September 19, 2023
Dear AO/PV blog readers:
We have shifted the public release of the Arctic Oscillation/Polar Vortex blog to Thursday.
For those who would like an early look on Tuesdays, we will be offering at a nominal price (US $50) a PDF version of the upcoming blog, and we will be rolling out access to the datasets used in the production of this blog. At present we plan to make available in comma-separated values the timeseries of the Polar Cap Height and the timeseries of the Wave Activity Flux (vertical component), though we would appreciate to hear your suggestions for additional data of interest to you all.
Dr. Judah Cohen from Atmospheric and Environmental Research (AER) embarked on an experimental process of regular research, review, and analysis of the Arctic Oscillation (AO) and Polar Vortex (PV). 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 we transition to a spring/summer schedule, which is once every two weeks. Snow accumulation forecasts will be replaced by precipitation forecasts. Also, there will be less emphasis on ice and snow boundary conditions and their influence on hemispheric weather. During the winter schedule the blog is updated once every week. Snow accumulation forecasts replace precipitation forecasts. Also, there is renewed emphasis on ice and snow boundary conditions and their influence on hemispheric weather.
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The AO/PV blog is partially supported by NSF grant AGS: 1657748.
- The Arctic Oscillation (AO) is currently neutral and is predicted to remain neutral to negative the next two weeks as pressure/geopotential height anomalies across the Arctic are currently mixed and are predicted to slowly turn mostly positive over the next two weeks. The North Atlantic Oscillation (NAO) is currently negative with mostly positive pressure/geopotential height anomalies across Greenland and the NAO is predicted to trend towards neutral the next two weeks as pressure/geopotential height anomalies will slowly become increasingly mixed across Greenland.
- Over the next two weeks, ridging/positive geopotential height anomalies across Greenland will help anchor troughing/negative geopotential height anomalies in the central North Atlantic further helping to anchor ridging/positive geopotential height anomalies across Europe with the exception of troughing/negative geopotential height anomalies in Western Europe this week only. This pattern will support normal to above to even well above normal temperatures across Europe the next two weeks including the United Kingdom (UK) but mostly next week. One exception is normal to below normal temperatures in Western Europe including the UK due to troughing this week.
- This week ridging/positive geopotential height anomalies are predicted to dominate Asia with the exceptions of troughing/negative geopotential height anomalies across Eastern Siberia and parts of Central Asia. Next week troughing/negative geopotential height anomalies will become focused across the Urals and Northern Siberia with ridging/positive geopotential height anomalies elsewhere across Asia but centered in East Asia. This pattern mostly favors normal to above normal temperatures widespread across Asia with normal to below normal temperatures across Eastern Siberia and parts of Central Asia this week and then normal to above normal temperatures again widespread across Asia with normal to below normal temperatures mostly limited to Northern Siberia.
- The general predicted pattern across North America the next two weeks is troughing/negative geopotential height anomalies across Alaska, the Gulf of Alaska and along the west coast of North America forcing ridging/positive geopotential height anomalies across eastern North America. This pattern generally favors normal to below normal temperatures for Alaska and the West Coasts of Canada and the United States (US) with normal to above normal temperatures widespread across most of Canada and the US east of the Rockies.
- In the Impacts section I continue to recap the Northern Hemisphere (NH) summer and pivot to next winter. As I like to describe below for now it is the red team vs. the blue team.
Plain Language Summary
Despite the calendar reading fall, summer is hanging on tough with record heat in Europe and the US ongoing. some relief is on the way by next week especially in the Eastern US. And the string of summer extreme weather continues with record heat and flooding rainfall. The summer temperature pattern was overall consistent with forecasts posted in the blog at the end of May (see Figure iii). Still too early to say much about this winter other than El Niño is a certainty. It is thought to favor an overall mild winter but not always.
This is the last time that I will discuss summer 2023. In Figure i, I show the two dynamical forecasts from the North American multi-Mold Ensemble (NMME - top panel), the C3S ensembles (Erupean multi model ensemble including ECMWF - second panel), the AER statistical model (third panel) and the observed surface temperature anomalies for June, July and August 2023 for the Northern Hemisphere (NH). The forecasts were posted in the 22 May 2023 blog. The observed temperatures are derived from NCEP/NCAR (NNR) reanalysis using a 30-year climatology of 1991-2020.
Figure i. The NMME surface temperature anomaly forecast for June, July and August 2023 from https://www.cpc.ncep.noaa.gov/products/NMME/ (top). The C3S surface temperature anomaly forecast forJune, July and August 2023 from https://www.copernicus.eu/en (center). c) The AER surface temperature anomaly forecast for June, July and August 2023 (third panel). d) Observed surface temperatrue anoamlies for June, July and August 2023 from the NCEP/NCAR reanalysis (bottom).
