February 19, 2018
Special blog on winter 2016/2017 retrospective can be found here - http://www.aer.com/winter2017
Special blog on winter 2015/2016 retrospective can be found here - http://www.aer.com/winter2016
Dr. Judah Cohen from Atmospheric and Environmental Research (AER) recently embarked on an experimental process of regular research, review, and analysis of the Arctic Oscillation (AO). This analysis is intended to provide researchers and practitioners real-time insights on one of North America’s and Europe’s leading drivers for extreme and persistent temperature patterns.
With the start of fall I will be transitioning to a fall/winter schedule, which is once every week starting sometime in October. Precipitation forecasts will be replaced by snow accumulation forecasts. Also there will be more emphasis on ice and snow boundary conditions (which are both now in their seasonal advance) and their influence on hemispheric weather.
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- The Arctic Oscillation (AO) is currently neutral and is predicted to remain neutral to slightly positive this week before trending negative. The turn to a negative AO coincides with the downward propagation of atmospheric circulation anomalies associated with the ongoing polar vortex (PV) disruption.
- The current neutral AO is reflective of mixed pressure/geopotential height anomalies across the Arctic and mixed pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is currently positive with mostly negative pressure/geopotential height anomalies across Greenland and Iceland and positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic.
- The models predict that over the next two weeks ridging/positive geopotential height anomalies will dominate the Eurasian side of the Arctic first focused in the Barents-Kars seas and then shifting to Greenland. This will force troughing/negative geopotential height anomalies further to the south across much of the continent. High pressure/geopotential heights to the north and low pressure/geopotential heights to the south favor cold temperatures expanding across much of the Asian continent including East Asia and Europe United Kingdom (UK) over the next two weeks.
- Currently amplified ridging/positive geopotential height anomalies stretches from the Gulf of Alaska north into Alaska and into the Central Arctic with deep troughing/negative geopotential height anomalies covering most of Canada and extending into the Northwestern United States (US) with more ridging along the Eastern US. This is resulting in warm temperatures for Alaska and along the Southern and Eastern US with cold temperatures across much of Canada and the Northwestern US.
- The pattern across North America will retrograde westward over the next two weeks. The ridging in the Gulf of Alaska is predicted to slides into the Central North Pacific. This will allow the trough in western North America move into the Gulf of Alaska and the ridging centered in the Southeastern US move in the Central US while weakening. This will result in a relaxation of the very cold pattern in Western Canada and the Northwestern US.
- Simultaneously the high heights from the Eurasian Arctic is predicted to slide to Greenland helping to force lower heights along the US East Coast. This will result in well above normal temperatures in the Eastern US cooling back towards more seasonable levels.
- The pattern of high heights in the Arctic with lower heights across the mid-latitudes in particular the Eurasian continent is the tropospheric response to the ongoing significant PV disruption. This pattern typically persists for four to six weeks following a PV disruption and is often accompanied by widespread cold temperatures across the continents with a focus across Eurasia.
As I have discussed in previous blogs there seems to me to be two responses to a significant PV disruption: an immediate response and a longer term response. When the PV split it created two sister vortices a dominant center over North America and a more minor center over Eurasia. In between the two PV centers high pressure filled the void but was shifted towards the Eurasian continent. Across Eurasia the immediate and longer term response seem to be consistent. The immediate tropospheric response or at least the tropospheric circulation related to the PV split has been high pressure/heights to the north, low pressure/heights to the south, predominant anomalous easterly flow and below normal temperatures across northern Eurasia.
In contrast the immediate and longer term response across North America do not seem to be the same. When the PV split into two pieces the dominant sister center formed over Western Canada and has been spinning in place in the polar stratosphere. It appears to me this has contributed or at least is related to troughing/negative geopotential height anomalies across Canada and then eventually into the Western US accompanied by colder temperatures. This in turn has forced further downstream across eastern North America ridging/positive geopotential height anomalies, southwesterly flow and mild even record warm temperatures.
