Dr Judah Cohen AER: Arctic Oscillation Analysis and Forecasts

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November 24, 2015

Summary

• The AO is currently positive and is predicted to remain positive, though the trend is towards neutral next week.
• The persistent positive AO conditions is reflective of the model forecast of low pressure/geopotential heights over the Arctic, especially the North Atlantic side, while high pressure/geopotential heights dominate the mid-latitudes.
• With positive AO conditions and mostly high geopotential heights entrenched across the mid-latitudes, the weather pattern should be relatively mild across the Northern Hemisphere continents including Northern Europe and North America, especially Canada.  One exception will be East Asia where cold temperatures will likely linger from the recent Arctic outbreak.
• Longer-term high latitude boundary conditions favor a negative bias to the AO including extensive Eurasian snow cover and low sea ice extent in the Barents-Kara seas.  The key will be how active the energy transfer from the troposphere to the stratosphere is in December.  Without which the polar vortex will remain strong, the AO mostly positive and mild temperatures will dominate the mid-latitudes.

Impacts

Despite a positive AO, temperatures are cold in Europe, East Asia and the Eastern United States.  However the models suggest that the AO will remain positive into the foreseeable future and this should result in an overall warming trend in these regions.  Though temperatures will be slowest o recover across East Asia.

The predicted persistent positive AO state is reflective of below normal geopotential height anomalies in the Arctic basin, though some predicted ridging extending northward from western North America will pull the AO back towards neutral.  From the end of June through the current period, high latitude blocking and positive geopotential height anomalies have dominated the Arctic and the high latitudes.  However so far this month, the atmosphere has clearly transitioned from one where the Arctic is dominated by high pressure/geopotential heights to one where the Arctic is dominated by low pressure/geopotential heights. In contrast to the Arctic, the models predict that the mid-latitudes will be dominated by high pressure/geopotential heights. Initially the three dominant centers of positive geopotential height anomalies will be the eastern North Pacific, the North Atlantic and Northern Asia.  A weaker region of ridging or positive geopotential heights is building in central North America.  However with time the models are suggesting that the dominant positive geopotential height centers will be eastern North America, Western Europe and northwest Asia and the western North Pacific. With the mid-latitudes dominated by ridging and positive geopotential heights, this will result in an overall mild pattern across the Northern Hemisphere mid-latitudes.  However exceptions to the mild weather pattern will be found downstream of two of the dominant positive geopotential height centers across East Asia and the Southeastern United States.

Looking longer term towards the winter, we follow Siberian October snow cover extent, Barents-Kara sea ice extent and tropospheric precursors to try to predict the winter mean AO state. October Eurasian snow cover extent was above normal.  Following above normal snow cover, we expect a northwestward strengthening of the Siberian high coupled with a deepening Aleutian low.  The Siberian high has strengthened to the north but is somewhat displaced eastwards than is typical. Arctic sea ice in the Barents Kara seas is below normal and certainly less extensive than last year at this time.  The persistent below normal sea ice in this region should help to build the Siberian further westward with time.  That coupled with a strengthening Aleutian Low, typical of El Niño winters should set up an atmospheric pattern that is favorable for increased energy transfer from the troposphere to the stratosphere that eventually warms the polar stratosphere and weakens the polar vortex and helps force a negative AO.  However if the energy transfer is not sufficiently strong over the next month to weaken the polar vortex, then the AO is likely to average positive in the mean and a mild weather pattern will dominate the md-latitude continents this upcoming winter.  This is especially true across North America where an El Niño driven southern Jet Stream will flood North America with mild Pacific air.

Recent and Very Near Term Conditions

Currently, the AO is positive and is predicted to remain positive over the next two weeks though the uncertainty becomes large after week one (Figure 1).  The predicted positive AO is reflective of several weak polar lows in the high latitudes  (Figure 2) and mostly positive geopotential height anomalies in the mid-latitude ocean basins.

Figure 1. The predicted daily-mean near-surface AO from the 00Z 24 November 2015 GFS ensemble. Gray lines indicate the AO index from each individual ensemble member, with the ensemble-mean AO index given by the red line with squares.

Currently there are several polar lows in the Arctic basin (Figure 2).  A fairly strong trough extends equatorward towards Europe from one polar low in the Kara Sea, bringing cold temperatures to much of Europe that extends even into western North Africa (Figure 3). Another vigorous trough extends equatorward into the Northeastern United States from another polar low near the North Pole (Figure 2) bringing cold temperatures to the Eastern United States (Figure 3).  However the most impressive region of below normal temperatures is associated with ridging and above normal geopotential heights in Central Siberia (Figure 2) with a surface reflection helping to strengthen the Siberian high and much colder temperatures to Central Siberia and East Asia (Figure 2).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 24 November 2015 at 00Z. Note the low heights over the Arctic and with high heights over the North Atlantic, Central Siberia and the eastern North Pacific.

