A Recap Of Recent Wintry Events_Lingering Color

The Winter Season Of 2019-20

The dawn of December will mark the beginning of Meteorological Winter 2019-20, so it is time to review some of the main players expected to impact the upcoming season (*).

*The following highlights some major forcing factors, but not nearly all the factors, and is given only as general guidance.

Beginning with the main climate system driver, the sun, Earth is currently within a deep solar minimum as 2019 is now poised to become one of the top years with respect to a lack of sunspots on the surface of the sun.

Track Sun Spot-Less Days On SpaceWeatherLive

*National Center for Atmospheric Research Staff (Eds). Last modified 20 Aug 2013. “The Climate Data Guide: QBO: Quasi-Biennial Oscillation.” Retrieved from https://climatedataguide.ucar.edu/climate-data/qbo-quasi-biennial-oscillation.

A -QBO phase in combination with a solar minimum enhances the potential for stratospheric warmings, with a major Southern Hemispheric warming having already been observed above Antarctica.

The above graphic shows the deviation of actual observed temperatures at 10 MB above the South Pole (black line) versus long-term climatological values (grey line). Warming began suddenly during late August-early September, fitting the name of Sudden Stratospheric Warming (SSW) which in this case was a major event.

This has been responsible, in part, for dryness and wild fires across Australia making current news headlines.

Initial warming above the North Pole during late autumn and early winter 2019-20 ended up being minor in nature, but with notable impacts on the hemispheric pattern.

Reference Early Winter 2019_High Knob Massif for more details.

A renewal of warming in the stratosphere is expected in early January, with time telling if this will be more significant with respect to impacting the Polar Vortex than the early season warming event.

Stratospheric warming is being driven by North Atlantic wave 1 forcing, with upward propagation and wave breaking (releasing mass, momentum, and energy) from the troposphere into the stratosphere in combination with upward wave activity flux (WAF) from Siberian snow cover as highlighted by Cohen to initiate the beginning of troposphere-stratosphere-troposphere coupling through coming weeks.

Judah Cohen highlights in his blog that snow cover extent during October was above average across Siberia, which favors stronger Siberian High Pressure and subsequent weakening of the polar vortex via upward wave activity flux (WAF) and feedback with tropospheric-stratospheric-tropospheric coupling leading to a negative Arctic Oscillation and colder than average conditions across mid-latitudes of the Northern Hemisphere.

Eurasian Snow Cover Variability And Links To Stratosphere-Troposphere Coupling

This is complicated and subject to variations depending upon the nature and positioning of forcing factors, both within and outside the arctic region and across the Northern Hemisphere.

As warming occurs, the polar vortex weakens and wind speeds decrease. During a Sudden Stratospheric Warming (SSW) event the direction shifts to easterly in direction. Warmings and SSW’s increase meridional flow across the Northern Hemisphere, enhancing transport of arctic air southward.

Although there is no official El Nino or La Nina running in the equatorial Pacific Ocean, the Southern Oscillation Index (SOI) has been consistently in decline as above average sea surface temperature (SST) anomalies are concentrated mostly in the central Pacific Ocean (versus eastern Pacific in classic +ENSO).

A rise in the daily SOI occurred through early December with a lag time correlated to a milder pattern developing in the eastern USA during mid-late December 2019.

A renewed and stronger mid-late December drop in the SOI may be signaling the return of a colder pattern in the eastern USA by early January 2020.

This is a Modoki El Nino (+ENSO) signal which has, from a climatological perspective, favored colder than average conditions in eastern North America and the eastern USA.

A positive Indian Ocean Dipole (+IOD) phase acts to enhance any positive ENSO that may be present during the same time.

The PNA has mostly been in its positive phase (+PNA) during November, which has been anomalously cold, following the mainly -PNA phase of October 2019 that featured much above average temperatures.

The most recent PNA trend has been negative and associated with milder conditions in the eastern USA. This trend is predicted to change back to positive as January 2020 approaches.

Warm sea surface temperatures, relative to average, within the Gulf of Alaska favors a positive PNA oscillation pattern and downstream troughing across the eastern USA, but the positioning and extent of western Pacific ridging is critical.

The December 1-19 pattern has been split between colder than average and near average to milder than average conditions across the southeastern USA.

The general December 1-19 pattern featured near to a little above average temperatures across much of the continental USA.

Blocking near the Aleutians and across Greenland was a persistent feature through Summer 2019 to the present, which contributed to major melting of the Greenland ice sheet (continental glacier).

The continuation of warm sea surface temperatures around Greenland would favor high latitude blocking to continue over this region of the North Atlantic.

Snow and ice extent across North America and Eurasia are near decadal averages, which is now ready for southward expansion of a more long-lived nature. Expansion across the northern USA will aid southward movement through the end of 2019, with initially wet and stormy conditions (featuring mainly rain) in the south.

If positive snow cover anomalies develop, as models currently predict, across eastern and southeastern Canada into December this will be a strong signal for later wintry conditions across the eastern USA and Appalachians versus if the main positive snow cover anomalies were to develop across western Canada.

Feedback associated with positive snow cover anomalies force a natural southward progression of the baroclinic zone (and main storm track) over time, since storm systems feed upon thermal contrasts developing across snow cover-bare ground gradients.

The Bottom Line

Many factors currently favor a colder than average winter season during 2019-20, with stormy conditions across much of the nation (especially in the Great Lakes to Appalachians corridor of the eastern-northeastern USA).

The potential for severe winter periods are likely given highlighted factors, but this will depend upon how the forcing from each of these different teleconnections (Modoki +ENSO, -QBO, SST Anomalies, Polar Vortex changes, snow cover, and more not highlighted here) interact at any given time during coming weeks to dictate the magnitude and extent of these periods.

Long-term forecasts remain elusive given complex feedbacks within and between components of the climate system, with short-term events in both space and time acting to alter conditions immediately that then can force subsequent changes which may skew a month or season away from a prediction made prior to development of the short-term event(s).

This is an evolving situation, as always during the Holiday season, so check back for later updates as conditions change across the Northern Hemisphere.