El Nino temperature anomalies in the eastern Pacific Ocean.
Jennifer Skene‘s post earlier this week here on the QUEST Community Science Blog about the potential effects of this winter’s La Niña is a great lead-in for discussing this climatic phenomenon in a bit more detail. As Jennifer noted, the La Niña weather pattern is the flip side of El Niño, which is when unusually warm waters of the eastern Pacific Ocean affect weather patterns for large swaths of North and South America. For California, El Niños typically result in increased precipitation in the winter months and La Niñas are characterized by drier conditions.
El Niño is the nickname of the climate pattern called the El Niño-Southern Oscillation, or ENSO. Although we don’t hear much about the Southern Oscillation part, it is the atmospheric component of this linked ocean-atmosphere phenomenon. Although the physics of ENSO is still not fully understood and the subject of current research, the regularity of the pattern is well documented. The image below is a time series plot of ENSO events for the past 60 years. The positive values filled in with red are the warm ENSO phase (El Niño) and the negative values in blue are the cool ENSO phase (La Niña). The regularity isn’t perfectly on beat — it varies from 3-7 years between measurable events. But this is enough regularity to make ENSO one of the more predictable patterns climate scientists have studied.
While the timing of ENSO cycles might have some predictability, the magnitude of ENSO (the height/depth of the peaks) can vary significantly. Some are weak while others are quite strong. The 1997-1998 El Niño is considered to be one of the strongest of the past 100 years and is still in the memory of many Californians because of the intense precipitation and subsequent flooding it unleashed.
What’s really interesting is that this 3-7 year pattern of alternating ENSO phases is just the shortest timescale in a climate phenomenon with multiple rhythms superimposed. Paleoclimate research has revealed that ENSO also beats at timescales of hundreds to thousands of years. The image below is very similar to the above diagram — it has time in years on the horizontal axis and occurrence of ENSO events on the vertical axis. (Important difference to note are that the present is on the left side on this plot instead of the right side and time is in ‘years ago’ and not a date.)
This plot goes back to 10,000 years ago and shows the variability in ENSO at a much longer timescale. Within those taller peaks in the plot are numerous individual El Niños that are grouped together in time. This doesn’t mean that every single El Niño is very strong — just that during a few hundred years there more of those strong El Niños. In addition to the peaks every several hundred years there is also an even longer-term trend of increasing ENSO events over 5,000 to 6,000 years.
Like a complex musical composition with multiple interacting rhythms, the interacting timescales of this climate phenomenon might result in weather patterns that defy our ability to predict confidently. The authors of the study looking at ENSO patterns for the past 10,000 conclude that bigger-scale changes in global climate (due to changes in the Earth’s orbit around the sun) are driving those longer-timescale changes. A big question right now is how modern global climate change will affect ENSO. A warming ocean suggests El Niños will get more intense, but perhaps there are some unanticipated effects from the multiple interacting factors that still needs to be studied. We are improving our understanding of the Earth’s climate systems but, as always, much more work needs to be done.
Images: (1) El Nino anomalies in the eastern Pacific Ocean; image from McPhaeden et al. of NOAA (2) ENSO Index from 1950-2010; image from McPhaeden et al. of NOAA; (3) Figure from Moy et al. (2002); Nature 420