There’s an important call regarding the Madden Julian Oscillation (MJO) transition over the next 1-2 weeks which will have an impact on potential California rainfall, a thermal pattern change for the U.S. and needed rainfall in Argentina. There are 2 schools of thought. The GFS holds the MJO in the central equatorial Pacific (Fig. 1) while the ECM is progressive across the equatorial Pacific into the Atlantic (Fig. 2).
Fig. 1-2: The GFS and ECM MJO forecasts through the next 2 weeks.
The 40-day tendency of the MJO (Fig. 3) has been quasi-stationary across the West Pacific and Indonesia where equatorial sea surface temperature anomalies (SSTA) are warmer than normal (and attracting the convective nature of the MJO pattern). Persistence lends support to the stalling GFS forecast.
Fig. 3: The 40-day persistence of MJO.
If the GFS is correct and MJO stalls near the Dateline (phase_6) the (NOAA) climatology for December favors a warm shift into the East and persistent dryness remaining across the West. On average, forecast models are generally in line with this idea for the 6-10 day period. If the MJO is more progressive through the East Pacific tropics to the equatorial Atlantic, California will gain storm risk. Additionally, a cold outbreak would be implied for the U.S.
So what’s the answer? The best estimate is that both forecasts are correct. In the short-term, Western warmth shifts eastward and this week’s eastern chill retreats to Canada. After the warm shift toward the East in the 6-10 day period the MJO becomes progressive leading to following cold as Christmas approaches. The cold could be extreme due to a stratospheric warming event in northern Canada. The wet risk to California is brief and centered around 10 days from now.
In Argentina there is interest in the MJO transition as an eastward shift across the Pacific supports needed wet weather. At midday operational forecast models point toward the previously described (MJO) transition. MJO stalls in the central Pacific much of the next week or so correlating to limited Argentina rainfall. Once the MJO shifts toward and into the tropical Atlantic in the 11-15 day period Argentina will turn wet.