Fig 1.
Model domain, depth of σ0 = 26.2 and wind stress.
(a) The release regions of the passive tracers are plotted as coloured lines parallel to the model boundaries (cf. Table 1). The surface and near-surface currents and the upwelling cells of the northern Benguela are redrawn from [21]. Please refer to the text for abbreviations. The depth [m] of the σ0 = 26.2-isopycnal which separates surface from central water [22] is shaded in yellow. The potential density was calculated from modelled temperature and salinity. (b) Wind stress [N m−2] is represented by vectors and wind stress curl [10−7 N m−3] is colour coded. Areas of negative wind stress curl are shaded in blue. Shown are model results from January of the monthly climatology from 2002 to 2016.
Table 1.
Passive tracers: Release region and density range of their release region.
Fig 2.
Zonal currents at 8°W and release regions of tropical passive tracers.
Positive zonal velocities, i.e. eastward, are shaded in red. The release regions R of the EUC, SEUC and SECC tracers are shown (cf. Table 1). The mixed layer depth is plotted as a green line, black lines denote potential density. January (a) and July (b) of the monthly climatology from 2002 to 2016.
Fig 3.
Currents in the subsurface layer.
Horizontal currents averaged from 50m to 200m depth from a monthly climatology from 2002 to 2016. Colours represent the vertical velocity at 50m depth. Positive vertical velocity (red) is directed upward. (a): April (b) October.
Fig 4.
Sverdrup balanced averaged transport.
Time averaged wind stress curl divided by density and Coriolis parameter (colour coded) and flow lines of the vertically integrated velocity. a) Flow lines of the Sverdrup balanced flow, b) Flow lines of the time averaged and depth integrated velocity (upper 500m) from the numerical circulation model.
Fig 5.
Hovmöller plot of horizontal currents at 5°E.
The magnitude and direction of the vertically averaged horizontal currents at a meridional section at 5°E. a) Surface currents averaged from 0m to 50m b) Subsurface currents averaged from 50m to 200m. Monthly climatology from 2002 to 2016.
Fig 6.
Tracer concentration isopycnicly averaged.
The tracers are released at 8°W, 50m to 200m. Concentrations averaged within the isopycnal range given in Table 1 [m−3] are shown for July 2000 (a few days after tracer initialisation), January 2001, July 2001, January 2002 and July 2016. The tracers are released between 2°N to 2°S (EUC, upper panel), 6°S to 2°S (SEUC, middle panel) and 10°S to 6°S (SECC, lower panel).
Fig 7.
The lines visualise only trajectories of particles upwelled in the northern Benguela upwelling system. Upper panel: upwelling happens within the five days prior to July 13, 2015, lower panel: upwelling within five days prior January 14, 2016. For both time periods, the four panels show those trajectories of upwelling particles that traversed one of the tracer release regions EUC, SEUC, SECC and SOUTH2 (Table 1). Locations of “coastal” and “oceanic” EUC upwelling trajectories are indicated by the thick black lines.
Fig 8.
Coastal and oceanic propagation of upwelling particles.
The number of upwelling particles per 5 d intersecting the release regions of the tracers show a strong seasonal variability. EUC upwelling trajectories were grouped as “oceanic” if they intersected a zonal section at 6°S between 10°W and 8°E, otherwise as “coastal” (cf. Fig 7). Data were smoothed with a running mean of 30d. Monthly climatologies of the number of EUC (green), SEUC (orange), SECC (blue) and BC (purple) upwelling particles per 5 d (2010 to 2016, axis on the left) and of the spatial average of the mixed layer depth in the northern Benguela (2002 to 2016, black dashed line, axis on the right) are shown in the upper left corner.
Fig 9.
Currents in upwelling cells of the northern Benguela upwelling system.
The currents [ms−1] at a zonal cross-section in the Kunene Cell (KC, 18°S; a and b), the Central Namibian Cell (CNC, 23°S; c and d) and the Lüderitz Cell (LC, 27°S; e and f). The vector arrows are composed of the zonal and vertical flow component. The vertical velocity component is enlarged for better visibility. Equatorward motion is shaded red. January (left) and July (right) from the monthly climatology from 2002 to 2016.
Fig 10.
Central water in the Kunene Cell.
The concentrations of tracers averaged over a cross-shore section from the coastline to the 200 m-isobath in the Kunene Cell at 18°S were computed with Eq 6. The tracers are released parallel to the western model boundary (EUC—green, SEUC—orange, SECC—blue) and parallel to the southern model boundary (BC—purple) between 50m to 200m. T300 is shown in turquoise. Additionally, the average concentrations of those five tracers are summed (black line in both panels). The corresponding release regions are given in Table 1.
Fig 11.
Central water in the Lüderitz Cell.
The concentrations of tracers averaged over a cross-shore section from the coastline to the 200 m-isobath in the Lüderitz Cell at 27°S were computed with Eq 6. The tracers are released parallel to the western model boundary (EUC—green, SEUC—orange, SECC—blue) and parallel to the southern model boundary (BC—purple) between 50m to 200m. T300 is shown in turquoise. Additionally, the average concentrations of those five tracers are summed (black line in both panels). The corresponding release regions are given in Table 1.
Fig 12.
Vertical average concentration of tracers in the mixed layer.
The average concentration of tracers from the western model boundary (a: sum over EUC, SEUC, SECC and WEST concentration), the southern model boundary (b: sum over concentrations from SOUTH 1 to SOUTH 4) and of T300 (c). The vertical average in the mixed layer of the sum over all nine tracer concentrations is shown in the right panel (d). Data were averaged from 11th to 15th of January, 2017.
Fig 13.
Schematic subsurface circulation.
The magnitude of horizontal currents averaged over 50m to 200m is shaded. The black vectors represent main current systems. The current directions were derived from Fig 3. April (a) and October (b) of the monthly climatology from 2002 to 2016.