Thursday, October 16, 2014

Long Range Discussion, Concerns About Winter (Part 1)

Update: Click Here for the new Part 2 Post (ideally read after reading this post).

Today, we'll go over the latest outlook for the long range, and go over some new concerns I have about the coming winter. This is the first post of the two, dealing with the long range forecast portion. The winter concern post will come out tomorrow.

Let's first discuss the Global Wind Oscillation and Atmospheric Angular Momentum (GWO and AAM) concepts.

The image above shows anomalies of the relative atmospheric angular momentum (AAM) since this past July to the present day. The image looks pretty complicated, and in reality it is, so we'll avoid any extremely difficult parts. In essence, positive AAM values can indicate enhanced areas of the jet stream, while negative values may indicate a weakened jet stream. As an example, take a look at the green swath around the 60N parallel in the first days after October. This green tells us of negative AAM anomalies in the area, so let's see if we can identify a cause behind it.

The graphic above shows geopotential height anomalies since the middle of this past June. The top panel gives an indication of these geopotential height anomalies on a time-by-height graph, where reds depict positive height anomalies (warm weather, high pressure) and blues indicate negative height anomalies (cold weather, low pressure). Check out that big swath of reds only a few days after the start of October. We saw significant ridging in the troposphere and even into the stratosphere, disrupting the jet stream and sending the Arctic Oscillation plummeting. As the jet stream significantly weakened with the ridging pushing through, the AAM reflected appropriately a negative AAM.

The overall AAM anomaly is shown by the bottom panel, and we can see how the AAM has been negative lately. However, it's been rising in recent days.

The image above now shows the tendency of the AAM; among other things, it tells us whether the AAM wants to be in positive or negative territory. Lately, the AAM tendency has been in a positive state, and this is likely what the rising relative AAM is caused by. The tendency will take on a more significant role later on in this post.

Take a look at the graphic above. Pretty complicated, right? Not quite. In the bottom left and right corners, as well as the top left and right corners, we see descriptions for regions that see convection in various stages of the Global Wind Oscillation (GWO). Phase 3 sees Indian Maritime convection, while the Dateline region observes tropical thunderstorms in Phase 5, and so on. More experienced weather junkies may realize these denotations are actually juxtaposing the Madden-Julian Oscillation (MJO) onto this GWO description.
As for the GWO itself, it does have eight phases, as the chart above entails. From another source in the ESRL, the GWO has been allotted into four primary phases. I've copied and pasted the four phase descriptions below:

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.
'Woah! Slow down! I don't know what this means!' is probably what some of you are thinking right now. Let's cut out the complicated parts and focus our attention on the underlined phrases above. In each stage, there's a description about the AAM and AAM tendency. We know what those mean, after discussing them above, so let's put it to use. Right now, the relative AAM is negative and the tendency of the AAM is positive. If we match that up with the underlined phrases, we find ourselves in a Stage 2 set-up, with the negative relative AAM described in Stage 1, but the positive AAM tendency described in Stage 2. Doing a quick evaluation, when we account for the fact that El Nino-like anomalies (Stage 3) are shown by high GWO values (Phases 5-8), and La Nina-like anomalies (Stage 1) display themselves in low GWO phases 1-4, we can estimate the GWO to currently be around Phase 3 or 4, given how Stage 2 (where we are now) is in between the high and low GWO phases.

As if that wasn't confusing enough already, we can actually forecast the AAM in the future!

The graphic above now shows transport of the AAM. As if there aren't enough ways to examine the AAM, the transport of the AAM can allow us to delineate how AAM anomalies are making their way to the upper latitudes; I've drawn two arrows to illustrate this above. Notice how we currently see decently-strong positive AAM transports pushing northward. This tells us that we can expect a strengthening jet stream in the near future (if we recall positive AAM shows a strong jet stream), something confirmed by the forecast of the Arctic Oscillation, as we see below.

Ensemble members have the Arctic Oscillation pushing positive, something I wouldn't be surprised to see with the positive AAM transports in coming days.


