MJO – Influences In Forecasting

29 Jun

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 weather forecasts pin-point those more specific areas at risk of say heavy rainfall and thunderstorms, to then the ensemble forecast information from the likes of the GFS and ECMWF which out to 2 weeks or more help to gauge the possible weather trends within the medium and longer term. When it comes to longer term forecasts there are some other factors which can be taken into consideration and one of these is the MJO – Madden Julian Oscillation

This tropical feature which is characterised by an eastward movement of rainfall across the Indian Ocean and into the Western Pacific whilst it may not seem to be of that much importance to the UK at first glance, does have significant and widespread affects on weather patterns across the globe and including the UK. There are many studies on the MJO that prove this to be true and one particular analysis and seminar by Christophe Cassou in 2008 provides significant and detailed information that the MJO can and does affect the weather across the UK and the North Atlantic area in general.

What I will say at this stage as a priority however, is that the MJO is not the “be all and end all” when it comes to medium and longer term forecasting. Just like any part of weather forecasting a variety of pieces of information need to be looked at and acknowledged when forecasting the weather and the MJO can be overridden by other large scale, global weather events and features.

However, I want to use this blog as an experiment, a forecasting experiment to see whether, on this one particular occasion, the MJO can indeed be a guide as to the longer term weather trends across the UK and the North Atlantic in general. Reanalysis of past weather data has lead to a series of pieces of information that correlate to the expected phase of the MJO, time of year and how approximately 10 to 12 days ahead (Cassou 2008) it may then influence the weather across the North Atlantic. Just like weather charts that show weather forecasters how areas of low pressure and high pressure move, there are forecast charts that show how the MJO may progress as well – http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/clivar_wh.shtml

So, using the last link and the forecast charts on that page you can see that the MJO Index is currently in Phase 8. For a June month, this correlates to the follow possible synoptic pattern within approximately 10 to 12 days;

JunePhase8500mb

What this is showing us is that the green areas are signalling areas where pressure is likely to be higher than average and the blue colours denoting lower than average pressure. It is interesting to note that on this chart there is a clear signal that for the UK and NW Europe pressure is likely to be high and the current expectations are that towards and beyond the opening week of July the UK will experience a progression towards high pressure leading to settled, summery and potentially very warm conditions. So this is one interesting correlation and example of how the MJO may well be highlighting/influencing this signal of better weather given the current phase of the MJO.

So lets look further ahead for a real challenge;

ALL_emean_phase_full

The latest forecast guidance regarding the progression of the MJO is that there is quite high confidence that by the time we move into early July (1st to the 3rd) the MJO will have progressed into Phase 1. Note on the above image how the forecast models (coloured lines) signal the progression of the MJO into the ‘1’ region. Now, using the reanalysis charts again as a possible guide to longer term trends, this is the following possible pattern for a July month given a Phase 1 of the MJO;

JulyPhase1500mb

Now, in this instance we have a totally different possible synoptic pattern across the UK. Remember that blue colours denote generally lower than average pressure and clearly there is a region of blue/lower than average pressure located across the UK.

So, using the above information and the latest forecast models regarding the track of the MJO, I’ll make a prediction that approximately 10 to 12 days after the opening few days of July the weather across the UK will become more unsettled as the high pressure and potentially very warm conditions from the 6th and 7th of July are replaced by cooler and more unsettled conditions as we progress towards mid-July (12th to the 15th approx).

I’ll come back to this blog and prediction around the middle of July to see whether it was accurate or totally wide of the mark, but it will be an interesting experiment to see, if on this one occasion, the MJO did have some influence on the weather longer term. If this prediction is also likely to be correct, it would seem that the possible settled and very warm conditions towards the 6th and 7th of July won’t be prolonged and lasting for less than a week.

Time will tell as ever!

Regard to all,

M.

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