Southern Annular Modes (past & present)

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RWood
Posts: 3745
Joined: Sat 24/01/2004 16:56
Location: Wellington

Southern Annular Modes (past & present)

Unread post by RWood »

spwill wrote:The Southern Annular Mode is running negative, the westerly pattern that normally sits south of NZ gets pushed north. Expect generally below average temperatures across NZ next two or three weeks, not to say there wont be some warm sunny days in the mix.

For Auckland the next few days look cloudy, possible light shower tomorrow, showers Thursday, fine weather for Friday/Saturday. Edit, looks like cloud will burn off quite a bit today.
Different every time though - there was period of negative SAM late 2014, and Wellington was dry and sunny until the SAM reversed. The last 4 months of the year here have in toto been very wet and very cloudy - I rate the period easily the worst since 1992.
tunster
Posts: 117
Joined: Thu 05/11/2009 13:55
Location: Kelburn, Wellington

Re: General December Weather

Unread post by tunster »

RWood wrote:
spwill wrote:The Southern Annular Mode is running negative, the westerly pattern that normally sits south of NZ gets pushed north. Expect generally below average temperatures across NZ next two or three weeks, not to say there wont be some warm sunny days in the mix.

For Auckland the next few days look cloudy, possible light shower tomorrow, showers Thursday, fine weather for Friday/Saturday. Edit, looks like cloud will burn off quite a bit today.
Different every time though - there was period of negative SAM late 2014, and Wellington was dry and sunny until the SAM reversed. The last 4 months of the year here have in toto been very wet and very cloudy - I rate the period easily the worst since 1992.
The SAM was strongly positive during September 2016, when much of the country was suffering below average temperatures and pretty grim weather.
The SAM is useful but also still cannot possibly tell the entire story (e.g. the lagged impact of the negative IOD might still be causing us trouble). In the NH they can run into similar issues with just using the AO and NAO.
spwill
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Joined: Sun 29/06/2003 22:39
Location: Mt Eden Auckland

Re: General December Weather

Unread post by spwill »

tunster wrote: The SAM was strongly positive during September 2016, when much of the country was suffering below average temperatures and pretty grim weather.
.
The niwa summary here, https://www.niwa.co.nz/files/Climate_Su ... r_2016.pdf
Warm for many areas and the pattern was different
spwill
Posts: 10079
Joined: Sun 29/06/2003 22:39
Location: Mt Eden Auckland

Re: General December Weather

Unread post by spwill »

tunster wrote:[
The SAM is useful but also still cannot possibly tell the entire story (e.g. the lagged impact of the negative IOD might still be causing us trouble). In the NH they can run into similar issues with just using the AO and NAO.
Yes there are many factors that influence our weather.
Metservice say the SAM is part of their toolbox when issuing the 4-week rural forecast
tunster
Posts: 117
Joined: Thu 05/11/2009 13:55
Location: Kelburn, Wellington

Re: General December Weather

Unread post by tunster »

spwill wrote:
tunster wrote: The SAM was strongly positive during September 2016, when much of the country was suffering below average temperatures and pretty grim weather.
.
The niwa summary here, https://www.niwa.co.nz/files/Climate_Su ... r_2016.pdf
Warm for many areas and the pattern was different
I mean that strong positive SAM events and strong negative SAM events can lead to mediocre weather in NZ. September this year was pretty cloudy for the NI, had some pitiful temperatures (there were a few days were Napier didn't even reach the teens) and was wet for the NI. That was a mostly strong positive SAM month.
Meanwhile November 2010 was a mostly strong positive SAM month - very different weather.
You can see in the attachments, two periods chosen when the AAO (SAM) was around 2, but the weather for NZ was completely different. November 2010, very settled, dry, just clearly anticyclone dominated. September 2016: none of the above.

What I'm most interested in is the longitude of long waves setting up. Why do we end up in long periods of planetary troughing over NZ? The SAM and IOD can help figure out some of it but I don't think they have the full answer. We seem to have been under a planetary trough dominant regime since August or September. Synoptic scale fronts and lows passing over NZ have a tendency to slow down and strengthen over us, hence the prolonged periods of unsettled conditions and cloud and rain. I would like to know why this happens! We don't have great predictors like conveniently located continents they get in the NH, but I'm sure it can't be down to chance. Maybe factors like the ice edge contribute - the retreat perhaps varies longitudinally year by year, maybe this has an impact on the east-west line up of lows and highs for us.
Interestingly, the ECMWF 32 day forecast can quite often give good hints as to where you might expect to see long wave troughs and ridges over the next month. It never gets the extremes right, but it can help with the general pattern.
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spwill
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Joined: Sun 29/06/2003 22:39
Location: Mt Eden Auckland

Re: General December Weather

Unread post by spwill »

tunster wrote:
I mean that strong positive SAM events and strong negative SAM events can lead to mediocre weather in NZ. September this year was pretty cloudy for the NI, had some pitiful temperatures (there were a few days were Napier didn't even reach the teens) and was wet for the NI. That was a mostly strong positive SAM month.
Meanwhile November 2010 was a mostly strong positive SAM month - very different weather.
You can see in the attachments, two periods chosen when the AAO (SAM) was around 2, but the weather for NZ was completely different. November 2010, very settled, dry, just clearly anticyclone dominated. September 2016: none of the above.

.
It is about the strength or weakness of westerly pattern affecting NZ . The two positive SAM months you mention did lack westerly pattern. September 2016 had a lot of easterly flow, Nov 2010, anticyclonic.
Edit, however I'm not suggesting every positive SAM event is dominated by weaker westerlies and every negative SAM event is dominated by stronger westerly pattern.
Last edited by spwill on Thu 29/12/2016 18:01, edited 1 time in total.
RWood
Posts: 3745
Joined: Sat 24/01/2004 16:56
Location: Wellington

Re: General December Weather

Unread post by RWood »

Re SAM - I was referring to the more recent times with negative SAM and contrasting with (about) November 2014.

September 2016 was a ghastly month here by any reckoning, and the Sept-Dec stats will show that period to be the worst since 1992. Spring was the wettest since 1875 if older city sites are considered, and Sept-Dec will have a sun deficit of about 150-170 hours.