| 11 Nov 2025
Physical and biological processes driving seasonal variability of Nitrate budget and biological productivity in the Congolese upwelling system
Abstract. The Congolese upwelling system, located in the southeastern Gulf of Guinea, is a highly productive marine ecosystem influenced by both local and remote physical forcing. This study investigates the seasonal variability of the nitrate budget and biological productivity in this region using a high-resolution (1/36°) coupled physical-biogeochemical simulation with the NEMO-PISCES model. The analysis highlights the relative contributions of physical and biological processes in modulating nitrate concentrations in both the mixed layer and the euphotic zone.
Results reveal a semi-annual cycle of nitrate, with two upwelling periods (May–August and December) and two downwelling periods (January–April and October–November). These cycles are primarily driven by the passage of coastal trapped waves forced by equatorial Kelvin waves, inducing vertical thermocline displacements and regulating nitrate availability in the euphotic zone. The nitrate budget analysis shows that the vertical advection, linked to the coastal trapped waves (CTWs), is the dominant process supplying nitrate to the mixed layer during the main upwelling season. However, near the Congo River mouth (5.5° S–6° S), the horizontal advection plays a key role, supplying significant amounts of nitrate through the river plume. In the lower euphotic layer, the vertical mixing contributes to the nitrate loss during the upwelling but becomes a source of nitrate during the downwelling periods. The seasonal cycle of the chlorophyll-a (CHLa) concentration follows that of nitrate, confirming that the primary production in this region is mainly driven by nitrate availability. The study also highlights the role of the Angola Current in transporting low-nitrate waters from the Equatorial Undercurrent, which influences the nitrate and CHLa balance in the Congolese upwelling system.
These findings provide new insights into the mechanisms governing nutrient dynamics and biological productivity in the Congolese upwelling system. Understanding these processes is crucial for assessing the impact of climate variability on the regional marine ecosystems and fisheries.
Publisher’s note: this comment was edited on 17 December 2025. The following text is not identical to the original comment, but the adjustments were minor without effect on the scientific meaning.
This study investigated physical and biogeochemical processes responsible for seasonal variability in nitrate and biological productivity in the Congolese upwelling system using a high-resolution ocean-BGC coupled model. The authors performed the detailed budget analysis for seasonal change in nitrate and ecosystem and suggested that vertical motion is responsible for the nitrate tendency in the upwelling season. This vertical motion also includes some clue of coastal Kelvin waves. On the other hand, around the Congo River mouth, horizontal advection is an important player in supplying nitrate provided by the riverine flux of the Congo. I found this work quite interesting and revealed details of physical and biogeochemical processes in the high-productivity region. However, one concern is that the mixed-layer depth defined in this study is a bit skeptical: in the entire year, the depth is 10m and almost constant (Figs.12-14). According to the definition in this study, I would think that the authors capture a “barrier layer” created by fresh water, not mixed-layer. Therefore, some budget analysis might not capture the processes in mixed-layer properly. I would recommend testing other criteria of mixed-layer depth and see the differences. Further comments are given as below.
List of comments
Line 109-111: better to be careful describing. According to those references, only N2O is addressed in the EBUS. Not sure CH4 flux is also active in the region. For CO2, I could imagine some CO2 flux due to rich DIC. But, cold SST can suppress the CO2 outgassing, so curious which factor is more responsible for CO2 flux there. Any observed data/indication available?
Line 111-112: do you mean such greenhouse gases are drivers of climate variability? If so, what type of variability is driven?
Line 113: Apart from?
Line 114: should be switched. Sea surface temperature (SST, ....).
Line 134: twitch =>which
Line 149-150: This sentence is partially finished? Or want to say "Whereas the total advection contribution is less important, it plays a secondary role in the mixed layer heat budget". ?
Line 170: reveal or other verbs sound better.
Line 209: year of 2011, I am wondering how the year of 2011 was in terms of trpical Atlnaitc ocean climate. Any strong Atlantic NIño/Niña and/or Benguela Niño/Niña? Maybe some brief information could added.
The end of Section 1: please give a summary of strcture of this paper in the last part of the Section 1.
Line 229: some typo?
Line 231: no comma?
Line 232: 5th daily?
