Effect of Solar Activity

Mendoza et al. (1991) analyzed the occurrence of El Niño events in coastal Peru as related to sunspot numbers (see rule 57: xls, doc). They found that, between 1700 and 1985, more El Niños (63%) occurred in sunspot minima or with negative gradients (descending phase) of the solar cycle, than in maxima or in the ascending phase (37%). Our research indicates that during the ascending phase of the solar cycle the odds are shifted in favor of La Niña events (rule 485: xls).

Solar Cycle 24, which officially started in January 2008, has been a subject of much debate due to competing forecasts on whether it will be a highly active or a quiet low cycle. Both camps agree, however, that the sooner the new cycle takes over the waning previous cycle, the more likely it will be a strong one. As of this writing (May 2008), solar activity remains at a very low level, with just a few sunspots, which some believe still belong to the “never ending Cycle 23” (see here). Solar cycle 24 is expected to peak in late 2011 or mid-2012.  If the patterns described in rules 57 and 485 hold true, the probability of La Niña events will remain relatively high and the background SST will be relatively low until about 2014. However, the next El Niña event (and possibly a strong one) is currently brewing (see below). 

Precursors

Recent work suggests that decadal changes in equatorial Pacific SST emanate from the tropical South Pacific (Giese et al., 2002). Subsurface temperature in the latter region leads the equatorial SST by about 7 years. Since the southern tropical Pacific temperature shows a distinct cooling since the early 1990’s, one can expect a similar cooling in the equatorial Pacific SST from the late 1990s onward. A similar situation existed in the early 1940’s when the equatorial Pacific experienced a transition to the generally cold regime of  1942–1976 (Zhang et al., 1997). 

Regime Shifts

Due to strong interannual variability in ENSO, the regime shift analysis of climatic variables in the equatorial Pacific is very difficult. Nevertheless, the regime shift detector was able to identify statistically significant shifts in some of the variables. For example, the March-April Multivariate ENSO Index (MEI) experienced regime shifts in 1977 and 1999, which were statistically significant at p < 0.001 and p < 0.01 respectively (Fig. 1, top). The same shifts were found in the March-April Southern Oscillation Index (SOI, not shown). In addition, the SOI experienced regime shifts in the time series for the prewinter (October-December) season (Fig. 1, bottom). For monthly SOI values, the regime shifts in 1976 and 1998 are detectable even at the lower target significance level of p* = 0.01 (Fig. 2). The actual significance levels for both these shifts p < 0.00000001.

	Fig. 1. Regime shifts in (top) March-April MEI and (bottom) October-December SOI, as obtained using the regime shift detector with the following parameters: Target significance level p* = 0.2, regime length l = 20 years, and Huber parameter h = 1.
	Fig. 2. As in Fig.1, except for monthly SOI values, with p* = 0.01 and l = 250 mos.

The regime shift in the late 1990s was also observed in the North Pacific (a review for that region is coming soon). It may serve as an additional confirmation of the shift in ENSO indices, since some of the decadal variability in the Tropics may have a subtropical or extratropical origin (see Power et al., 2006 for a review).

ENSO Evolution

The 1976/77 regime shift was characterized not only by an increase in the background SST, but also by a change in the ENSO evolution pattern. Before the shift El Niño events tended to develop first along the coast of South America and then spread westward. This is known as the “canonical” ENSO scenario described by Rasmusson and Carpenter (1982). After the 1976/77 shift, ENSO events developed first in the central Pacific and then spread eastward (e.g., Wang 1995). Trenberth and Stepaniak (2001) introduced the Trans-Niño Index (TNI) and used it in conjunction with the more traditional Nino 3.4 index (N3.4) to trace the evolution of ENSO events. They found that the TNI lead N3.4 by 3 to 12 months prior to the climate shift in 1976/77 and also followed N3.4 but with opposite sign 3 to 12 months later. However, after the 1976/77 shift, the signs of the TNI leads and lags were reversed. 

It appears that the evolution of ENSO events is now returning to its canonical scenario. The recent weak to moderate El Niño events of 2002/03, 2004/05, and 2006/07 were all preceded by peaks in TNI (8, 9, and 11 months earlier, respectively). Similarly, the strong La Niña event of 2007/08 was preceded by a minimum in the TNI 8 months earlier (Fig. 3) 

Fig. 3. The Niño 3.4 and Trans-Niño unsmoothed monthly indices, January 1960 - April 2008.

Next El Niño

If the above assumption is true and ENSO evolution indeed switched back to its pre-1977 pattern, then the recent sharp increase in the TNI index (Fig. 3) may be a harbinger of an El Niño event several months later. Another indication of a developing El Niño event is a record high warm water volume in the western Pacific. The higher the heat content, the stronger the subsequent El Niño warming. The occurrences of warm pool heat content maxima precede the maxima in Niño 3 SST anomalies by 12-24 months (rule 11: xls, doc). Within this interval, the El Niño will come probably earlier than later, because of the low level of solar activity. As shown by Enfield et al. (1991), low level of solar activity is supposed to leave the El Niños free to develop at intervals near 2-3 years, the recurrence interval favored by the internal dynamics of the Southern Oscillation system.       

Conclusion

The climate regime that dominated in the equatorial Pacific since the late 1970s appears to have ended in the late 1990s. The shift to a new regime was not as apparent as the previous shift in the late 1970s (which was quite unique), and the current conditions are not necessarily close in every aspect to those during the pre-1977 period. Nevertheless, it is expected that the new regime, which will probably last until about 2014, will be characterized by relatively low background SSTs and more frequent La Niña events. This may also weaken the global warming signal (the global trend analysis is coming soon). The evolution of El Niño events will follow the “canonical” scenario, first developing along the South American coast and then moving westward. The next El Niño (and, possibly a strong one, similar to the 1972/73 event) is expected in 2009, but may come as early as the end of this year.

References

Enfield, D. B., S. Cid, and Luis, 1991: Low-frequency changes in El Nino-Southern Oscillation, J. Climate, 4, 1137-1146.

Giese, B. S., S. C. Urizar, and N. S. Fuckar, 2002: Southern Hemisphere Origins of the 1976 Climate Shift, Geophys. Res. Lett., 29, doi:10.1029/2001GL013268.(1014).

Mendoza, B., R. Perez-Enriquez, and M. Alvarez, 1991: Analysis of solar activity conditions during periods of El Nino events, Ann. Geophysicae, 9, 50-54.

Power, S., M. Haylock, R. Colman, and X. D. Wang, 2006: The predictability of interdecadal changes in ENSO activity and ENSO teleconnections, J. Climate, 19, 4755-4771.

Rasmusson, E. M. and T. N. Carpenter, 1982: Variations in tropical sea surface temperature and surface wind fields associated with the Southern Oscillation/ El Nino, Mon. Wea. Rev., 110, 354-384.

Trenberth, K. E. and D. P. Stepaniak, 2001: Indices of El Niño Evolution, J. Climate, 14, 1697-1701.

Wang, B., 1995: Interdecadal changes in El Niño onset in the last four decades, J. Climate, 8, 267.

Zhang, Y., J. M. Wallace, and D. S. Battisti, 1997: ENSO-like interdecadal variability: 1900-93, J. Climate, 10, 1004-1020.

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