Could we feel a jetlag-equivalent effect when we travel a long distance in the North-South direction? We would be moving from summer to winter in one day. Doesn't this affect our biological clocks?
Maybe you won't have jet lag but still there might be problems.
If it's a redeye then you will miss a good night's sleep.
Say FRA-CPT is easily 12 hours during which you will mostly be sitting in a dry, pressurized aluminium tube with limited mobility. If you doze off a bit it might upset your sleep schedule, depending. Also, you might just find the whole experience just exhausting.
So: jetlag no but you still might need a day to recover.
There's no jetlag when you're travelling within the same timezone (or +/- 1 hour) in a north-south axis.
For other effects it depends a lot on your constitution and physical fitness, for some even a minor change in climate is felt very strongly for others not at all. I travelled to Argentina from a North American winter, from -20 to +35 and I didn't get sick, my girlfriend on the other hand recently got home sick after a trip to Peru. So it depends
Change of temperature alone probably won't affect your internal clock, you have to look at altitude, humidity, time difference and exposure to daylight among other factors to get a better idea of how a trip might affect you.
Can you get "jetlag", not really because there is no major time change.
BUT depending on how far south you started and how far north you are going and what time of year it is, your body's clock can be upset because of differences in sunrise and sunset.
As an example, in summer or winter if you flew between South Africa and Norway you would go from a short day to almost 24 hours of daylight. And since sun plays a role in our body clock, you would have some disruptions to your sleep patterns. But if you made the same trip in spring or fall, then the sun differences would be more minor.