The Grand North-South Asymmetry
The Antarctic Circumpolar Current
We have been mainly discussing the Northern Hemisphere, which is appropriate since people live on land, and most of the land is in the northern hemisphere. What about the southern hemisphere? Can we use the insights gained from studying the situation in the north to predict changes in the south?
Nowhere in the Southern Hemisphere is there a great heating machine like the Iceland Low. In fact, the only other feature on Earth like the Iceland Low is the Aleutian Low, also located in the Northern Hemisphere. It depends on the Kuroshio (the current off Japan that is analogous to the Gulf Stream) and associated currents and winds for heat supply. The fundamental difference in the southern hemisphere is that we have the great Circumpolar Current, which is like a wall, preventing warm waters from penetrating into cold regions around Antarctica. Thus, there is no comparable warm current fueling a stationary low-pressure cell in high latitudes, such as the Kuroshio does for the Aleutian Low, or as the Gulf Stream does for the Iceland Low.
Diagram illustrating southern hemisphere and its Antarctic Circumpolar Current (in red arrows), which creates a “wall” that prevents warm water from reaching Antarctica.
North-South Circulation Asymmetry
This asymmetry between the Northern and Southern hemispheres is readily appreciated when looking at the geography of equivalent latitudes in the North and South. What do we have at 45°N in western Europe? Wine country around Bordeaux. What do we have in New Zealand at 45°S? Glaciers and giant fern forests!
In the southern hemisphere, at 70°S, we find ourselves either in pack ice at the rocky shores of Antarctica (in the company of penguins and seals), or on top of the enormously thick continental ice sheet that covers Antarctica. Certainly these are not good places for a university campus (as is that case for Tromsoe, Norway, located at 70°N).
Why the Difference?
There are at least two reasons for the north-south asymmetry. First of all, the northern hemisphere has a lot more land than the southern hemisphere. As a result, the northern hemisphere heats up more readily during its summer than does the southern hemisphere during its summer. Land surfaces heat quickly, water surface slowly. As the land surfaces heat up, the air overlying them rises and is replaced by air pulled in from adjacent regions, including from south of the equator. Thus, the North steals some of the warm tropical air from the South, a process made evident in the displacement of the "Intertropical Convergence Zone" (ITCZ) to positions north of the equator (See Lesson 5 for a review of the ITCZ; also see the glossary).
In addition to this grand-scale asymmetry in landmass distribution, there is the fact that in the northern hemisphere, roughly north of 70°N, there is an ocean basin centered on the pole, while in the southern hemisphere, roughly south of 70°S, there is a large landmass. Most of the ice (some 85 percent of it) therefore is located in the south, specifically in the thick ice cap of Antarctica. The presence of this ice mass has the effect of fixing freezing temperatures farther away from the pole than is the case on the northern hemisphere. Ultimately, this fact, in combination with the fast-running circumpolar current, pushes climate belts closer to the equator, and encourages transfer of heat to the north.
North-South Asymmetry and Climate Change
During glacial periods in the geologic past, the northern hemisphere has acquired ice masses that exceed those of the south. The asymmetry is thereby destroyed and there is no heat transfer from south to north. How will global warming impact this North-South asymmetry? Again, this is entirely open, but we can make some guesses. In principle, as we have seen, monsoon circulation is likely to get stronger at the expense of zonal circulation. Thus, since the monsoon effects are responsible for stealing heat from the south in the first place, the delivery of heat from south to north is likely to increase, at least as far as the atmospheric circulation. If the production of North Atlantic Deep Water is reduced as well, the heat transfer associated with the Atlantic Conveyor would also be reduced. We might also speculate that the effects of global warming and of reduced heat piracy might cancel, in the North Atlantic realm (at least for a while). However, it turns out this is unlikely since the effects are not precisely opposite but concern different regions.