Teleconnections

Circulation patterns like NAO are often associated with global teleconnections through propagation of Rossby wave trains.  El Nino-Southern Oscillation (ENSO) events, Sudden Stratospheric Warmings (SSW) and pronounced Madden-Julian Oscillation (MJO) events have been found to enhance the predictability and skill of forecast circulations in the North Atlantic/European area.

In particular, the presence of and the phases of significant MJO events can be linked to an increase of skill in forecasting NAO– circulation patterns  Fig8.2.8-1 compares the bivariate correlation (NAO & BLO) against analysis for forecasts initiated with and without an MJO event.  Forecasts initiated with an MJO event show higher skill (an improvement of the order of one day) between Day8 and Day15.  Correlations are significantly increased for Day11 and Day12 at a 90% confidence level, and for Day10 to Day13 at an 80% confidence level.

Inspection of Hovmoeller and Wheeler-Hendon diagrams can give a guide towards the consistency and intensity of an MJO event.  The distortion of the upper flow associated with an MJO affects monsoon activity and, equally important, modifies the northern and southern hemisphere mid-latitude jets, impacting on predictability of extra-tropical patterns. This can be expressed in terms of weather regime impacts. 

Some broad remarks can be made:

• The probability of the development of a positive NAO signal is significantly increased about ten days after the MJO is in phase 3 (i.e. enhanced convection over the Indian Ocean), and significantly decreased about ten days after the MJO is in phase 6 (i.e. enhanced convection over the Western Pacific and suppressed convection over Indian Ocean).
• The probability of the development of a negative NAO signal is significantly decreased about ten days after the MJO is in phase 3 (i.e. enhanced convection over the Indian Ocean), and significantly increased about ten days after the MJO is in phase 6 (i.e. enhanced convection over the Western Pacific and suppressed convection over Indian Ocean).
• The impact of the MJO on two other Euro-Atlantic weather regimes (the Atlantic Ridge and Scandinavian blocking) is much weaker.

Research has shown that in the winter half of the year the reliability of 2m temperature forecasts for Europe is influenced by whether or not there is a substantive MJO at analysis time.  The IFS can model the evolution of a pre-existing MJO quite well and also can also capture the consequent influence at mid-latitude effects.  However, the IFS has trouble generating new, substantive MJOs in the right place, which then impedes the predictive skill when there is no MJO at start time.

Thus the 2m temperature anomaly forecasts for Europe tend to be:

• more reliable if there is a substantive MJO at analysis time, (i.e. lying outside the circle on the Wheeler-Hendon diagram).
• less reliable if there is not a substantive MJO at analysis time.

Forecasters should be able to use these results to good effect when examining the monthly forecasts.

Fig8.2.8-1: Bivariate correlation (NAO & BLO) for forecasts with (red) and without (black) an MJO event in the initial conditions.  Colours refer to initial conditions for the forecasts:

• black - no initial MJO.
• red - initiated with an MJO (shows higher skill between Day8 and Day15).

Fig8.2.8-2: Brier Skill Scores (BSS) for NAO– predictions according to the initial MJO phase.  Colours refer to initial conditions for the forecasts:

• black - no initial MJO.
• red - MJO phases 2–3 (enhanced convection over the Indian Ocean).
• blue - MJO phases 4–5 (enhanced convection over the Maritime Continent).
• green - MJO phases 6–7 (enhanced convection over the western Pacific).
• brown - MJO phases 1 and 8 (suppressed convection over the Maritime Continent).

The MJO influences skill in forecasts concerning NAO– circulation pattern:

• During MJO phases 4&5 skill is increased for the Day13-20 period.
• During MJO phases 6&7 skill is increased for the Day8-13 period.   

The corresponding increases in skill for NAO+, BLO+ and BLO– are small.

Fig8.2.8-3: Variation of the MJO index bivariate correlation with forecast lead-time for two older ECMWF model cycles (40r1 and 40r3).  Beyond Day20 correlation falls below 0.7. By Day 27 it falls below 0.6, implying marginal skill.

MJO Teleconnection Example

The forecast based on 00UTC 25 Feb 2019 illustrates the tele-connection effect of tropical deep convection over the Indian Ocean upon subsequent downstream developments of NAO+ type over the North Atlantic/Europe. 

Fig8.2.8-4: MJO Wheeler-Hendon diagram for the monthly forecast based on DT 00UTC 25 February 2019.   The colours represent ENS forecasts at various lead times as given by the key above the diagram.  Initially the MJO lies within Sector1 (Western Indian Ocean) and is forecast to progress into Sector2 (Eastern Indian Ocean) by Day5 with a fairly limited spread among ENS members. The mean position of the ENS forecasts during the subsequent ten days show continuing westward progress although with an increasing spread before weakening (moving into the central circle) by Day20.