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[font=Arial, Helvetica, sans-serif]Arctic Oscillation Analysis and Forecasts[/font]
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AER scientists provide researchers and enthusiasts real-time insights on one of North America’s and Europe’s leading drivers for extreme and persistent temperature patterns. Authors Judah Cohen, Ph.D., and Jason Furtado, Ph.D., work at Atmospheric and Environmental Research (AER), a division of Verisk Climate.
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January 8, 2015
Summary
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- A positive to strongly positive AO will continue for the next week along with a positive North Atlantic Oscillation (NAO). Meanwhile, higher in the atmosphere, a minor sudden stratospheric warming (SSW) event has occurred with a split in the stratospheric polar vortex into two distinct centers.
- Extreme cold over the Central and Eastern US in the near term should relax to more seasonable cold next week before milder weather takes hold at the end of week 2. Extreme cold across Eastern Europe will also moderate, and a more seasonable regime will take hold across the continent.
- The downward propagation of negative AO conditions in the stratosphere will be slow to influence the tropospheric circulation, particularly if vortex recovery is slow as forecasted to be. Therefore, we anticipate near-neutral AO conditions to persist for much of the second half of January. This will promote a progressive flow pattern and periods of warmth and seasonable coolness hemispherically for the remainder of the month.
- Prospects for a negative AO remain fairly high into early February given the recent minor SSW, though some uncertainty on timing and the eventual strength of the negative state still remains.
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Impacts
Positive to strongly positive AO conditions will prevail for the next week to 10 days before a trend back to near-neutral conditions occurs. Despite the positive AO, conditions across the Central and Eastern US are extremely cold this week, and cold weather should continue into this weekend. The combination of a positive AO/positive NAO will create a very active storm track over northern Europe with the coldest air held in Eastern Europe under an anomalously deep trough and in far northern Scandinavia. The Eastern Europe trough will eventually swing eastward in response to a strong westerly jet stream across the continent.
In the meantime, a minor sudden stratospheric warming (SSW) has taken place in the polar stratosphere. This event has caused the stratospheric polar vortex to briefly split, with one center over north central Canada and the other on the northern Siberian coast. Models show that the vortex will reform over the next week, with a shift in the center of the polar vortex into the Western Hemisphere. This should allow slightly cooler than normal weather to prevail over North America into week 2, while a warming trend takes hold across Eurasia. However, continued recovery of the stratospheric polar vortex will be slow because of continual weak wave driving. The other impact of this minor SSW will be its influence in weakening of the positive tropospheric AO signature to near-neutral conditions, combined with other tropospheric processes. This change in the AO will deamplify the hemispheric flow pattern at the end of week 2 into week 3 and allow for a warming trend (potentially significant) over much of North America and eastern Asia. Persistent troughing in Barents-Kara (BK) Seas may retrograde and keep cold air locked across northern Scandinavia and far northern Eurasia.
Longer-term prospects for the AO are tied to this minor SSW event. The downward propagation of the negative AO conditions from the stratosphere into the troposphere should cause a shift in phase of the tropospheric AO. Timing of this change is now seen towards the end of January/early February, particularly given the weak recovery of the vortex. Another element adding uncertainty to the forecast is the recent volatility and biases from the models. For example, the GFS/GEFS has held a negative AO bias (and also cold bias for North America and Europe) over the last month, while the ECMWF ensemble has had a slight positive bias in both features. Thus, we favor a neutral or slightly positive AO for the second half of January transitioning to possibly weakly negative in early February.
Recent and Very Near Term Conditions
The AO is strongly positive and will remain that way into early next week (Figure 1). The North Atlantic Oscillation (NAO) will also have the same positive trend (not shown). The tropospheric circulation pattern, however, will not be wholly characteristic of a positive AO regime. That is, deep troughing will take hold over Central and Eastern North America, delivering extremely cold weather for the remainder of this week. However, the positive NAO, with a very potent jet stream across the North Atlantic, will deliver very mild air over most of Europe except for Eastern Europe, where a deep trough will keep cold and stormy weather in place. This cold weather will subside into the weekend, however, and even Eastern Europe should begin a warming trend.