Let's take a step away from the AAM now and move on to other long range factors.

The graphic above shows a 500mb mean height anomaly forecast from the combined GFS ensemble members, with individual ensemble member height contours shown in the smaller panels. On this graphic, valid for 60 hours out, we can see a strong trough dropping into the Gulf of Alaska, as the deep blue colors indicate. A small cut-off low occupies the Southwest, with a weak ridge persisting in the Plains. A trough is on its way to the Atlantic, as seen in the Northeast in the graphic above.
If you were to examine a loop of the forecast from these GFS ensembles, you would see several bursts of ridging in the West and Central US that initially appear strong, but quickly weaken and push east. If you're a close reader, you'll take a look at the description of the GWO Stage 2 and realize that these quick-dissipating ridges are actually the 'Fast Rossby Wave dispersion events'.

The atmosphere's just one big web of connections and correlations...

Fast forwarding to the GFS ensemble height anomaly forecast valid 264 hours out, we now see that Gulf of Alaskan troughing has pushed east into the West Coast, thanks to sustained ridging in the Bering Sea. As such, the ridging previously in the West/Central US is now finding a home further east, in the Plains. The New England region is seen basking in below-normal temperatures with the negative height anomalies.
Take a look back at our AAM section and give Stage 3 a look. As I had indicated, we're forecasted to head into positive AAM territory, which is the equivalent of Stage 3 in the GWO. Note how the Stage 3 description tells of an extended Pacific Jet Stream, as well as high-impact weather events in the West US. It's no mistake that the long range GFS ensembles are showing a stormy West Coast, fitting in with that Stage 3 description.

I'll use these forecast images again in tomorrow's Part 2 post, which will go over my concerns about next winter; my post here is already too long to extend it into the second topic.

Tropical Tidbits
Let's give our brains a break and use some of our simpler tools in the long range. The image above shows the ECMWF 500mb height anomaly forecast over the Western Pacific, valid this morning (Thursday). Note the trough digging into Japan. Using the Typhoon Rule and 6-10 day correlation, we can expect a stormy period in the US 6-10 days from today, in an October 22nd to 26th period.

Tropical Tidbits
But all's not well that ends well, winter weather fans. The graphic above shows the same ECMWF height anomaly forecast in the West Pacific, now valid for 10 days out. We see a massive - emphasis on massive - ridge forecasted to cover the eastern part of Asia, including over Japan. If this forecast comes to fruition, we might expect to see a prolonged period of significant warmth in the November 1st to November 5th timeframe. Again, that's if this comes to fruition.

One final piece concerning this forecast. I mentioned the Madden-Julian Oscillation (MJO) earlier in this post, and wanted to discuss it for a moment. In the image above, we see Outgoing Longwave Radiation (OLR) anomalies in the color shadings, with 200mb velocity potential contours and divergence with the arrows. Negative OLR anomalies indicate stormy weather, and are displayed as blues on this graphic. Oranges depict positive OLR anomalies, highlighting quieter than normal tropical convection. Notice how the strongest divergence is located just west of South America... and it doesn't even appear to be focused along the Equator! This tells us that although tropical forcing is strongest in the Western Hemisphere, the MJO is anomalously weak, as it has been for the last several weeks, per the graphic below.

40 day observed MJO
The MJO isn't expected to be a significant factor now or in the near future, per model forecasts.

Long Range Forecast Summary

- The AAM is expected to shift to positive in coming days, as the AAM tendency remains positive and positive AAM transports are pushing to the upper latitudes.
- Due to the positive AAM occupying the upper latitudes, strengthening of the jet stream is expected. This will result in a positive AO, and thus warmer weather.
- The GWO is expected to push into Stage 3 with the positive AAM, which will allow an El Nino-like set-up to evolve. Stormy weather in the West US, with warmth in the Central can be expected in coming weeks.
- East Asian signals tell us of a brief stormy period to end October, with potential significant warmth to start November.

Remember to look for Part 2 tomorrow afternoon!