Ridging and above normal geopotential heights dominate central North America (Figure 2). That coupled with a trough just off the West Coast is resulting in a general west to southwest flow of air, with temperatures averaging above normal for the Central United States and Canada (Figure 3).   However cold air is building in northwest Canada and as the trough comes ashore it will drag Arctic air from northern Canada into the Western United States.

Figure 3. Surface temperatures (°C; shading) on 24 November 2015 at 00Z. Note the cold temperatures extending from Central Siberia towards East Asia, Europe and the Eastern United States with warm temperatures across much of central North America and western Asia.

1-2 week

The general positive AO conditions are predicted to continue this period (Figure 1) as low pressure and below normal geopotential heights continues to dominate the North Atlantic side of the Arctic (Figure 4).  With low pressure dominating the high latitudes, high pressure will continue to dominate the mid-latitudes.  The models are predicting a wave four configuration for the mid-latitudes.  The dominant mid-tropospheric height center this period is a positive geopotential height anomaly centered over Central Canada (Figure 4).  Also for the first time this season, the models are predicting a deep Aleutian low upstream of the Canadian ridging (Figure 4).  This coupling is more typical atmospheric pattern associated with El Niño. This will result in mostly above normal temperatures for Canada, though temperatures will likely be closer to seasonable as an active Jet Stream pulls south modified Arctic air behind traversing storm systems (Figure 5).

Figure 4. (left) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 30 November – 4 December 2015. (right) Same as (left) except averaged from 5–9 December 2015. The forecasts are from the 00z 2 November 2015 GFS ensemble.

Across Eurasia, there are two predicted dominant mid-tropospheric height centers this period one covering much of Europe and the second covering northwest Asia.  In concert, this will help to maintain positive geopotential heights over much Eurasia (Figure 4). With overall above normal geopotential heights, western and Central Eurasia, including Europe, should experience temperatures that are average to above normal (Figure 5).  Exceptions to the overall ridging pattern are troughs downstream of the positive height centers with one in the eastern Mediterranean and the other in East Asia (Figure 4). The two troughs will result in below normal temperatures for the Mediterranean/Southern Europe and East Asia respectively (Figure 5).

Figure 5. Forecasted surface temperatures (°C; shading) from 30 November – 4 December 2015. Note the warm temperatures across much of North America and northern Eurasia with cold temperatures in East Asia and North Africa. The forecasts are from the 00Z 24 November 2015 GFS ensemble.

3-4 week

Looking at the end of the Week 2 period, many of the atmospheric features from the previous period remain.   The dominant geopotential positive height center continues to cover much of Canada (Figure 4).  This will result in temperatures well above normal for much of Canada (Figure 6).  Temperatures are predicted to be mild for the United States as well but an active storm track will likely help to advect at least some modified Arctic air into the United States behind departing storms. Eurasia is also predicted to be dominated by positive geopotential height anomalies with centers of positive geopotential heights across Europe and central/Southeast Asia.  Downstream of the mid-latitude positive height centers some weak troughing is also predicted in southwest Asia, which extends southwestward into the Mediterranean, and northeast Asia (Figure 4).  This should result in a mostly mild weather pattern for Eurasia, though colder temperatures are predicted for the Mediterranean/Southern Europe and East Asia respectively (Figure 6).

Figure 6. Forecasted surface temperatures (°C; shading) from 5–9 December 2015. Note the predicted continuation of the cold temperatures for East Asia, Southern Europe, North Africa and the southeastern United States with warm temperatures across much of North America Northern Europe and western Asia. The forecasts are from the 00Z 24 November 2015 GFS ensemble.

30–day

Longer-term weather prediction will likely require a correct forecast of the strength of the stratospheric polar vortex. Currently the polar vortex is quite strong and as long as it remains strong, the overall hemispheric pattern will likely remain mild.  A favorite diagnostic of ours to analyze and anticipate troposphere-stratosphere coupling is the polar cap geopotential height (PCH) anomaly plot.  Consistent with the predicted positive AO, the models predict that over the next two weeks the polar cap geopotential height anomalies will remain mostly normal to below normal for much of the troposphere and well below normal for the stratosphere (Figure 7).  Previously we discussed the strong Siberian high observed so far this month that is favorable for wave activity flux (WAF) to become more active.  By the end of the week the model is predicting a strong upward pulse of energy into the stratosphere (Figure 8).  However one WAF pulse in November is not sufficient to weaken such a strong polar vortex.  The WAF forecast is for a second pulse the end of week two but this is highly speculative at this point.

