April 21, 2017
Special blog on winter 2015/16 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 spring I will be transitioning to a spring/summer schedule, which is once every two weeks. Snow accumulation forecasts will be replaced by precipitation accumulation forecasts. Also there will be less emphasis on ice and snow boundary conditions (which are both now in their seasonal decline) and their influence on hemispheric weather.
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Currently pressure/geopotential height anomalies are mostly positive on the North Pacific side of the Arctic but mostly negative across the North Atlantic side of the Arctic with mostly negative pressure/geopotential height anomalies across the mid-latitude ocean basins. This is resulting in a negative Arctic Oscillation (AO) but a positive North Atlantic Oscillation (NAO).
Despite the positive NAO, temperatures are below normal across western Eurasia including much of Europe as a strong block/high pressure has developed in the eastern North Atlantic with a cold, northerly flow downstream of the block across Europe.
The blocking high in the eastern North Atlantic is predicted to drift northward contributing to a negative bias to the AO and eventually the NAO over the next two weeks. Therefore, the pattern of cool temperatures across western Eurasia including Europe looks to continue into the foreseeable future.
There are currently also cool temperatures across parts of Canada and the Northeastern United States (US) associated with negative pressure/geopotential height anomalies. However mostly positive pressure/geopotential height anomalies are predicted across the US with mild temperatures into the foreseeable future.
The models are predicting that pressure/geopotential height anomalies could soar to well above values across the Eastern US in week two resulting in well above normal temperatures for the end of April and into early May.
Concurrently pressure/geopotential height anomalies are expected to fall across the Western US resulting in a stretch of below normal temperatures.
Across East Asia variable pressure/geopotential height anomalies on either side of normal will likely result in variable temperatures on either side of normal.
The current weather pattern across the Northern Hemisphere but in particular across the North Atlantic sector is a good example of why teleconnection indices are often overly simplistic in describing regional weather conditions. Typically, Europe is the region that is most sensitive to the phase of the NAO with above normal temperatures expected when the NAO is positive and below normal temperatures when the NAO is negative. Currently the NAO is positive in contrast to the AO which is currently negative. A positive AO and a negative NAO would be favorable for above normal temperatures across Europe and possibly below normal temperatures across the Eastern US (positive geopotential height anomalies are necessary in the North Pacific side of the Arctic including Alaska to force a negative AO when the NAO is positive, which favors cold temperatures downstream across the Eastern US). However, over the next two weeks the models are predicting the exact opposite.
Despite negative geopotential height anomalies across Greenland and Iceland and thus a positive NAO, a strong block/center of positive geopotential height anomalies has developed across the eastern North Atlantic that is predicted to drift north but mostly persist across the northern North Atlantic. The block favors northerly flow and/or negative geopotential height anomalies downstream across Europe and below normal temperatures. The block will dominate the large-scale weather pattern across Europe over the next two weeks.
The major weather story across the Eastern US will also be associated with a ridge/center of positive geopotential height anomalies but with the opposite outcome. Spokes of energy will spin off from an unseasonable deep Aleutian low helping to carve out a deep trough/negative geopotential height anomalies in the Western US over the next week. The trough in the Western US will help build an unseasonably early summer geopotential height anomalies. Because the ridge axis will be centered right over the Eastern US, high heights and southwesterly flow of air will transport unseasonably warm temperatures across the Eastern US. This will likely result in above normal to possibly well above normal temperatures for the Eastern US for late April and early May. Eventually energy from the Western US trough will break down the Eastern US ridge with moderating temperatures.
Near Term Conditions
The AO is currently negative (Figure 1), reflective of positive geopotential height anomalies across the North Pacific side of the Arctic and mostly negative geopotential height anomalies across the mid-latitude ocean basins (Figure 2). Geopotential height anomalies are negative near Greenland and Iceland (Figure 2), and therefore the NAO is positive.
Figure 1. (a) The predicted daily-mean near-surface AO from the 00Z 21 April 2017 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.
Strong blocking high/positive geopotential height anomalies centered in the eastern North Atlantic and the United Kingdom is forcing troughing/negative geopotential height anomalies downstream with two closed low centers one over Northern Scandinavia and northwest Russia and the other over the Balkans (Figure 2). Northerly flow between the ridge in the eastern North Atlantic and the tough over Eastern Europe and Western Asia, is predicted to persist cold temperatures across much of Europe and Western Asia (Figure 3). Much of eastern Asia will be dominated by ridging/positive geopotential height anomalies downstream across East Asia (Figure 2). With much of East Asia predicted to be dominated by ridging/positive geopotential height anomalies, above normal temperatures are favored for much of East Asia and especially Siberia (Figure 3). Some troughing/negative geopotential height anomalies are predicted across Southeast Asia (Figure 2) resulting in normal to below normal temperatures for Southeast Asia (Figure 3).
Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 21 April 2017 at 00Z. Note the high heights over the United Kingdom, the Beaufort Sea and most of North America with low heights over Western Asia and the Balkans.
Much of North America is dominated by ridging/positive geopotential height anomalies (Figure 2). This is contributing to an overall mild pattern for much of Western Canada and the US including Alaska (Figure 3). However northerly flow between positive geopotential height anomalies centered north of Alaska and troughing/negative geopotential height anomalies in Hudson Bay is bring below normal temperatures to Central and Eastern Canada (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 22 – 26 March 2017. Note the warm temperatures across Eastern Asia and the US with cold temperatures in Europe, Western Asia and Canada. The forecast is from the 00Z 21 April 2017 GFS ensemble.
Low pressures in Scandinavia and Southeastern Europe respectively could bring heavy precipitation including snow to the Alps, the Balkans and Scandinavia this week (Figure 4). A couple of storms swinging through the Eastern US this week will bring above normal precipitation to the Eastern US (Figure 4).
Figure 4. Forecasted precipitation anomalies (mm/day; shading) from 22 – 26 April 2017. Note the rainfall over Southeastern Europe, the Alps, and the Eastern US. The forecasts are from the 00Z 21 April 2017 GFS ensemble.
The AO is predicted to turn positive next week as negative geopotential height anomalies dominate much of the Arctic, especially on the North Atlantic side (Figure 5a). And with negative geopotential height anomalies stretching from Greenland to Iceland, the NAO will likely remain positive as well.
Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 27 April – 1 May 2017. (b) Same as (a) except averaged from 2 – 6 May 2017. The forecasts are from the 21 April 2017 00z GFS ensemble.
The ridging ridging/positive geopotential height anomalies centered near the United Kingdom last period will weaken and drift north towards the Barents-Kara Seas (Figure 5a). This will favor troughing/negative geopotential height anomalies south across much of Europe (Figure 5a), persisting widespread below normal temperatures across most of Europe and even into Western Asia (Figure 6). Some of the negative geopotential height anomalies in the Arctic will extend southward into Western Siberia (Figure 5a) resulting in below normal temperatures in Western Siberia (Figure 6). Downstream of the European troughing, ridging/positive geopotential height anomalies will stretch across Western and Central Asia (Figure 5a). High heights and west to southwesterly flow will result in mild temperatures across much of Central and East Asia especially Central and Eastern Siberia (Figure 6). Persistent troughing/negative geopotential height anomalies across Southeast Asia (Figure 5a) will bring normal to below normal temperatures to the region (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 27 April – 1 May 2017. Note the warm temperatures across East Asia, and much of Canada and the US with cool temperatures in Europe, Western Asia and Western Siberia. The forecasts are from the 00Z 21 April 2017 GFS ensemble.
Energy from a deep Aleutian low pressure will propagate eastwards and carve out troughing/negative geopotential height anomalies from across Western Canada and the Western US (Figure 5a). Below normal geopotential heights will favors near seasonable temperatures across western North America (Figure 6). Troughing/negative geopotential in the Western US will contribute to strong ridging/positive geopotential height anomalies in eastern North America, centered on the Northeastern US (Figure 5a). Strong positive height anomalies and southwesterly flow will likely result in above to well above normal temperatures in the Eastern US and Southeastern Canada (Figure 6).
Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 27 April – 1 May 2017. Note the rainfall over Southeastern Europe, the Western and the Mississippi Valley. The forecasts are from the 00Z 21 April 2017 GFS ensemble.
Troughing In Europe will support more heavy precipitation in the southwesterly flow across Eastern Europe (Figure 7). Similarly, the trough in the Western US and an active Jet Stream in between the cooler air in the Western US and warmer air in the Eastern US will support heavy rainfall in the Western and Central US (Figure 7). Along the dividing line between cooler and warmer temperatures severe weather is also possible.
Positive pressure/geopotential height anomalies in the Barents-Kara seas last period will amplify and become more widespread across the Arctic this period (Figure 6b) forcing a negative trend in the AO (Figure 1). With positive pressure/geopotential height anomalies across Greenland and Iceland, the NAO will also likely trend negative this period as well (Figure 1).
