The Circumglobal Teleconnection and its Links to Seasonal Forecast Skill for the European Summer

Email: j.beverley@pgr.reading.ac.uk

Recent extreme weather events such as the central European heatwave in 2003, flooding in the UK in 2007, and even the recent dry summer in the UK in 2018, have highlighted the need for more accurate long-range forecasts for the European summer. Recent research has led to improvements in European winter seasonal forecasts, however summer forecast skill remains relatively low. One potential source of predictability for Europe is the Indian summer monsoon, which can affect European weather via a global wave train known as the “Circumglobal Teleconnection” (CGT).

figure1
Figure 1: One-point correlation between 200 hPa geopotential height at the base point (35°-40°N, 60°-70°E) and 200 hPa geopotential height elsewhere in the ERA-Interim (1981–2014) reanalysis dataset, for August. The boxes indicate the regions defined as the “centres of action” of the CGT – these are North Pacific (NPAC), North America (NAM), Northwest Europe (NWEUR), Ding and Wang (D&W) and East Asia (EASIA).

The CGT was first identified by Ding and Wang (2005) as having a major role in modulating observed weather patterns in the Northern Hemisphere summer. Using a 200 hPa geopotential height index centred in west-central Asia (35°-40°N, 60°-70°E), they constructed a one-point correlation map of geopotential height with reference to this index (reproduced in Figure 1). From this, they identified a wavenumber-5 structure where the pressure variations over the Northeast Atlantic, East Asia, North Pacific and North America are all nearly in phase with the variations over west-central Asia (these are known as the “centres of action”). They also showed that the CGT is associated with significant temperature and precipitation anomalies in Europe, so accurate representation this mechanism in seasonal forecast models could provide an important source of subseasonal to seasonal forecast skill.

The model used here is a version of the European Centre for Medium-Range Weather Forecasts (ECMWF)’s coupled seasonal forecast model. Reforecasts are initialised on 1st May and are run for four months, so cover May-August, with start dates from 1981-2014. The skill of the model 200 hPa geopotential height is shown in Figure 2, defined as the correlation between the model ensemble mean and ERA-Interim. The model has good skill in May (to be expected given that the reforecasts are initialised in May) but in June, July and August areas of zero or negative correlation develop across much of the northern hemisphere extratropics. The areas of reduced skill align closely with the location of the centres of action of the CGT shown in Figure 1, suggesting that there is a link between the model skill and the model representation of the CGT.

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Figure 2: Model ensemble mean skill for 200 hPa geopotential height as defined as the correlation between ERA-Interim and model ensemble mean for (a) May (b) June (c) July and (d) August

To determine how well the model represents the CGT, Figure 3 shows the correlation between the D&W region and the other centres of action of the CGT, as defined in Figure 1. Focussing on August (as August has the strongest CGT pattern) it can be seen that the model correlations, indicated by the box and whisker plots, are weaker than in observations (red diamond) for the D&W vs. North Pacific (NPAC), North America (NAM) and Northwest Europe (NWEUR) regions. This indicates that the model has a weak representation of the wavetrain associated with the CGT.

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Figure 3: Distribution of correlation coefficients for the D&W Index correlated against the other centres of action of the CGT. The box plots represent the upper and lower quartiles, and the whiskers extend to the 5th and 95th percentiles. The black horizontal line represents the median value and the red diamond the observed correlation coefficient from ERA-Interim.

There are likely to be several reasons for the weak representation of the CGT in the model. One important factor is the presence of a northerly jet bias in the model across much of the Northern Hemisphere. This can be seen in Figure 4, which shows the model jet biases relative to ERA-Interim in the coloured contours, and the observed zonal wind in the black contours. The dipole structure of the biases which exists across much of the hemisphere, particularly in June, July and August, indicates that the model jet stream is located too far to the north. This means that Rossby waves forced in this region will have different wave propagation characteristics to reality – they may propagate at the incorrect speed, in the wrong direction or may not propagate at all, and this is likely to be an important factor in the weak representation of the CGT in the model.

figure4
Figure 4: Model 200 hPa zonal wind bias (filled contours, m/s), defined as the model ensemble mean minus ERA-Interim zonal wind, and ERA-I 200 hPa zonal wind (black contours) for (a) May (b) June (c) July and (d) August. The location of the centres of action of the CGT are marked with white crosses.

