During periods where the La Nina/Neutral mode of the ENSO dominates, the trade winds blow strongly from east to west across the equatorial Pacific Ocean. The presence of these strong winds results in a corresponding strengthening of the Walker circulation cell. Note that the trade winds are generally stronger during the La Nina phase compared to the Neutral phase.
During periods where the El Nino mode of the ENSO dominates, the east to west flow of the trade winds across the equatorial Pacific Ocean substantially weaken. This results in a corresponding weakening of the Walker circulation cell.
Let's consider the possibility that the natural forcing factors that produce a La Nina/Neutral mode in the ENSO are not the same as those that produce an El Nino mode. If this is true, then the main interaction between these two phenomena would just be that the presence of one would (by necessity) preclude the presence of the other.
Hence, a more realistic investigation of the factors driving the La Nina/Neutral phenomenon could be carried out if the La Nina/Neutral component of an ENSO index time series could be isolated from its EL Nino component. One such index is the monthly Nino3.4 SST anomaly which exceeds 0.8 C when the ENSO in the equatorial Pacific Ocean is in an El Nino state.
Clearly, there is no easy way to fully isolate the ENSO La Nina/Neutral state, however, it could be crudely done by simply setting all the monthly
Nino3.4 SST anomalies above 0.8 C to zero. This would have the gross effect of partially subduing spectral component associated with the El Nino state. In the following, this time series will be referred to as the monthly
Nina3.4 SST anomaly.
The top plot in figure 1 (below) shows the monthly Nina3.4 SST anomaly from 1950 to 2017, with all SST anomalies > 0.8 C set to 0.0 C (black curve). The x-axis shows the number of months since the start of 1950.
Superimposed on the top plot in figure 1 is the 11-year component (red curve) obtained from Singular Spectral Analysis (SSA) of the monthly Nina3.4 SST anomaly time series (see figure 3 below for the SSA plot). A comparison between the red and black curves shows that, except for a brief period around 1955 (i.e. ~ 65 months), the 11-year SSA component seems to match the smoothed long-term variations of the monthly Nina3.4 SST anomaly data.
The bottom plot in figure 1 shows the monthly sunspot number (SSN) between 1950 and 2017, also plotted against the number of months since 1950. As you can see, there is a good phase match between the 11-year SSA spectral component and the 11-year cycle in the SSN.
Note that the data in figure 1 implies that there is a weakening (or slow down) of the Walker Circulation at times near solar maximum. This is in agreement with a recently published paper:
Slowdown of the Walker circulation at solar cycle maximum
Stergios Misios, Lesley J. Gray, Mads F. Knudsen, Christoffer Karoff, Hauke Schmidt, and Joanna D. Haigh
The authors of this paper provide robust evidence that the solar (sunspot) cycle affects decadal variability in the tropical Pacific. Using the analysis of independent observations, they demonstrate a slowdown of the Pacific Walker Circulation (PWC) at solar cycle maximum.
Figure 1
Confirmation of the presence of a periodic 11-year cycle in the monthly Nina3.4 SST anomalies time series is shown in figure2. This plot is a Fast Fourier Transform (FFT) of the time series. It shows that there are four periods in the monthly Nina3.4 SST anomalies that have a significance greater than 95 % (assuming AR1 noise) [Note: These four periods are
NOT found in the El Nino component of the monthly Nino3.4 SST anomaly series]:
a) the 1.0-year period associated with the seasonal cycle
b) the 1.125-year period associated with the 1.127-year Full Moon Cycle (FMC)
c) the 0.950-year period associated with the 0.949-year (lunar) Draconic year cycle
d) the 11.24-year period tentatively associated with the 11.2-year Schwabe Cycle in the SSN.
The presence of spectral peaks at periods associated with the FMC and the (lunar) Draconic year indicates that there must a lunar tidal influence upon the timing of the La Nina/Neutral component of the ENSO phenomenon.
Figure 2
Figure 3 shows the SSA of the monthly Nina3.4 SST anomalies presence of:
a) the 1.0-year period associated with the seasonal cycle
b) the 11.127-year period associated with the (lunar tidal) FMC
c) the 11.9-year period tentatively associated with the 11.2-year Schwabe Cycle in the SSN.
d) the 3.62-year period tentatively associated with the 3.73-year 1/3rd Schwabe Cycle.
Note that 3.62-year peak is most likely the merged peak of the 1/3rd Schwabe SSN cycle (at 3.747 years) with a peak at three times the Chandler Wobble (3.555 years = 3 x 1.1850 years) such that:
3.651 years = (3.555 + 3.747 years)/2
Figure 3
