In this study, the long-term changes in the ABF are explored and the main processes driving these changes and their seasonal dependence are investigated. To do so, the OGCM NEMO3.6 model is used in its Tropical Atlantic configuration. In the first step, the model SST trend is compared with the observational data over the 1986-2015 period on both annual and seasonal scales. On the annual scale, the model represents the warming trend in the Angolan sector well.
In Northern Benguela, the model shows a cooling trend while the observations show a warming
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trend. However, the cooling and warming patterns in NEMO outputs are consistent with the trend in GODAS, ORA-S4, and CRCM reported in Figure 2 in the study of Vizy et al. (2018). At the seasonal scale, the NEMO SST trend is consistent with the observed SST trend, despite the cooling trend in the model being stronger than the cooling trend in the observations. This is most likely due to an unrealistically higher than the normal trend in wind forcing that can increase the upwelling and the equatorward horizontal motion of cold water to the north a bit too strongly.
In the second step, the long-term mean of different terms contributing to the temperature rates changes (see Eq 2.1) is evaluated. This analysis contributes to determine the potential important heat budget terms involved in the long-term changes of mixed layer temperature or SST. These important heat budget terms are the net surface heating term, the horizontal advection of heat and the vertical advection of heat. From the climatological point of view, the net surface heating term warms the coastal ocean of Angola and Northern Namibia while the horizontal advection and vertical advection cool the coastal ocean of Angola and Northern Namibia. The lateral diffusion, vertical diffusion and vertical entrainment contribution are extremely weak and do not account much for the upper-ocean heat budget climatology.
Trends analysis of different parameters involved in the most important heat budget term contributing to the long-term SST change is investigated to understand the mechanism behind the warming or cooling trends in Angolan and Northern Namibia coast at seasonal scale. The results suggest that the change along the Angolan and Namibian coasts is explained by more than one mechanism acting at different time scales. For instance, in austral autumn the warming trend observed along the Angolan coast is associated with a positive trend in Qnet (Figure 4.7) and the weakening of vertical flow (Figure 4.13) associated with the upwelling of cooler water to the surface. However, the weakening of vertical flow is not statistically significant. In early austral summer (November-January), the warming trend observed along the Angolan and Namibian coasts is primarily associated with the intensification of the poleward flow (Figure 4.11) which brings more warm water from the tropics into the area of interest. It is also attributed to the weakening of the vertical flow (Figure 4.13), while locally, Qnet trend tends to cool the ocean. This result contradicts the result of Vizy and Cook (2016) who suggested that the austral SST warming trend along the Angolan and Namibian coast is associated with an increase in the net downward
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atmospheric heat flux. Notably, the strong poleward flow in austral summer along the Angolan and Namibian coast is consistent with the result of Koseki et al. (2019) who attributed the poleward flow to the process of frontogenesis of the ABF. Furthermore, the poleward intensification of the Angola current is attributed to the presence and intensification of the Angola Dome. Moreover, the poleward intensification of the Angola Current suggests an intensification and poleward shift of the front as observed by Vizy et al. (2018). I also notice that the warming trend observed along the Angolan and Namibian coast reaches 100m depth in early austral summer, while it is noticeable only in the upper mixed layer for the rest of the year. At the annual scale, the subsurface water shows a cooling trend, as the cooling trend is observed for most of the year along the Angolan and Namibian coasts. This result also contradicts the conclusions of Vizy et al. (2018), who suggested that the warming of the subsurface ocean under the upper mixed layer was due partly to an increase of the Agulhas leakage (Beal et al., 2011;
Lübbecke et al., 2015). The cooling trend that occurred south of the ABF, especially in austral winter and early spring, is primarily associated with the horizontal currents that advect cooler water from the east and south and intensify the upwelling of cold water from the subsurface to the surface. These changes are consistent with the changes in the wind field (not shown), which enhances the offshore ocean currents off the Namibian coast.
In the next chapter, I will study the decadal variability in the Benguela Upwelling system to see whether the 10-14 year and 23–28-year periodicity found in chapter 3 and also mentioned in the literature for South Atlantic variability for the 10-14 year are found using longer duration datasets. I also explore potential links between decadal variability and well-known climate modes or global oceanic region.
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