(Editor's Note: This article was originally published on May 28, 2008. You comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
It's Hot, and that's Cool!
One of the most intriguing climate paradoxes is that Antarctica (see photo at right) holds the title of coldest place on Earth, while the southern hemisphere winters are generally warmer than the northern hemisphere winters. The reason for this lies in the difference between water and land. Land warms and cools about 4 times faster than water, so large bodies of water can serve to moderate temperatures. In my own greenhouse, I found that I could use this knowledge to help keep temperatures more moderate at night in winter. To accomplish this, I placed large containers of water underneath my benches. During the day, as the air warms inside, the water also warms. At night, the warmed water gives up its heat throughout the night and helps keep the air warmer than it would be otherwise. Plus, the higher humidity helps bounce around the infrared radiation, delaying heat loss to the outside air. Without the water, the air would cool much faster.
This is similar to what happens in the southern hemisphere. The amount of land surface there is much smaller than the amount of land surface in the northern hemisphere. All that ocean water can store an amazing amount of heat energy. As a result, even places you'd expect to be very cold, like southern Australia and New Zealand, are actually more temperate in climate than you'd expect from their latitude. For example, Minnesota, USA (approx. 45 degrees latitude N) is far colder in winter than southern New Zealand (approx. 45 degrees latitude S), even though both are at a comparatively equal distance from the equator. This is important information to remember when considering whether a particular southern hemisphere plant will survive in the northern hemisphere. Latitude alone is not a reliable reference for hardiness from southern hemisphere to north.
A mile-thick layer of ice in the desert
Imagine a desert and I'll bet you envision dry sand, sweltering hot temperatures, no clouds, and no water in sight anywhere. That is one example, but how about a place where rain is almost nonexistent, yet under your feet is over a mile of ice? That describes parts of Antarctica; I write "parts" because the ice is much thicker than that in places. The tremendous amount of ice there means that on clear summer days, most of the sunlight is reflected off the ice. Therefore, little warming will take place, and the warming that does occur will escape rapidly into space once the Antarctic night commences. However, when a layer of ice covers open ocean, it actually acts more like a blanket, delaying the loss of heat from the ocean water.
Paradoxically, when the weather cools globally is when Antarctica warms up. This is so because the cooling results in cloud cover over the south polar continent, which retards the loss of what meager warmth is there in summertime.
Another intriguing aspect of Antarctica is the Southern Ocean, the ocean that surrounds the continent. This ocean is unique in that it is the only ocean that circles the globe without being blocked by any continent. The Southern Ocean is home to the Antarctic Circumpolar Current, which has a critical influence on much of the worlds' climate. This current effectively connects and mixes water from the Pacific, Indian, and Atlantic ocean basins, thereby influencing patterns of temperature and rainfall. For more details, see The Southern Ocean and global climate from the Australian Academy of Science.
Little Girls and Boys
You've heard weather forecasters talk about the "El Niño" or "La Niña" ("The Boy Child" or "The Girl Child") events as being the reason for particular weather phenomena in different areas of the world. These two terms actually refer to parts of what is known as the Southern Oscillation, or ENSO (El Niño/Southern Oscillation), a repeating cycle in the equatorial Pacific Ocean involving changes in sea surface temperatures and atmospheric pressures over a broad area. One entire cycle can occur over a period of from 2 to 7 years. This cycle is important because of the dramatic weather changes that can result when one or both of the parts are particularly intense. Essentially, an "El Niño" is occurring when the ocean waters of the equatorial eastern and central Pacific are abnormally warm (2 or more degrees C above average), while a "La Niña" is taking place when the same ocean waters are abnormally cool. An "El Niño" is visualized in the graphic at left; the cream/white tongue extending west from central and south America is the warm water zone. Extremes in either of the two portions of the cycle can result in droughts, west coast rain, more Atlantic hurricanes (La Niña), or flooding, more tornadoes, and less Atlantic hurricanes (El Niño), A paradox here is that when a "La Niña" (an ocean cooling event) is underway, most of the United States will be warmer than average.
About half of the planet is significantly affected when a strong event of this type is underway. The only climate events more far-reaching that this one are the seasonal changes, so that gives you an idea of how important this cycle is to our weather.
For further details on this phenomenon, see the El Niño, La Niña, and ENSO Public Factsheet from the National Weather Service.
Photo credit: NASA and NOAA public domain images