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Explora: Environment
and Resource Stratification and mixed layer deepening

For example, during a semester marked by El Niño, 50 years. The strengthened vertical density gradients limit
stratification is likely to increase, and this trend may persist mixing between upper and deep ocean layers.
into the next semester due to the average 9-month duration Parallel to this phenomenon, Sallée et al. observed a
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of El Niño events. Similarly, during a La Niña phase, consistent deepening of the summer surface layer at a rate
stratification tends to decrease, with a higher probability of 2.9 ± 0.5%/decade, corresponding to 5 – 10 m/decade,
that this trend will continue into subsequent months. Given depending on the region. The winter mixed layer also
the high frequency of such events – covering 58.5% of the appears to be deepening on a global scale, though this
1950 – 2023 period – this phenomenon provides a sufficient finding is based on shorter time series and carries greater
explanation for the pronounced Markov 1 lengthening uncertainty. Sallée et al. attribute this deepening primarily
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behavior observed in the biannual stratification patterns of to the mechanical action of increasing ocean winds.
the UOS and the 0–2000 m ocean layer.
Young et al. estimated a global wind speed increase
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3.3. Mechanisms linked to ocean stratification and of approximately 0.25%/year during the period 1991 –
deepening of the mixed layer 2008, while Zheng et al. reported a significant rate of
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The biannual alternation in ocean stratification, closely 3.35 cm/s/year for the period 1988 – 2011. However,
linked to El Niño-Southern Oscillation (ENSO) events, reservations regarding the reliability of these data have
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explains a significant portion of the observed signals been raised by Zieger et al. and Wohland et al. Even
presented in Table 1. However, ENSO alone does not assuming their accuracy, such wind speed increases appear
account for the observed global increase in ocean insufficient to account for the observed deepening rates of
stratification, which has risen by approximately 1%/decade 5 – 10 m/decade.
since the 1960s. For example, the correlation coefficient Given that the average wind speed at the ocean surface
1,2
between the annual ONI index of ENSO and the annual is approximately 7 m/s, an increase of 3.35 cm/s/year
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averages of N² stratification anomalies from Zeltz is 0.13, corresponds to a cumulative increase of approximately
1
a value too low to indicate a significant correlation. 5% over a decade. This increase is inadequate to drive
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Conversely, a strong correlation (coefficient = 0.84) sustained additional mixing at the bottom of the mixed
exists between semiannual stratification changes and layer sufficient to cause the pycnocline to descend by 5 –
thermal energy in the UOS for the period 1955–2023, 10 m/decade.
using data from Zeltz and NOAA (downloadable from the Surface winds generate ocean currents at approximately
1
link https://www.climate.gov/media/13603). This finding 2% of their speed, meaning a decadal increase of 3.35 cm/s
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suggests that additional heat from global warming is the in wind speed results in a surface current increase of
primary driver of increased stratification. The underlying less than 1 mm/s. This current increment diminishes
mechanisms are elaborated below. as it propagates to lower layers, ultimately reaching the
Water stratification arises from differences in water pycnocline. In addition, the Coriolis effect introduces a
density, which are directly linked to temperature gradients. spiral flow (Ekman spiral phenomenon), which further
Significant warming of the UOS – caused by atmospheric dissipates the energy transferred to deeper layers. 19
heat and solar radiation – disrupts the balance of densities, In specific cases, such as in the tropical Atlantic basin, a
enhancing stratification. As the heat gradient intensifies, linear relationship is often observed between surface wind
so does the density gradient, leading to increased speed and mixed layer deepening. 20,21 Episodes of intense
stratification. winds, such as tropical cyclones, are increasing in frequency
Without ENSO, stratification signals would closely and intensity due to climate change. These extreme events
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mirror ocean heat data, exhibiting Markov-1 alternating significantly influence mixed layer deepening, especially
behavior, as stratification strongly depends on thermal on large spatial scales. However, under typical conditions,
energy. However, ENSO’s long cycles and extensive the increase in wind-driven mixing near the thermocline
influence modify these patterns, lengthening alternation is minimal. Extreme storm events are the exception, with
periods. In summary, ENSO lengthens alternation periods, their wind power capable of deepening the mixed layer
while additional heat in the UOS increases stratification. enough to contribute to global averages. Furthermore,
these events are correlated with increases in sea surface
The increase in stratification directly impacts the 23,24
thermocline, which increasingly acts as a barrier to heat temperature and thermal energy in the UOS.
transfer into the deep ocean. According to Sallée et al., the To model mixed layer deepening (Section 3.8), we
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density contrast at the base of the UOS during summer propose a thermal energy threshold of 2 × 10 J entering
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has increased at a rate of 8.9 ± 2.7%/decade over the past the UOS. Beyond this threshold, wind contributions to

Volume 1 Issue 1 (2024) 6 doi: 10.36922/eer.4578