Page 107 - EER-1-1

Explora: Environment
and Resource Stratification and mixed layer deepening

the depth of the mixed layer (approximately 3 cm) will be equivalent to a deepening of 5 – 10 m/decade, depending
included. This threshold accounts for approximately 8% of on the region. For this analysis, we adopt the median value
the average monthly thermal energy received by the UOS of the range, that is, 7.5 ± 1.3 m/decade, corresponding to
and approximately 10% of the average monthly deepening. 75 ± 13 cm/year. Furthermore, the current thermocline
These values were chosen to reflect the minor role of wind strength evaluation standard follows the Standard of
in mixed layer deepening and to ensure the model aligns Marine Survey, where the intensity threshold is 0.2 °C/m
with the observations of Sallée et al. 2 for shallow water (depths <200 m) and 0.05°C/m for
The remaining 90% of the observed deepening must be deeper waters (depths >200 m). 29
attributed to other mechanisms. It is unlikely that this can For oceanic thermoclines, an intermediate value of
be explained by thermal expansion of water or the influx of 0.1°C/m is assumed. Consequently, for a 75 cm deepening,
freshwater from melting ice, as these processes caused sea the temperature gradient corresponds to 0.075°C. The
level rises of only 3.1 ± 0.7 mm/year during 1993 – 2003. 25 volume of water affected by this change is approximately:
Analogous phenomena observed in large freshwater V = 0.75 × S (I)
systems, such as the Great American Lakes during summer where S (≈ 3.6 × 10 m²) represents the total surface of
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heatwaves, 26-28 suggest that the primary driver of mixed the oceans. Thus, V ≈ 2.7 × 10 m .
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3
layer deepening is the additional thermal energy entering
the layer. The mass of seawater at the relevant temperatures is
approximately 1027 kg/m (as per Copin-Montégut). The
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3
Excess thermal energy in the UOS resulting from global
warming is dissipated through three main mechanisms: thermal energy required to heat this volume V by 0.075°C
is given by:
(i) Heating the mixed layer.
(ii) Evaporation into the atmosphere as latent heat. Q ≈ 1027 × C × V × 0.075 (II)
(iii) Conduction into the thermocline, which does not act where C ≈ 4 × 10 J/K/kg is the specific heat capacity of
3
as a perfectly isolated “lid” from the mixing layer. liquid water. Substituting values yields Q ≈ 1027 × 4 ×10
3
31
19
The thermocline’s upper layer, in direct contact with × 2.7 × 10 × 0.075 ≈ 8.32 × 10 J.
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the mixing layer exposed to excess heat, absorbs part of Over the studied period, the increase in thermal
this energy through conduction. Over time, this layer energy in the UOS is approximately 3.3 × 10 J/decade,
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warms enough to integrate into the mixing layer. This according to Zeltz , or 3.3 × 10 J/year. This means around
21
7
process repeats incrementally, with successive layers of 2.5% of the additional energy contributes to thermocline
the thermocline warming and integrating, always through deepening. Further, calculations address the distribution
conduction. The deepest thermocline layer, while remaining of the remaining 97.5%.
the coldest, develops an accentuated temperature gradient
relative to the deep ocean. The observed thermal expansion due to ocean warming
primarily affects the upper layer, with an increase attributed
This gradient eventually results in the formation of to this phenomenon estimated at 0.89 ± 0.05 mm/year
a new layer at the boundary of the deep ocean and the for the 0 – 700 m layer between 1970 and 2015. 32,33 This
thermocline. This layer absorbs some heat, warms slightly, estimate assumes no trend below 2000 m depth before
detaches from the deep layer, and becomes part of the 1992 and incorporates the modest contribution from the
thermocline. If excess heat continues to penetrate the UOS, 700 – 2000 m layer. According to Desbruyères et al.,
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this process repeats, ultimately leading to the increase in this contribution increased by 10% during 2006 – 2015,
the thickness of the mixed layer and the observed summer consistent with the deepening of the thermocline.
deepening of the thermocline, as reported by Sallée et al. 2
To calculate the corresponding temperature increase
For the remainder of this study, the dissipation of T , we use Equation III:
excess heat from the mixing layer will be analyzed in terms [0–700]
of ratios representing the contributions of these three 700 × d water × T [0–700] = 0.89 × 10 −3 m /year (III)
mechanisms: conduction into the thermocline, heating of where d is the thermal expansion coefficient of water.
water
the mixture layer, and evaporation as latent heat into the Asper Thomas, d ≈ 2.6 × 10 −4 m /°C. Solving for T :
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atmosphere. water [0–700]
T [0–700] = (0.89 × 10 )/(700 × d water ) (IV)
−3
3.4. Estimation of ratios which gives T ≈ 0.00489°C/year. This result is
[0–700]
According to Sallée et al., over the period 1970 – 2018, consistent with the estimate of 0.005°C/year provided
2
the thermocline depth increased by 2.9 ± 0.5%/decade, by Hasselmann. For the 0–200 m mixing layer, the
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Volume 1 Issue 1 (2024) 7 doi: 10.36922/eer.4578