Figure 1. The predicted daily-mean near-surface AO from the 00Z 6 January 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.
Higher in the atmosphere, a minor SSW event is underway with a split vortex seen at 10 hPa (Figure 2, left), generating negative AO conditions as seen with the positive polar cap heights (Figure 3). These minor events typically have a downward influence on the lower stratospheric and the tropospheric circulation starting about 10-14 days later with persistence of negative surface AO conditions possible thereafter. The potential for this persistence of the AO is discussed further in the 3-4 Week and 30 Day sections.
Figure 2. (left) 10 hPa geopotential heights (dam; contours) and temperature anomalies (°C) across the Northern Hemisphere at 00Z 6 January 2015. A minor sudden stratospheric warming event is underway. The stratospheric polar vortex is currently split into two centers – one over north central Canada and the other across northern Eurasia. (right) The predicted 10 hPa geopotential heights and temperature anomalies from the GFS ensemble for 16 January 2015. Note that the vortex is beginning to recover, though warm anomalies persist in the Eurasian sector. Also not that the vortex center lies more on the North American side of the Arctic, which will be coincident with the predicted lingering troughing across northeastern North America.
Figure 3. Observed and predicted daily polar cap height (i.e., geopotential heights averaged poleward of 60°N) standardized anomalies. Forecast from the 00Z 6 January 2015 GFS operational model.
Near Term
1-2 week
The relatively strong positive AO expected into early next week will trend negative starting in week 2. Part of this negative trend is likely attributed to the minor SSW, though other tropospheric processes (e.g., tropical convection) are also contributing to the negative trend. Nevertheless, a progressive flow pattern should hemispherically and start a large warming trend across most of the continents. The exception may be across northeastern North America, where troughing should linger and thus keep conditions colder than normal until the end of week 2. The delay in warming across eastern Canada and the Northeast US is related to the reformation of the stratospheric polar vortex towards the North American side of the Arctic (Figure 2, right). There will also be residual cold in the Southeastern US thanks to a potentially active southern part of the polar jet. Across Eurasia, relatively mild weather will spread across much of the continent as the jet stream relaxes to a more climatological position. Low pressure will remain anchored across Scandinavia and the BK Seas area, however, which should keep colder than normal weather across far northern Europe and the Eurasian/Arctic coast.
The return to near-neutral AO conditions also means weaker than average vertical wave driving to occur in the polar stratosphere (Figure 4). Though the vortex is expected to strengthen with time, it will be prevented from making a quick recovery because of these wave pulses. This is evident in the polar cap height projections, which anticipate little recovery of the stratospheric polar vortex over the next two weeks (Figure 3).
Figure 4. Observed and predicted daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N. Forecast from the 00Z 6 January 2014 GFS operational model.
3-4 week
The longer-term prospects for the AO are complex for a few reasons. Typically following a SSW, the AO turns more negative in the lower stratosphere and eventually throughout the troposphere within 2 weeks. However, given that this warming was a minor SSW and that the vortex will be slow to recover, the transition to a tropospheric negative AO likely will be delayed, potentially until week 4. In terms of tropospheric forcing influencing the AO this period, current tropical convection across the Maritime Continent is anticipated to move into the western Pacific but then possibly weaken. The current convection pattern promotes a warm signal for much of North America and a positive NAO for Europe into weeks 2 and 3 (there is a 8-14 day lag in the signal based on the literature). Thereafter, if the convection does indeed weaken, the impact of the tropics on the polar jet stream will weaken and thus allow the AO to shift to more neutral conditions. Thus, we foresee that the AO should average near neutral or slightly positive for week 3 and part of week 4 before turning more negative with the downward influence of the SSW.
