A Connection Between El Nino And Europe Heat Waves?

Written by Mark Vogan on August 11, 2015 in Rest of Europe, Summer 2015, United Kingdom & Ireland with 0 Comments

In today’s post I wanted to share with you an interesting article I came across on Netweather.tv. A theory about a very possible connection between El Nino and European heat waves. Given that I base my own forecasts on connections with major global drivers such as El Nino and SSTA’s, I very much believe there’s a connection just like I forecasted a cool summer based on the cool Atlantic and the wet too based on the El Nino. It’s always good to see other folks theories and ideas.

By Vorticity0123 Netweather.tv

ORIGINAL LINK https://forum.netweather.tv/topic/83676-model-output-discussion-1st-august-00z/page-12#entry3243571

Where the UK continues to be affected by periods of unsettled weather associated with active troughing and over the eastern Atlantic ocean, parts of central and northern Europe are experiencing much hotter and drier weather. In fact, Germany touched upon its national heat record from 2003 (40.3*C). This weather is associated with anomalous ridging (high pressure activity) both near the surface and at 500 hPa over Eastern Europe. On the last few pages of this thread an interesting discussion has been going on about this ridge and the connection to El Nino. In order to explore this link, and (hopefully) find a few answers, I will examine some teleconnections, as well as have a look at the general circulation pattern.

Bridging the gap

A nice bridge to my previous post (discussing the ocean and the NAO in connection with troughing over the UK), and the current topic is that we are now not looking at the Eastern Atlantic trough and small sea surface temperature anomalies in the Atlantic, but the neighboring ridge and a much greater phenomenon – ENSO. Here is the link to my previous post:

https://forum.netwea…rds-18z/page-47

Furthermore, on the same page, Knocker posed a key question for the current discussion, and a first start in our search for an answer:

Quote

Vorticity.

Is the heat in Central and Eastern Europe and our cooler temps linked to the current El Nino?

Michael Ventrice ‏@MJVentrice 5h5 hours ago

It’s interesting other El Nino states similar to the current event saw more heat waves o/central-eastern Europe

Attached Thumbnails

To clarify: Michael Ventrice is a leading scientist in the field of teleconnections related to weather. And the image shown in the quote is an ECMWF forecast for 30 days out of surface temperature anomalies, NOT a representation of average surface temperatures during an El Nino event.

Link real or nonsense?

Validating the link is crucial here, so to test that below are 500 hPa height anomalies during El Nino events in summer:

500 hPa height anomalies during an El Nino summer, composed by averaging over various El Nino years. Courtesy: NOAA.

Unexpectedly (at least for me), there are negative rather than positive height anomalies present over central and eastern Europe.This means that on avearge, during an El Nino summer, the 500 hPa heights are lower than usual. This would coincide with more low pressure activity than usual. In short, this is contradicting what I initially expected (namely that an El Nino summer correlates positively with 500 hPa height anomalies over central Europe).

Does this mean the answer is that there is no link? It is good to realize that we are currently facing a rather strong summer El Nino, and in the plot above also weak El Ninos are taken into account. Therefore, it might pay off to look only at strong El Nino events (although one could question the value of comparing only one year). Below are the 500 hPa height anomalies for the summer of the El Nino event of 1997:

500 hPa height anomalies during 1997 between June and August. Courtesy: NOAA.

In this particular El Nino event, we do see strong positive 500 hPa height anomalies covering the whole of Europe. However, does one year’s match make the link to be true? This is a difficult question, where further examination of other strong El Nino years could be very valueable.

Finding an explanation for a phenomenon that may or may not be there is even more hazardous. Therefore, for the remainder of this post I will attempt to explain the current anomalous ridging over central Europe without looking at past analogues.

A rather amplified atmospheric state

In order to explain the current ridging over central Europe, it is good to take a look at the big picture. As such, below are the 100 hPa heights of the Northern Hemisphere (in order to retain the most clean view):

ECMWF height analysis at 100 hPa as of 06-08 12 UTC.

Note that even though the given level is near the stratosphere, the general pattern nicely matches the pattern at 500 hPa. The red lines indicate the position ridges, whereas the blue lines indicate the position of troughs.

