Before quantifying the contribution of net surface heat flux (Qnet) and the mixed layer depths changes to SST trends at the seasonal scale, I first present in Figure 4.6 the maps of the seasonal SST trends shown in Figure 4.3. Warming trends up to 0.8°C per decade occur along the Angola-Namibia coast as well as in the open ocean in early austral summer from November to December. These warming trends are not statistically significant south of 21oS. Warming trends also occur in late austral summer, from February to April, in the Angolan sector and offshore of Namibia while negative trends occur along the Namibian coast. The cooling or warming trends along the Angolan and Namibian coasts in late austral summer are not statistically significant. In austral winter, from May to August, cooling trends up to -0.4°C per decade, are observed in most parts of the ocean along the Angolan and Namibian coasts as well as in the open ocean. However, weak warming trends of about 0.2°C occur along the Angolan coast in May and August. In early
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austral spring, cooling occurs in the open ocean and along the Namibia coast while warming trends up to 0.5°C per decade are found in the open ocean and along the Angolan coast. The cooling trends in the Namibian sector are only statistically significant along the coast.
Figure 4.6: Monthly decadal trends of SST (oC per decade) from NEMO over the period 1986- 2015. Dashed contour lines denote statistically significant values at the 95% confidence level using Student’s test based on linear regression.
Secondly, I quantify the contribution of the Qnet to the SST trend. To do so, I estimate the month to month SST change (dSST) derived from Qnet (see section 2.2.4 in Chapter 2 for details) and then I analyse the trends of dSST (Figure 4.7). Note that the spatial patterns of the seasonal
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Qnet trends match very well with the spatial patterns of the seasonal dSST trends. This suggests that the trends in dSST are due to change in Qnet and MLD does not play a role in dSST trend.
The mixed layer depth trends are then investigated at the end of this section.
Figure 4.7: Monthly decadal trends of NEMO dSST (oC per decade) estimated from Qnet (see section 2.2.4). Dashed contour lines denote statistically significant values at the 95% confidence level using Student’s test based on linear regression.
The trend analysis of dSST, reveals that in the Angolan sector (north of 15oS), dSST shows a positive trend, up to 0.8oC per decade in late austral summer to early Autumn (February to April) and late austral winter to early spring (July to October) meaning that positive trends in Qnet
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contribute to the positive trends in SST. The biggest contributions to positive trends in Qnet occur from July to August, when the SST shows cooling in July or weaker warming trends in August (Figure 4.6). Positive trends in dSST are also observed in the Angolan open ocean in early austral summer (November, December, January). However, along the Angolan coast north of 15oS, negative trends in dSST, up to -0.5oC per decade, occur in the same period. This suggests that in austral summer, Qnet has decreased over the 1986-2015 period which would cool down the SST.
However, as the SST shows a pronounced warming trend in the region (Figure 4.6), the cooling due to Qnet is compensated by something else. In other words, the negative trend of Qnet in the Angolan sector suggests that changes in Qnet are not responsible for the warming trend along the Angolan coast in austral summer. In the ABF, close to the coast, the trends in dSST are positive all year round except in November and December when negative trends are observed. The biggest values of the dSST trend are observed from February to April when they exceed 0.9oC, suggesting that the trend in Qnet would have contributed to the warming trend of the ABF coastal zone all year round, except in early summer when negative trends in Qnet are found. In February- March, despite the important contribution of the trend in Qnet to the positive trend in dSST, the SST in the ABF coastal zone (Figure 4.6) shows a negative trend. Furthermore, in austral winter dSST shows positive trends in the ABF coastal zone while SST shows negative trends. Again, this suggests that Qnet is not the main factor for the SST change in the ABF coastal zone over the 1986- 2015 period. In the Namibian sector (south of 19oS), the dSST trend is positive throughout the entire year. However, the positive trend is weak in austral winter, less than 0.3oC per decade, and not statistically significant.
As the net surface heating term is also a function of the mixed layer depth, I also analyse its contribution to the SST trend. Figure 4.8 shows the seasonal mixed layer depth trends of the model. Figure 4.8 indicates negative mixed layer depth trends at rates of -1 to -5 m per decade through most months of the year in the Angola region. Relatively small positive mixed layer depth trends of up to 1.5 m per decade are found in early summer (October to December) off northern Angolan coast to about 9°S in late summer (January-February) in the open ocean of Angola region. However, these positive trends are not statistically significant. The maximum values of negative mixed layer depth trend in the Angola region are observed in the winter period. In
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northern Namibia, along the coast, the mixed layer depth is shoaling at the rate of up to -2 m per decade in summer (November to April) while relatively small positive deepening mixed layer depth trends up to 2 m per decade is found in winter. However, these positive or negative trends are not statistically significant at 95 % level. Overall, in Northern Namibia, negative mixed layer depth trends are found from October to May and positive trends with the same magnitude are found in the rest of the year making the annual trend almost null (not shown) while in the Angolan region negative trend is observed almost year-round making the annual trend negative with the rate up to -2 m per decade. The shoaling of the mixed layer depth contributes to the warming associated with the net surface heating term while a deepening of the mixed layer depth contributes to the cooling associated with the net surface heating term. However, a comparison of the annual cycle of the mixed layer trend (Figure 4.8) and dSST trend (Figure 4.7) does not show a similar pattern in most parts of the study region. For instance, dSST shows a negative trend in the open ocean from May to June while the mixed layer depth contributes to the positive trend of dSST. Other examples are observed in October in the Namibia region and November- December along the Southern Angolan coast. These contradictory results confirm as I mentioned earlier that the MLD does not play a role in dSST. The trends in dSST are due to change in Qnet as the spatial pattern of seasonal Qnet trend (not shown) matches very well with the spatial patterns of the seasonal dSST trends.
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Figure 4.8: Monthly decadal trends of NEMO mixed layer depth (m per decade). Dashed contour lines denote statistically significant values at the 95% confidence level using Student’s test based on linear regression.
In summary, trends in dSST are due to changes in Qnet. The long-term mean mixed-layer heat budget analysis appears to demonstrate that the net surface heating term is the main driver of the warming trend observed in the Angolan and ABF areas. The seasonal trend analysis of dSST in Figure 4.6 demonstrates that trends in Qnet cannot sufficiently explain the SST change in the Angolan and Namibian coastal zones, especially in early austral summer. Indeed, the SST trends have the highest values in austral summer although the Qnet trends tend to cool the SST.
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