MJO

When it comes to weather forecasting there are a variety of weather models that are used to give insight as to the possible weather. Both higher resolution short term models help to pin-point those more specific areas at risk of say heavy rainfall and thunderstorms, to then the ensemble forecasts from models such as the GFS & ECMWF which help to gauge the possible weather trends in the medium to longer term. When it comes to longer term forecasts (15-30 days) there are some other factors which can be taken into consideration and one of these is the Madden Julian Oscillation, also known as the MJO.

This tropical feature which is characterised by an eastward propagation of rainfall across the Indian Ocean and into the Western Pacific. Whilst it may not seem to be of much importance to Europe at first glance, there have many numerous studies to suggest that it has significant and widespread effects on weather patterns across the globe, including Europe. See below the eastward propagation of anomalous rainfall.

One important paper by Christophe Cassou (2008) provides significant and detailed analysis to show the MJO does affect the weather across Europe. He found that positive NAO events mostly respond to a mid-latitude low-frequency wave train initiated by the MJO in the western–central tropical Pacific and propagating eastwards. Precursors for negative NAO events are found in the eastern tropical Pacific–western Atlantic, which lead to changes in the North Atlantic storm track.

The phase of the MJO is used as a guide to intra-seasonal forecasting across the North Atlantic & Europe in general. Reanalysis of past weather data has lead to a series of informations that correlate to the expected phase of the MJO, time of year and how approximately 10 to 12 days ahead (Cassou, 2008).

Cassou found that the anomalous precipitation affects the Rossby wave train, which tends to affect Europe around 10-14 days later. As you can see in the graphic below, there seems to be a positive correlation between phase 3 of the MJO and a positive NAO. Similarly, phase 7 & 8 correlate to a negative NAO.

Example:

On March 13th, the MJO entered into phase 7. There is a strong projection on the MJO where the amplitude is greater than one. This below composite shows 500mb geopotential height (GPH) anomalies for the Northern Hemisphere, for phase 7 of the MJO where the amplitude is greater than one.

MJO phase 7

This composite for phase 7 of the MJO for March, the yellows & greens would indicate higher than normal heights across northern Europe, and hence higher pressure at the surface. It also indicates lower than normal heights close to Iberia, and therefore lower than average pressure there at the surface.

This 1-5 day period GPH anomaly as per the ECMWF for the period of 19th-25th March agree with the above composite for higher pressure across northern Europe (bar the far north of Scandinavia), and lower pressure near Iberia. There are some slight variances, but the overall pattern is similar.

ECM 1-5 GPH anom

This pattern is indicative of a negative NAO. According to Cassou, phase 7 should demonstrate a negative NAO phase which is in agreement with the observations in this time frame as per NAO index.

NAO-

It is clear to see the MJO can be used an important source of predictability for Europe. It should only be used as a guide, but it can be very useful for medium range forecasting.

Useful websites:

  1. Cassou gave a presentation at the ECMWF on ‘Euro-Atlantic Regimes and their teleconnections.’ http://old.ecmwf.int/newsevents/meetings/annual_seminar/2010/presentations/Cassou.pdf
  2. This is the most useful website in terms on deciding the most favourable pressure pattern and temperature anomalies by phase of the MJO & by month. http://www.americanwx.com/raleighwx/MJO/MJO.html
  3. Phase Plots of MJO Index Forecasts Wheeler & Hendon plot. http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/clivar_wh.shtm