Influence of position and strength of westerlies and trades on Agulhas leakage and South Benguela Upwelling

The westerlies and trade winds over the South Atlantic and Indian Ocean are important drivers of the regional oceanography around Southern Africa, including features such as the Agulhas current, the Agulhas leakage and the Benguela upwelling. The Agulhas leakage is the transport of warm and saline water from the Indian Ocean into the South Atlantic. The leakage is stronger during intensified westerlies and probably also when the wind systems are shifted poleward. Here we analyzed the wind stress of different observational and modelled atmospheric data sets (covering the last two millennia, the 5 recent decades and the 21st century) with regard to the intensity and position of the south-easterly trades and the westerlies. The analysis reveals that variations of both wind systems go hand in hand. A poleward shift and intensification of westerlies and trades took place during the recent decades. Furthermore, the upwelling in South Benguela slightly intensified and the characteristics of the water masses fed into the upwelling region changed with a poleward shift of the trades. Projections for strength and position of the westerlies in the 21st century depend on assumedCO2 emissions. In the strongest emission scenario 10 a further southward displacement will occur, whereas a northward shift is modelled in the weakest emission scenario, possibly due to the dominating driving effect of ozone recovery. Thus, the Agulhas leakage has intensified during the last decades and is projected to increase if greenhouse gas emission are not reduced. This will have a small impact on Benguela upwelling strength, but will have consequences for water mass characteristics in the upwelling region. An increased contribution of Agulhas water to the upwelling feed water masses will import more preformed nutrients and oxygen into the upwelling region. 15


Introduction
The regional oceanography around Southern Africa includes phenomena with larger scale implications.They are the Agulhas Current, the Agulhas leakage and the Benguela upwelling.All three phenomena are to a large extent influenced by one of the two wind regimes in this region, the westerlies and the trades.Here, we analyze several observational data sets and model sim-reanalysis data set (European Centre for Medium-Range Weather Forecasts re-analysis for the 20th century) (Poli et al., 2016) to cover a longer time period from the last century (01/1900-12/2010) with a spatial resolution of T159 (125 km).
We additionally analyze two simulations with the Max-Planck-Institute Earth System Model (MPI-ESM-MR) (Giorgetta et al., 2013) for the historical period .The two simulations only differ in their initial conditions.We analyze three future scenarios (MPI-ESM-LR) with different strength in greenhouse gas forcing, representative concentration pathways rcp2.6,rcp4.5, and rcp8.5,where the numbers indicate the anthropogenic radiative forcing in Wm −2 reached by the year 2100 (Taylor et al., 2012).Tropospheric ozone concentrations are the same for all three rcp scenarios (Giorgetta et al., 2013).Furthermore, we analyze the variations of the wind stress during the last two millennia in a simulation with the Earth System Model MPI-ESM-P (ECHAM2k) (S.Wagner, pers. comm.).
For our study of the trade winds and the westerly wind band we analyze the averages of the austral summer season (December-January-February; DJF) of zonal wind stress over the whole South Atlantic and South Indian Ocean (40 • W -100 • E, 15 • S -60 • S) (Fig. 1).From these data, we calculated the temporal dependency of the latitude of maximum westerly winds (hereafter, position of westerlies) and the latitude of maximum wind stress curl (the latitude where winds change from westerlies to easterly trades, hereafter position of trades).Furthermore, we calculated the intensity of the westerlies as the meridional mean between 60 • S and the latitude where winds change from westerlies to trades to analyze the intensity of the wind stress in addition to the position of the wind systems.For the trades, we calculated the meridional mean between the latitude where winds change from westerlies to trades and 15 • S. We focus on DJF because it is the season of maximum upwelling in South Benguela (Tim et al., 2015).To investigate whether there is an impact of El Niño-Southern Oscillation (ENSO), we used the Multivariate-ENSO-Index (MEI, Wolter and Timlin (1993)), a bimonthly time series  including not only the SSTs and the sea level pressure (SLP), but also additional atmospheric variables for calculating the index.For ENSO, the seasonal mean of DJF was calculated by averaging the bimonthly means of December-January and January-February.
Vertical velocity in the ocean as simulated by the global ocean simulation INALT20 driven by COREv2 atmospheric forcing (Schwarzkopf et al., 2019) is used as an upwelling index.The data was selected at 100 m depth, in the region between 27 • S-34 • S, over a corridor of 100 km along the coast from the Cap Basin (34 • S) to Lüderitz (27 • S) (Tim et al., 2015).
For the statistical significance of the linear trends in the wind stress position and intensity and the linear correlations a significance level of p = 0.05 was adopted.
For the COREv2 data set we used the wind stress as seen by INALT20: this wind stress data set is the COREv2 data interpolated to the higher model grid of INALT20, at 1/20 • resolution and considering ocean velocities (relative winds).Since the position and intensity of the data sets COREv2, NCEP, and ECHAM6XR are significantly correlated (between r=0.85 and r=0.94), taking the ocean velocity into account when calculating the wind stress does not impact the conclusions of this study.

