The Southern Alps effect on Chch rainfall

[Cook]

Hokitika Airport and Christchurch Airport have similar annual mean temperatures, so the net affect of the Alps on the annual mean temperature is possibly small.

Consider why it is cold in CH when it is cold there? It is a mainland effect, with cold winds blowing off the land in winter. The land mass to the west of CH is large so this mainland effect would be maintained, even without the Alps.

Why does CH get hot days? Because of foehn winds blowing off the Alps in a westerly flow. Without the Alps, no foehn, so not so warm. I therefore expect the mean temperature in CH to be lower without the Alps.

Why isn't it similarly cold in Hokitika because of the mainland effect? Because it is closer to the sea and an onshore flow (westerlies) is more common there than it is in CH. If you only consider warming from the sea it SHOULD be warmier in Hokitika than CH. The reason it is not is probably the warm northwesterlies blowing off the Alps. Take away those warm down-slope winds and it will get colder in CH so I maintain that the net effect of removing the Alps should be a cooler mean temperature in CH.

[Cook]

Christchuch probably wouldnt get NE unless true flow,the wind directions would be more varied on the whole and have no shelter any direction though less showers than Holkiatika in westerlies.

CH would continue to get a NE sea breeze.

In the winter CH should have less showers than Hokitika in westerlies. In the summer, perhaps not. In the winter the warm sea is driving the showers while in summer it is the warm land. During the summer with westerlies of the right strength across the South Island a sea breaze convergence line could be set up along the east coast and this could help to generate some intense showers in CH while the west coast gets nothing.

[Razor]

Ok so if we level the Southern alps for a gap to the west about the size of Cook Strait ChCh will get NWers that will blow us to the chatams, and some kick arse thunderstorms???

[NZstorm]

The dewpoint temperatures would be a lot higher over Canterbury if there was no Alps and this would have the affect of raising the average minimum temperature somewhat. Its hard to quantify that affect but it would definitely by higher.

I believe the mean temperature could go up a bit for Canterbury.

[David]

The dewpoint temperatures would be a lot higher over Canterbury if there was no Alps and this would have the affect of raising the average minimum temperature somewhat. Its hard to quantify that affect but it would definitely by higher.

I believe the mean temperature could go up a bit for Canterbury.

I agree. Chch would experience fewer frosts as a result

[Cook]

The dewpoint temperatures would be a lot higher over Canterbury if there was no Alps and this would have the affect of raising the average minimum temperature somewhat. Its hard to quantify that affect but it would definitely by higher.

I believe the mean temperature could go up a bit for Canterbury.

The Chatham Islands are at about the same latitude as CH. There are no Alps there and the dewpoint temperatures are relatively high. So following that line of thinking the temperature there would definitely be higher?

The annual mean at the Chathams is 12.4C while the annual mean in CH is 13.2C. The mean annual temperature in Hokitika is 12.7C. Both Hokitika and the Chathams are more humid AND colder than CH. Now, why do you think that is? What ingredient is there that causes CH to be warmer? the Alps perhaps???

[spwill]

You need to consider landmass size and fohn affect

[RWood]

The dewpoint temperatures would be a lot higher over Canterbury if there was no Alps and this would have the affect of raising the average minimum temperature somewhat. Its hard to quantify that affect but it would definitely by higher.

I believe the mean temperature could go up a bit for Canterbury.

The Chatham Islands are at about the same latitude as CH. There are no Alps there and the dewpoint temperatures are relatively high. So following that line of thinking the temperature there would definitely be higher?

The annual mean at the Chathams is 12.4C while the annual mean in CH is 13.2C. The mean annual temperature in Hokitika is 12.7C. Both Hokitika and the Chathams are more humid AND colder than CH. Now, why do you think that is? What ingredient is there that causes CH to be warmer? the Alps perhaps???

Where are you getting these numbers from?

From CliFlo database, annual means 1971-2000 (if there are 1981-2010 ones available, they won't be much different, certainly won't change the relationships):

Hokitika Aero 11.7C Ch'ch Aero 11.6C Ch'ch Gardens 12.2C Chathams Is 11.5C

Just looked up 1981-2010: All 4 means same as before.

Given the more urban nature of the Gardens site, and the Aero values, I can't see a very serious claim for Ch'ch being warmer than those other locations.

[Cook]

NIWA, 1971-2000

I don't know how serious NIWA is....

[RWood]

NIWA, 1971-2000

I don't know how serious NIWA is....

Well mine are from NIWA's CliFlo, and the 2010 updates simply confirm them. Whatever you linked to is incorrect.

[Cook]

http://www.niwa.co.nz/education-and-tra ... eanairtemp

The data presented here is incorrect? Please inform NIWA so they can correct it.

I would like something more concrete from those who think the mean temperature in CH would rise in the absence of the Alps. I find the higher dewpoint argument rather vague.

