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CAPE

Winter 2020-21 Discussion

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A good read -  posted by bluewave yesterday from a recent study on the winter  + NAO and the near record + IOD last November. 

 

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A recent study linked  the very strong +NAO last winter to near record +IOD in November. The +IOD was associated with the standing wave in the MJO 1-2 which was observed as early as last October and November. Those October MJO phases were similar to the other La Niña and Niña-like below normal snowfall seasons in 19-20, 18-19, 11-12, and 07-08. They mention a stratospheric pathway linking the flat ridge north of Hawaii and the more +NAO in the Atlantic. The normal to above normal La Niña snowfall seasons since 2000 had stronger MJO 5 forcing in October. Those seasons were followed by intervals of -EPO and -NAO blocking. So this may potentially be how we saw the October to winter MJO relationship since 2000. But as always, there are no guarantees this relationship will work every season. 

https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/asl.1005

Predictability of European winter 2019/20: Indian Ocean dipole impacts on the NAO

3.3 Stratospheric pathway

As alluded to in the previous section, the stratospheric teleconnection pathway of the IOD to the Atlantic involves the Aleutian cyclone, and is similar to that already documented for ENSO (Manzini et al., 2006; Ineson and Scaife, 2009). Demonstrated best by the IOD experiment in the month of December, Figure 3cshows a Rossby wave train emerging from the Indian Ocean that leads to poleward flow in the North Pacific near the dateline, and equatorward flow to the east of this. These two responses combine to give anomalously positive MSLP just south of Alaska and the Aleutian Islands. Figure 4a shows that this MSLP anomaly in DJF is in a very similar location to that due to ENSO (shown also in fig. 1 of Ineson and Scaife, 2009). For the single case of winter 2019/20, this signal is evident in early winter, as discussed below, and again in the DJF mean (Figure 3b) due to a particularly strong signal in January and February.

As described in Ineson and Scaife (2009), the positive MSLP anomaly (Figure 4a) acts to reduce the strength of the climatological Aleutian cyclone and, thereby, reduces the amplitude of planetary waves propagating upwards into the stratosphere. Figure 4b demonstrates this reduction in planetary wavenumber 1 amplitude (diagnosed using geopotential height at 100 hPa, area averaged 40–80°N, and then Fourier decomposed; Hardiman et al., 2008) in the IOD experiment, and is consistent with fig. 10b of Fletcher and Cassou (2015).

Reduced planetary wave driving in the stratosphere leads to an anomalously strong stratospheric polar vortex (defined by U(60°N, 10 hPa) in Figure 4c, and see also fig. 10 of Fletcher and Cassou, 2015). Anomalously strong vortex signals propagate downwards into the troposphere, resulting in a positive NAO at the surface approximately 1 month later (Baldwin and Dunkerton, 1999; Kidston et al., 2015).

In fact, this positive MSLP anomaly in the Aleutian region occurs also in November, so reduced wave driving (Figure 4b) and an anomalously strong stratospheric polar vortex (Figure 4c) are already apparent in November. Indeed, the November polar vortex strength is anomalously positive in IOD composites, the IOD experiment, ERA‐5, and all forecast systems (not shown). Fig. 4 of Nie et al. (2019) demonstrated that an early winter preconditioning of the stratospheric polar vortex in November descends through the stratosphere and troposphere in the following winter months, projecting onto an anomalously positive NAO in DJF.

The winter of 2019/20 was anomalously warm and wet across the UK and Northern Europe, due to a strongly positive NAO. The winter was well forecast by the C3S and the Met Office DP3 seasonal forecast systems. Even the details of individual months, such as the transition from the negative pressure anomaly west of the UK in December to a positive NAO in January/February, were well forecast by all seasonal systems. Such remarkable agreement amongst systems is suggestive of the positive NAO being strongly driven by global influences, and predictable in this case. In this paper, composite analysis and numerical experiments are used to identify the very strong positive IOD event at the start of the winter as the key driver.

Two teleconnection pathways are identified using an experiment in which two ensemble forecasts, one with the observed November 2019 Indian Ocean SST anomalies, and one with the negative of these anomalies, are produced using DP3. The difference in the ensemble mean response, shows a Rossby wave train originating in the Indian Ocean and propagating across the Pacific and Atlantic Oceans. In the Atlantic, this wave train projects directly onto the observed Atlantic MSLP anomalies. In the Pacific, the wave train acts to reduce the amplitude of the Aleutian cyclone and therefore the amplitude of planetary waves propagating into the stratosphere. This results in an anomalously strong stratospheric polar vortex, projecting onto an anomalously positive NAO. This numerical experiment shows good agreement with both the ERA‐5 reanalysis data and the C3S multi‐model seasonal forecasts in terms of the details of both teleconnection pathways. Furthermore, both pathways are very similar to the well documented tropospheric and stratospheric teleconnection pathways whereby ENSO impacts the north Atlantic MSLP (Hardiman et al., 2019).

The impact of the IOD on the Atlantic jet stream and associated precipitation anomalies is a northward shift in the jet latitude, a slight increase in the jet strength, and anomalously high precipitation over the UK and northern Europe, as shown in Figure 5. This is consistent with an anomalously positive NAO and agrees well with the features observed in winter 2019/20. There is a remarkable agreement between the IOD experiment and the C3S multi‐model mean forecast. The signal in the December observations is noisier (Figures 3a and 5a), but this is expected, being only a single realisation of a single month.