Position and intensity of westerlies and trades
The position of westerlies and trades (DJF means) over the South Atlantic and Indian Ocean are calculated based on the wind stress of the data sets NCEP1, COREv2, JRA-55, ERA20C, and ECHAM6XR.The interannual variation in the positions of these two variables (Fig. 1) are significantly correlated with correlation coefficients between r=0.65 and r=0.78.
The width of the westerlies band is defined here as the distance between the latitude of the maximum of the westerlies and the equatorward latitude where the wind stress changes to easterly trades.This width is negatively correlated to the position of the westerlies.Thus, a more southerly latitude of the maximum winds tends to occur simultaneously with a broader subtropical belt of westerlies.Both trades and westerlies tend to shift poleward or equatorward simultaneously, but the amplitude of westerlies displacement is stronger, leading to a correlation between the latitudinal position of the wind systems and the width of the subtropical belt of westerlies.
All five data sets show highly significant correlations as described above.Figure 2 shows the wind stress of the COREv2 data set for the year 1964/1965 at the beginning of the time series (a) and in the year 2007/2008 at the end of the analysis period (b).The intensification, poleward displacement and widening are well borne out.
We also investigate the variations in the intensity of the westerlies and the trade winds.Intensity of the westerlies is defined here as the zonal wind meridionally averaged between 60 • S and the latitude of maximum wind stress curl; intensity of the trade winds is defined as the zonal wind meridionally averaged between the maximum wind stress curl and 15 • S. As expected, the intensity and position of the westerlies are highly negatively correlated, so that a more southerly position of the wind maximum  is accompanied by stronger westerlies, and vice versa.Furthermore, there is also a positive correlation between the width of the westerlies band and the intensity of the westerlies, so that the width of the westerly band (equatorward of the maximum) is broader when westerlies are stronger (for ERA20C and ECHAM6XR).
The correlation of the intensity of westerlies and trades is, although insignificant for NCEP1 and COREv2, negative (between r=-0.21 and r=-0.62).These negative correlations indicate a tendency of both wind systems to intensify or weaken at the same time (Fig. 3), especially in the model simulation ECHAM6XR and the reanalyses data sets with higher spatial resolution than NCEP1 and COREv2.The correlations are numerically negative because westerlies and trades are contrarily directed.Thus, both subtropical wind systems tend to vary coherently and a more poleward position goes with stronger westerlies and a broader subtropical belt of westerlies, a more equatorward position goes with weaker and narrower belt of westerlies.
A trend analysis over the recent decades (1948-2012CE, 1958-2013CE, 1958-2009CE, and 1948-2015 CE) reveals a shift of both westerlies and trades to more poleward positions in all data sets (Table 1).Trends in the position of the westerlies are stronger than those of the trades, and are significant in all but one data set (position of trades in COREv2).The strength of the westerlies is also increasing; all data sets provide significant positive trends.The intensity of the trade winds shows a significant (positive) trend in JRA-55 and ECHAM6XR.(strengthening) of both wind systems, westerlies and trades.
The poleward shift and intensification of the westerlies is linked to trends in the Southern Annular Mode (SAM).This index is defined as the difference of sea level pressure anomaly between 40 • S and 60 • S, here calculated from the COREv2 data set.
Correlations reveal a strong and significant link between SAM and the position of the trades and westerlies, as well as with the intensity of the westerlies.The trends identified in the wind systems as previously described are related to the positive trend in SAM over the recent decades (1958( -2009 CE) CE).
Expanding our research area to the whole southern hemisphere, again using the wind stress over the ocean between 15 • S -60 • S derived from the NCEP1 and COREv2 data sets, reveals the comparable correlations and trends in both data sets as in the previous analysis that was limited to the Atlantic and Indian Ocean sector.The position and intensity of the westerlies display significant trends, whereas the trades do not.
A candidate forcing mechanism for the trends and variability of the westerlies may involve the latitudinal surface temperature gradient (via the thermal wind equation).To test this hypothesis, we calculate the southern hemisphere meridional temperature gradient between 30 • S and 80 • S from the NCEP1 data.This gradient was estimated with a linear fit of the zonal mean temperature against latitude for each December-to-February season, thus yielding one value of the gradient per year.Correlations between the latitudinal temperature gradient at the surface and the wind stress reveal a strong temperature gradient being associated with a southerly position of the westerlies, a broader westerlies band and stronger westerlies.The estimated impact of the meridional temperature gradient on the zonal wind stress profile between 45 • S and 65 • S is depicted in Fig. 4, along  with the climatological meridional profile of the wind stress.The impact of the temperature gradient is estimated from a linear regression between the meridional temperature gradient in this region (predictor) and the zonal wind stress at each latitude band (predictand): wind_stress(latitude, time) = wind_stress_climatology(latitude)+α * temperature_gradient(time)+residual(latitude, time) (1) Figure 4 shows with red and blue lines the deviations of the climatological wind stress profile caused by a temperature gradient 5 steeper of flatter than the climatology by one standard deviation.