I use the NNR reanalysis because that is what is used to generate the AER temperature forecast to be consistent, but it clearly has issues, the most obvious being too cold. I showed the observed temperatures from CFSR in last week’s blog (5 Sept 2023) and in Figure ii I show the global plot from Copernicus, which is very similar to the CFSR plot. Above normal temperatures were almost universal most notably in western North America, Southern and Eastern Europe, the Middle East, North Africa and Central Asia (see Figure iii). Some regional exceptions were below normal temperatures in the Eastern US, Western Russia, parts of China and along the India-Pakistan border. I think that the AER model was very competitive with the dynamical forecasts systems, but all the models did a credible job, and each did better and worse in certain regions.
Figure iii. Observed surface temperature anomalies (°C; shading) across the globe and Europe from 1 June – 31 August 2023 from the ERA5 and downloaded from https://climate.copernicus.eu/surface-air-temperature-august-2023
As we look ahead to October, summer or at least above to even well above normal temperatures are showing no signs of quitting or certainly leaving gracefully. We have several strong ridges spread across the NH, including Eastern Europe/Western Russia, East Asia but the most impressive by far is in Eastern Canada (see Figures 5 and 8). This will ensure above to even above normal temperatures for much of the NH including the US, Europe and large parts of Asia. In Eastern Canada I would expect record warmth and for sea ice formation in Hudson Bay to be significantly delayed.
You might think with heat domes centered over the continents and not over the Arctic Ocean this would favor preserving Arctic sea ice and even support the beginning of the sea ice growth season, but from NSIDC still no sign that the Arctic sea ice minimum has been reached. Looks like the minima will be close to the recent years of 2019 and 2020 and potentially in the top five lowest sea ice extents on record. Typically, the sea ice minima is observed in mid-September, therefore it appears to me to be a bit delayed.
Figure iii. Observed Arctic sea ice extent on 18 September 2023 (white). Orange line shows climatological extent of sea ice based on the years 1981-2010. Image from the National Snow and Ice Data Center (NSIDC) https://nsidc.org/arcticseaicenews/
Looking at the current Arctic sea ice extent, it is almost completely shifted onto the North Atlantic side with the North Pacific side almost ice-free (see Figure iii). But overall low Arctic sea ice in the fall months favor a weaker polar vortex (PV) and cold temperatures in winter, at least regionally. However, research shows that it is sea ice absence only in the North Atlantic side favors a weak PV, but the extent can and will shift around. Though low sea ice on the North Pacific side is associated with a stronger PV it is also associated with colder temperatures in North America.
Like in the previous blog, if there is one feature that is giving me hope that a turn to fall and eventually winter weather are indeed coming, it’s the predicted troughing (Figure 8) coupled with colder temperatures (Figure 9) and even snow for Siberia (see Figure iv). Snow cover extent is currently above normal for Siberia (not shown) but so far nothing impressive is apparent to me. And the predicted Siberian troughing and cold temperatures are not impressive. However, if the Ural blocking can become more amplified and persistent than predicted, then the outlook might look different. I argue that above normal snow cover in Siberia in the fall but especially October favors a weaker PV and a higher probability of severe winter weather across the NH continents.
Figure iv. Forecasted snowfall rate (mm/day; shading) from 30 September – 4 October 2023. The forecast is from the 00Z 19 September 2023 GFS ensemble.
El Niño is all but certain for this winter and the one remaining question is how strong. A lot of the dynamical models are predicting a strong El Niño this winter. There are not that many historical data points for strong El Niño’s but all have them have featured widespread mild temperatures. Many of the statistical models are predicting a moderate El Niño this winter and there have been more interesting winters during moderate El Niño. Also positive is the Indian Ocean Dipole (IOD). In late 2019 the IOD was strongly positive, and the winter of 2019/20 featured a record strong PV, record positive AO and very mild temperatures across the NH continents.
In summary the tropics (red team) are so far favoring a mild winter of 2023/24 but if the El Niño and positive IOD can remain more in the moderate range their influence could be moderated. Arctic boundary conditions (blue team) favor a weak PV and a colder winter, but it is very early and so far, the signals are weak. I think lots of uncertainty for now and how boundary conditions evolve this fall are critical.
Not much new from Tuesday. The pattern across North America remains hostile to a robust transition from summer to fall and eventually winter. That ridge in Eastern Canada (see Figure v) remains tenacious with record warmth likely, the delay in ice growth in Hudson Bay and retarding the advance of snow cover extent in eastern North America.
Figure v. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 2 – 6 October 2023. The forecasts are from the 00z 21 September 2023 GFS ensemble.
The only sign of a seasonal transition remains Siberia. A possibility that I discussed on Tuesday is becoming more likely, more amplified and persistent ridging is predicted near the Urals coupled with higher geopotential heights in the Central Arctic (see Figure v). This should support cold temperatures at least for parts of Siberia and the continued advance of snow cover extent across the region (see Figure vi). Certainly not the perfect pattern for the establishment of cold and snow across Siberia given the predicted ridging in Eastern Siberia but for winter weather enthusiasts that’s the best we have right now with Canada stuck in reverse (summer).