The general pattern of cold western Eurasia and warm eastern Eurasia looks to continue this period as well. Above normal geopotential heights across the Northern North Atlantic will continue to favor troughing/below normal geopotential heights across Europe (Figure 5b). Therefore the stretch of below normal temperatures across Europe looks to continue into the first week of May (Figure 8). Amplifying ridging anchored in the Barents Kara Seas also contributes to deepening negative geopotential height anomalies across Western Asia (Figure 5b). Strengthening northerly flow is predicted to amplify negative temperature departures across Western Asia (Figure 8). Further upstream, ridging/positive geopotential height anomalies are predicted for much of East Asia (Figure 5a) favoring mostly above normal temperatures for East Asia including Eastern Siberia (Figure 8).
Figure 8. Forecasted surface temperature anomalies (°C; shading) from 2 – 6 April 2017. Note the warm temperatures across East Asia, and much of Canada and the US with cool temperatures in Europe, Western Asia and Western and Central Siberia. The forecasts are from the 00Z 21 April 2017 GFS ensemble.
The models are predicting a much less amplified flow across North America this period. Some of the energy from the Western US trough will propagate eastwards, weakening the Western US trough and also helping to break down the Eastern US trough resulting in mostly normal to above normal geopotential heights across much of North America (Figure 5b). This pattern favors normal to above normal temperatures for much of Canada and the US (Figure 8). One possible exception is far Northern Canada as a deepening trough in response to the amplifying ridge in the Barents Kara Seas and strengthening northerly flow (Figure 5b) could result in below normal temperatures (Figure 8).
Figure 9. Forecasted precipitation anomalies (mm/day; shading) from 2 – 6 April 2017. Note the above normal rainfall over the Eastern US. The forecasts are from the 00Z 21 April 2017 GFS ensemble.
As energy from the Western US trough tries to break down the Eastern US ridge it could result in above normal rainfall in the Eastern US (Figure 9). Southwesterly flow ahead of the northwestern Asia trough could bring above normal rainfall to north-central Asia (Figure 9).
The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows mostly above normal PCHs in the troposphere and below normal PCHs in the stratosphere (Figure 10). I expect PCHs to remain below normal in the stratosphere through the summer as increasing greenhouse gases favor a colder stratosphere in the absence of upwelling energy from the troposphere. In contrast variability is predicted in the tropospheric PCHs, with the positive/warm PCHs transitioning first to negative/cold PCHs and then back to positive/warm PCHs consistent with variability in the AO from first negative back to positive and then back to negative again (Figure 1). Europe is predicted to be insensitive to the oscillations in the tropospheric PCHs as regional blocking favors cooler than normal temperatures over the next two weeks. However the Eastern US may be more sensitive to PCH variability. The peak in the anticipated warmth will likely coincide with the coldest tropospheric PCHs the very end of April into very early May. Then as the tropospheric PCHs warm the first week of April will likely favor a moderation of the very warm temperatures. How quickly Eastern US temperatures return to normal is likely dependent on the tropospheric PCHs forecast with warmer PCHs favoring cooler temperatures.
Figure 10. Observed and predicted daily polar cap height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecast is from the 00Z 21 April 2017 GFS ensemble.
Longer-term above normal temperatures are favored across the Northern Hemisphere NH. For now this is most likely across the US where mostly warm temperatures are predicted over the next two weeks. It will be interesting to monitor NH snow cover over the coming weeks. Recently, typically starting now NH spring snow cover collapses. But currently snow cover and snow depth remain above normal. If snow cover can somehow persist longer than what is the “new normal” this spring that could be a cool bias to NH land temperatures.
Surface Boundary Conditions
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) have warmed from earlier in the winter and SSTs are above normal in much of the tropical and subtropical Pacific (Figure 11). If the warming continues through the spring and summer, El Niño is possible next winter.
Figure 12. The latest weekly-mean global SST anomalies (ending 20 April 2017). Data from NOAA OI High-Resolution dataset. NH SSTs are mostly above normal, most notably STTs in the eastern equatorial Pacific.
Currently no phase is favored for the Madden Julian Oscillation (MJO) but the MJO is predicted to merge into phases eight and then one (Figure 12). These two phases favor cool and dry conditions in the Eastern US, neither of which is consistent with the forecasts. Therefore I don’t expect the MJO to have much influence on North American weather in the near term.
Figure 12. Past and forecast values of the MJO index. Forecast values from the 00Z 21 April 2017 ECMWF model. Yellow lines indicate individual ensemble-member forecasts, with the green line showing the ensemble-mean. A measure of the model “spread” is denoted by the gray shading. 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/facstaff/roundy/waves/phasediags.html.