Variations in the past and in future climates
The global ECHAM2k simulation is a coupled atmosphere-ocean simulation with a state-of-the art Earth System Model driven by external climate drivers (solar variations, volcanic activity and greenhouse gases).The simulation covers the last two millennia, for which statistical analyses of trends and variability of the wind systems were conducted similarly to those for reanalysis 10 data in the previous section.In general, the correlations of position and intensity of trades and westerlies support the inferences based on the reanalysis data sets and the global downscaling product ECHAM6XR.Variations in the latitudinal position and strength of westerlies and trades tend to be simultaneous and the westerlies band is broader when the westerly maximum has a more poleward position.Regarding centennial trends, the results are not unambiguous: calculated for each century separately, they suggest that winds are stronger when located more poleward in 16 (14) centuries out of 21 for the westerlies (trades).During the 16th century, a particularly cold century during the Little Ice Age between the 15th and the 19th centuries, westerlies significantly weakened and shifted equatorward.This is supported by the studies by Peeters et al. (2004) and Granger et al. (2018), who also found that the winds shifted equatorward during glacial periods.
The MPI-ESM simulations for the historical period (1850-2005 CE) and the future (2006-2100 CE) reveal the same coherences of westerlies and trades as the other data sets (NCEP1,COREv2, JRA-55, ERA20C, and ECHAM6XR).However, a significant difference is that the intensity of trades and westerlies are not correlated in the model results of the rcp4.5 and rcp8.5 scenarios.Also, the trends for most parts are insignificant over the historical period and only partly significant in scenarios of future radiative forcing (Table 1).For the future, these trends indicate a poleward shift and intensification of westerlies (and a poleward shift and weakening of the trades) only in the rcp8.5.In the weaker emission scenario, by contrast, significant trends mark a northward shift of the westerlies and a weakening of trades and westerlies.As prescribed ozone concentrations are the same in all three scenarios, we speculate that ozone recovery drives the simulated future evolution and offsets effects of weak greenhouse gas emissions.Under the high emission scenario, the increase in greenhouse gases leads to a further poleward shift The comparison of the time series of the COREv2 to the MPI-ESM historical runs in the period 1958-2009 reveal that the position of the westerlies is more to the north during the last 20 years in the coupled model simulation than in the reanalysis data set (Fig. 5) and at a similar latitudinal position as the reanalysis in the beginning of the simulation time period.This leads to the insignificant trends in the historical period in the MPI-ESM simulations.Therefore, the response of the wind stress to the increase in greenhouse gases and decrease in ozone is weaker in the MPI-ESM simulation than in the observational based COREv2 data.This weaker sensitivity, detected in the historical period, may be relevant for the real future trends.Thus, the simulated decrease of the westerlies in the scenario rcp2.6 and its increase under scenario rcp8.5 may be an underestimation of the expected response.

