Amir Mohsen
متخصص بخش هواشناسی
Arctic Oscillation Analysis and Forecasts
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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[h=3]December 30, 2014 [h=4]Summary
However longer-term prospects for the AO continue to hinge on the anomalous polar low in the Barents Kara-seas region that has temporarily weakened. It is predicted to strengthen again across the north slope of Siberia and eventually move back over the Barents Kara seas region. Perhaps the current temporary relaxation is a foreshadowing of a longer term weakening trend but for as long as the Barents-Kara seas low persists, it destructively interferes with an externally forced negative AO (from the SSW). The biggest impact would to inhibit the flow of Siberian air into Europe. It’s impact would be less on North American temperatures where in the near term, ridging across western North America and a polar low in eastern Canada will direct arctic air into the lower 48.
[h=4]Recent and Very Near Term Conditions The AO is currently negative. The GFS ensemble-mean forecast shows that the AO should reverse and spike positive after the New Year (Figure 1). The result will be a brief warm-up along the East Coast of the United States and probably a more meaningful warm-up for Europe. The negative NAO has resulted in cold and snowy weather for much of Europe and the reversal in phase signals a return to more seasonable weather. In East Asia the very cold temperatures have abated and the more seasonable temperatures in place should continue. Often the largest negative temperature departures across East Asia precede a stratospheric warming relative to post the stratospheric warming. Therefore the most persistent cold weather may be over in East Asia for the winter. This is consistent with our seasonal forecast model that predicts December as the coldest winter month relative to normal for East Asia.
The strong spike in the AO is consistent with the cold polar cap heights (PCH) in the lower troposphere right at the start of the New Year (Figure 2) and the strong upward pulse in the vertical Wave Activity Flux (WAFz) also during the first week of January (Figure 3). Strong upward WAfz warms the polar stratosphere and increases the PCH in the stratosphere (as seen in Figure 2) but also forces cold/lower PCH in the troposphere.
[h=4]Near Term
1-2 week As has been the case for most of the winter so far there is considerable in the GFS ensemble-mean near-surface AO forecast for the this time period. However the ensemble-mean shows an overall positive AO is predicted this period (Figure 1). But of greater importance to longer-term trends in the AO is the sudden stratospheric warming (SSW) predicted to take place by the models the end of the first week of January (Figure 4). As we have been discussing for over a month, based on the extensive Eurasian snow cover observed this past October we considered a SSW in early January as highly probable and that the tropospheric wave anomalies favor a polar vortex split. Both the GFS and ECMWF weather models predict a SSW and polar vortex split as seen in the model forecasts for 10 hPa geopotential heights/temperatures for January 5, 2015 (Figure 4).
The models do disagree on the strength of the polar vortex weakening. The GFS predicts a relatively quick polar vortex recovery and is predicting that the polar vortex will return to normal and even slightly above normal strength by the end of the period (Figure 2). The quick recovery in the polar vortex is directly attributable to the large deficits in the WAFz predicted by the GFS this period (Figure 3). The ECMWF in contrast shows continued wave driving after January 5 that continues to perturb and weaken the polar vortex even beyond January 5. Based on the ECMWF forecast, the SSW will get close to a major SSW (defined as a zonal mean easterly wind at 10 hPa and 60°N) and would likely achieve major SSW strength with further wave driving. The GFS has displayed a bias all winter of under-predicting the vertical WAFz and over-predicting polar vortex strength. Therefore we favor the ECMWF forecast of a more perturbed and longer duration polar vortex weakening.
Regardless, with positive AO conditions at the surface temperatures should be seasonable to even seasonably mild across Europe and East Asia. However the Central and Eastern US will be cold as the developing stratospheric warming preferentially builds heights over the Bering Sea region forcing a ridge over Alaska and the West Coast of North America that drives Arctic air into the lower 48.
