Tropical Atlantic

The variability observed in the D354 dataset is very small compared to that seen in the D361 Tropical Atlantic, data. This data was collected in the region

of ocean adjacent to the plume of atmospheric dust that originates from the Sahel region of Africa. This atmospheric source is one which varies hugely depending on meteorological and terrestrial conditions across North Africa. Within the model the process of atmospheric supply of dust is provided with monthly average modelled deposition fields. The process of averaging monthly fields removes the deposition model’s ability to resolve variations on shorter timescales. Consequently, large sporadic depositions caused by large weather systems may be missed by the MITgcm. The zonal section at 11◦N from the Tropical Atlantic data, figure 4.14, shows the magnitude of this atmospheric deposition being particularly strong close to the African Continent.

−28 −26 −24 −22 −20 −18 −16 −250 −200 −150 −100 −50 0 1 2 x 10−6 Depth (m) Longitude oN −28 −26 −24 −22 −20 −18 −16 −3500 −3000 −2500 −2000 −1500 −1000 −500 0 0.5 1 1.5 2 x 10−6

Figure 4.14: MITgcm new closure 11◦N dissolved iron distributions, mol m−3_{. The}

contoured section is model output; data values are scattered onto the model field.

This region at the surface is reasonably well captured by the model, however the model’s surface layer performance worsens with distance from Africa. The data suggest a low iron surface layer, whereas the model predicts that the

surface layer is the only depth which concentrations greater than 0.6nM are seen. This statement also highlights the failing of the model to capture a plume of iron emanating from the continent westwards at 500 to 1000m depth. This may be the model lacking the suitable structure to resolve some small circulatory features or, more likely, suggests that local extremes of supply can cause significant variations from the annual average iron concentrations, particularly when considering the work of Ye et al. (2011) who highlight the vitality of timing in determining the exact response of the system to an iron source perturbation. The modelled deep water away from the continental shelf is well matched to the observations with concentrations below 0.8nM

replicated in the model. A patch of particularly low iron in the west most station below 1500m is not seen in the model. This minimum could be a second order response of the system to the high deposition of particles. A section taken

−15 −10 −5 0 5 10 15 20 25 −250 −200 −150 −100 −50 0 1 2 x 10−6 Depth (m) Latitude oN −15 −10 −5 0 5 10 15 20 25 −3500 −3000 −2500 −2000 −1500 −1000 −500 0 0.5 1 1.5 2 x 10−6

Figure 4.15: MITgcm new closure 28◦W dissolved iron distributions, mol m−3_{. The}

through the tropical region of the Atlantic at approximately 28◦_{W shows how}

the model’s meridional distribution fares through the dust deposition plume of the mid-Atlantic. The problem of the model failing to capture the low surface iron concentrations continues into this section. Despite the model identifying the latitudes between 10 and 20◦_{N as a region of strong iron supply, and}

resultant iron concentration, these high concentrations are not seen in the observations. As mentioned previously, Ye et al. (2011) talk about the ability of atmospheric dust supply acting as a sink of iron due to particle scavenging and a lack of iron-binding ligands. This mechanism for deposition instigating removal involving variable ligand concentrations is not in the MITgcm iron closure. Alternatively, the natural variability of deposition, that is lacking in the MITgcm, could provide an explanation for the low concentrations of iron in the observations from the surface. If the deposition was particularly short-ranged, or anomalously deposited at higher or lower latitudes to those shown here, the surface supply of iron could be missed by the geographical range of the observations. The region to the south of the deposition plume in the model is well replicated, both the data and model concentrations are particularly low, from 0.1 to 0.3 nM. This region represents the South Subtropical Gyre of the Atlantic. The deeper waters in this section show strong similarities. With the exception of the patch of high iron, 1.2 to 1.5 nM at 500 to 1000m, between 5 and 15◦N, the deep iron concentrations consistent, observational data tends to be slightly higher, but the homogeneity and magnitude of these measurements support the model’s representation of the deep water.

The phosphate distributions produced by the original iron closure were reasonable and it was important to avoid perturbing this regime. However, the distributions of phosphate in the Tropical Atlantic resulting from the new iron closure have improved the phosphate concentrations quite considerably, Figure 4.16. The new iron closure maintained the qualitative skill of the model and improved the phosphate concentrations within the model to match the data observations. The surface depletion and subsurface nutrient maximum of phosphate concentrations were maintained and the gradient of phosphate at depths greater than 2000m was reduced in line with the observed distribution.

Although the aim was to try and maintain the phosphate distributions as they were prior to adjusting the iron closure, the changes made have improved the skill of the model at resolving phosphate distributions in the Tropical Atlantic.

−15 −10 −5 0 5 10 15 20 25 −250 −200 −150 −100 −50 0 1 2 x 10−3 Depth (m) Latitude oN −15 −10 −5 0 5 10 15 20 25 −2000 −1500 −1000 −500 0 0.5 1 1.5 2 2.5 x 10−3

Figure 4.16: MITgcm new closure 28◦_{W phosphate distributions, mol m}−3_{. The}

contoured section is model output; data values are scattered onto the model field.

The biogeochemical regimes present in the Tropical Atlantic dataset were described in section 2.5.4. The iron closure has qualitatively replicated the macronutrient and micronutrient regimes. The supply of iron to the surface of the Tropical Atlantic is able to encourage biological activity, which allows the regeneration of organic material and the remineralisation of scavenged iron to resupply iron into the subsurface waters. The plume of iron resulting from these processes augments the intermediate and deep water masses with iron. The supply of iron within the North Atlantic Deep Water, is resolved well in the model. There are still aspects of the regimes that are not well simulated. The concentrations of iron in the surface water of the model are far too high. The model is still unable to rapidly remove this strong atmospheric source. The capacity for the model to retain iron in the water column is also lower than seen in the data. The constant total ligand concentration effectively sets a limit to the amount of iron the model can retain in the subsurface ocean. This parameter precludes any higher concentrations of iron being maintained by a supply of organic ligands from the atmosphere or another source.

The steps forward made in the resolution of the biogeochemical regimes of the Tropical Atlantic will help to provide meaningful and reliable inferences from the model output. It is clear the processes important in this region are simulated qualitatively within the new iron closure, in contrast to the limited range of processes previously resolved.