The impact of the position of westerlies and trades on the South Benguela upwelling
For this section we limit your analysis to the COREv2 data set and use the simulated vertical velocity of the COREv2-driven INALT20 ocean simulation to investigate the link between the westerlies and upwelling intensity in the Benguela upwelling system.The correlation between the upwelling (simulated vertical velocity at 100 m depth), spatially averaged over the South Benguela upwelling region (27 • S-34 • S, 100 km width band off the coast), and the position of the trades is insignificant, but shows a weak tendency towards stronger upwelling when the trades are located further south.In contrast to their position, the intensity of the trades does show a significant positive correlation with upwelling.
Although the link between the position of the trades and upwelling is weak, there is a clear correlation between position of the trades and SSTs (Fig. 6).A northerly position of the trades is linked to negative SST anomalies west of the southern tip of the African continent and in the offshore part of the South Benguela upwelling region, but not directly at the coast.The correlation of the position of the trades and the spatial average of the SST over the upwelling region is significant and positive (r=0.43).
Thus, a northerly position of the trade winds is associated with warm SSTs in the upwelling region and with cooler SSTs further offshore (Fig. 6).This is explained by the relationships between the position of the trades and intensity of upwelling on the one hand, and by the relationship between the position of the trades and the Agulhas leakage on the other hand.First, warm SSTs in the upwelling region indicate that upwelling is weakened during a more northerly position of the trades.Secondly, the Agulhas leakage is weakened during weaker westerlies.Since the position of the trades and the westerlies is significantly correlated, a more northerly position of the trades leads to weaker westerlies.Furthermore, since westerlies and trades in general shift into the same direction, the low SST offshore of the upwelling on the shelf are caused by a weak Agulhas leakage and a reduced transport of warm water from the Indian Ocean into the Cape Basin.
ENSO has been found to have some influence on the South Atlantic trade winds, on the westerlies and also on the Benguela upwelling (Philippon et al., 2012;Tim et al., 2015).The correlation between ENSO and the trades is strong and significant, whereas the correlation between ENSO and the westerlies is weak and not significant.is the same, so that ENSO does not disrupt the tendency of the trades and westerlies to intensify or weaken at the same time.The position of trades is displaced to the north, trades are weak during a positive ENSO phase (El Niño) and Benguela upwelling is slightly reduced (Tim et al., 2015).Westerlies also tend to be located further north during an El Niño event (although this link is statistically weaker) and the Agulhas leakage is reduced, leading to cooler SSTs in the Cape Basin and offshore of the upwelling region.