Eventually however the Eurasian PV sister center is predicted to weaken and dissipate leaving just one PV center over Western Canada. That PV center is predicted to make its way back to the North Pole or alternatively there are some model forecasts of the PV center being further displaced towards Eurasia.
Longer term the tropospheric response seems to be less about the initial displacement and the associated circulation around the respective PV centers and more about the warming and high pressure/heights related to that warming. The corresponding tropospheric response is high pressure and relatively warm temperatures over the Arctic. With respect to the ongoing event the high pressure and warm temperatures in the polar stratosphere are centered over Greenland and therefore it seems likewise in the troposphere the high pressure/heights and warm temperatures will be centered over Greenland. This transfer of high pressure/heights and warm temperatures over the Arctic is seen in the apparent downward propagation of positive/warm polar cap geopotential heights and/or a negative AO from the mid-stratosphere eventually down to the surface. On average this downward propagation or transfer takes about two weeks.
Therefore in summary based on my reasoning, the immediate response to a PV disruption is somewhat random dependent on the displacement of the PV center(s) and the circulation around the PV center(s). For the current event the immediate tropospheric response related to the location and circulation of the North American sister vortex favors relatively cold temperatures in western North America and mild temperatures in eastern North America. However the tropospheric response could have just as likely been the opposite favoring relatively mild temperatures in western North America and cold temperatures in eastern North America. Across Eurasia the immediate response favors relatively cold across northern Eurasia and mild temperatures across southern Eurasia; though it does seem that the immediate response across Eurasia is less random than for North America for reasons that I don't fully understand.
The longer term response or legacy however to a PV disruption is less random and is not as dependent on the location and circulation of the PV center(s) but rather on the warming and building of high pressure/heights across the Arctic which shows greater similarity across PV disruption events. High pressure/heights and warm temperatures favor colder temperatures in preferential locations: the Eastern US, Northern Europe and East Asia resulting in a warm Arctic/cold continents pattern. Therefore my expectations of the longer term response to the ongoing PV disruption is the same - a preference for relatively cold temperatures in the Eastern US, Northern Europe and East Asia over the coming four to six weeks starting the very end of February or the beginning of March.
Near Term Conditions
The AO is currently slightly positive (Figure 1), reflective of mostly negative geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). Geopotential height anomalies are negative near Iceland and Greenland and positive across the mid-latitudes of the North Atlantic (Figure 2), and therefore the NAO is positive.
Currently ridging/positive geopotential height anomalies are positive across the entire Eurasian side of the Arctic contributing to troughing/negative geopotential height anomalies further south across Europe, Northern, Eastern and Southwestern Asia (Figure 2). Induced northerly and easterly flow by high pressure to the north and low pressure to the south favors widespread normal to below temperatures for Europe including the UK, Northern and Eastern Asia with normal to above normal temperatures for Southern Asia (Figure 3). Southwest winds on thedownstream side of the European troughing are predicted to usher mild temperatures to Southeastern Europe (Figure 3).
Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 19 February 2018.
Amplified ridging/positive geopotential height anomalies in the Gulf of Alaska and Alaska are forcing downstream a deep trough/negative geopotential height anomalies across Canada and the Western USwith highly amplified ridging along the US East Coast (Figure 2). This patternwill favor normal to below normal temperatures stretching across much of Canada and into the Northwestern US (Figure 3). Meanwhile predictedridging/positive geopotential height anomalies across Alaska and the US East Coast (Figure 2) favor normal to above normal temperatures for Alaska, the Eastern US and Southeastern Canada (Figure 3).
Troughing and/or cold temperatures are predicted to yield new snowfall for parts of Europe, Siberia, East Asia, Alaska, Canada and the Northwestern US (Figure 4). In contrast, southerly flow and warmer temperatures will result in snowmelt across Southwestern Asia and the Northeastern US (Figure 4).
The AO is predicted to turn negative next week (Figure 1) with strongly positive geopotential height anomalies on the North Atlantic side of the Arctic and negative geopotential height anomalies across the North Atlantic mid-latitudes (Figure 5a). And with strongly positive geopotential height anomalies stretching from Greenland to Iceland, the NAO will likely be negative next week as well.