A knowledge of the teleconnection pathways between the IOD and the North Atlantic gives greater confidence in the seasonal forecast skill they offer. The frequency of positive IOD events has doubled in the 20th century, and their intensity has also increased, with this trend projected to continue (Abram et al., 2020). It is likely, therefore, that such connections will become increasingly important for seasonal forecasting of European winters during the rest of the 21st century.

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@frd that would bode well for this winter. Still not great as we typically struggle even in a better Nina but it would suggest we might avoid total pac ridge hell like last year. I’m still not sold one way or another. Lots of conflicting tea leaves. 
555A9020-ADAC-4172-BAFE-672AC3824CE2.gif.aef0b6ddbd4cd278da96289bc18b0b02.gif

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I imagine this might be useful in winter, or anytime. Great resource material from Simon. 

Check it out. 

 

 

 

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It seems like whatever condition...Nina, Nino, neutral..the temps always show some shade of orange for winter outlooks in our region.  It’s baked into the cake. 

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6 hours ago, WinterWxLuvr said:

Isn’t that a typical Nina look?

Boilerplate. I guess it's as good as anything else for a general winter outlook.

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1 hour ago, leesburg 04 said:

Winter must be coming the squirrel's are furiously gathering stormchaserchucks to hunker down

Must be big axx  squirrels ! 

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Never seen 75% on the CPC's 3-month Winter before. (edit: that was before). Weak-weather.gov continues to play behind on the most basic weather-observation sense.. I doubt the NW will verify this Winter. 

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5 hours ago, leesburg 04 said:

Winter must be coming the squirrel's are furiously gathering stormchaserchucks to hunker down

My cats gained like 7 pounds recently.. I think that’s a sign or something 

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28 minutes ago, NorthArlington101 said:

Guess we might need a LR thread soon, but the GFS has consistently  been throwing out some pretty serious cold for this time of year around Halloween.  

Maybe you all will get paid back for the snowstorm that misses you back in December of 2018. And we get even more rain down here.

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14 hours ago, NorthArlington101 said:

Guess we might need a LR thread soon, but the GFS has consistently  been throwing out some pretty serious cold for this time of year around Halloween.  

It likes to do this in the LR.

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Looking at the latest LR global means, some significant changes up top. Lots of blue, and CFS weeklies continue that through November. The -NAO period was possibly just a transitional thing. The CFS run of indicating HL help for the winter months seems to have finally ended. Its h5 depiction for DJF aligns with most other climate/seasonals now.  Has a +AO/NAO throughout. The mean EPAC ridge location/orientation could be worse though. Actually has hints of an EPO ridge, esp for late winter. fwiw.

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16 hours ago, frd said:

Nina results vary,  but cool to see the various temp profiles going back to 1925. 

 

Back in July, never in a million years did I think this Niña event would ever approach strong, but it appears to be doing just that. The models continue to get stronger with it. It’s now fully coupled, ENSO region 4 has dropped like a rock. There are also shades of the 88-89 strong La Niña showing up, which HM and Ben Noll have pointed out: 

 

 

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Simply the fact the Nina is strong doesn’t necessarily mean a torch winter. There have been some cold or somewhat snowy strong Nina’s. However, there are plenty of other factors that in conjunction with a strong Nina aren’t good. 

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1 hour ago, psuhoffman said:

Simply the fact the Nina is strong doesn’t necessarily mean a torch winter. There have been some cold or somewhat snowy strong Nina’s. However, there are plenty of other factors that in conjunction with a strong Nina aren’t good. 

As Uncle W just pointed out, 73-74 was the strongest La Niña in history and had a severely negative PDO yet it was a decent winter

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59 minutes ago, snowman19 said:

As Uncle W just pointed out, 73-74 was the strongest La Niña in history and had a severely negative PDO yet it was a decent winter

Significant HL blocking can mitigate a lot of bad elsewhere. Unfortunately we are on a bad run in that dept.

I suppose we are due. :rolleyes:

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57 minutes ago, CAPE said:

Significant HL blocking can mitigate a lot of bad elsewhere. Unfortunately we are on a bad run in that dept.

I suppose we are due. :rolleyes:

Very true. I guess you also have to consider that the 73-74 super La Niña occurred during a totally different, cooler climate era. I kind of doubt that even with a complete carbon copy of 73-74 this winter, you’d have the same results in the temperature and snowfall departments....

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20 hours ago, CAPE said:

Significant HL blocking can mitigate a lot of bad elsewhere. Unfortunately we are on a bad run in that dept.

I suppose we are due. :rolleyes:

This part of the equation is where my pessimism lies. We can and have had decent snowfall in a strong Nina. But every instance took high latitude blocking. Not some. Not most EVERY SINGLE one. I can’t stress that enough. Since 1948 Baltimore hasn’t had a single warning level snowfall during either a strong Nina or a central pacific ridge (predominant effect if said Nina) without blocking. Even when we include non Nina years that featured a Central pac ridge I found not one single significant snowfall with that pac pattern that didn’t include blocking. 
 

I get the sense some people think we might luck our way into a snowstorm even with a strong pac ridge and +NAO. But history suggests we won’t. That if we do get the currently expected pacific and Atlantic patterns we are just as much toast as we were last year. 
 

I’m not saying we won’t get snow. But I’m saying we need one of those two patterns to break our way. Not the whole winter. Look at 2000. We only got the high latitudes to cooperate for a few weeks but that was enough to save us.  But if we do get a wall to wall +NAO like we’ve seen for years now...I am not optimistic “luck” saves us. 

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