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coastal region. Contrary to the Southern Benguela, the poleward shift of SASH decreases the wind speed in the Northern Benguela which warms up the SST. In austral winter JJA, the 1°C positive anomaly in CB SST corresponds to a maximum of northwesterly wind anomaly of ~1.4-1.8 m.s-1 in the CB region. The negative correlation between CB SST and CB wind speed is about - 0.6- 0.7.
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Figure 3.14: Austral summer (DJFMA) long-term mean of 1000 geopotential height (m) over the period 1982-1999 (left panel), the period 2000-2017 (middle panel), and the difference of the long-term mean between the period 2000-2017 and 1982-1999 for top to bottom: ERA5, Era- Interim, MERRA-2, and NCEP2 reanalyses respectively. The white and black cross lines represent the position of the maximum in mean geopotential height for the period 1982-1999 and 2000- 2017 respectively. The magenta contour lines denote where the long-term means of geopotential height of the two periods are significantly different at the 95%-level according to Welch’s t-test.
In austral summer (DJFMA), the two long-term means show that the SASH is confined within the South Atlantic basin between 15°S and 40°S latitude and 36°W and 18°E longitude.
The centre of the SASH is about 30.5°S± 0.5° and 4.5°W± 0.5° for the first period 1982-2017 and shifts poleward by about 1°in the second period 2000-2017 for all datasets except NCEP2 which
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has too coarse a resolution (2.5°×2.5°)to pick up a 1° displacement. Moreover, the core of the SASH in the period 2000-2017 is expanded to the south in all datasets. The difference of the long- term mean between 2000-2017 and 1982-2017 (Figure 3.14, right panel) reveals that the long- term mean geopotential height has increased in the second half period 2000-2017 by up to 10 m over the ocean south of 18°S in all the datasets, while a decrease of long-term mean geopotential height in the second half period up to -4 m is observed in the equatorial part of the ocean (9°S to 9°N) and along the Angolan and Namibian coasts in most datasets. This suggests an intensification of the SASH in the south more than a shift, which is consistent with the increase in upwelling-favourable wind in the south. The increase of geopotential height in the south would have led to an increase of the pressure gradient from southwest to northeast, leading to the observed south to south-east increase in wind. The increase of geopotential height over the open ocean is more pronounced south of 30°S; it is statistically significant and may be a sign of intensification and poleward extension of the SASH. MERRA-2 shows less augmentation of the geopotential height over this period compared to other datasets which are consistent with MERRA-2, having a smaller increase in wind speed. Disagreements among the datasets are observed over land. A statistically significant increase of geopotential height up to 10 m, over the period 2000-2017 compared to the period 1982-1999, is also observed on the continent in the central part of Africa in all datasets while a statistically significant decrease of geopotential height up to -4 m over the same period 2000-2017 is observed in the equatorial part of the continent in most datasets, except Era Interim. There is no increase or a slight non-significant decrease of geopotential height in the south of the continent, which again is consistent with the increase in wind speed south of the Benguela Upwelling System and decrease of SST. As the land temperature increases due to global warming, one would expect the surface pressure to decrease. It seems therefore that the increase in wind speed is due to change in the SASH rather than change over the land in austral summer.
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Figure 3.15: same as Figure 3.14 but for austral winter period (JJASO).