Line 247-248, Eq.1: Perhaps, this term is missing in the equation (1). In the equaiton, 4th terms looks like "vertical" diffusion.
Line 262: italic?
Line 295-303: perhaps, better to refer each panel of Fig.2.
Line 306: (e,f) is missing
Line 313: In the model, it looks like that the coastal upwelling is not very realistic as seen by warm SST. The cold SST is mainly coming from the Congo river plume. I am wondering how JRA-55's performance in reproducing coastal level jet. Could you find any clues on this from this paper?, https://journals.ametsoc.org/view/journals/clim/31/4/jcli-d-17-0395.1.xml
Line 313-314: Also I am curious how the NEMO's performance in ocean current system in this region. From nitrate and SST distribution, south of Congo river mouth, nitrate is quite poor compared to observation (perhaps, less upwelling). On the other hand, north of the River Mouth, coastal nitrate is enriched even though SST is warmer indicating the weak upwelling. Maybe this high concentration in the north of River Mouth is influenced by the current?
Line 317: how about in the observation?
Line 318: dot is missing.
Figure4: Why the observation CARS does not have a peak around 6S close to the Congo River Mouth? Due to the relatively coarse resolution or NEMO-PISCES has a large bias of nitrate input of riverine flux?
Line 341: seasonal variability
Line 344: December-to-January(or February)?
Line 352-356: This argument makes sense. Here, linking to Fig.4, I am wondering if upwelling due to CTWs is too strong in the model as I can see the 15mm/m3 line is too shallow compared to the observation. Any argument is possible?
Line 367-369: Sound a bit repetitive with lines 352-356?
Line 372: "deepen" sounds better than lower.
Fig.6: no any description on wind stress. In October, the alongshore wind stress is stronger than other months. But, SST is warm and CHL-a is less (NO3 as well). Does this indicate that offshore Ekman transport is not responsible for BGC process here? Or, should we consider some time lag between wind-stress and BGC response?
Fig.7: not sure what is the main philosophy to show this plot here. For example, latitude-time section of zonal current. Are the authors interested in vorticity associated with the current? How about if the authors show the latitude-time section of u-v vectors like Fig.6.
Fig7a and b: might be better to rotate the plots by 90 degrees to see the longitude as x-axis.
Line 399: The plot is near surface, right? So, a bit hard to see undercurrent signature in the plot, I’d suppose.
Line 407: "Like" indicates the two seasonal cycle is similar/identical for me. But, Fig.8a and b have 45degree shift. So, "Corresponding to" or other expression could be better here.
Line 407: a bit unclear expression. “the seasonal cycle of nitrate tendency”.
Line 408: August => July?
Line 409: July => June?
Line 409: September => August?
Figure7: Before plotting this Hovmöller, better to show horizontal current plot how the surface current is going on. Otherwise, a bit hard to focus on which characteristics.
Figure7: does it make sense if x-y axes flip? As a and c use longitude in y-axis, a bit hard to follow the plot.
Figure8: Fig.8c and d saturate its color, perhaps better to have a bit larger color scale in positive and negative.
Figure8: As commented above, Fig.7 could show the lat-time hovmöller of u-v vector so that readers can compare Fig.7 and Fig.8 and have some indication (and a good flow to the next subsection).
Fig9c and Line 432: as the color is saturated, not veyr clear what the authors argue here.
Line 434-435: A bit confused. In the nitrate budget, the zonal advection is internal component in the ocean. My understanding is that the riverine flux is external inputs in the budget.
Figure 9e: As commented above, Fig.7 could be a hovmöller plot of u-v vectors. Then, readers can understand this U-adv and V-adv contribution well.
Line 440-441: I am wondering the intense negative tendecy by vertical component. If this latitude is under the effect of Congo River discharge, the vertical motion could be suppressed due to fresh water input, thus, more stratification. Why in such situation, the vertical downward advection is strong?
Line 443: Maybe this argument is an answer to my previous question. However, mixed-layer mean value of nitrate looks identical or slightly smaller than that of subsurface in Fig.4b. So, I wouldn't imagine such large negative value of vertical advection.
Line 451-452: which figures mentioned?
