Different types of troughs and ridges can occur. IE - you can have summer surface troughs that point to the south, but these generally occur in warmer areas (and in summer) in the southern hemisphere. These are often associated with heat troughs in central Australia (due to the very strong heating, the air mass rises and an area of low pressure is formed).
Below is an example of a surface summer trough (Nov 5, 06z, 2000).
This type of trough wouldn't occur much down in your parts of the woods.
The types of troughs that you guys would be used to would be winter troughs, they point to the north, and are often associated with low pressure systems further to their south, and sometimes have cold fronts associated with them.
Below is an example of a winter surface trough (June 12, 2001, 18Z)
So why do we see an increasing number of "winter like surface troughs" (ie, their axis points northward) as you ascend higher into the atmosphere? Summer troughs, even at 300mb still exist though, an example is below: (May 06, 2001 00z)
Well - we know that troughs are a region of lower pressure. So what causes lower pressure in the upper atmosphere? Cold air does - it is more dense that warm air, thus it takes up less space than warm air. So there's lower pressure heights, and the pressure is lower.
So - where does the cold air come from? Well, this tends to come back to a fundemental question - what is weather? Weather is the ongoing balance of the uneven surface heating on the Earth. The Equatorial regions receive high amounts of surface heating, and the Polar regions receive small amounts. Essentially, the cold air from the poles move towards the Equator (in the north in the Southern Hemisphere), and the warm air from the Equator moves polewards (to the south in the Southern Hemisphere).
So if the upper level troughs require cold air, then it would make sense that the bases of the upper level troughs in the Southern Hemisphere commence on towards the south, and point to the north. (IE, the cold air intrustion is to the north, with its base, or open end towards the south).
Upper level ridges would be the opposite of this, as they would require warm air.
The question then arises why do the winter troughs (northward pointing ones) compared to summer troughs (southward pointing ones) occur more often as you ascend into the atmosphere?
Well - we can see that the surface is much more variable than the upper levels. We can readily get summer surface troughs. And they are often reflected in the lower layers (ie, 925mb, 850mb and 700mb). Often they are the result of heating lows and troughs (the summer troughs at those levels). But to then answer your question of where does the change from a mixture of winter and summer troughs, to generally only winter troughs (with the exceptions to the rules) occur? Generally around 700mb - after this, the winter troughs tend to dominate. And the only reason why you'll get a summer trough, is if a cut-off upper level develops (ie, the example I showed before at 300mb), and extends colder air to its south towards the poles due to it being completely enclosed by warmer surrounding air.
Hope this helps answer your question!