Discussion and conclusions
We analyzed the intensity and position of the trade winds and of the belt of westerlies in the South Atlantic and South Indian Ocean with regard to implications for the intensity of the Agulhas leakage and the South Benguela upwelling, during austral summer (December-February) as modelled for the last two millennia, the last century, the past 60 years (∼1950-2010 CE) and projected for the 21st century.
Our conclusions are: -Interannual latitudinal shifts in the position and intensity of trades and westerlies go hand in hand.Both systems tend to simultaneously shift latitudinally.When the shift is poleward they also tend to become stronger.These results are confirmed by the analyses of the position and intensity of the wind stress over the entire southern ocean between 80 • S and 30 • S. The earth system model MPI-ESM provides results that agree with the reanalysis data sets (NCEP1, COREv2, JRA-55, ERA20C) and the global high-resolution simulation ECHAM6XR, with significant correlations between the positions and strengths of the westerlies and the trades.One driving mechanisms appears to be the surface air temperature difference between the subtropics and midlatitudes, as position and intensity of the wind systems are correlated with the meridional temperature gradient: the stronger the gradient, the further south and stronger are the westerlies.Correlations with the Southern Annular Mode (SAM) reveal a strong connection to the sea level pressure difference between the subtropics and midlatitudes as well.
-During the recent decades the westerlies have shifted poleward and have become more intense (in the reanalysis data sets).However, the trends of the MPI-ESM for the historical time period (1850( -2005 CE) CE) are weaker than in the reanalysis data sets and mainly insignificant (even when analyzing a time span comparable to that covered by the other data sets).
Swart and Fyfe (2012) also found an intensification and poleward shift of the southern hemisphere surface westerly wind stress jet in various Coupled Model Intercomparison Project model simulations (CMIP3 and CMIP5) and reanalysis data sets.Furthermore, there are reasons that suggest that the trends in the reanalysis are not artifacts.NCEP1 has been shown to have an above-average trend of SAM (Marshall, 2003) (Solomon et al., 2016).The trends in the intensity and position of the westerlies have been found to cause an intensification of the Agulhas leakage in ocean simulations driven by atmospheric meteorological reanalysis (Biastoch et al., 2009).Durgadoo et al. (2013) and Loveday et al. (2014) showed that Agulhas leakage and Agulhas Current are decoupled, but that both respond to the intensification of westerlies and trades, respectively.Thus, the coherent variability and trends in the two wind systems cause an intensification of both oceanic components, Agulhas Current and Agulhas leakage.
-The meridional shifts of the trades and the westerlies and the associated variations in their intensity has an impact on the SSTs around Southern Africa.The position of the winds is only weakly correlated with South Benguela upwelling intensity (stronger upwelling when trades are shifted poleward).In contrast, the strength of the trades is significantly correlated with Benguela upwelling, with more intense trades being linked to stronger upwelling in South Benguela.
Furthermore, a more southerly position of westerlies and trades leads to positive SST anomalies in the Cape Basin and the offshore part of the South Benguela upwelling region.This SST anomaly pattern cannot be explained by a more intense upwelling, which is usually linked to more intense trades and westerlies.Instead, the physical mechanisms linking variations in strength and position of the wind systems to SST in the South Atlantic is the Agulhas leakage, which modulates the advection of warm Indian Ocean water into the South Atlantic and into the Benguela upwelling system.This agrees with proxy-based findings of Granger et al. (2018) for the late-Holocene.They found that during a northward shift of the westerlies, SSTs have been lower in South Benguela, not caused by intensified upwelling but due to changes in the advected water masses.
-Regarding the centennial time scales over the past two millennia, the analysis of the wind stress variations (of the general circulation model ECHAM2k) is consistent with the results obtained for the observational period.Both wind systems, westerlies and trade winds, are stronger when located further south in most of the centuries and vice versa.Furthermore, the northward shift and weakening of the westerlies during the cooler Little Ice Age in the 16th century in these simulations is consistent with evidence from sediment cores for the same period (Granger et al., 2018) 2k-PMIP3 group, 2015).Despite these uncertainties, in general the temperature levels have been lower during the period of the Little Ice Age in the Southern Hemisphere for both, proxy (Granger et al., 2018) and model simulations.
-The future evolution of the wind stress was analyzed with three emission scenarios of the MPI-ESM, differing in the prescribed strength of greenhouse gas forcing.Future trends of rcp2.6, the weakest scenario, indicate a northward shift and weakening of the westerlies, while trends derived from scenario rcp8.5, the strongest scenario, indicate a southward shift and strengthening.The scenario with moderate emissions, rcp4.5, has no significant trend.As ozone concentration changes are equal in the three scenarios, these different trajectories lead us to conclude that the ozone pool will be the dominant driver, if greenhouse gas emissions are drastically reduced.In the more probable business-as-usual scenario (rcp8.5) the increase in atmospheric greenhouse gas concentrations will override the ozone effect and will lead to a further poleward shift and strengthening of the westerlies.Such a poleward shift and intensification of the westerlies will enhance the Agulhas leakage and, therefore, cause a larger transport of warm Indian Ocean water into the South Atlantic.This is likely to affect the Benguela upwelling system in several ways, and may have done so in the recent and geological past.A stronger leakage may increase the volume and changes the properties of the Eastern South Atlantic Central Water, containing Agulhas water that enters the Benguela upwelling system (Tim et al., 2018).This water mass will become younger, warmer, richer in oxygen, and its higher volume increases the share of preformed nutrients in the South Benguela system.In contrast, a weakened Agulhas leakage due to a northward shift of wind systems reduces the contribution of Agulhas water in the upwelling region, possibly to be compensated by the inflow of an older intermediate water mass (the South Atlantic Central Water dominant in the northern Benguela upwelling system) with low oxygen, high CO 2 and nutrient concentrations.We postulate that variability in the positions and strengths of dominant wind systems -remote westerlies and the trade winds -set the pace for the Agulhas contribution to the upwelling feed water masses in the Benguela upwelling system, impacting the productivity of the region, its CO 2 balance, ecosystems, and living natural resources.

