It's been a while since any of us has posted to the blog and wanted to add some recent discussion brought up by Jake concerning the unique weather pattern most of us have experienced over the past few months. Jake's opened ended question has opened up some great discussion between himself, Dima, and I along with many forecasters around the country. Here are a few of the emails that have been sent thus far and I will add Dimas response after he has time to send out his response:
From Jake Beitlich:
In your own words...what is going on this year...? It seems like this entire winter has been "northern jet stream across southern Canada, with a cutoff low over the desert southwest." If you get a chance, could you please explain this in 101 terms concerning the NAO, PNA, and La Nina?
Here's my take.....
- La Nina means weaker than normal subtropical jet...therefore the southern stream has a harder time phasing with the northern stream (a.k.a cutoffs over 4corners that slowly meander eastward toward Louisiana and don't emerge as the panhandle hooker as major cyclones.)
- Meanwhile strong westerly flow over Alberta...aka lee side warming, keeps the cold arctic air to the north (not sure how this is anomalous from previous winters).
- I don't exactly know how the NAO and PNA are related.....
- I also don't know how to explain why this 500mb image I sent seems to be the most common weather map of the season.
Looking forward to your thoughts,
My response:
I love this question Jake. This exact question has been the talk of the weather community here in the Desert Southwest for the last 3 months. I’ll try and elaborate on this topic as best I can (a limited understanding of the underpinnings of the large scale teleconnections at work, ENSO, MJO, NAO, PNA, and the AO).
Building a little on Jake’s take with the help of some colleagues:
1. Weak La Nina phase of ENSO: It’s well documented that this past summer/fall into the start of the winter season has been dominated by La Nina conditions especially noted over the Nino 3.4 region (central equatorial pacific). Typically moderate to strong La Ninas favor upper level ridging over the western half of US (drier and warmer conditions) with large scale troughing over the central/eastern US (cool and wetter conditions). However, weak La Nina to ENSO neutral conditions (like what we are currently seeing) complicate the large scale pattern and can allow for periods where the western ridge breaks down for periods of time during the winter months. The climatological storm track across the Western US during the phase mentioned above is more conducive to a prolonged cut-off low season across the southwest.
2. Madden Julian Oscillation (MJO): 30-60 day Tropical Kelvin Wave that traverses the equator with strongest signal in tropical Pacific during weak La Nina/ENSO neutral conditions. Can have significant impact on the upper level storm track across the Western US. Impacts include large scale variation in temperature, winds, and precipitation patterns which are similar to moderate El Nino but with a shorter duration. I’ve included a figure from a presentation that Mike Baker (NWS Denver/Boulder) gave back in December showing the November-December storm tracks which were possibly aided by a positive phase in the MJO.
3. NAO and PNA: Going to defer to Dima on this one as I don’t really know enough to make any strong points. About all I can say especially towards the NAO is that it blends with the Arctic Oscillation (what I discuss in limited detail below). Dima I know there is large disagreement within the climatological community as to which oscillation plays a more dominant role and thus gives a more representative depiction of the large scale/dynamical pattern.
4. Arctic Oscillation (AO): Arctic climate index with positive and negative phases that represent the mean upper level pattern over the Arctic Latitudes. In the positive phase, generally lower than normal pressure over the polar region and higher pressure over mid-latitudes steering the majority of Pacific storms northward. Frigid winter time temperatures don’t extend as far south into central US. The negative phase is a general reversal of the pattern just described. Our buddies over at the NWS PHX office had a nice little presentation covering this a few weeks back. I’ve attached a slide from their presentation that gets more into what a Positive AO means for the Southwest.
My General “101 style” take away thoughts (please feel free to correct me if I’m making any misguided statements):
- Weak La Nina to ENSO neutral conditions favor sporadic but prolonged breakdowns of the western ridge normally seen during more moderate La Nina type winters.
- A Positive MJO index (November-December, now slightly negative) aided in transport of moisture rich tropical air across southwestern US by way of subtropical jet and enhancement of Western Pacific troughing pattern during that time.
- A positive AO which favors low pressure over Southern California in November and December despite the bulk of Polar Jet energy being steered into Central and northern Canada i.e. all of the cut off low scenarios we’ve seen the last 60 days. A positive AO also typically keeps temperatures much warmer and conditions drier to the east of the Colorado Rockies which we’ve also seen across the Plains and Midwest (it’s unbelievable how warm it has been thus far).
As best as I can understand it in terms of the large scale pattern, it seems like a pretty unique coupling of several dominant teleconnections has allowed for our interesting start to the winter. I honestly can’t remember seeing anything quite like this in the 10 or so years of weather forecasting that I’ve been a part of!!!
I’m really looking forward to hearing our expert Climatologist’s thoughts. It’s such an interesting topic and directly related to forecasting challenges Jake and I have seen over the last two months.
This post will be updated in the coming days as further responses become available:
Dan aka Squall Line
No comments:
Post a Comment