Other potential factors involved are a poor representation of the link between monsoon precipitation and the geopotential height in west-central Asia (which was shown by Ding and Wang (2007) to be important in the maintenance of the CGT) and errors in the forcing of Rossby waves associated with the monsoon. For a more detailed explanation of these, see my paper in Climate Dynamics (Beverley et al. 2018). It seems likely that the pattern of reduced skill in Figure 2, with negative correlations located at the centres of action of the CGT, including over Europe, is related to the poor representation of the CGT in the model. This raises the question of whether an improvement in the model’s representation of the CGT would lead to an improvement in forecast skill for the European summer. To address this question, sensitivity experiments have been carried out, in which the observed circulation is imposed in several centres of action along the CGT pathway to explore the impact on forecast skill for European summer weather.

References

Beverley, J. D., S. J. Woolnough, L. H. Baker, S. J. Johnson and A. Weisheimer, 2018: The northern hemisphere circumglobal teleconnection in a seasonal forecast model and its relationship to European summer forecast skill. Clim. Dyn. https://doi.org/10.1007/s00382-018-4371-4

Ding, Q., and B. Wang, 2005: Circumglobal teleconnection in the northern hemisphere summer. J. Clim. 18, 3483–3505.  https://doi.org/10.1175/JCLI3473.1

Ding, Q., and B. Wang, 2007: Intraseasonal teleconnection between the summer Eurasian wave train and the Indian monsoon. J. Clim. 20, 3751-3767. https://doi.org/10.1175/JCLI4221.1

International Conferences on Subseasonal to Decadal Prediction

I was recently fortunate enough to attend the International Conferences on Subseasonal to Decadal Prediction in Boulder, Colorado. This was a week-long event organised by the World Climate Research Programme (WCRP) and was a joint meeting with two conferences taking place simultaneously: the Second International Conference on Subseasonal to Seasonal Prediction (S2S) and the Second International Conference on Seasonal to Decadal Prediction (S2D). There were also joint sessions addressing common issues surrounding prediction on these timescales.

Weather and climate variations on subseasonal to seasonal (from around 2 weeks to a season) to decadal timescales can have enormous social, economic, and environmental impacts, making skillful predictions on these timescales a valuable tool for policymakers. As a result, there is an increasingly large interest within the scientific and operational forecasting communities in developing forecasts to improve our ability to predict severe weather events. On S2S timescales, these include high-impact meteorological events such as tropical cyclones, floods, droughts, and heat and cold waves. On S2D timescales, while the focus broadly remains on similar events (such as precipitation and surface temperatures), deciphering the roles of internal and externally-forced variability in forecasts also becomes important.

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Attendees of the International Conferences on Subseasonal to Decadal Prediction

The conferences were attended by nearly 350 people, of which 92 were Early Career Scientists (either current PhD students or those who completed their PhD within the last 5-7 years), from 38 different countries. There were both oral and poster presentations on a wide variety of topics, including mechanisms of S2S and S2D predictability (e.g. the stratosphere and tropical-extratropical teleconnections) and current modelling issues in S2S and S2D prediction. I was fortunate to be able to give an oral presentation about some of my recently published work, in which we examine the performance of the ECMWF seasonal forecast model at representing a teleconnection mechanism which links Indian monsoon precipitation to weather and climate variations across the Northern Hemisphere. After my talk I spoke to several other people who are working on similar topics, which was very beneficial and helped give me ideas for analysis that I could carry out as part of my own research.

One of the best things about attending an international conference is the networking opportunities that it presents, both with people you already know and with potential future collaborators from other institutions. This conference was no exception, and as well as lunch and coffee breaks there was an Early Career Scientists evening meal. This gave me a chance to meet scientists from all over the world who are at a similar stage of their career to myself.

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The view from the NCAR Mesa Lab

Boulder is located at the foot of the Rocky Mountains, so after the conference I took the opportunity to do some hiking on a few of the many trails that lead out from the city. I also took a trip up to NCAR’s Mesa Lab, which is located up the hillside away from the city and has spectacular views across Boulder and the high plains of Colorado, as well as a visitor centre with meteorological exhibits. It was a great experience to attend this conference and I am very grateful to NERC and the SummerTIME project for funding my travel and accommodation.

Email: j.beverley@pgr.reading.ac.uk