This post first appeared on the climate.gov ENSO blog and was written by Nat Johnson
As the 2020ā21 La NiƱa has come to an end, leaving us with neutral conditions in the tropical Pacific, we now wonder if we have seen the last of La NiƱa for a while or if we will see another dip into La NiƱa conditions by next fall. In the world of the El NiƱo-Southern Oscillation (ENSO), double-dipping is not a party foulāitās actually quite common for La NiƱa to occur in consecutive winters (not El NiƱo, though). If youāre wondering why then this is the blog post for you!
Mirror, mirror on the wall
To understand why La NiƱa commonly double dips, we first need some basic understanding of ENSO asymmetry. Often, we think of El NiƱo and La NiƱa as mirror oppositesāfor example, that the warmer-than-average east-central tropical Pacific conditions during El NiƱo are exactly matched by the cooler-than-average conditions of La NiƱa. This mirror-opposites perspective is a pretty good approximation, but itās not perfect!
Upon closer inspection, we see some subtle but important differences between El NiƱo and La NiƱa in terms of their patterns and behavior. First, the sea surface temperature anomalies (difference from the long-term average) during El NiƱo tend to be centered farther east than for La NiƱa, especially for the stronger El NiƱo episodes. Second, the strongest El NiƱos tend to be stronger than the strongest La NiƱas. We can see these differences in the maps above. The warm anomalies in the eastern Pacific for the El NiƱo composite are between 2.25 and 2.75 degrees Celsius (4 and 5 degrees Fahrenheit), while the cool anomalies in the east-central Pacific for the La NiƱa composite top out at 2.25 degrees C.
We also see notable differences in how sea surface temperatures in the NiƱo 3.4 region of the Pacific change over time for first-year El NiƱos (meaning, the previous winter did not feature El NiƱo) and first-year La NiƱas. After the peak of El NiƱo, which typically occurs in late fall or winter, NiƱo3.4 surface temperatures usually decline rapidly to neutral conditions by the following spring. In the ensuing summer through winter, ENSO neutral conditions continue, or La Nina develops, but the occurrence of El NiƱo in a second consecutive winter is uncommon.
For La NiƱa, in contrast, the return to ENSO neutral after the late fall peak is usually more gradual, and, as Emily recently reminded us, the transition to El NiƱo after the first winter of La NiƱa rarely occurs. Instead, we generally see ENSO-neutral or a transition back into La NiƱa conditions by the following fall, as shown by that second dip in average NiƱo3.4 temperatures in the plot above. To reiterate what Emily already noted, of the 12 first-year La NiƱa events on record, 8 were followed by La NiƱa the next winter, 2 by neutral, and 2 by El NiƱo.
All things being unequal
Things would be simpler if El NiƱo and La NiƱa behaved similarly, so why are the transitions out of El NiƱo and La NiƱa so different? The answers lie in the processes that cause El NiƱo and La NiƱa to strengthen and weaken. Several months ago, Michelle introduced us to the essential processes for ENSO, the fun-to-say Bjerknes feedbacks. Essentially, Bjerknes feedbacks describe the reinforcing interactions (positive feedbacks) between the ocean and atmosphere that cause El NiƱo and La NiƱa to grow: changes in the tropical Pacific ocean temperatures cause changes in the overlying trade winds, which then cause additional, reinforcing changes in the ocean temperature (please see Michelleās post for more of the details!).
If all essential processes that comprise this feedback process were equal but opposite for El NiƱo and La NiƱa, then we might expect mirror opposite patterns. However, this isnāt the case. For example, the response of the trade winds to the NiƱo3.4 surface temperatures is unequal between warmer and cooler surface conditions. We see this above in the scatter plot of monthly MayāSeptember zonal (east-west direction) wind stress anomalies (1) in the equatorial Pacific versus the NiƱo3.4 surface temperature anomalies. As we expect, warmer NiƱo3.4 surface temperatures that are tied to El NiƱo conditions bring weaker trade winds and a weaker Walker circulation (positive wind stress anomalies), whereas cooler NiƱo3.4 conditions bring stronger trade winds (negative wind stress anomalies) connected with a stronger Walker circulation.
