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Explora: Environment
and Resource Statistical analysis of climate time series

yearly data? significant and cold current equivalent to the flow of about
• Question 4: Why do the monthly evolutions of the two 150 Amazon Rivers, has its core along the thermocline
layers oscillate between a Markov-0 binomial type and and generally moves eastward due to the stress exerted
a faintly Markov-1 lengthening type? by westward-flowing trade winds. This creates a stratified
The answer to Question 1 is provided in detail in shear flow, giving the main thermocline its characteristic
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section 3.1 of Zeltz, reproduced in full in Appendix File . slope. However, when the trade winds weaken, the slope
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In summary: of the main thermocline is modified, causing the EUC to
change direction. This, in turn, modifies the stratification
As indicated by Sallée et al., a seasonal summer-winter of the upper 0 – 2000 m layer, which contains the
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alternation exists, particularly in temperate and cold zones, thermocline located below the UOS. Given the scale of the
where a seasonal thermocline forms in summer (reinforced phenomenon, its impact is readily observed in the global
by the ice melting and weakens in winter ). Likewise, average stratification of the layer. Moreover, this effect
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the Intertropical Convergence Zone (ITCZ) contributes manifests more quickly than temperature changes induced
to a biannual alternation. If only these two were at play, by El Niño-related water movement, which take longer to
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a biannual Markov-1 alternating signal would emerge. develop due to the thermal inertia involved.
However, as demonstrated by Zeltz, particularly through Hence, shortly after the establishment of this new
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the index studied by Trenberth the dominant influence regimen, changes in stratification are felt in the 0 –
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of ENSO with its El Niño and La Niña phenomena, which 2000 m layer, becoming clearly visible during the current
explains that the Markov 1 lengthening type signal.
quarter and likely persisting into the following quarter.
For Question 2, we propose the following explanation: This explains the strongly “lengthening” character
Unlike the semesters, over an entire year, the impacts of the quarterly stratification signal for this layer.
of the ITCZ and the stratification caused by summer heat However, as the El Niño event reaches its full strength,
in the two hemispheres are balanced and neutralized. As stratification changes in this layer gradually return to
for the “lengthening” impact, the average duration of the standard behavior, causing the signal to quickly lose its
events involved (9 months for El Niño and La Niña, and “lengthening” characteristic at the half-year level for the
6 months for a neutral period), is too short to significantly upper 0 – 2000 m layer. Conversely, the UOS layer begins
increase the probability of repetition in the following year. to be progressively affected by temperature changes that
Therefore, the annual fluctuations in the stratification of modify its stratification, eventually displaying a more
the UOS and the 0 – 2000 m layer exhibit a Markov-0 pronounced “lengthening” characteristic at the half-year
binomial-type behavior. level. This described behavior of the EUC is certainly not
a unique case; similar phenomena, such as those observed
For Question 3, we propose the following explanation: in the Weddel Sea under the influence of ENSO, further

For the upper 0 – 2000 m layer, the quarterly corroborate this pattern. 45
stratification data strongly exhibit a Markov-1 lengthening For La Niña, unlike El Niño, deep convection is
type behavior, whereas this is not as pronounced for the reinforced in the west of the basin while the trade winds
UOS layer. To clarify, this does not mean that stratification gain intensity. Therefore, the inclination of the thermocline
increases more strongly on a quarterly basis in the further increases compared to normal conditions, which
0 – 2000 m layer; rather, it means that if stratification strengthens the EUC in the eastward direction. This results
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increases (or decreases) in one quarter, the probability that in cold temperature anomalies in the eastern Pacific and
it will continue to increase (or decrease) in the next quarter warm surface waters near the Asian coasts. These changes
is higher than vice versa. occur with a similar lag as in El Niño events, where the purely
The explanation for this pattern is still largely linked to dynamic and mechanical modifications to stratification at
the ENSO phenomenon and this is how: the thermocline appear relatively quickly, while those of
thermal origin take longer to manifest in the UOS.
At the core of El Niño and La Niña events, there is a
disruption of the trade winds compared to their ordinary And finally here is our answer to the Question 4:
regime. During an El Niño event, these winds weaken Whether dynamic or thermal in nature, the effects on
significantly due to an abnormally weak anticyclone, a stratification of events such as El Niño or La Niña may not
phenomenon linked to the Walker convective loop but not be sufficiently established to appear clearly in stratification
yet fully understood. 39-42 This disruption has repercussions signals during the 1 month following their onset in the
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on the circulation of the equatorial underlying current Pacific zone. Furthermore, over the 100 consecutive
(EUC) which usually flows in the same area. The EUC, a months considered, there are long sub-periods without
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Volume 2 Issue 1 (2025) 10 doi: 10.36922/eer.6109