Turbidity directly affects the physical properties of the global ocean through the absorption of solar radiation by the upper layers. In the third and fourth experiments, the depth of penetration depends on the surface chlorophyll concentration, where the third experiment uses remote sensing chlorophyll measurements and the fourth experiment uses chlorophyll produced by the biochemical model. The current thesis focuses on the feedback mechanism between different parameters of penetrating solar radiation on the sea surface circulation and temperature of the equatorial Atlantic and Pacific oceans and the feedback between sea surface temperature and circulation and wind stress.
The equatorial regions of the Indian Ocean were not investigated because the dominant presence of monsoons introduces a seasonal bias in the annual average of the ocean's thermal structure and circulation. Turbidity is essentially an optical property of the ocean that changes the absorption of incoming solar radiation. The exponential decay of I0 expressed by 1.1 is a poor approximation in the upper 5 meters of the ocean, due to the selective absorption of the short and long wavelengths of solar radiation by the water.
The resolution of the configuration is 1o×1o in the horizontal (362×332 grid points) and 75 unevenly spaced vertical levels and for surface heat.
Experiments
It should be noted that biologically produced chlorophyll concentrations from PISCES (hereinafter referred to as PISCES-chlorophyll) in the TYPE I and TYPE III experiments are independent of any observations and depend only on the temperature fields induced by the oceanic component of the ocean circulation. model. PISCES chlorophyll in the RGB experiment is also independent of any chlorophyll observations, although the ocean circulation model in this experiment uses satellite observations to calculate solar radiation penetration depth. The PISCES chlorophyll in the COUPLED experiment is the same as that used by the ocean circulation model to calculate the solar radiation penetration depth using the Lengaigne method (same as in the RGB experiment).
To see if the model has reached steady state, the time series for the global Figures 2.3a,b show that trends in kinetic energy and potential temperature are significantly small (m2/s2/50 years and +0.06oC/100 years respectively) so it is safe to assume that the model has reached steady state.
Results
Impacts of different turbidity parameteriza- tions on biologically produced chlorophyll
- Differences in chlorophyll concentrations between ex- periments
In the equatorial region, there is an overestimation of chlorophyll concentration values in the eastern equatorial Pacific near the coast of Peru and an underestimation in the eastern equatorial Atlantic region near the Gulf of Guinea. Although the equator is the area of interest here, in polar regions where the presence of LIM increases the complexity of tuning with OPA and FISH, the chlorophyll concentration is poorly reproduced. There is a clear pattern in the differences between the two chlorophyll fields in the equatorial regions where there are river outlets (Congo, Senegal, Amazon).
In figure 3.3a, the equatorial Pacific has increased values of chlorophyll concentrations, suggesting that increased turbidity in TYPE III increases the chlorophyll concentration in the Pacific equatorial upwelling region. There is a small tendency for the opposite behavior in the equatorial Atlantic, where the more turbid TYPE III shows reduced chlorophyll concentration compared to the less turbid TYPE I, although this negative difference between them is smaller in magnitude than the equivalent positive one in the Pacific Ocean. Figures 3.3b,c show the same but smaller in magnitude pattern in the differences between the experiments.
Figure 3.3d shows the differences between the RGB and COUPLED experiments, showing slightly positive but smaller differences in magnitude in both the equatorial Pacific and the Atlantic Ocean away from the coast. At the equator, Figure 3.4 shows that ¯ξRGB is larger than ¯ξCOU P LED, which means that in the COUPLED experiment the equatorial Pacific and Atlantic Oceans are cloudier. The increased equatorial turbidity in the COUPLED experiment compared to that of the RGB is also reflected in the time series of sea surface chlorophyll concentration in the 2.5oN - 2.5oS band for both oceans (Figure 3.5a,b).
While in the first 15 common years of the experiments (70–85 years of simulations) there is no clear difference between RGB and COUPLED, in the last 15 years these differences clearly show that this equatorial band COUPLED is cloudier than RGB. Time series of PISCES chlorophyll in the equatorial band show in the Atlantic that TYPE I has more chlorophyll than TYPE III. In the equatorial Pacific, TYPE III has more chlorophyll than TYPE I except for year 97, when both experiments had the same chlorophyll concentration.