[h=4]3-4 week Typically the circulation anomalies associated with a SSW, high PCH, an equatorward displaced Jet Stream and negative AO conditions propagate down from the middle stratosphere to the surface in one to two week’s time. Based on the SSW alone we would expect an overall mean negative AO this period with cold temperatures across the Eastern United States and northern Eurasia. Though even based on the polar vortex split there is still much uncertainty which regions will turn cold that will in part be determined by the evolution of the polar vortex post the stratospheric warming and not all regions are cold (Europe, eastern United States, East Asia) post a sudden SSW even a major SSW.
However this winter there has been an additional level of uncertainty due to anomalous area of low pressure in the Barents–Kara seas region, which has destructively interfered with the negative AO forced/associated with high Eurasian snow cover. The models last week were predicting that low pressure to split into two with each piece being absorbed into the climatologically favored polar lows in East Siberia and Hudson Bay. Though this has happened, allowing for increased wave driving and wintery weather across Europe, the models now forecast for the low pressure to reform, though based on the vortex split likely on the inland side of the Eurasian north slope.
It is hard to predict how long this area of low pressure will persist, which is likely to be at least somewhat supported by the relatively extensive North Atlantic sea ice extent compared to recent years and to the North Pacific side of the Arctic. As long as low pressure remains near the Barents-Kara seas any tropospheric influences from the stratosphere this time period will be favored across North America relative to Europe. However if a piece of the polar can move over Europe then this could help force much colder weather into Europe.
[h=4]Longer Term
30–day Typically mean negative surface AO conditions are favored 1-2 weeks following a SSW for up to six more weeks. Therefore based on the predicted SSW during the first week of January, mean negative AO conditions are favored through the end of January and into early February. However as discussed above the persistent polar low in the Barents-Kara seas region is destructively interfering with any externally forced negative AO, introducing uncertainty into the forecast of a negative AO and cold temperatures across the Northern Hemisphere continents.
[h=4]Winter AO Outlook
Arctic Sea Ice We have discussed that Arctic sea ice this fall and winter is not likely to contribute to a negative winter AO as sea ice were near normal levels in the North Atlantic sector, including the Barents-Kara Seas. Low pressure has been unexpectedly strong in the Barents-Kara seas region and it is plausible that the relatively high sea ice in the region has contributed to maintaining low pressure and thus favoring the positive NAO observed over the past month. Instead the largest ice deficits have been in the North Pacific region (Figure 5) where high pressure has been favored. It is plausible that the sea ice anomalies have contributed to the observed dipole across the Arctic that has destructively interfered with negative AO conditions so far this winter in the North Atlantic sector.
[h=4]El Niño/Southern Oscillation and the Pacific Basin Weak El Niño conditions continue to persist in the Pacific, with anomalies growing mainly in the Niño 4 region with even a slight cooling trend apparent for the far eastern tropical Pacific (Figure 6). This pattern resembles more of a weak Central Pacific El Niño event. We expect weak El Niño conditions to persist into the coming weeks, but atmospheric coupling remains uncertain. Convection associated with Madden-Julian Oscillation (MJO), has been weak and disorganized (Figure 7). The lack of the convection tracking over the central equatorial Pacific has therefore not yielded the coupling needed to increase the impact of ENSO on the NH extratropical circulation. Convection is currently over the Maritime Continent and the ECMWF model predicts that convection will push eastward into Phase 5 (Figure 7). However, the convection is not strong, thus, confidence on the future of tropical convection impacts on ENSO evolution or on the NH extratropics remains low.
In the North Pacific, the positive Pacific Decadal Oscillation (PDO) pattern continues to strengthen with anomalously warm waters off of western North America and cooler waters in the central North Pacific (Figure 6). The PDO should continue to remain positive and likely strengthen in magnitude.
In the North Pacific, the positive Pacific Decadal Oscillation (PDO) pattern continues to strengthen with anomalously warm waters off of western North America and cooler waters in the central North Pacific (Figure 6). The PDO should continue to remain positive and likely strengthen in magnitude.
