Volatile Vortex

Video of nullschool wind patterns from October 2017 up to yesterday, showing the vortex splitting as described below.

Dr. Judah Cohen covers the latest vortex shanigans and implications for future weather at his always informative website Arctic Oscillation and Polar Vortex Analysis and Forecasts February 19, 2018. Excerpts below with my bolds.

The stratospheric PV remains split into two pieces with one dominant center over Western Canada and a second much weaker center over northwestern Europe (Figure 12). The Eurasian center is predicted to retrograde westward and dissipate while the North American center slowly drifts north towards the North Pole and even possibly into Eurasia. The most persistent legacy of the PV spit is above normal geopotential heights and warm temperatures in the polar stratosphere. This is reflected in the stratospheric AO, which is predicted to remain negative over the next two weeks, though slowly trend back to neutral (Figure 1).

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.

Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 19 February 2018.

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.

Figure 5. (a) Forecasted average 500 mb geopotental heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 25 February – 1 March 2018. (b) Same as (a) except averaged from 2 – 6 March 2018. The forecasts are from the 19 February 2018 00z GFS ensemble.

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.

Forecast for the next month from Environment Canada.


via Science Matters


February 20, 2018 at 01:58PM

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