| 14 Apr 2026
Chlorophyll-a concentration effects on equatorial Atlantic Ocean mean-state and interannual variability
Abstract. Chlorophyll-a concentration is known to influence the mean-state and interannual sea surface temperature (SST) variability of the tropics. Here, we investigate this effect in the equatorial Atlantic Ocean using a suite of ocean model simulations. In these simulations, the prescribed monthly climatology of chlorophyll-a concentration is multiplied by a factor ranging from 0.01 to 2. We find that a 'clear-ocean' simulation, i.e an ocean simulation with the monthly climatology of chlorophyll-a concentration multiplied by 0.01, results in a significantly warmer (+0.15 °C) eastern equatorial Atlantic SST and in a reduced (14 %) amplitude of SST seasonal cycle when compared against a simulation with realistic chlorophyll-a levels. Additionally, the equatorial Atlantic vertical temperature gradient is weakened, the mixed-layer and thermocline are deepened, and equatorial upwelling is reduced. These changes in the mean-state of the 'clear-ocean' simulation lead to a significant reduction (12.9 %) in eastern equatorial Atlantic SST variability. We also show that when the prescribed monthly climatology of chlorophyll-a concentration is scaled by 0.01, 0.5, 1, 1.5, and 2, the eastern equatorial Atlantic SST variability responds non-linearly. Our results also suggest that the ongoing observed decrease in tropical Atlantic chlorophyll-a concentration may weaken the interannual variability of SST.
Title:
Chlorophyll-a concentration effects on equatorial Atlantic Ocean mean-state and interannual variability
Authors
Arthur Prigent, Riccardo Farneti , Manfredi Manizza and Rodrigue Anicet Imbol Koungue
A well written relatively short paper discussing the effect of plankton, represented by the concentration of chlorophyll-a, on the surface layers of the equatorial Atlantic. However the paper does appear to have one major fault in that, in a system where SST and near surface stratification is important, the paper does not consider the interaction of the ocean and atmospheric temperature fields. I discuss the problem more below.
This review concentrates primarily on the ocean physics aspects of the study. I do not have the right background to review the comparisons with studies in the Pacific or with observations of Atlantic upwelling systems.
Detailed Comments
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Abstract
Line 7:
This does not explain which part of the 'vertical temperature gradient' is weakened. Later are you referring to the surface thermocline or the main oceanic thermocline.
Line 8
Equatorial upwelling is not reduced. If the winds used in the different runs are the same then the Ekman transports will be the same, as will equatorial upwelling.
Line 10
'non-linearly'. After reading the rest of the paper I find this bland statement misleading. It needs something which implies that a reduction in chlorophyll has a large effect - and so will be the focus of the paper - with increases in concentration have a smaller effect.
Introduction
Line 14
'The underlying dynamics ...'. I am not sure why 'underlying' is needed. Given that the best predictions of El Ninos are produced by computers and nobody understands the 'underlying' 'why', this is a slightly dangerous statement.
Line 33
'well observed' is a bit clumsy - how about something like 'accurately and easily measured'.
Line 59
After a detailed discussion of previous studies, the paper states some aims of the study - but I felt it would help the reader to have some clues along the lines of "As will be shown ... ".
Data and Methods
Line 90
It would be cleaner just to say that in the model the incident radiation is split into three components, infra-red, red visible and green visible, in the ratios 0.58, 0.21 and 021.
Line 95
Given the importance of the depth of heating in the following analysis, it would help to give the e-folding distance (i.e. 1/k) for each of the three 'k'. For the two which depend on concentration and the seasonal cycle, minimum and maximum depths would also be important.
Line 99
The paper states "this climatology is considered vertically uniform". It then states
"This is certainly a limitation as the maximum on Chi-a is usually found close to the nutricline".
The phrase "considered vertically uniform" is too vague. I presume that this means the chlorophyll field is independent of depth in the model.
But I think a more serious problem is that the paragraph hints that the model is not suitable for many studies involving chlorophyll. In reality the chlorophyll and nutrient distribution will depend on surface mixed layer depth and stratification. It therefor needs to explain why the model is suitable for the different runs. I can understand it may have been tuned for the standard case and still be valid with no chlorophyll, but with double chlorophyll one might expect additional feedbacks.
Methods
Line 108
It would help to state here that the paper will focus on the changes in the low chlorophyll run.
Line 118
This needs to be clear about the difference between the near surface thermocline and main oceanic thermocline.
Line 122
The null and alternative hypotheses statements are misleading. The null hypothesis should probably be that on the basis of the evidence the signal cannot be distinguished from zero. But I don't than that a yes/no argument is valid. What the paper discusses later is the 'probability' that the signal is different from zero.
The bootstrap method of resampling with replacement is one way of doing this. I think the paragraph needs a good rewrite, with some good references describing the method, the quality of the results and how it compares with other methods for estimating the probability that results are significant.
Results
Line 132
The paper appears to make a serious mistake in not considering the strong interactions between air temperature and sea surface temperature.
It states that the JRA-55 atmospheric fields have been used. As well as the wind stress fields and the downward radiation fluxes JRA-55 also includes the 10m air temperature and humidity. Within the ocean model these variables will normally be used to calculate the sensible and latent heat fluxes across the air sea interface.
If this does not happen in the version of Mom used, they my following comments should be ignored and the paper corrected with a better explanation of the way the ocean model handles the JRA-55 data.
The sensible heat flux depends just on the air-sea temperature difference. The latent heat flux is a function of both the temperature difference and the humidity of the atmosphere. Ocean models usually include code to represent these processes and I would be surprised in Mom is different. The fluxes are not small and in reality this means that SST and the surface atmospheric temperature are usually close.
In the present case, where the atmospheric temperature is fixed, it means that the SST will always be close to the imposed air temperature. This helps explains why the two functions in Fig. 4c stay so close over such a long period of time. The SST differences seen in Fig 4a, are thus really a measure of how hard to system is trying to get away from the imposed air temperature. In a fully coupled system the differences will be larger.
I am not sure how the surface boundary condition affects the rest of the results. I suspect that they also show the direction in which changes will occur in a fully coupled system - but this is really up to the authors to think through.
Lines 165-180
The statement of line 170 correctly implies that because the winds are the same in the different runs, the Ekman net transport will be the same. This further means that Ekman driven upwelling at the Equator will also be the same. So nothing can "strengthen equatorial upwelling through poleward volume transports"
What can change is the vertical distribution of the Ekman transport and this will change the velocities at each level. The changed transport will also affect the vertical structure of equatorial upwelling.
As an example of the Ekman transport profile changing, over much of the ocean a large fraction of the Ekman transports is usually found in the surface mixed layer. In this case if the mixed layer shallows but the transport in the layer stays the same, then velocities in the layer will increase to give the same total transport.
Anyway both paragraphs need to be rewritten as do any similar statements elsewhere in the paper.
Line 200
"weakened equatorial upwelling". See above.
Line 206
"MJJ", "ND". Expand.