[h=4]Northern Hemisphere Snow Cover Expansion of Northern Hemisphere, total snow cover extent has been anemic most of the month and has fallen back to close to 10-year averages. Snow cover across Eurasia is near normal extent (Figure 8; top), while the milder pattern across North America has resulted in below normal snow cover (Figure 8; bottom). With the negative AO, Eurasian snow cover has spiked higher due to an advance across Europe but with a reversal of the NAO expected the snow cover could contract somewhat. With colder temperatures expected across the United States the first two weeks of January, North American snow cover should finally begin to advance.
The high October snow cover extent for Eurasia (the second highest monthly total on record) favors a SSW warming followed by generally a negative AO for the winter season. It is looking likely that a SSW will occur in early January. Therefore we do not see any more value in using Eurasian snow cover to predict winter weather other than to influence local conditions due to thermodynamic influences.
[h=4]Quasi Biennial Oscillation (QBO) The QBO continues to trend more easterly in the lower stratosphere, with winds turning easterly in the tropics at 50 mb at the end of November. Winds at 50 mb may have peaked at their easterly intensity already and will weaken over the next month. Overall, the easterly winds descending into the lower tropical stratosphere is continually favorable for redirecting tropospheric waves anomalously poleward, which increases the odds of interaction with the stratospheric polar vortex and hence increases the chances of a SSW event this winter. The lack of interaction this winter so far, therefore, is more so with the lack of significant wave driving from the extratropics, not the base state of the stratosphere to support the anomalous wave propagation.
[h=4]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 shows the general progression of the temperature anomalies across the Northern Hemisphere based on October Eurasian snow cover variability alone.
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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.
Do you want to be notified when there's an update to this AO analysis? Use this form and put "AO updates" in the field labeled Requests or Comments.
[h=3]December 30, 2014 [h=4]Summary
- A positive AO and North Atlantic Oscillation (NAO) trend are expected in the short term. This is associated with a strong pulse in wave driving.
- In the short term temperatures will turn mild across Eurasia with the positive trend in the AO.
- The strong wave driving will result in a sudden stratospheric warming (SSW) and a splitting of the polar vortex. In the near term this will help build ridging across Western North America and cold temperatures across the United States after the New Year.
- Following a SSW a negative AO is favored for up to 60 days. However anomalous low pressure across the Barents-Kara seas and northern Eurasia destructively interferes with externally forced negative AO conditions and introduces uncertainty longer term.
However longer-term prospects for the AO continue to hinge on the anomalous polar low in the Barents Kara-seas region that has temporarily weakened. It is predicted to strengthen again across the north slope of Siberia and eventually move back over the Barents Kara seas region. Perhaps the current temporary relaxation is a foreshadowing of a longer term weakening trend but for as long as the Barents-Kara seas low persists, it destructively interferes with an externally forced negative AO (from the SSW). The biggest impact would to inhibit the flow of Siberian air into Europe. It’s impact would be less on North American temperatures where in the near term, ridging across western North America and a polar low in eastern Canada will direct arctic air into the lower 48.
[h=4]Recent and Very Near Term Conditions The AO is currently negative. The GFS ensemble-mean forecast shows that the AO should reverse and spike positive after the New Year (Figure 1). The result will be a brief warm-up along the East Coast of the United States and probably a more meaningful warm-up for Europe. The negative NAO has resulted in cold and snowy weather for much of Europe and the reversal in phase signals a return to more seasonable weather. In East Asia the very cold temperatures have abated and the more seasonable temperatures in place should continue. Often the largest negative temperature departures across East Asia precede a stratospheric warming relative to post the stratospheric warming. Therefore the most persistent cold weather may be over in East Asia for the winter. This is consistent with our seasonal forecast model that predicts December as the coldest winter month relative to normal for East Asia.
