The details of the sampling strategy, analytical methods, data quality control and calibration procedures employed for taking the diverse set of biogeochemical and physical measurements and recovered observational data at the KI CELAS and PAP observatories (Figure 1) have been reported by [16] [17] and also available on the EuroSITES data documentation (http://www.eurosites.info/).

The numerical model used for this research is the MIT Ocean General Circulation Model (MITgcm) regionally configured for the North Atlantic Ocean with computational domain 20˚S 81.5˚N, and a horizontal resolution of 0.5˚ longitude and 0.5˚ latitude. The MITgcm is a physical-biogeochemical-ecosystem model designed for study of the atmosphere, ocean, and climate. Details of this model have been described in previous papers [9] [18] , and the principal features of the physical part of the model are presented in Table 1.

The model incorporates nutrient cycling using an ecosystem model of intermediate complexity [10] and carbonate chemistry cycling [22] that models a pelagic ecosystem with one zooplankton class and two phytoplankton classes (diatoms and small phytoplankton) [10] [18] . The atmospheric pCO₂ forcing in the model is from Mauna Loa Observations [23] and takes into account the annual seasonal pCO₂ cycle. A schematic presentation of the ecosystem part of the MITgcm is as illustrated (Figure 2).

In this section, the mixed layer depth profile of partial pressure of CO₂ (pCO₂), thermal (pCO₂-T) and nonthermal (pCO₂-nonT) composites of pCO₂, physical and biogeochemical parameters such as temperature, chlorophyll-a and salinity are estimated with the primary objective of reenacting scenarios that would have been obtainable in the absence of in situ measurements. Therefore, the following diagrams depict the characteristic mo- del simulations of pCO₂, pCO₂-T, pCO₂-nonT, MLD temperature, chl-a, and salinity profiles at the subpolar NE Atlantic Ocean time series PAP location (Figures 3-8).

The model mixed layer pCO₂ (Figure 3) indicated a distinct temporal variability as the season changes coinciding with the change in sea surface temperature. Also illustrated in Figure 3 is the fact that the model pCO₂ generally showed relatively high pCO₂ with increasing depth of the ocean. The oceanic pCO₂ during the summertime as expected was characteristically low at the surface mixed layer and may be attributed to stratification of the mixed layer depth, and relatively higher during the wintertime following deep convective mixing. Although displaying characteristics associated with increased biological activity, the mesopelagic level of pCO₂ was relatively high. This appears to be as a result of a possible nutrient drawdown and other inorganic and organic carbon producing processes such as respiration, remineralisation, among others. The thermal (pCO₂-T) and nonthermal (pCO₂-nonT) composites of pCO₂ (Figure 4 and Figure 5), further elucidate the driving forces governing the seasonal variability of pCO₂. The pCO₂-T showed a characteristic maximum pCO₂ levels in sum- mer and minimum in winter, while the pCO₂-nonT component indicated a marked seasonal summertime minimum to late wintertime maximum with phenomenally high pCO₂ during the springtime. The enhanced surface mixed layer pCO₂ is indicative of possible spring bloom estimation or entrainment of dissolved carbon and nutrients rich water to the upper mixed layer. In an oceanographic region considered as a perennial sink for atmospheric CO₂, mixing processes are capable of entraining CO₂―enriched bottom mixed layer seawaters into the upper euphotic layer, thereby resulting in enhanced pCO₂.

The model mixed layer temperature profile suggests that the ocean system at the PAP site had a temperature-induced (warming) stratified mixed layer during summertime that tended to extend into early fall, followed by a rapid wind-driven mixing of subsurface waters into the mixed layers through the wintertime (Figure 6). However, this is followed by a re-stratification period, which starts as early as May and lasts the summertime following the warming of the ocean. Warming of ocean euphotic zone has been reported to induce stratification. This oceanic process is reportedly governed by a combination of positive physical forcing fueled by temperature changes [24] . The profile estimated by the model showed a season-dependent change in mixed layer temperature, with sea surface temperature indicating relatively high estimates during the summertime to early fall (Figure 6). This is expected but however, the marked decline in temperature down the mixed layer especially along stratified gradient in the summer months, separating the specific thermoclines of the oceanic layer is worthy of note.

Model output for the PAP site indicated that maximum levels of chlorophyll-a concentration (maximum levels of phytoplankton) were observed during the late spring through early summer. This is in fair agreement with the observed PAP data, which showed a characteristic regional summertime maximum, wintertime minimum variability (Figure 7). It is worthy of note that the model output indicated chlorophyll-a concentrations from the surface microlayer extending to about 45 m depth, with moderately elevated concentrations simulated within the 0 - 25 m depth. This enhanced chlorophyll-a concentration might have been filliped by entrenchment of nutrients, and could have coincided with a period of shallower mixed layer, which later disappeared following the outset of a possible deep mixing period. According to [16] , this is partly due to dilution in addition to possible export. Also, model oceanic mixed layer salinity profile corresponding to the period of deployments at the PAP observatory, clearly shows slight to moderate seasonal variability in salinity gradient throughout the period (Figure 8). Based on monthly averages, the salinity distribution could have been influenced by convective currents in the region. Salinity changes are also closely associated with rainfall events. However, much variability in seawater salinity could simply be attributed to physical processes rather than other events, and is not caused by a salinity change in the water itself.

In this section, the profiles of pCO₂, pCO₂-T, pCO₂-nonT, mixed layer temperature, and chlorophyll-a are simulated depth wise using the MITgcm numerical model and observed oceanographic data obtained at the K1 site. Figures 9-13 show model simulations of pCO₂, pCO₂-T, pCO₂-nonT, MLD temperature and chl-a depth integrated profiles respectively at the subpolar NW KI Central Labrador Sea time series site. Model integrated pCO₂ simulation (Figure 9) indicates a general trend towards higher pCO₂ at low temperatures and vice versa. This is consistent with observational data summertime pCO₂ minimum and wintertime maximum.

The ocean system at the K1 Central Labrador Sea site as simulated witnessed a temperature-induced stratified mixed layer during the summertime and tended to extend deeper down the subsurface as fall sets in. This was followed by a possible wind-driven mixing of subsurface waters into the mixed layers through the wintertime. The convective entrainment of subsurface nutrients and export production in the model shows a clear increase in seawater surface pCO₂. This estimate is also consistent with the observed data. However, as depicted in the plot, a re-stratification period appears to have preceded the convective mixing of subsurface seawater [24] .

The oceanic pCO₂-T model (Figure 10) depicted a stratified mixed layer that spans all year round. Characteristically, summertime high pCO₂ coincided with increase in temperature, while wintertime relatively low temperatures were marked by low pCO₂ at the seawater surface. This is expected because warm summer surface waters are known to promote enhanced pCO₂. In the winter, mixed layer tends to cool and deepens (Figure 12). This is attributed to net heat lost at the ocean surface and mixed-layer bottom [25] . During this time, intense

thermal convection might be responsible for the mixed-layer deepening. Model chlorophyll-a profile at the K1 indicated significant biological signal during the summer months with intense productivity extending down to about 25 m depth. Therefore, increase in mixed layer chlorophyll-a concentrations coincided with summertime. This is expected because biological production reduces pCO₂. The model output generally showed a characteristic regional interannual variability with a summertime maximum and wintertime minimum chlorophyll-a concentration (Figure 13). This work has provided a means of simulating parameters that could reflects oceanographic observations, determines the approximated values of the poorly known parameters, and provides insight into which physico-biogeochemical parameters are constrained by the model [26] - [28] .