Why is the jet stream stronger in winter?
I'll look at the climates of the town of Inuvik In Canada's Northwest Territory at a latitude of 68°21′42″N against that of Mexico City at a latitude of 19°26'N. In July, the daily mean temperatures are 14.1°C in Inuvik and 18.2°C in Mexico City — a difference of 4.1°C.

Compare that small difference with the daily mean temperatures in January of -26.9°C in Inuvik and 14.6°C in Mexico City – a huge difference of 41.5°. That factor of ten increase in the temperature difference in winter over summer drives a lot more north-south circulation in the winter compared to the summer, which in turn makes the jet streams stronger in winter compared to summer.