Monday, March 23, 2015

Long Range - Monthly Outlook: April 2015

This is the long range/Monthly outlook for late March into April 2015.

We'll first begin with a diagnosis of the atmosphere. For those who find the technical jargon a little overwhelming, summaries to the post are provided at the bottom of the page.

The above image shows relative atmospheric angular momentum anomalies on the top panel, with the global anomaly on the bottom panel. According to the latest data, we are in a globally positive-AAM (+AAM) state, mainly due to a nice surge of +AAM to the equator. This has pushed the atmosphere into an El Nino mindset, which has shown up in our atmosphere. We'll go more in-depth to that later on.

The graphic above shows the tendency anomalies of the atmospheric angular momentum on the top panel, with the net tendency on the bottom panel. Notice how we're currently recovering from a massive +AAM tendency spike, and are now in a net negative AAM tendency. The cause of this tendency is not the thing we will analyze; right now, we are noting that we're in a +AAM state with -AAM tendencies.
This fits well into the four stage descriptions of the Global Wind Oscillation (GWO), which takes those above graphics and makes them into useful data. Here's the descriptions of all four stages of the GWO; pay careful attention to the first and last sentences of each stage.

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.
What did you notice in each stage description? Did you notice how each description began with details on how the net (global relative) AAM or AAM tendency was positive or negative? Let's apply that here.
We saw earlier how we are in a +AAM / -AAM tendency phase right now. Using those particular sentences in each description, it appears we're in Stage 3 or Stage 4 of the Global Wind Oscillation. Let's investigate each description closer. In Stage 3, among other details, the Pacific jet stream is extended (can stretch across the Pacific), and active weather impacts the West US. In Stage 4, the subtropical jet stream is strengthened, and closed lows in the subtropics can permit heavy rainfall over the Southwest.

Do we have any of those factors?

The answer is yes. The graphic above shows 200 hPa heights and wind speeds in ~4 day periods, with the least recent panel in the top left and the most recent panel in the bottom left. Notice how the Pacific jet stream is clearly extended into the North Pacific, fulfilling our Stage 3 ideals. If you'll look back at the stage description for Stage 3, you'll notice how it says 'El-Nino Like'. This means that conditions in this stage are most likely to provoke an El Nino-like response in the atmosphere. We're already seeing that response, namely in the extended Pacific jet and the AAM charts. I expect this to continue throughout April due to ensemble agreement.

The temperature anomaly charts in the US came back very cold for the next week or so. Each individual member is shown on those smaller boxes, while the large box contains the mean temperature departure. There is very good agreement on this cold, as well, valid on March 28th.

Lastly, let's go over the tropics. We're currently seeing a very strong tropical forcing episode over Phase 8 of the Madden-Julian Oscillation, a signal that has been created thanks to a powerful convectively coupled Kelvin Wave (CCKW). The long range ECMWF ensembles believe we see this current wave dissipate before we dip back into the circle of the unknown, and this proposal is backed up by the GFS model. This renders tropical forcing useful until just about March 28th, before the wave weakens into the Circle of Death on the MJO chart. Phase 8 and Phase 1 events in this time of year routinely create warm and cold conditions, respectively, in the Midwest, Central US, and East US. I have a good feeling that the Phase 8 activity will far outpace the Phase 1 activity, keeping us in the El Nino mindset well into April.

To summarize the forecast:

- The last days of March (24-30) may see sustained cooler than average temperatures. This may persist even longer in the Northeast.
- Early April will see continued threats to the West for heavy rainfall and moisture. A warm-up is expected for this time period.
- Mid and Late April can expect more El Nino conditions, with warm weather in the Morth and stormy/cool weather in the South.