# Why is the pressure on the poles higher in Summer and lower in Winter?

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

https://en.wikipedia.org/wiki/Rabat#Climate

compared to

https://en.wikipedia.org/wiki/Tallahassee,_Florida#Climate

https://en.wikipedia.org/wiki/Shanghai#Climate

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

• Comments are not for extended discussion; this conversation has been moved to chat. – gerrit Mar 29 at 18:45
• Others have been asking you for sources without much success, I will just come out and say it: The statement "as the prevailing winds causing rainfall on Eastern coasts come from the East, they must have been moving equatorwards due to the coriolis force (Trade Winds)" is demonstrably false. The prevailing winds in temperate zones, even on the east coast, are westerly. – Spencer Apr 13 at 14:52
• (Cont): Since you say you're only interested in temperate zones, it's meaningless to talk about trade winds, which are subtropical or tropical. From the Wikipedia article on Trade Winds: "The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region (between 30°N and 30°S latitudes)." – Spencer Apr 13 at 14:53
• @Spencer I understand thanks for the correction. Still, even the most cursory check by going through climate data validates the claim about WInter rainfall on West coasts and Summer rainfall on East coasts (no source required). So, what causes this phenomenon if it was not what I had said? I admit my conclusions and reasoning may be wrong, the underlying observation is correct which is why I felt frustrated with the people who were flat out saying I was wrong. After all, people are encouraged to try to make their own conclusions on StackExchange as it shows research effort, that's what I did. – aman Apr 14 at 6:04
• @aman It is good to be curious and we'll be happy to answer (that's partly why we're here ;-)), but the assertions in the first four paragraphs of your question are not in line with general observation and that makes a meaningful too tedious. Furthermore, Earth's poles couldn't be more different than they are, there's one over sea with continents around subject to changing weather, and another one is a continent girdled round by the sea and largely isolated from the global circulation. "A clear explanation of seasonal variation" is too difficult (at least for me). – a_donda Apr 14 at 12:10

My answer will only relate to the question in the title: Why is pressure on the poles higher in summer and lower in winter?

Due to the axis tilt of the earth the winter hemisphere gets less heated by solar radiation. This leads to a high temperature gradient from equator to the winter hemisphere pole (strong baroclinicity). In contrast we have a low temperature gradient from equator to the summer hemisphere pole. The missing solar heating in the winter hemisphere additionally leads to descending air at the pole leading to low pressure at jet stream level (around 300 hPa). On a non rotating planet the air would of course directly flow from the high pressure (equator) to the low pressure (pole) to remove the pressure gradient. As warm air would flow to the cold pole air, this would also weaken the temperature gradient. But due to the rotation of the earth, air flows around the area of low pressure. This flow gets increasingly stronger as the meridional temperature gradient strengthens.

This maybe seen from thermal wind balance, which tells us that the geostrophic wind $$V_g$$ is related to the temperature gradient $$\nabla T$$ by $$\frac{\partial V_g}{\partial \ln p} = -\frac{R}{f}k \times \nabla T$$. Here $$p$$ is pressure, $$R$$ is the gas constant for dry air, $$f$$ is the Coriolis parameter and $$k$$ is vertical unit vector. Assuming there is no zonal temperature gradient, we can conclude that a strong meridional temperature gradient leads to an increasing change of $$V_g$$ with height. We can observe this strong wind as the strong/weak jet stream in the winter/summer hemisphere. What does all of this have to with the pressure?

The jet stream acts as a barrier, dividing the warm and cold air masses. The Temperature will keep getting lower which will also increase the pressure and temperature gradient. In return the barrier (jet stream) gets stronger. This positive feedback mechanism remains until after winter solstice. Then the radiation reaching the pole will increase again, warming the air masses. Which will then lead to a weakening of the gradients.

So summarizing the above one could say, the missing radiation cools the air leading to a low pressure system at jet stream level. The jet stream itself gets stronger as the temperature gradient increases and acts as a barrier for warm and cold air masses and therefore maintains the pressure difference.

• As the low pressure zone develops in the subpolar or jet stream level, air flows from the subtropical high to the subpolar low? – aman Apr 30 at 2:30
• Yes, that makes sense. There is a very clear pressure gradient in the Winter. In the Summer, on the other hand, there is no clear gradient mapsofworld.com/world-maps/… mapsofworld.com/world-maps/… – aman Apr 30 at 3:32
• No The low pressure zone is not the jet stream region. The jet will form where the pressure gradient is highest. That is somewhere in between the high and low pressure zone. Additionally it is not true that at the pole there is high pressure. When talking about high and low pressure systems you always need to consider the height you are talking about. The polar low forms because cold air is descending. Imagine you are at a certain level of height, say 3km. The pressure is just given by the air mass above you. If air subsides and there is no inflow the mass above you decreases. – J. Fregin Apr 30 at 8:25
• Yes the air is supposed to flow from the subtropical (relative) high to the polar low, however the Coriolis force does a tremendous job at preventing that. Notice that every low pressure system outside the equatorial region needs friction to equilibrate the pressure. If there was no friction air would just keep circling around the pressure systems. – J. Fregin Apr 30 at 8:29
• Ok, so geostrophic balance is not present due to friction, so the air does end up flowing polewards. Also, at the surface level, descending air should mean high pressure at the surface right? You said air descended at the poles, so I am a bit confused – aman Apr 30 at 13:29