The almost concomitance of the advection of warm water in the central-eastern basin of the Pacific Ocean and the onset of ENSO events highlights a positive feedback between baroclinic ocean waves and atmosphere during these events. Warming of the upper part of the ocean promotes deep convection[i] in the atmosphere, thus weakening the Walker circulation. Conversely, attenuation of winds promotes convection processes in the atmosphere and, as a corollary, the process of convection in the mixed layer of the ocean due to the release of latent heat[ii] that cools the surface.
The end of the El Niño event leads to La Niña as soon as the westward phase propagation of the quadrennial quasi-stationary wave (QSW) occurs, which strengthens upwelling off the South American coast. The cold ascending column reduces evaporation process, which causes an increase in atmospheric pressure and the resumption of trade winds, due to the strengthening of the Walker circulation and expansion of the Hadley cell[iii]. In the central Pacific the Rossby wave reinforces mixing of shallow water, which reduces the sea surface temperature and also reduces the evaporation process.
Thus the uprising of the thermocline during El Niño events, which accompanies the recession to the west of the quadrennial QSW, leads to the depletion of warm surface waters and La Niña takes over. In turn, easterlies favor the westward spread of the quadrennial QSW. Strengthening winds during La Niña reduces cloud cover in the tropical Pacific, which allows shortwave (visible sunlight) to warm the ocean. Thus, La Niña helps recharge the warm water pool in the Western Pacific. It is this mass of warm water, which is under a thermal barrier formed by a water layer of low density (less salty), which will arouse the following El Niño episode, transport to the central-eastern Pacific being performed by a Kelvin wave resulting from the coupling of the annual and quadrennial basin modes. The quadrennial sub-harmonic appears as self-sustained because forcing associated with El Niño and La Niña occurs during a critical phase of its evolution, which explains the high variability of its period in the absence of a periodic forcing external to El Niño (Pinault, 2015).
To be convinced of the leading role of the quadrennial QSW in the genesis and in the evolution of ENSO, just characterize the events based on the date they are mature. Indeed, both the upsurge and the decay phases of ENSO are closely related to their time lag that is represented relative to the central value of successive intervals of 4 years length, e.g. 01/1868 to 01/1872, 01/1872 to 01/1876…, 01/1996 to 01/2000…
Two signals are widely used to characterize ENSO, the Southern Oscillation Index (SOI) and the Sea Surface Temperature (SST) anomaly between 5°N-5°S in latitude and 170°W-120°W in longitude (Nino 3.4 index) that has been proposed by Trenberth, 1997. To determine accurately the time of occurrence of ENSO, from which is deduced the lag, the signal has to be filtered with a low-pass filter to remove the noise and to prevent having too frequent events.
The SST anomaly in the central-eastern Pacific results from both the strip closest to the equator in the northern hemisphere of the annual QSW and the central-eastern antinode of the quadrennial QSW. So the SST anomaly primarily results from the annual QSW. When an El Niño event is occurring the SST anomaly reflecting the eastward phase propagation of the quadrennial QSW follows the anomaly related to the annual QSW. Indeed, supposing the geostrophic forces of the tropical basin permit, a Kelvin wave is formed in the most western part of the basin as soon as the modulated current of the North Equatorial Counter Current (NECC), which is a node of both the annual and the quadrennial QSWs, accelerates while approaching the equator. The fast eastward propagation of the Kelvin wave causes the SST anomaly related to the quadrennial QSW follows the anomaly related to the annual QSW, which produces a single unresolved peak. Thus, the peaks characterizing such events are recognizable by their width, but filtering of the signal has no sense.
Characterization of ENSO events according to their lag determined from the date of occurrence of the minima of SOI filtered in the band 1.5/15 yrs (Pinault, 2016)
By choosing a cutoff frequency of SOI in order to select the 38 most significant events observed between 1866 and 2016, the distribution of the events versus their lag is represented in Table 1. They are very unevenly distributed. But the highest frequencies correspond to the lags -1.5,-1 yr, -0.5, 0 yr, 0.5, 1 yr and 1.5, 2 yr, which suggests a strong interaction between the annual and quadrennial QSWs. Thus 34 % of the events are in phase with the annual QSW (they occur between July and December when the eastward velocity of the modulated current at the southern node of the annual QSW reaches its maximum) while being unlagged. 34 % of the events are in phase with the annual QSW while being weakly lagged, 11% are in phase with the annual QSW while being strongly lagged and 21% are out of phase with the annual QSW. Such events are necessarily lagged since no event occurs in the interval 0, 0.5 yr.
The dynamics of the quadrennial QSW according to the lag and to its phase with the annual QSW is illustrated in the following three videos. The first represents an unlagged event, the second a weakly lagged event in phase with the annual QSW, the third an event out of phase with the annual QSW.