However, the plot also reveals that the relationship between the warm (El NiƱo) and cool (La NiƱa) sea surface temperature anomalies and the wind anomalies they generate are not equal. El NiƱoās warm water anomalies generally produce wind stress anomalies that are stronger and farther east than those produced by cool La NiƱa anomalies of equal strength. The stronger response is demonstrated by the steeper slope of the red line versus the blue line. This difference may seem a bit subtle, but it can have big consequences for ENSO asymmetry (2).
The strength of the coupling between the winds and the upper ocean matters because it not only strengthens El NiƱo and La NiƱa, but it also sets the wheels in motion for the ultimate demise of each event. The strong coupling between ocean and atmosphere and the more eastward wind stress anomalies during El NiƱo contributes to a robust poleward discharge of equatorial Pacific upper-ocean heat, typically by early spring, that ends the El NiƱo event and sets the stage for the next La NiƱa. The weaker coupling and more westward wind stress anomalies during La NiƱa mean that the corresponding ārechargeā of heat is also weaker, and so the ocean is not as primed for a transition to El NiƱo. Instead, if we experience the right sequence of tropical weather, the second winter of La NiƱa may return instead (3).
The scenario I have described is far from complete, as there are many other atmospheric and oceanic (and even biological!) processes that likely contribute to ENSO asymmetry and the tendency for La NiƱa to persist for more than one winter (4). There is still considerable debate about which processes are most important for these El NiƱo/La NiƱa differences (still so much unsettled ENSO science!).
A heads up?
Can we predict when La NiƱa will double-dip? Some recent studies suggest that we may be able to predict (in a probabilistic sense) more than a year in advance the likelihood of two-year La NiƱa based on the strength of the preceding El NiƱo and poleward discharge of equatorial heat (5). However, the most recent La NiƱa is a bit unusual because El NiƱo did not immediately precede it, and so it is difficult to identify any clear indicators that La NiƱa will return next fall at this time. Although we must await further guidance to get a better handle on the forecast, we at least can say that both history and the current ENSO forecast suggest that El NiƱo is unlikely to return in the near future.
Special thanks to Dr. Andrew Wittenberg for guidance and helpful comments while preparing this blog post!
Footnotes
(1)Ā Ā Wind stress measures the lateral force per unit area that the wind exerts on the ocean surface. A āpositive zonal wind stress anomalyā corresponds to an anomalous eastward force on the ocean surface, exerted by a stronger-than-average eastward component of the winds; this weakens the normally westward force exerted by the trade winds. Conversely, negative zonal wind stress anomalies indicate a stronger westward force on the surface ocean, exerted by stronger-than-average westward trade winds.
(2)Ā Ā The zonal wind stress analysis of this post is modeled after Choi et al. (2013). That study provides conceptual and theoretical support for the hypothesis that the asymmetry in zonal wind stress response to tropical Pacific sea surface temperatures, like what is shown in the scatter plot of this post, is sufficient for explaining many of the asymmetries between El NiƱo and La NiƱa, including the tendency for La NiƱa to persist for more than one winter (see the next footnote for additional details).
Choi, K.-Y., G. A. Vecchi, and A. T. Wittenberg, 2013:Ā ENSO transition, duration, and amplitude asymmetries: Role of the nonlinear wind stress coupling in a conceptual model. Journal of Climate, 36, 9462-9476.Ā https://doi.org/10.1175/JCLI-D-13-00045.1
(3)Ā Ā For those of you who are interested in more of the scientific details about how this wind stress asymmetry between El NiƱo and La NiƱa may contribute to the tendency for La NiƱa to double-dip, this footnote provides additional, more technical discussion, courtesy of Dr. Andrew Wittenberg.