Figure 7. Observed and predicted daily polar cap height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies. Forecast from the 00Z 24 November 2015 GFS operational model.

Given the above normal Eurasian snow cover this October and ongoing below normal sea ice extent in the Barents-Kara seas, we continue to favor building of high pressure across northwest Asia and the adjacent Arctic with downstream troughing into East Asia and the North Pacific.  This should both force the PCH warmer or positive in the troposphere and increase the WAF. Therefore we continue to anticipate the active transfer of energy from the troposphere to the stratosphere and a weakening of the polar vortex later in December and or early January.  Following the weakened polar vortex we would further expect a negative AO and cold temperatures across the Northern Hemisphere continents.  However if the WAF stays close to or below normal then the polar vortex will remain strong, the AO positive and the overall weather pattern mild to even very mild.

Figure 8. Observed and predicted daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N. Forecast from the 00Z 24 November 2015 GFS operational model.

Surface Boundary Conditions

Arctic Sea Ice

Arctic sea ice extent is expanding but continues to track well below normal (Figure 9).  The largest negative sea ice extent anomalies are now clearly in the Barents-Kara seas as expected. In contrast to last year, the sea ice is more extensive on the North Pacific side and less on the North Atlantic side.  Negative sea ice anomalies in the Barents-Kara seas region could eventually force further north the positive geopotential height anomaly currently over Europe and a colder pattern across Europe.  In addition based on some recent research low Arctic sea ice especially in the Barents-Kara seas favors a negative winter AO.

Figure 8. Observed Arctic sea ice extent on 22 November 2015 (white). Orange line shows climatological extent of sea ice based on the years 1981-2010. Image courtesy of National Snow and Ice Data Center (NSIDC).

El Niño/Southern Oscillation and the Pacific Basin

Figure 10 shows the weekly global sea surface temperature (SST) anomalies. A strong El Niño is still apparent, and both dynamical and statistical models continue to indicate a strong El Niño for this winter. The trend of strengthening positive SST anomalies towards the dateline and away from the South American coast continues.  If during the winter, the strongest positive SSTS are in the Central Pacific, this it could have important implications for the winter weather across North America.  Also of note, the models are predicting a fairly rapid weakening of El Niño through the winter months.  Another key feature in the North Pacific basin is the very warm waters in the eastern North Pacific and the north central North Pacific.

Figure 9. The latest weekly-mean global SST anomalies (ending 22 November 2015). Data from NOAA OI High-Resolution dataset. The tropical Pacific shows the El Niño SST structure with warm waters near the equator in the eastern and central tropical Pacific. Warmer than normal waters also extend into the subtropical North Pacific and along the western coast of North America and the north-central North Pacific.  Cooler than normal waters extend across the subpolar North Atlantic from the Canadian Maritimes to Great Britain.

Tropical convection is currently in amplitude phase two (Figure 11) and is predicted to head into phase three and then weaken.   During phase three convection is near the Maritime continent. There is likely to be little influence on mid-latitude weather from tropical convection.

Figure 10. Past and forecast values of the MJO index. Forecast values from the 00Z 23 November 2015 ECMWF model. Yellow lines indicate individual ensemble-member forecasts, with the green line showing the ensemble-mean. A measure of the model “spread” is denoted by the gray shading. Sector numbers indicate the phase of the MJO, with geographical labels indicating where anomalous convection occurs during that phase. Image source: http://www.atmos.albany.edu/facstaff/roundy/waves/phasediags.html.

Northern Hemisphere Snow Cover

Seasonal snow cover continues to advance both across Eurasia and North America, though so far snow cover has advanced more rapidly across Eurasia.  Eurasian snow cover is above normal and is close to one of the highest values observed for this time of year over the past decade (Figure 12). Eurasian snow cover extent for the month of October was above normal. Above normal Eurasian snow cover extent for the month of October favors a negative winter AO.  Also above normal snow cover favors more active energy transfer from the troposphere to the stratosphere in the six weeks from mid-November until the end of December, which often results in a weakening of the polar vortex in January.  More active energy transfer is predicted this week by the models and we expect that trend to continue for now.

Figure 11. Observed Eurasian (top) and North American (bottom) snow cover extent through 23 November 2015. Image source:

http://www.star.nesdis.noaa.gov/smcd/emb/snow/HTML/snow_extent_plots.html

North American snow cover advance has been slower and is closer to normal.  However with an active storm track predicted across the United States, snow cover advance is expected to continue.