|Modeling pco2 variability in the Gulf of Mexico|
By Xue et al.
This manuscript presents the surface pco2 in the Gulf of Mexico from a coupled physical-biogeochemical model. The focus is on the annual mean and the seasonal cycle of surface pco2 and air-sea CO2 flux for the northern (Louisiana-Texas shelf) and the deep part (water depth >200m) of the gulf. Some discussion is also made for other areas such as the western Florida shelf and Mexican shelf.
The over-all results seem reasonable in that the modeled surface pco2 and air-sea CO2 fluxes agree reasonably well with available data. The open ocean flux, however, is about 2-3 times of the estimate by Robbins et al. (2014). In addition, it seems the model is unable to reproduce the large inter-annual variability of surface pCO2 (hence likely air-sea flux as well) as seen in the pco2 data, also reported by Huang et al. (2013, 2015a,b).
The biggest issues appear to be 1) a lack of key information, and 2) that some statements/conclusions need better justification. For example,
1) Page 5-8, description of the model setup is not detailed enough and at times, confusing. For example, there is no description of how carbon is being cycled through the food web and in the water column (uptake, sinking, remineralization etc.). Also is organic carbon input included in the river discharges or along the open boundaries? And if yes, what are the data sources and processing details?
2) Page 10-13, it will be helpful to include some information regarding the spatial distribution and seasonal cycle of new productivity (or net community productivity) since it is concluded that biological removal is the dominant driver for air-sea CO2 flux in the gulf (see, e.g. abstract). In order to evaluate the impacts of biological removal, a map showing the differences of pco2 and air-sea flux between exp1 and exp2 (similar to Figure 8) will be useful as well.
3) Page 10, section 4.1. In addition to temporal comparison, a spatial comparison is needed to verify the spatial pco2 pattern, e.g., by binning the data into 0.5x0.5 boxes and then making a 1 to 1 correlation analysis.
4) Page 18, for the carbon budget, some calculation of production, respiration (it was asserted respiration is the dominant process within the Mississippi river plume), and export (vertical and horizontal) for NGoM or entire gulf will be useful as well
5) Page 14-17, section 5.1. This is a rather long section but the discussion is rather meandering and sometime contradicting. For example (line 359-360), why the high pco2 in the plume and on the shelf is a result of respiration? Just two paragraphs above (lines 333-334), the authors also state that there is rich DIC input from the river, which would presumably bring high pco2 along with the plume. The bottom line is that there is a lack of quantitative information regarding the roles of biological production/removal and respiration. Also, why is the river plume “light-limited”?
6) Page 17, lines 368-370. This statement appears to conflict with Figure 6 (bottom panel), which indicates the biological removal strongly affects surface pco2 and air-sea flux throughout the year.
7) Page 18, last paragraph, what exactly are model uncertainties?
The writing seems confusing or prone to grammatical errors. Here are some examples,
1) page 5, line 95, pco2 -> CO2
2) page 6, line 123, we found -> we found that
3) page 6, line 124, were -> was
4) page 6, line 126, limited in -> limited to
5) page 8, line 155, Talk+50 is assigned as river DIC. This sounds like a strong assumption that needs to have a better justification.
6) Page 8, line 160-167. Grammar error.
7) Page 8, line 165-168. I assume the authors simply mapped the 1904-1910 conditions to 2004-2010 with a day to day match (e.g. Jan 20, 1904 will be corresponding to Jan 20, 2004). But, yes or no, this needs to be explicitly stated. Also, why 1904-1910? Any special reason to choose this period?
8) Page 9, line 178-182. How exactly is the data being processed? I assume the author use a 10-day binning for temporal processing. How about spatial means? Were the data within one domain (say NGoM) being summed up and divided by the total number of valid data points?
9) Page 9, line 188, fell in -> fell within
10) Page 10, line 217-218. I think this statement is not accurate. The pco2 maxima in the NGoM appear to be about 50 ppm higher. But this does not seem to be the case for low end pco2.
11) Page 13, line 271, wrong numbers.
12) Page 13, line 287, were simulated -> were
13) page 15, line 333-334, why this is true? What is the correlation relationship between Talk-sal (henceforth DIC-sal), Talk-DIN, and DIN-sal in the MS river?
14) Page 16, line 354-355. What does the bifurcation exactly imply?