Sensible weather impacts for the long term will be a progressive flow pattern across the hemisphere and overall variable temperature regimes. Models continue to show mild to potentially warm conditions across North America and much of Eurasia for week 3. Colder air should begin to seep into northern Europe and Asia during the end of week 3 and into week 4. This change is typical of AO regime changes to a negative phase – i.e., the cold air typically enters Eurasia first with a dip in the jet and then changes in the jet stream occur downstream.
Longer Term
30–day
We continue to anticipate that the AO will turn more negative going into early February, though the timing of this change will depend on how quickly the stratospheric polar vortex recovers and also whether or not persistent troughing in the BK Seas region eventually weakens. The long-range models have had significant trouble with this latter feature, and hence its eventual demise is uncertain. The shift to a negative AO will increase chances for more cold air outbreaks into Europe, much of Asia, and Central and Eastern North America during February, along with a stormier regime as well.
Winter AO Outlook
Arctic Sea Ice
Arctic sea ice extent continues below normal in the Chukchi Sea and also in portions of the BK Seas region (Figure 5). The ice anomalies have been concomitant with generally high pressure over the North Pacific and low pressure across the North Atlantic and BK Seas area, respectively. Whether there is an ocean-atmosphere feedback in place is unknown, but so long as the atmospheric pattern remains as such, the contribution will be toward a more neutral AO state.
Figure 5. Observed Arctic sea ice extent on 5 January 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
Weak Central Pacific El Niño conditions persist (at least in the ocean – atmospheric coupling has been indiscernible with this event) with a positive PDO-like sea surface temperature (SST) pattern in the North Pacific (Figure 6).
Figure 6. The latest weekly global SST anomalies (ending 5 January 2015). Image courtesy of NOAA/ESRL/PSD.
More importantly, the MJO is presently in Phase 5 and expected to move into Phase 6 before weakening (Figure 7). The response of this will be for a deepening trough across the Northeast Pacific and ridging over North America starting in week 2. The NAO may also strengthen briefly during week 2 and 3 in response to this convection. Thereafter, the forecasted neutral MJO conditions at the end of the period will remove its influence on the jet stream, allowing for the downward-propagating negative AO signal from the stratosphere to have more influence by the end of January.
Figure 7. Past and forecast values of the MJO index. Forecast values from the 00Z 6 January 2015 ECWMF model. 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/fa…ndy/waves/phasediags.html.
Northern Hemisphere Snow Cover
Snow cover has increased over the Northern Hemisphere and especially North America as colder and a somewhat stormier pattern has taken over of late (Figure 8). Hemispheric totals are higher than they were last year, thanks to growth in snow cover across Eastern Europe and parts of North America. The near term should feature a continued uptick in snow cover, especially across the United States. The progressive and mild regime predicted after mid-month, however, may cause a steady trend in snow cover or even a decrease across North America, particularly in the Plains States.
Figure 8. Observed Northern Hemisphere (top) and North American (bottom) snow cover extent through 5 January 2015 for this season (red curve) and the past 9 seasons (other colored curves). Image source:http://www.star.nesdis.noaa.gov/smcd/emb/snow/HTML/snow_extent_plots.html.
Quasi Biennial Oscillation (QBO)
Winds in the lower tropical stratosphere (e.g., 50 hPa) continue to trend more negative (i.e., easterly) and should continue that trend over the next month or two, while the easterly winds have peaked in intensity at 30 hPa. The easterly QBO signature continues to be favorable for changing the tropospheric wave guide and keeping wave energy anomalously directed toward the polar stratosphere. This element was a factor in producing the minor SSW and may still play a role in any future wave driving if it materializes. Note that signs of the westerly phase of the QBO are already appearing higher in the stratosphere and will propagate downwards over the next 6-9 months.
Temperature Anomaly Animation
Below we show the daily temperature difference between those with observed high October Eurasian snow cover minus low October Eurasian snow cover from September 1 through February 28 in degrees Celsius. Temperature differences are shown in shading and those differences that are found to be statistically significant are outlined by a black contour. This is not meant to be a forecast but instead[/font] shows the general progression of the temperature anomalies across the Northern Hemisphere based on October Eurasian snow cover variability alone.
