Exciting ENSO News: More Evidence of a Solar Trigger
White, W.B. and Liu, Z. 2008. Non-linear alignment of El Niño to the 11-yr solar cycle. Geophysical Research Letters 35: L19607, doi:10.1029/2008GL034831.
The El Niño (warm East Pacific tropical water temperatures) / La Niña (cold East Pacific tropical water temperatures) phenomenon is known to occur in a quasi-cyclic fashion repeating every 2-7 years. It is the leading reason for the global variation in temperatures and precipitation on an interannual time scale. Conceptual models and general circulation models (GCMs) have been used to hypothesize that El Niño may arise as a result of internal non-linear processes.
A paper by White and Liu (2008) finds that El Niño / La Niña pairs may be "phase locked" to the quasi-decadal oscillation (QDO), which is linked to the 11-year solar cycle. Phase locking means that two different harmonics vary in the same way with respect to each other. The simplest example of this is pure "constructive" or "destructive" interference. Specifically, White and Liu performed harmonic analysis as a diagnostic on a time series of Pacific region sea surface temperatures (SSTs) from 1895-2005. They also gathered 110 years of data from a multi-century run of a coupled atmosphere-ocean GCM corresponding to the observations. And what did they find?
The authors report the existence of an 11-year QDO cycle in the observed record as well as strong peaks in the years associated with El Niño, especially at 3.6 and 2.2 years. When the authors ran the GCM, however, without the 11-year solar forcing, the computer model could not reproduce the QDO in its SST record. But, when the GCM included this forcing, the model not only reproduced the QDO, but also the strong peaks in the 3.6 and 2.2 year period similar to observations.
They also found a "phase locking" of the 11-year cycle with the 3.6 and 2.2 year cycles in both the model and observations. This means that the higher frequency oscillations had higher amplitudes in step with the lower frequency one and are "hitting" a maximum (minimum) roughly in correspondence with the low frequency cycle.
White and Liu go further, however, and take the nine 11 year cycles found in each record and "composite" these (Fig. 1). In both the model and observations, similar behavior is observed. When the 11, 3.6 and 2.2 (ridge - warm - El Niño / trough - cold - La Niña) year cycles are added together, and superimposed on the 11-year cycle as a visual aide it is apparent that El Niño / La Niña couplets occurred together on the ascending and descending side of the QDO, but that a strong El Niño (La Niña) can also occur at the peak (in the valley) of the QDO.
Figure 1: Adapted from Fig. 1 White and Liu (2008). The nine member composites of each 11-year QDO cycle for the observations (obs) and the GCM (FOAM): a) is the QDO (thin) and 2.2 year El Niño (thick), b) is the same as a) except for the 3.6 year cycle, and c) is the sum of the three cycles (thick) shown against the 11 year cycle (thin). For d) and e) the figure shows the sum of the 2.2 and 3.6 year cycle against the entire record and the 11 year cycle for the d) model, and e) observations.
Finally, the authors used the previously derived QDO model of Jin (1997) and incorporated their findings here. They report a similar pattern to that shown in Fig. 1. They also used the 3.6 and 2.2 year harmonics to compare to the observed record with the 11 year cycle filtered out. They found that this combination identifies reliably 26 of 32 El Niño events from 1895-2005.
In conclusion, the authors provide evidence that the 11 year solar cycle may be the trigger for El Niño/La Niña events. By using harmonic analysis on observed and model data, they found similar El Niño-related behavior in each, meaning that "the solar forced QDO forces the ~3.6 year ENSO signal; which in turn forces the ~2.2 year ENSO signal, and so on." There are two important results to take away from this work; a) that models which include solar forcing have become more proficient at capturing interannual variability, and b) that El Niño and La Niña onsets may be somewhat predictable even 10 years in advance. Such developments would be a boon for long-range forecasting, and thus provide society with the ability to make longer range plans.
Jin, F.F. 1997. An equatorial ocean recharge paradigm for ENSO. Part I: Conceptual Model. Journal of Atmospheric Science 54: 811-829.