A triggering factor of the eastward propagation of the Madden–Julian oscillation (MJO) is proposed from a case study of the first MJO generated in late October during CINDY2011. The proposed scenario is that the eastward propagation of the MJO is triggered by an extratropical cyclone in the Southern Hemisphere. The ridge and trough pair meridionally extending between 30°S~15°N accompanied by the extratropical cyclone was found to be completely synchronized with the eastward propagating MJO convection. The ascending areas of the cold front extending from the extratropical cyclone partially combined with those of the MJO and the large-scale ridge and trough below 500 hPa were formed across the midlatitudes in the Southern Hemisphere and the tropics. The convection center of the MJO shifted eastward as a result of the westerly winds in the tropics, expanding eastward by the zonal pressure gradient force between the ridge and trough.

1. MOTEKI QoosakuJAMSTEC/DCOP
MJO
A triggering factor of the eastward propagation of
CINDY2011
during
observed in late October

2. The 1st MJO
during
CINDY2011
[10/29-11/4]
6m/s

3. 1st MJO during CINDY2011/DYNAMO

4. The eastward
propagation of
this MJO
could be ...
due to this
extratropical
cyclone?
6m/s
6m/s

5. 6m/s
6m/sdue to the
traveling
anticyclone?

6. Hypothesis:
The ETC triggers
the MJO propagation.
6m/s
6m/s
NZA850 (color) SLP (blue) OLR (red) UV vector (westerlies only)

7. Question:
Are there any
coupling processes
between
the MJO and ETC?
6m/s
6m/s
NZA850 (color) SLP (blue) OLR (red) UV vector (westerlies only)

8. Z anomaly =
Z Z averaged between 40-140E
Z variance between 40-140E
ー
normalized Z anomaly
for the meridional amplitude difference

9. JRA55dx, dy: 1.25°
dt: 6h
38 levels (1-1000hPa)
The Japanese 55-year reanalysis for 1958−2012

10. 10N-30N 10S-10N 30S-10S
NZA (color)
OLR-240W/m2 (red)
for 10S-10N
positive U (black)

12. NZA850 (color) SLP (blue) OLR (red) UV vector (westerlies only)
R T
Meridional trough
combined
the MJO and ETC
Following ridge
behind the ETC

13. NZA850 (color) SLP (blue) OLR (red) UV vector (westerlies only)
Westerly winds
behind the MJO
Southwesterly winds
behind the cold front

14. ω850 (color) SLP (blue) OLR (red) UV vector (southerlies only)
Ascending area junction

15. u∇PT850 (color) SLP (blue) OLR (red) UV vector (southerlies only)
Southerly
cold advection
into the Tropics

16. NZA850 (color) SLP (blue) OLR (red) UV vector (westerlies only)
R T
A pair of meridional
trough and ridge
shifted eastward.
PGF
Westerly winds
expands eastward by
PGF between R/T

17. ω850 (color) SLP (blue) OLR (red) UV vector (westerlies only)
Ascending area junction

18. LH
R T MJO
PGF
If coupling occurs,
eastward propagation is natural.

19. MJO
Eurasia High
Mascarene High
usually denies the coupling with the ETC.

20. LH
R T MJO
PGF
After Mascarene High decays,
ETC can travel into subtropics.

21. LH
R T MJO
PGF
After cold advection intrudes into the Tropics,
the MJO is pushed by PGF between R/T.

22. Hypothesis on the MJO propagation
The ETC is a triggering factor.

23. The MJO onset after decay of Mascarene High

24. u∇PT850 (color) SLP (blue) OLR (red) UV vector (southerlies only)
Southerly
cold advection
into the Tropics