Just to give context, this was not my original question but one that I ended up stuck with while trying to find a cause for the following observation (which is almost universally true for temperate climates):
Places on the WEST coast of continents tend to have a higher precipitation in WINTER, while places on the EAST coast of continents have a higher precipitation in SUMMER
To see this view the following maps http://www.city-data.com/forum/weather/2895681-driest-wettest-months-year-world-maps.html
If the region between 30 degrees and 60 degrees from the equator is the one that comes under the the influence of Westerlies, this is the relevant region to be considered on the map.
To view just a couple examples of this phenomenon, see the precipitation pattern in
The flow of moisture laden air is the primary determinant of precipitation. Due to the coriolis force, the air in the Northern hemisphere moves clockwise, and the air in the Southern hemisphere moves counter-clockwise.
As the main prevailing winds causing rainfall on the WEST coast of continents is coming from the WEST , it must have been moving polewards, then bent East due to the coriolis force (Westerlies)
Thus, there must be some reason why the Westerlies are stronger in Winter. The only possible explanation I could think of is that the pressure over the poles is higher in Summer (causing weaker pleward westerlies) and lower in Winter (causing stronger poleward westerlies)
This claim is in fact true, as is substantiated by Wikipedia.
https://en.wikipedia.org/wiki/Westerlies This page states that the pressure over the poles is lower during Winter causing stronger Westerlies:
"The westerlies are strongest in the winter hemisphere and times when the pressure is lower over the poles, while they are weakest in the summer hemisphere and when pressures are higher over the poles."
On the same wikipedia page, it states the claim I am asking an explanation for: " This occurs when the Arctic oscillation is positive, and during winter low pressure near the poles is stronger than it would be during the summer
I would like an explanation of this phenomenon, ideally explaining why the polar pressure varies to such an extent...
Also, I would like to know if the logic I have used to reach these conclusion is valid