Changes in meridional overturning circulation

MOC is defined as:

ψ(y, z) =− Z z

H

Z xw

xe

v(x, y, z)dxdz

The integral of this equation shows the strength of a large scale meridional mass transport. In Figures 3.17 and 3.18 the relative differences between the four ex- periments are presented. Figure 3.17a shows that when thurbidity increases, the poleward mass transport near the surface in the equatorial region increases signifi- cantly. The same pattern is present in the differences of the other three experiments.

This indicates the increase of the tropical cells in the Atlantic ocean, as discussed in the previous section. Similarly to the Atlantic ocean, increased poleward mass transport is observed in the equatorial Pacific ocean (Figure 3.18). The magnitude of the differences in the equatorial Pacific is significantly larger compared to the mag- nitude of differences in the equatorial Atlantic. Although, in the RGB-COUPLED differences (Figure 3.18d), the magnitude of these differences is quite small.

The strengthening of MOC on the Pacific ocean comes in agreement with the results of a previous study by Sweeney et al. (2005) who studied the effects of two different surface chlorophyll concentration dependent parameterizations of solar

irradiance. They showed that when the penetration depth of the incoming solar ra- diation increases, an SST warming occurs on the equatorial regions and that changes in mixed layer due to different parameterizations of penetrative solar radiation off the equator result in a slowdown on the MOC.

Figure 3.17: Meridional overturning circulation differences between the experiments close to the equator in the atlantic ocean.

Figure 3.18: Meridional overturning circulation differences between the experiments close to the equator in the pacific ocean.

Chapter 4

Summary and conclusions

In the present thesis, the possible feedback mechanisms between different penetra- tive solar parameterizations and the equatorial Atlantic and Pacific circulation and sea surface temperature and the feedback between the circulation and sea surface temperature and circulation and the wind stress are examined. To this end, four experiments were performed and the differences between them are examined. Their differences indicate that when turbidity increases the upper layers are more stabi- lized preventing upwelling everywhere except the equatorial regions. In the equa- torial Pacific and Atlantic the tropical cells appear enhanced, leading to weakened stratification and SST cooling exactly on the equator. The SST cooling has a direct impact of the wind stress. SST cooling makes the above layer of the atmosphere more stable, leading to decreased wind stress magnitude. The decreased wind stress magnitude should result in a weakening of the surface divergence and upwelling, the results show that decreased wind stress magnitude causes a very localized decrease of the surface circulation exactly on the equator, while the surface circulation off the equator and the tropical cells increase in both the Atlantic and the Pacific oceans.

This implies that differences on the local wind stress magnitude do not drive the increase in the upwelling and in the SST cooling on the equator.

A future work could focus on the feedback that increased off-equatorial wind stress could have on the enhanced equatorial circulation. It is well known that the meridional circulation carries the off-equatorial changes of the thermocline temper- ature to tropical the mixed layer(Schneider and Zhu, 1997; Lin et al., 2008; Kara et al.,2003). Sweeney et al. (2005) showed that changes on the off-equatorial mixed layer depth directly affect the equatorial circulation. But still the effect of the wind stress in the subtropical regions is not yet thoroughly studied.

Also, it would be very interesting to repeat the same experiments and examine their differences seasonally. In this way, the case of the equatorial Indian ocean could be studied and provide a better insight on how turbidity generally affects the equa- torial circulation. A very interesting work could be conducting experiments vertical profile of chlorophyll. This could give an insight on how chlorophyll concentrations in deeper layers affect the ocean dynamics and thermodynamics. Finally, the possi- ble feedback of the ocean biology to the earth’s atmosphere could be studied using coupled ocean-atmosphere simulations. Timmermann and Jin (2002) suggest that increased upwelling equatorial upwelling affect the temperature of the air masses just above the Pacific ocean leading to possible effects on the El Nino-Souther Equatorial Oscillation (ENSO).

Appendix A NOOA SST

Figure A.1: Sea surface temperature from NOOA (https://www.nodc.noaa.gov/cgi- bin/OC5/woa13fv2/woa13fv2.pl)

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