The strong spike in the AO is consistent with the cold polar cap heights (PCH) in the lower troposphere right at the start of the New Year (Figure 2) and the strong upward pulse in the vertical Wave Activity Flux (WAFz) also during the first week of January (Figure 3). Strong upward WAfz warms the polar stratosphere and increases the PCH in the stratosphere (as seen in Figure 2) but also forces cold/lower PCH in the troposphere.
[h=4]Near Term
1-2 week As has been the case for most of the winter so far there is considerable in the GFS ensemble-mean near-surface AO forecast for the this time period. However the ensemble-mean shows an overall positive AO is predicted this period (Figure 1). But of greater importance to longer-term trends in the AO is the sudden stratospheric warming (SSW) predicted to take place by the models the end of the first week of January (Figure 4). As we have been discussing for over a month, based on the extensive Eurasian snow cover observed this past October we considered a SSW in early January as highly probable and that the tropospheric wave anomalies favor a polar vortex split. Both the GFS and ECMWF weather models predict a SSW and polar vortex split as seen in the model forecasts for 10 hPa geopotential heights/temperatures for January 5, 2015 (Figure 4).
The models do disagree on the strength of the polar vortex weakening. The GFS predicts a relatively quick polar vortex recovery and is predicting that the polar vortex will return to normal and even slightly above normal strength by the end of the period (Figure 2). The quick recovery in the polar vortex is directly attributable to the large deficits in the WAFz predicted by the GFS this period (Figure 3). The ECMWF in contrast shows continued wave driving after January 5 that continues to perturb and weaken the polar vortex even beyond January 5. Based on the ECMWF forecast, the SSW will get close to a major SSW (defined as a zonal mean easterly wind at 10 hPa and 60°N) and would likely achieve major SSW strength with further wave driving. The GFS has displayed a bias all winter of under-predicting the vertical WAFz and over-predicting polar vortex strength. Therefore we favor the ECMWF forecast of a more perturbed and longer duration polar vortex weakening.
Regardless, with positive AO conditions at the surface temperatures should be seasonable to even seasonably mild across Europe and East Asia. However the Central and Eastern US will be cold as the developing stratospheric warming preferentially builds heights over the Bering Sea region forcing a ridge over Alaska and the West Coast of North America that drives Arctic air into the lower 48.
[h=4]3-4 week Typically the circulation anomalies associated with a SSW, high PCH, an equatorward displaced Jet Stream and negative AO conditions propagate down from the middle stratosphere to the surface in one to two week’s time. Based on the SSW alone we would expect an overall mean negative AO this period with cold temperatures across the Eastern United States and northern Eurasia. Though even based on the polar vortex split there is still much uncertainty which regions will turn cold that will in part be determined by the evolution of the polar vortex post the stratospheric warming and not all regions are cold (Europe, eastern United States, East Asia) post a sudden SSW even a major SSW.
However this winter there has been an additional level of uncertainty due to anomalous area of low pressure in the Barents–Kara seas region, which has destructively interfered with the negative AO forced/associated with high Eurasian snow cover. The models last week were predicting that low pressure to split into two with each piece being absorbed into the climatologically favored polar lows in East Siberia and Hudson Bay. Though this has happened, allowing for increased wave driving and wintery weather across Europe, the models now forecast for the low pressure to reform, though based on the vortex split likely on the inland side of the Eurasian north slope.
It is hard to predict how long this area of low pressure will persist, which is likely to be at least somewhat supported by the relatively extensive North Atlantic sea ice extent compared to recent years and to the North Pacific side of the Arctic. As long as low pressure remains near the Barents-Kara seas any tropospheric influences from the stratosphere this time period will be favored across North America relative to Europe. However if a piece of the polar can move over Europe then this could help force much colder weather into Europe.
[h=4]Longer Term
30–day Typically mean negative surface AO conditions are favored 1-2 weeks following a SSW for up to six more weeks. Therefore based on the predicted SSW during the first week of January, mean negative AO conditions are favored through the end of January and into early February. However as discussed above the persistent polar low in the Barents-Kara seas region is destructively interfering with any externally forced negative AO, introducing uncertainty into the forecast of a negative AO and cold temperatures across the Northern Hemisphere continents.