Figure 1 .
Figure 1.Temporal evolution of the December-February zonal mean (40 • W-100 • E) zonal wind stress [N/m2] (shaded color) and the meridional position of maximum of westerlies (light pink curve) and of wind stress curl (blue curve) in the period 1958-2009 in the COREv2 data set.The linear trends in their positions are also displayed.There is a significant trend of -0.52 • /decade in the position of the westerlies and an insignificant trend for the trades.

Figure 2 .
Figure 2. climatological mean of the December-February wind stress [N/m2] in the periods 1964/1965 (a) and 2007/2008 (b) derived from the COREv2 data set.Contours and shading represent the same variable.

Figure 3 .
Figure 3. Temporal evolution of the intensity of wind stress of westerlies (mean between 60 • S and of the latitude where wind stress turns eastward) and trades (mean between latitude where wind stress turns eastward and 15 • S) and their corresponding linear trends in December-February derived from the COREv2 data set.

Table 1 .
Trends of the position ( • /decade) and intensity (Nm-2/decade) of westerlies and trades for the five observational based data sets (NCEP1, COREv2, JRA-55, ERA20C, ECHAM6XR), for two free-running MPI-ESM simulations (r2 and r3) for the historical period, and for two simulations (r2 and r3) for three future scenarios.Statistically significant trends at the 95 % level are marked with an asterisk.Data set position of westerlies position of trades intensity of westerlies intensity of trades

Figure 4 .
Figure 4. Impact of the meridional temperature gradient on the southern hemisphere wind stress intensity and position.Black curve shows the climatological mean of the zonal wind stress at the latitudinal range 45 • S -65 • S in December-February (seasonal mean over the period 01/1948-08/2012) with NCEP1.The blue and red curves show the estimated alteration of the wind stress profile due to a one-standarddeviation steeper (red line) or one-standard-deviation flatter (blue line) surface temperature gradient between 30 • S -80 • S. The meridional temperature gradient was estimated as the slope of a linear fit of the zonal mean temperature against latitude.

Figure 5 .
Figure 5. Meridional position of the maximum intensity of westerlies in December-February derived from the COREv2 reanalysis and the MPI-ESM historical simulations r2 and r3.

Figure 6 .
Figure 6.Correlation pattern of the sea surface temperature in the INALT20 simulation and the position of the trades of COREv2 reanalysis in December-February over the period 1958-2009.
and, therefore, probably in other parameters dependent on sea level pressure in that region.COREv2 is based on NCEP1.The global simulation with the atmospheric general circulation model ECHAM6XR is only dependent on NCEP1 and shows even stronger trends than the reanalysis data sets.Also Earth Syst.Dynam.Discuss., https://doi.org/10.5194/esd-2019-16Manuscript under review for journal Earth Syst.Dynam.Discussion started: 18 April 2019 c Author(s) 2019.CC BY 4.0 License.analyzing the more recent data sets JRA-55 and ERA20C confirms our results of a poleward shift and intensification of the westerlies and the trades over the South Atlantic and Indian Ocean.Thus, these long-term trends are present across different reanalysis data sets that assimilate different observations.Watson et al. (2012) and McLandress et al. (2011) found that increasing greenhouse gas emissions and the ozone depletion drive the trends in position and intensity of the westerlies over the past decades.The ozone concentration has not fully recovered yet from the minimum attained in the 1990s, and a slightly positive trend in observations since the year 2000 is overlain by large interannual variability Competing interests.The authors declare that they have no conflict of interest.Earth Syst.Dynam.Discuss., https://doi.org/10.5194/esd-2019-16Manuscript under review for journal Earth Syst.Dynam.Discussion started: 18 April 2019 c Author(s) 2019.CC BY 4.0 License.Earth Syst.Dynam.Discuss., https://doi.org/10.5194/esd-2019-16Manuscript under review for journal Earth Syst.Dynam.Discussion started: 18 April 2019 c Author(s) 2019.CC BY 4.0 License.