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Where is ENSO Likely to Go?

By Joe D'Aleo
Monday, September 1, 2008

Last winter was a La Nina Winter. In fact by late winter its reached strong La Nina status according to the MEI, one of the ENSO measures discussed in an earlier story. It followed moderately strong though brief El Nino the prior fall and early winter.



The La Nina weakened rapidly during the spring and by this summer was smack dab in the middle of the neutral range.

Most stronger La Ninas weakened to some degree in the summer only to either return the following winter to La Nina or stay neutral. The major exception was 1964/65, which actually peaked in the fall of 1964 and by mid winter was already a neutral event and became a rather strong El Nino by the late spring and summer.



CDC left out 1998/99 and 1961/62 weaker first year La Ninas which also both went on to a second La Nina winter.  I added those two years and recreated this graph. As you can see, most La Ninas went on to be two year events. The exceptions being that 1964/65 event and 1988/89 which never quite made it to a  stable El Nino state (until 1991)



Again 1964/65 sticks out with its reversal, already a strong El Nino at this time. We are flat lining near 0.


What about the ENSO regions? The central Pacific remains colder than normal though anomalies have weakened but eastern areas have warmed.





How do the ENSO models look for the upcoming winter?  Here is a compilation from IRI of 23 statistical and dynamical ENSO models updated this month. Most remain in the -0.5 to +0.75 MEI range for the winter DJF (neutral to weak El Nino).



Of those ENSO models that extend that far, the majority (14) keep the current event in a neutral state next winter. 4 have a weak El Nino, 1 a moderate one and 1 a weak La Nina.


Normally a west QBO La Nina (west QBO is the case this winter), would show a stronger southeast ridge than last year dominating, with cold mainly in the west. The Arctic Oscillation tends to be more positive. East QBO La Ninas have flatter ridges and more high latitude blocking near the Davis Straits and Greenland with a distinctively negative Arctic Oscillation AO. That was precisely the case last winter, an East QBO La Nina.


Weak La Ninas are often more variable and would produce some intense cold intrusions into the east.




A neutral ENSO may produce similar results as the atmosphere carries a memory of the prior ENSO state.  An intervening weak El Nino this fall as some models show even if brief would negate that La Nina memory, in fact suggest the reverse pattern (left side below).



Note how the warmth though is rapidly diminishing in the ocean cross sections




If El Nino thresholds are met (which is a good possibility), it may only be briefly. The Pacific as a whole remains cold as evidenced by the PDO which remains at 2 STD negative levels which might reinforce the La Nina memory or a return quickly to a La Nina state.



Note the predominance of La Ninas in a negative cold PDO modes just as we saw a predominance of El Ninas when the PDO was positive. One could argue chicken and egg here as some believe the ENSOs drive the mean PDO state and vice versa. I believe the sun drives the ocean and ENSO and PDO are the mechanisms to achieve compensation for excess or deficit heat build up in the tropical Pacific (a story for another day).




Nonetheless this would imply any El nino would we relatively weak and La Nina returns again soon. This is not of course an official WSI forecast but some early scenario thoughts based on ENSO’s possible states. WSI runs its own proprietary climate models and has statistical models that utilize ENSO, PDO and numerous other parameters and official forecasts are based on a blend of those.