ENSO events are sparked by a disequilibrium between two coupled ocean/atmosphere time scales in the response to wind changes: a fast equatorial adjustment (oceanic equatorial Kelvin waves + Bjerknes feedbacks) and a slow off-equatorial adjustment (curl-induced oceanic Rossby waves and delayed negative feedbacks).
Equatorial wind anomalies that are located farther east (as during a strong El NiƱo) induce more transient growth, disequilibrium, and overshoot into the opposite phase for 2 key reasons.Ā (1) Their resulting zonal current anomalies & upwelling anomalies are located closer to the warm pool edge in the central Pacific and shallow thermocline of the East Pacific.Ā This strengthens the transient growth via fast equatorial processes (zonal advective feedback and Ekman feedback, two of the important Bjerknes feedbacks), which amps the wind anomalies so much that they can over-discharge the equatorial thermocline several months later.Ā (2) When the off-equatorial wind stress curl anomalies are located farther east, theyāre also farther from the western boundary, and so their resulting off-equatorial oceanic Rossby wave trains have farther/longer to travel before they can reflect back onto the equator and start the turnabout of the ENSO event.Ā This enables the equatorial disequilibrium to grow more strongly before being impeded & eventually reversed by the delayed negative feedback.Ā Thus, the more eastward-shifted zonal wind stress anomalies during strong El NiƱos enable stronger transient growth & disequilibrium, increasing the over-discharging and subsequent overshoot.
In short, under the āzonal wind stress asymmetry hypothesisā of Choi et al. (2013), La NiƱa is more likely to double-dip because its winds stress anomaly is so far from the āsea surface temperature-ticklishā zone of the central/eastern Pacific, and so close to the western boundary ātransfer stationā for the off-equatorial feedbacks, that La NiƱa just canāt get as much of a disequilibrium going ā and hence canāt over-charge the equator enough to guarantee an overshoot into El NiƱo.
(4)Ā Ā For a comprehensive review of existing hypotheses for ENSO asymmetry, I recommend An et al. (2020).
An, S.-I.,Ā E. Tziperman, Y. Okumura, and T.Ā Li,Ā 2020:Ā ENSO irregularity and asymmetry. InĀ A. Santoso,Ā M. McPhadenĀ &Ā W. CaiĀ (Eds.),Ā El NiƱo Southern Oscillation in a changing climateĀ (pp.Ā 153āĀ 172). John Wiley & Sons.
(5)Ā Ā For example, a few recent studies, including two led by Dr. Pedro DiNezio, suggest that the probability of a multi-year La NiƱa may be skillfully predicted 18-24 months in advance, given a preceding El NiƱo. The sources of skill are rooted in the strength of the El NiƱo and the magnitude of poleward heat discharge 6 months after the peak of El NiƱo.
DiNezio,Ā P. N.,Ā C.Ā Deser,Ā Y. M.Ā Okumura, andĀ A.Ā Karspeck,Ā 2017a:Ā Predictability of 2-year La NiƱa events in a coupled general circulation model.Ā Climate Dyn.,Ā 49,Ā 4237ā4261,Ā https://doi.org/10.1007/S00382-017-3575-3.
DiNezio,Ā P. N., C. Deser, A. Karspeck, S. Yeager, Y. Okumura, G. Danabasoglu, N. Rosenbloom, J. Caron,Ā and G. A. Meehl, 2017b:Ā A 2-year forecast for a 60ā80% chance of La NiƱa in 2017ā2018.Ā Geophys. Res. Lett.,Ā 44,Ā 11 624ā11 635,Ā https://doi.org/10.1002/2017GL074904.
Wu, X., Y. K. Okumura, C. Deser, and P. N. DiNezio, 2021: Two-year dynamical predictions of ENSO event duration during 1954ā2015. J. Climate, 34, 4069-4087, https://doi.org/10.1175/JCLI-D-20-0619.1.