Figure 10: Fig.10 c and d. The color scale is saturated. Better to have a wider scale.
Line 472: better to say with "km" not with degree.
Line 476: while the...
Line 480: look at
Figure 12: The black dashed line is referred as mixed-layer depth in the caption, but it looks fixed value in the entire year. Is this reasonable?
Figure12: Which region is taken for this analysis? Better to show a box in other figure and clarify it in the caption.
Line 490-497: I would guess the mixed-layer depth is mis-plotted in Fig.12 and the I cannot follow the argument here. According to the definition of MLD in this study, density at 10m is taken as reference and estimate 0.03kg/m3 exceeding level. However, as stated, if the estimated MLD in this study is 10m and almost constant (as commented in Figs.12,13, and, 14), I have an impression that 10m depth of the MLD here is not MLD, but barrier layer. I'd suggest that a different criteria might be implemented to estimate the MLD to avoid the strong influence of the fresh water and associated barrier layer.
Figure13: Same as Fig.12, the mixed-layer depth looks constant.
Line 515-516: might be kind for readers if the author give a supplemental plot of CTW properties here. Even if the previous study using the same modeled data already studied the CTWs in this region, maybe better to show a plot in a different way as supplement. Otherwise, this statement is a bit unclear.
Line531-533: I cannot grasp the fundamental philosophy why the advection term is subdivided into the components. (also mean <> and * what?). Need to clarify it more in this section. Therefore, a bit hard to follow the discussion in this subsection.
Line 544: not sure what is this term expresses.
Figure 14: Better to provide an equation that are shown in Fig.14.
Line 544: correlation with what?
Line 629: greater=> warmer
Line630-631: any references?
Line 633: but, XU et al. (2014) used a different coupled model. There are many causalities of the bias in this region.
Suggest see also
Voldire et al., 2019, https://link.springer.com/article/10.1007/s00382-019-04717-0
Koseki et al., 2018, https://link.springer.com/article/10.1007/s00382-017-3896-2
Koubanova et al., 2018, https://link.springer.com/article/10.1007/s00382-018-4197-0
Cabos et al., 2017, https://link.springer.com/article/10.1007/s00382-016-3319-9
Line 642: the CARS climatology
Line 642: But => better to replace with “however” or “on the other hand”.
Line 643: sounds a bit strange as here you refer to climatologicla distribution and seasonal cycle. maybe delete “Similarly”
Line 648-652: not sure if this sentence is necessary.
Line 657: underestimation
Line 666: better to plot horizontal current in some figures as commented above.
Line 668: CHLa
Line 673: budget
Line 675-676: As commented, I am skeptical about the MLD in the plots (too constant). Be careful for plotting and corresponding arguements.
Line 677-678 and 697-699: As commented, the estimated 10km MLD seems a barrier layer. What if a different definition of MLD is used?
Line 701-702: remove “In this section, we will….in the euphonic layer”
Figure A1: There is no lable in the y-axis. At the base of the puphotic layer (the pink dashed-line) the signals are quite weak, so I am not sure if the authors' argument here is reasonable.
Line 726: which box?
Line 731-732: Not only the nitrate meridional gradient, but also vertical motion's meridional gradient could be a source of nitrate frontogenesis: if vertical motion is more upward in the south than in the north, more nitrate is supplied in the south than in the north, consequently, the nitrate meridional gradient is negative like Fig.15b.
Line 732-733: Did the author show this plot? Same as CTWs, it would be helpful if any plots of connection to the equatorial part is shown.
Line 734-737: I'm curious the horizontal map of this argument: subsurface budget plots could be shown here (as supplemental information)?
Line 742/743: Current.
Line 746-748: From Fig.A1, it is not easy to see what the author mention here. As commented above, any horizontal plot at the subsurface might be helpful.
Line 754-792: It seems that the authors mix many words related to primary production (TPP, NPP, Net TPP, etc?). Therefore, it is hard to follow the statement in this part. Please be careful in re-wording.
Line 758: net primary production (NPP)
Line 771: Net TPP => NPP?
Line 772: fig.18 => Fig.18
Line 772-773: not sure what is “the difference between Net NPP and NP”
Line 783-784: 10m looks like barrier layer