[h=4]Winter AO Outlook
Arctic Sea Ice We have discussed that Arctic sea ice this fall and winter is not likely to contribute to a negative winter AO as sea ice were near normal levels in the North Atlantic sector, including the Barents-Kara Seas. Low pressure has been unexpectedly strong in the Barents-Kara seas region and it is plausible that the relatively high sea ice in the region has contributed to maintaining low pressure and thus favoring the positive NAO observed over the past month. Instead the largest ice deficits have been in the North Pacific region (Figure 5) where high pressure has been favored. It is plausible that the sea ice anomalies have contributed to the observed dipole across the Arctic that has destructively interfered with negative AO conditions so far this winter in the North Atlantic sector.
[h=4]El Niño/Southern Oscillation and the Pacific Basin Weak El Niño conditions continue to persist in the Pacific, with anomalies growing mainly in the Niño 4 region with even a slight cooling trend apparent for the far eastern tropical Pacific (Figure 6). This pattern resembles more of a weak Central Pacific El Niño event. We expect weak El Niño conditions to persist into the coming weeks, but atmospheric coupling remains uncertain. Convection associated with Madden-Julian Oscillation (MJO), has been weak and disorganized (Figure 7). The lack of the convection tracking over the central equatorial Pacific has therefore not yielded the coupling needed to increase the impact of ENSO on the NH extratropical circulation. Convection is currently over the Maritime Continent and the ECMWF model predicts that convection will push eastward into Phase 5 (Figure 7). However, the convection is not strong, thus, confidence on the future of tropical convection impacts on ENSO evolution or on the NH extratropics remains low.
In the North Pacific, the positive Pacific Decadal Oscillation (PDO) pattern continues to strengthen with anomalously warm waters off of western North America and cooler waters in the central North Pacific (Figure 6). The PDO should continue to remain positive and likely strengthen in magnitude.
In the North Pacific, the positive Pacific Decadal Oscillation (PDO) pattern continues to strengthen with anomalously warm waters off of western North America and cooler waters in the central North Pacific (Figure 6). The PDO should continue to remain positive and likely strengthen in magnitude.
[h=4]Northern Hemisphere Snow Cover Expansion of Northern Hemisphere, total snow cover extent has been anemic most of the month and has fallen back to close to 10-year averages. Snow cover across Eurasia is near normal extent (Figure 8; top), while the milder pattern across North America has resulted in below normal snow cover (Figure 8; bottom). With the negative AO, Eurasian snow cover has spiked higher due to an advance across Europe but with a reversal of the NAO expected the snow cover could contract somewhat. With colder temperatures expected across the United States the first two weeks of January, North American snow cover should finally begin to advance.
The high October snow cover extent for Eurasia (the second highest monthly total on record) favors a SSW warming followed by generally a negative AO for the winter season. It is looking likely that a SSW will occur in early January. Therefore we do not see any more value in using Eurasian snow cover to predict winter weather other than to influence local conditions due to thermodynamic influences.
[h=4]Quasi Biennial Oscillation (QBO) The QBO continues to trend more easterly in the lower stratosphere, with winds turning easterly in the tropics at 50 mb at the end of November. Winds at 50 mb may have peaked at their easterly intensity already and will weaken over the next month. Overall, the easterly winds descending into the lower tropical stratosphere is continually favorable for redirecting tropospheric waves anomalously poleward, which increases the odds of interaction with the stratospheric polar vortex and hence increases the chances of a SSW event this winter. The lack of interaction this winter so far, therefore, is more so with the lack of significant wave driving from the extratropics, not the base state of the stratosphere to support the anomalous wave propagation.
[h=4]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 shows the general progression of the temperature anomalies across the Northern Hemisphere based on October Eurasian snow cover variability alone.
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