Predictability of ENSO, a phenomenon of amazing simplicity? (Pinault, 2017)
This challenge, which may appear to be provocative, is based on an observation devoid of any ambiguity. Just compare indeed the mean subsurface water temperature measured between 100 and 200 m in depth at 2 ° N 137 ° E to the Southern Oscillation Index (SOI). Excluding the two periods marked by the resumption of La Niña following the El Niño events mature in 11/1997 and 09/2015, the SOI’s signature is reflected very faithfully and in every detail by the subsurface water temperature in the westernmost part of the equatorial Pacific.
These observations suggest that the discharge of the warm water pool of the tropical Pacific in its western part and the transfer of warm water to the east induce a straightforward ocean-atmosphere interaction. In other words, given the linearity of the interaction, the warm water is entirely involved in the evaporative processes controlling the Walker circulation, whatever the intensity and the time of occurrence of the resulting ENSO. The only deviation from this causal relationship of incredible simplicity appears when upwelling in the eastern Pacific is strongly stimulated during the westward phase propagation of the quadrennial Quasi-Stationary Wave (QSW) so that the cold water that extends on the surface generates non-linear processes leading to the amplification of easterlies during La Niña recovery.
The quadrennial QSW shares the tropical Pacific into western and eastern antinodes, both of them being nearly in opposite phase. The western antinodes result from off-equatorial Rossby waves. They have a warm water storage function. The subsurface water temperature at 2 ° N 137 ° E reflects the discharge of this reservoir, i.e. the gradual replacement of warm water with cold water as warm water is transferred to the east. By cons, the subsurface water temperature at the central-eastern antinode, which produces ENSO, reflects its recharge.
At the longitude 180°W, i.e. at the most westerly tip of the equatorial central-eastern antinode, the subsurface water temperature anomalies precede by 8-7 months the maturation phase of ENSO. That is the limit beyond which it is theoretically impossible to predict ENSO from direct observations because the central-eastern antinode vanishes west of 180°W. But along the equator the southernmost antinode, situated in the northern hemisphere, of the annual QSW is still perceptible. Resonantly forced by easterlies, this QSW has a large amplitude in the northern hemisphere, exhibiting two antinodes nearly parallel to the equator, around 8-10 ° N and 0-4 ° N. By cons, the two antinodes located in the southern hemisphere display a weaker amplitude, with the exception of their western part. At 2 ° S 180 ° W the measurement of subsurface water temperature exclusively reflects the quadrennial QSW that, as for it, is symmetrical with respect to the equator.
The correlation between the minimum SOI and the monthly subsurface water temperature at 2 ° S 180 ° W averaged between 125 and 200 m in depth, from 8 to 7 months before the minimum SOI, founds the coefficient of determination is 0.96, which reflects both the strong coherency between that temperature and the minimum SOI, and the accuracy of measurements when the SOI is filtered within the band 1.5/15 years in order to highlight the amplitude and the location of the successive minima that characterize the ENSO events. The correlation, which is optimal when the subsurface water temperature is measured between 8 and 7 months before the ENSO is mature, strongly depends on the offset. So, a significant correlation is obtained when the temperature reaches its maximum, which occurs during the eastward propagation phase of the quadrennial QSW. Since then, the maturation phase of the ENSO event will occur 8 to 7 months later while the temperature gradually decreases.
The subsurface water temperature measured in the central-eastern antinode of the quadrennial QSW 2 ° S 180 ° W allows getting out of the interference that results from the annual QSW for predicting the date of occurrence as well as the amplitude of ENSO events 8-7 months in advance. This apparent simplicity reflects the ocean-atmosphere interactions in the central-eastern Pacific induce a strong coherence between the subsurface water temperatures and the Walker circulation, when they are considered globally. By not taking into account the SST explicitly (Trenberth, 1997), the prediction of ENSO is carried out without involving the lag of the events, which ensures the generality of the method.
Pinault J.L. (2015) Long Wave Resonance in Tropical Oceans and Implications on Climate: the Pacific Ocean, Pure and Applied Geophysics, DOI: 10.1007/s00024-015-1212-9
Pinault, J.-L. (2016) Anticipation of ENSO: what teach us the resonantly forced baroclinic waves, Geophysical & Astrophysical Fluid Dynamics,
Pinault J.L. (2017) El Niño, a phenomenon of amazing simplicity?, Geophysical & Astrophysical Fluid Dynamics, submitted
Trenberth, K., 1997: The definition of El Nino. Bull. Amer. Meteorological. Soc., 78, 2771-2777.
[i] Deep atmospheric convection mainly occurs in tropical regions where it forms the ascending column of the Hadley circulation. It results from a strong coupling between the surface and the upper atmosphere.
[ii] The latent heat is heat-exchanged in the change of state of the seawater during the process of vaporization.
[iii] Hadley Cell
At the equator, the heat provided by the sun warms the air near the surface. This air rises into the atmosphere and this upward movement leads to the creation of an area of low pressure. Air is sucked into this low pressure area, resulting in the trade winds in the two hemispheres.
[i] SOI (Southern Oscillation Index)
The SOI is the amplitude of the Southern Oscillation; it is a measure of the monthly change in the normalized atmospheric pressure difference at sea level between Tahiti and Darwin (Australia).