April 23, 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.
Subscribe to our email list or follow me on Twitter (@judah47) for notification of updates.
- The Arctic Oscillation (AO) is currently positive and is predicted to trend further positive to strongly positive by the end of the week before trending slowly negative next week back towards neutral.
- The current positive AO is reflective of mostly negative pressure/geopotential height anomalies across the Arctic and mostly positive pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is currently positive with negative pressure/geopotential height anomalies across Greenland and mostly positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic. The forecasts are for the NAO to trend slowly negative for the next two weeks as geopotential height anomalies rise across Greenland and Iceland.
- Persistent troughing/negative geopotential height anomalies across western Siberia favors ridging/positive geopotential height anomalies both upstream across Europe and downstream across East Asia. This will result initially in relatively cool temperatures in the interior of Eurasia with relatively warmer temperatures on the edges including much of Europe (including the United Kingdom (UK)) and East Asia.
- However ridging/positive geopotential height anomalies now over Eastern Canada are predicted to propagate eastward into the central North Atlantic which will promote deepening troughing/negative geopotential height anomalies downstream across Western Europe resulting in a cooling trend this week and especially next week.
- This week, ridging/positive geopotential height anomalies are predicted to consolidate across Western North America favoring deepening troughing/negative geopotential height anomalies downstream across eastern North America including the Eastern United States (US). This pattern in general favors normal to above normal temperatures for western North America and normal to below normal temperatures in eastern North America including the Eastern US.
- The Global Forecast System (GFS) is predicting that the ridging/positive geopotential height anomalies in western North America will be progressive and slide into eastern North America resulting in normal to above normal temperatures for most of the continent. However the European Centre for Medium Range Forecasts (ECMWF) model is predicting that the ridging/positive geopotential height anomalies across western North America will remain semi stationary and therefore the downstream troughing/negative geopotential height anomalies downstream across eastern North America will likely be more persistent. The ECMWF solution favors normal to above normal temperatures across western North America similar to the GFS but normal to below normal temperatures across eastern North America including the Eastern US in early May.
In general spring has advanced aggressively across much of the Northern Hemisphere (NH) mid-latitude continents and I see little reason to deviate from those expectation for this spring. However there has been one regional exception to the generally recent warm springs and that is Eastern Canada that has experienced below normal temperatures over the past four April through Mays. A composite of the past four April through Mays (Figure i) shows generally above normal temperatures across the NH with the notable exception of below normal temperatures across eastern Canada.
Figure i. Surface temperature anomalies (using a 1981-2010 climatology) for the Northern Hemisphere composited for April and May 2014, 2015, 2016 and 2017.
One season that you don’t hear much about sea ice loss and mid-latitude weather is the spring. But the area that experienced the greatest warming the past four springs has been the North Pacific side of the Arctic. Warming in this region is related to ridging/positive geopotential height anomalies across Northwestern North America and into the Beaufort sea that would favor downstream troughing/negative geopotential height anomalies and relatively cool temperatures in Eastern Canada. This past winter was characterized by record low sea ice in the Bering and Chukchi seas that has continued unabated into the spring (see sea ice plot below). It is possible that record low sea ice in the Bering and Chukchi seas this spring is helping to anchor ridging/positive geopotential height anomalies across Northwestern North America with downstream troughing/negative geopotential height anomalies and relatively cool temperatures in eastern North America. That coupled with the unusually late retreat of eastern North America snow cover has contributed to a relatively cool spring across eastern North America and in some regions has been record cold.
A positive or warm phase of the Pacific Decadal Oscillation (PDO) can also be seen from Figure i, which may also be playing a role in the surface temperature pattern across North America. However the current sea surface temperature pattern in the North Pacific looks somewhat different.
Of course my thinking is speculative but could be one factor in the late start to spring across eastern North America. But based on this thinking it is plausible that sea ice can play a role in a delayed onset of seasonally warm temperatures for even a few more weeks.
Near Term Conditions
The AO is currently positive (Figure 1), reflective of mostly negative geopotential height anomalies across the Arctic and mostly positive geopotential height anomalies across the mid-latitudes of the North Atlantic (Figure 2). Geopotential height anomalies are negative near Greenland and positive across the mid-latitudes of the North Atlantic (Figure 2), and therefore the NAO is also positive.
Figure 1. (a) The predicted daily-mean AO at 1000 hPa from the 00Z 23 April 2018 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.
Troughing/negative geopotential height anomalies remain entrenched across Western Siberia (Figure 2) as has been the case for much of the spring. To the west and east, ridging/positive geopotential height anomalies dominate Europe and East Asia (Figure 2). This pattern favors widespread normal to below temperatures for much of Western Asia with normal to above normal temperatures both across Europe including the UK and East Asia (Figure 3).
Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 23 April 2018.
Ridging/positive geopotential height anomalies are predicted to consolidate across western North America this week, which will contribute to deepening troughing/negative geopotential height anomalies in eastern North America (Figure 2). This pattern will favor normal to above normal temperatures for much of Alaska, Western Canada and much of the Western and Central US (Figure 3). More seasonable temperatures are expected in Eastern Canada and the Eastern US with the best chance of below normal temperatures being in the Southeastern US (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 April 2018. The forecast is from the 00Z 23 April 2018 GFS ensemble.
Precipitation is predicted to be near normal across much of Eurasia (Figure 4). Precipitation is predicted to be near normal across western North America but troughing is predicted to result in above normal precipitation in eastern Canada and especially the Eastern US (Figure 4).
Figure 4. Forecasted precipitation anomalies (mm/day; shading) from 24 – 28 April 2018. The forecast is from the 00Z 23 April 2018 GFS ensemble.
The AO is predicted to trend negative next week but overall remain positive (Figure 1) as negative geopotential height anomalies dominate the Arctic with mostly positive geopotential height anomalies across the mid-latitudes (Figure 5a). And with geopotential height anomalies remaining negative stretching from Greenland to Iceland, the NAO will likely remain positive next week as well.