What can be seen is that, if one follows the 1648 dam line, the pattern is rather wavy/amplified. In other words, there are a lot of rather deep troughs (isolines pointing towards the equator) and strong ridges (isolines pointing towards the poles) present. In my post on the 25th of June, I treated that subject in somewhat more detail, that post can be found here.

If we look into somewhat more detail (towards our region) we can see a deep trough extending southward just west of mainland Europe, while a potent ridge is positioned all the way up from central Europe to Siberia. This is a pattern that has been observed a lot during this summer.

One key conclusion can be driven from this is that the ridge over central and eastern Europe is present throughout the troposphere, and that it may be seen as a blockade in the atmosphere. The same applies for the trough just west of Europe.

Arctic oscillation as a measure: high pressure over the poles

Another way to look at this ‘waviness’ is the arctic oscillation. When this oscillation is positive, little ‘waves’ are present in the atmosphere resulting in a generally east-west circulation with weak to no dominant ridges or troughs. On the other hand, when the AO turns negative, lots of meanders (visualized by troughs and ridges) are present, resulting in lots of blockades and north/south orientated flow.

During a large part of the summer, the AO has been negative:

Arctic oscillation trends over the past few months (courtesy: NOAA)

What can be seen is that the AO has been negative a lot, indicative of a rather amplified flow. Therefore, anomalous ridges and troughs are more often than not occurring. Linking back to the ridge over Europe, the negative AO nicely coincides with the anomalous ridge present over central Europe.

In terms of the 500 hPa pattern over the pole itself, this pattern could be explained by the fact that the North Pole has seen higher than average heights (so higher than average pressure) during most of summer. This acted as a catalyst of the amplified flow. More about this can be found here:

https://www.aer.com/…tic-oscillation

What can be seen in the AO forecast in the image and the article above (the red lines are a forecast ensemble of the GFS model), is that the AO is going to turn positive. In other words, the flow at the midlatitudes is forecast to become more zonal (east-west oriented, with less strong ridges/troughs)

Europe ridge not responding

So that would mean easing of the ridge over central and eastern Europe? Well, this is not the case. In fact, the ridging only seems to stubbornly maintain itself, only changing some in shape. This is illustrated in the GFS ensemble 500 hPa heights chart below:

GFS ensemble 500 hPa heights and surface level pressure as of 18Z 07-08 T+240.

As can be seen here, there is still a strong ridge present at 500 hPa (orange and red colours) over Scandinavia.

Concluding, the decreased ‘waviness’ of the atmosphere does not seem to lead to a relaxation of the ridge over central Europe. The ‘waviness’ of the atmosphere can therefore not be seen as a lead contributor to the ridge.

Taking a different approach: Atmospheric Angular Momentum linked to El Nino as a key player?

For a further search towards an answer, we might want to turn our eyes on the AAM budget, reflected in the Global Wind Oscillation (GWO). The AAM is, in short, the velocity of the movement of the atmosphere relative to the earth.

It is well-known that during an El Nino event, there are often higher than usual AAM values. This is also the case as of speaking, and will remain the case for the next weeks:

GWO analysis and forecast (in green). Courtesy: University of Albany.

The upper part of the diagram indicates high values of AAM, while the lower part of the diagram indicates lower values of AAM. As can be seen here, the AAM is currently positive, and will remain positive, despite the GWO making some orbits.

Unfortunately, this is as far as my knowledge allows me to go. Possibly somebody else can make or break the theory that this ridging over central Europe is associated to El Nino via the AAM budget, of course assuming that the link exists at all. It would be greatly appreciated!

Conclusion

In this post I came to the conclusion that the anomalous ridge over central Europe may not be directly coupled to El Nino, despite me initially thinking otherwise. Furthermore, we have seen that a rather amplified flow has been associated to the ridge, yet this waviness of the atmosphere did not seem to be directly linked. This was because even though forecasts pointed towards less ‘waviness’ in the atmosphere, the ridge over central Europe was not weakening as would be expected. Finally, I touched upon the atmospheric angular momentum and the link with El Nino as being a possible driver.

And yet, the only thing this post seems to do is raise more questions, rather than a definite answer to the question whether El Nino is related to anomalous ridging over Europe. This is the interesting part of science, though, as unveiling one aspect of a phenomenon reveals even more parts to discover! Hopefully, despite the lack of a definite answer, this post is an interesting read, and more contributions/theories/corrections are greatly appreciated as always :smile: .