In the austral winter period (JJASO, see Figure 3.15), the SASH is more intense and extends toward the adjacent continent compared to the summer period (DJFMA, see Figure 3.14). The SASH in JJASO is connected with the Mascarene high over the Indian Ocean through Southern Africa when land surface temperatures over the latter are cooler than over the adjacent South Atlantic (Sun et al., 2016). The SASH is also closer to the equator in JJASO compare to DJFMA. The centre is located at 28°S± 0.5° latitude and 4.5°W ± 0.75° of longitude in the first half period 1982-1999 and shifts by about 0.5° poleward in ERA5 and Era-interim. No poleward change is observed with MERRA-2 and NCEP2. However, all datasets show a westward displacement of
~0.5° to 2.5o depending on the datasets. Similar to the austral summer, the difference of the long- term means between 2000-2017 and 1982-2017 in JJASO shows that the long-term mean
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geopotential height has increased up to 10 m in the second period 2000-2017 over the ocean south of 9°S in all datasets (Figure 3.15, right panel). This increase of the long-term mean geopotential height in the second period is more pronounced south of 27°S suggesting an extension southward of the SASH. A decrease of JJASO long-term geopotential height, up to -4 m, in the second period 2000-2017, is also observed along the equator and on the continent in most datasets except Era Interim which manifests a statistically significant increase of the austral winter long-term geopotential height over the continent during the period 2000-2017.
Next, I analyze the linear trend of the latitudinal position and magnitude of the center of the SASH (Figure 3.16 and 3.17). The SASH center is determined each month by the position of the maximum of the geopotential height in the South Atlantic region within the 45°S-0°S - 40°W- 15°E region. Figure 3.16 shows the timeseries of the latitudinal position (left panel) and magnitude (right panel) of the SASH centre for ERA5, Era-Interim, MERRA-2, and NCEP2 reanalyses. The black solid lines denote the linear trends. The trend analysis results are summarized in Table 3.2. All datasets agree on intensification and a poleward displacement of the SASH with positive trends in magnitude and negative trends in latitudinal position of the SASH centre. The positive trends in the magnitude of the SASH centre are statistically significant at the 95% level in all datasets. The trend is 0.165m per decade, 0.216m per decade, 0.166m per decade and 0.299m per decade for ERA5, Era-Interim, MERRA-2 and NCEP2, respectively. The positive trends in the magnitude of the SASH centre are observed all year round in all datasets (Figure 3.17). The positive trends in the magnitude of the SASH centre are statistically significant at the 95% level in January, July, and September in most datasets (Figure 3.17). The negative trends observed in the latitudinal position are also statistically significant at the 95% level in most of the datasets except Era-Interim where it is statistically significant at a 90% level (Figure 3.16). The trend value is -0.029o per decade, -0.014o per decade, -0.015o per decade and -0.02o per decade for ERA5, Era-Interim, MERRA-2 and NCEP2, respectively. The negative trends in the latitudinal position of the SASH centre are observed in most months of the year in most datasets (Figure 3.17).
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Table 3.2: Linear trend analysis results of latitudinal position and magnitude of the SASH centre for each different dataset: Era5, Era-Interim, MERRA-2 and NCEP2. T-stat indicate the student test statistic. Significant trends are shaded in grey.
Decadal trend
Decadal trend unit
Standard Error
t-stat P-value R2
Latitudinal position of SASH centre Era5 -0.029
°/decade
0.001 -2.893 0.004 0.020
Era-Int -0.014 0.0008 -1.719 0.086 0.006
MERRA- 2
-0.016 0.0009 -1.837 0.066 0.007
NCEP2 -0.020 0.001 -2.066 0.039 0.010
SASH centre Magnitude Era5 0.165
m/decade
0.004 4.143 <0.001 0.040
Era-Int 0.216 0.005 4.346 <0.001 0.042
MERRA- 2
0.166 0.005 3.274 0.001 0.024
NCEP2 0.299 0.004 6.051 <0.001 0.079
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Figure 3.16: Anomalies of the meridional position (SASHy) (left panel) and magnitude (right panel) of the SASH center for the period 1982-2017 for, top to bottom: ERA5, Era-Interim, MERRA-2, and NCEP2, respectively. Solid black lines indicate linear trends.
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Figure 3.17: Linear trends (° per decade) in the meridional position of the SASH (left panel) and its magnitude (right panel) for each month of the year (blue bars) over the period 1982-2017 for (top to bottom): ERA5, Era-Interim, MERRA-2 and NCEP2, respectively. The red stars indicate statistically significant trends at the 95 % level using student’s test based on linear regression.