (8.1) (the ''ideal gas law'') shows generally that temperatures and pressures proportionally increase and decrease together:

where P is gas pressure; V, gas volume; n the amount of gas; R, a constant; and T gas temperature. An ''ideal gas'' involves molecular collisions with negligible cohesive forces between the molecules.

As opposed to a stationary body, however, the Earth is spinning around its axis (Figs. I and 7.2) at approximately 1670 kilometers per hour at the equator, com pleting one rotation every 24 hours (Chapter 1: Global Dimensions). This daily rotational period is the same at all latitudes. However, given that distances around the Earth decrease north and south of the equator, the apparent speeds of rotation decrease toward higher latitudes, such that the Earth is only moving half as fast at 60° latitude.

This spinning of the Earth not only influences day and night periods throughout the year, but also influences the trajectory of air and water masses around the planet. As a simple experiment: draw a line across a piece of paper. Now draw the same line while somebody rotates the piece of paper. The curvature of your line is analogous to the directional changes imposed on all air and water masses that are in motion across the planet. Similarly, from the perspective of outer space, moving fluids are deflected in opposite directions in the two hemispheres—curving to the right of their original path in the Northern Hemisphere and to the left in the Southern Hemisphere. This impact of a rotating Earth, which causes the apparent trajectory of a moving fluid to be deflected at right angles to its original path, is known as the ''Coriolis effect'' after the Frenchman Gaspard Gustav de Coriolis (1792-1843), who proposed the concept in the early 19th century.

Like a marble rolling down an incline, air masses will move from high to low pressures—gaining velocity in proportion to the slope of the pressure gradient. Greater than 1000 meters above the Earth, atmospheric motion also will be unimpeded by surface frictional forces—resulting in winds that flow eastward in the direction of the Earth's rotation.

Around low-pressure regions, air masses will converge and spiral inward to form cyclones that gyrate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Conversely, air masses will diverge and spiral outward from high-pressure regions to form anticyclones that circulate in reverse directions.

As the coldest region on the planet, Antarctica has the lowest regional atmospheric pressures on Earth [seeEq. (8.1) about the temperature-pressure relationship]. Moreover, around 5000 meters altitude (which is well above Antarctic surface elevations), atmospheric pressures decrease continuously toward the high southern latitudes—producing a latitudinal pressure gradient (Fig. 8.2) that mirrors the temperature gradient at the sea surface (Fig. 8.1). This barometric gradient persists throughout the year, during both winter and summer seasons, because of the enormous thermal inertia generated by Antarctica and its vast ice reservoir. Moreover, this zonal pressure gradient has been maintained over millions of years—as long as Antarctica has been a global heat sink surrounded by warmer regions (Fig. 6.3, Table 6.1).

Because fluids move from high to low pressures, the atmospheric pressure gradient toward Antarctica creates winds that are acted upon by the Earth's rotation, generating an enormous cyclonic circulation that exists across half of the Southern Hemisphere. Clockwise movement of this polar vortex generates the West Wind Drift (Fig. 8.3).

Summer Winter

Bandera Onu Colorear

FIGURE 8.2 Circumpolar bands of atmospheric pressures at the 500 millibar height (about 5000 meters above sea level), which includes the atmosphere over the entire continent, during the summer (January) and winter (July). Note that atmospheric pressures are lower during the cooler winter months [as described in Eq. (8.1)]. Progressive increase in atmospheric pressures with decreasing latitude establishes the zonal pressure gradient that generates the giant cyclonic circulation of the West Wind Drift across half of the southern hemisphere (Fig. 8.3). Parallel patterns of atmospheric pressures, seasonal sea surface temperatures, and sea ice extent (Fig. 8.1) are influenced by the annual solar radiation cycle (Fig. 7.1). Modified from Schwerdtfeger (1970).

FIGURE 8.2 Circumpolar bands of atmospheric pressures at the 500 millibar height (about 5000 meters above sea level), which includes the atmosphere over the entire continent, during the summer (January) and winter (July). Note that atmospheric pressures are lower during the cooler winter months [as described in Eq. (8.1)]. Progressive increase in atmospheric pressures with decreasing latitude establishes the zonal pressure gradient that generates the giant cyclonic circulation of the West Wind Drift across half of the southern hemisphere (Fig. 8.3). Parallel patterns of atmospheric pressures, seasonal sea surface temperatures, and sea ice extent (Fig. 8.1) are influenced by the annual solar radiation cycle (Fig. 7.1). Modified from Schwerdtfeger (1970).

Like the oceanic density gradients that drive water-mass circulations (Chapter7: Flowing Planet), atmospheric pressure gradients generate wind-driven currents. Because of its persistent frictional drag on the sea surface, the West Wind Drift produces a clockwise circulation in the underlying Southern Ocean. This coupling between the atmosphere and ocean has produced the largest current system on Earth—the Antarctic Circumpolar Current—which flows around the continent at an average rate of 120 million cubic meters every second. This volume transport of the ocean around Antarctica is two to three times that of the Gulf Stream in the North Atlantic.

The high volume transport of the Antarctic Circumpolar Current is not due to its velocities—only a tenth of those for the Gulf Stream, which travels around 200 centimeters per second. Rather, the massive transport of the Antarctic Circumpolar Current is due to its immense breadth (up to 1000 kilometers wide), depth (over 2000 meters deep), and circumpolar trajectory, which is virtually unobstructed by continental land masses.

Between the Antarctic Circumpolar Current and the continent is the counterclockwise East Wind Drift (Fig. 8.3). The East Wind Drift creates the Antarctic Coastal Current, which flows around the continent from the Antarctic Peninsula along a narrow band near the continent before being incorporated into the northward-flowing Weddell Sea Drift.

water in, water out

Because of differential heating across the planet (Fig. 7.1) and the "Coriolis effect,'' there are six wind belts around the Earth. These wind zones are formed by three convection cells of converging (flowing together) and diverging (flowing apart) air masses in each hemisphere (Fig. 8.4).

Regions where air masses converge at the surface and diverge aloft produce zones of ascending air. In these regions, warm air rises and then cools at higher altitudes as atmospheric pressures decrease. Owing to the general pressure-temperature relationship of gases [Eq. (8.1)], temperatures of rising air masses will decrease by around 10°C every 1000 meters. In addition, because warm air can retain more moisture than cold air, the moisture in the rising air masses will condense at higher altitudes. For these reason, zones of ascending air—which occur at the equator and at 60° latitude in both hemispheres—are characterized by high levels of precipitation (Fig. 8.5).

Conversely, regions where air masses converge aloft and diverge at the surface represent zones of descending air. At higher altitudes, cold air retains small volumes of moisture. As it descends, atmospheric pressures increase and the cold, dry air warms [Eq. (8.1)]. This warmed dry air then contacts the Earth's sur-

which are mirror images in both hemispheres, are influenced by this relative heating of the atmosphere and the ''Coriolis effect'' associated with the Earth's rotation (see text).

Water in, Water out S

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