The temperature is raised in CH relative to what it would be without the Alps on many days a year due to the foehn effect. I still do not see any mechanism that will raise the temperature in CH if the Alps are taken away. Higher dewpoints will stabilize the temperature and lead to less swings, up or down but to have a higher mean you need to have some source of energy. Condensation over the Alps releases a great deal of energy and this serves to warm CH up. What will make up for the loss of this if the Alps are gone?

[RWood]

http://www.niwa.co.nz/education-and-tra ... eanairtemp

The data presented here is incorrect? Please inform NIWA so they can correct it.

I would like something more concrete from those who think the mean temperature in CH would rise in the absence of the Alps. I find the higher dewpoint argument rather vague.

The temperature is raised in CH relative to what it would be without the Alps on many days a year due to the foehn effect. I still do not see any mechanism that will raise the temperature in CH if the Alps are taken away. Higher dewpoints will stabilize the temperature and lead to less swings, up or down but to have a higher mean you need to have some source of energy. Condensation over the Alps releases a great deal of energy and this serves to warm CH up. What will make up for the loss of this if the Alps are gone?

Those numbers are the same as the ones I gave, except that a couple of them are rounded downwards instead of up! I have no opinion on this actual argument, but your claim for Ch'ch being significantly warmer than the other sites clearly doesn't hold up.

[Cook]

http://www.niwa.co.nz/education-and-tra ... eanairtemp

The data presented here is incorrect? Please inform NIWA so they can correct it.

I would like something more concrete from those who think the mean temperature in CH would rise in the absence of the Alps. I find the higher dewpoint argument rather vague.

The temperature is raised in CH relative to what it would be without the Alps on many days a year due to the foehn effect. I still do not see any mechanism that will raise the temperature in CH if the Alps are taken away. Higher dewpoints will stabilize the temperature and lead to less swings, up or down but to have a higher mean you need to have some source of energy. Condensation over the Alps releases a great deal of energy and this serves to warm CH up. What will make up for the loss of this if the Alps are gone?

Those numbers are the same as the ones I gave, except that a couple of them are rounded downwards instead of up! I have no opinion on this actual argument, but your claim for Ch'ch being significantly warmer than the other sites clearly doesn't hold up.

You mean NIWA's claim.

http://www.niwa.co.nz/education-and-tra ... eanairtemp

[Manukau heads obs]

even if the south island was relatively flat, there is still going to be a minor rainfall shadow and minor warming effect in a NW wind, and generaly warmer weather than on the west coast maybe?
(e.g look at South east UK for comparison)

[RWood]

http://www.niwa.co.nz/education-and-tra ... eanairtemp

Why are you requoting? Ive already looked those numers - they are the same, at the risk of repeating myself. If NIWA were "claiming" anything, it would be that Ch'ch Aero has about the same means as the other 2 places, and it's more urbanised site is a little warmer - that's all!

[RWood]

even if the south island was relatively flat, there is still going to be a minor rainfall shadow and minor warming effect in a NW wind, and generaly warmer weather than on the west coast maybe?
(e.g look at South east UK for comparison)

If you look at New Plymouth and Napier, the latter has a small temp. average advantage though slightly further south.

[Manukau heads obs]

a reasonable comparison, you would assume there would be some sort of range of hills in place of the southern alps
all theoretical of course and just an interesting discussion thats all

[Cook]

even if the south island was relatively flat, there is still going to be a minor rainfall shadow and minor warming effect in a NW wind, and generaly warmer weather than on the west coast maybe?
(e.g look at South east UK for comparison)

Exactly, minor. Not major as it is now with the Alps in place. Warmer than the west coast in summer but colder in the winter.

Apologies to Rwood. I did not look at the numbers correctly. The means are indeed 12.1C for Christchurch, 11.4C for the Chathams and 11.7C for Hokitika.

[RWood]

As further confirmation of the insignificance of the differences, the 1981-2010 normal quoted for non-urban Lincoln is 11.6C, the same as Ch'ch Aero and virtually the same as Hokitika and the Chathams.

[spwill]

I still do not see any mechanism that will raise the temperature in CH if the Alps are taken away. ?

There will be less night time cooling due to increased dewpoint and a wetter environment .

Would there be more west/northwest wind and less NE wind for Chch if the Alps were gone ?.

[Manukau heads obs]

oh, yes, good point

[Cook]

I still do not see any mechanism that will raise the temperature in CH if the Alps are taken away. ?

There will be less night time cooling due to increased dewpoint and a wetter environment .

Would there be more west/northwest wind and less NE wind for Chch if the Alps were gone ?.

I am with you there. It would be warmer at night and colder during the day.

I am not sure what the answer is to the question about the winds. Perhaps NIWA or the Metservice would like to run a computer simulation of the South Island with no Alps? That would be interesting.

[Michael]

Its likened to Whangarei in a W or SW flow,its less showery than Dargaville so reason chch be similar in relation to Hokitika though it be cooler in the last instance.Northland probably only area which experiences a climate in the given topic.

even if the south island was relatively flat, there is still going to be a minor rainfall shadow and minor warming effect in a NW wind, and generaly warmer weather than on the west coast maybe?
(e.g look at South east UK for comparison)