Impacts of different turbidity parameteriza- tions on temperature
The effect of turbidity changes on SST is shown by the differences of SST between the experiments (Figure 3.7). Differences between TYPE III and TYPE I show that when turbidity increases, SST values increase everywhere in the ocean except the equatorial upwelling regions (Figure 3.7a). Increasing turbidity by reducing the penetration depth (TYPE III) should generally cause a warming of the ocean surface layers, which should lead to warmer SSTs and thus increased stratification.
A decrease in temperature along the equator with increased turbidity means an increase in equatorial upwelling, which is associated with an increase in tropical cell circulation (Perez and Kessler 2009 and references therein). The comparison between the RGB-COUPLED experiments (Figure 3.7d) shows SST differences of smaller magnitude than the differences between the other three experiments (Figure 3.7a,b,c) for the equator. The fact that PISCES chlorophyll is greatest in TYPE I in the equatorial Atlantic Ocean can be explained by the way temperature or other mechanisms besides nutrients affect chlorophyll production in FISH.
In the equatorial Pacific, TYPE I is again the warmest, but also the one with the lowest chlorophyll concentrations, which in both cases implies a weaker upwelling than TYPE III (Figure 3.9). Even more importantly, Figure 3.10a shows that at depths between 50 and 200m a cooling of the seawater occurs. This is explained because when turbidity increases, the stratification of the upper layers also increases, preventing upwelling.
Right in the equatorial upwelling region, the temperature cooling from the surface to 50.0 meters further shows the increase in upwelling (Figure 3.9). The same vertical temperature distribution is shown in Figures 3.9b,c, albeit at a smaller magnitude than that of Figure 3.9a. The differences between the RGB-COUPLED experiments indicate a weakening of upwelling in the equatorial Pacific (Figure 3.9d).
It should be noted that the cooling of the ocean waters, as shown in Figure 3.11a,b, extends to greater depths than in the case of the Atlantic Ocean.
Effects of turbidity on wind stress and circu- lation
Warmer SSTs will act to intensify the surface wind stress, while colder SSTs will act to weaken the wind stress. Comparing Figures 3.7 and 3.13, negative SST differences coincide with weakening of the magnitude of the equatorial wind stress. In Figure 3.13(a), when comparing the magnitude of the wind stress with the magnitude of the differences, it is shown that the differences are at most ~ 10% of the field's actual values at the equator.
Figure 3.14a shows that along the equatorial Pacific and Atlantic oceans the surface circulation increases, but exactly at the equator it appears to weaken. This means that the reduction in wind stress has a very localized effect on the equatorial surface circulation. In the TYPE III-RGB case, increased turbidity results in a decrease in wind stress with a decrease in SST, but the surface circulation between the two oceans responds differently to the aforementioned change.
Figure 3.14b shows that a decrease in wind stress causes a very localized decrease in the easterly equatorial zonal circulation in the Pacific Ocean, but the small decrease in wind stress in the equatorial Atlantic is not sufficient to cause such an increase cause. When comparing RGB-COUPLED, weaker negative differences in the magnitude of wind stress are shown over both the equatorial Pacific and Atlantic Oceans. But positive differences outside the equator are relatively large compared to the field wind stress values.
At the same time, changes in wind stress in the case of RGB-COUPLED can partially explain the circulation patterns (Figure 3.14d). Figure 3.15a shows the strengthening of poleward surface velocities as well as the strengthening of deeper equatorward returning currents. Similar to the Atlantic Ocean, the equatorial Pacific tropical cells also increased (Figure 3.16).
Changes in meridional overturning circulation
They showed that when the penetration depth of the incoming solar radiation increases, a SST warming occurs on the equatorial regions and that changes in mixed layer due to different parameterizations of penetrative solar radiation from the equator result in a slowdown on the MOC. SST cooling makes the upper layer of the atmosphere more stable, leading to reduced wind stress. The reduced wind stress magnitude should result in a weakening of the surface divergence and upwelling, the results show that reduced wind stress magnitude causes a very localized decrease in the surface circulation precisely on the equator, while the surface circulation causes an equatorward and tropical cells away. increase in both the Atlantic and Pacific oceans.
But the effect of wind stress in subtropical areas has still not been thoroughly studied. In this way, the case of the equatorial Indian Ocean could be studied and a better understanding of how turbidity in general affects the equatorial circulation could be obtained. Finally, the possible feedback from ocean biology to Earth's atmosphere could be studied using coupled ocean-atmosphere simulations.
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