Browsing by Author "Mongwe, Ndunisani Precious"

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    The seasonal cycle of CO₂ fluxes in the Southern Ocean: a model spatial scale sensitivity analysis
    (2014) Mongwe, Ndunisani Precious; Monteiro, Pedro M S
    A recent study by Lenton et al., 2013, compared the mean seasonal cycle of air-sea CO₂ flux in the Southern Ocean(SO) over 1990 – 2009 period using five ocean biogeochemical models(BGMs) and atmospheric and ocean inversion models with monthly mean observations for the year 2000. This was done using a set of geographic boundaries to defined sub-domains of the SO consistent with the Regional Carbon Cycle and Assessment and Processes (RECCAP) protocol. Lenton et al., 2013 found that the seasonal cycle anomaly of the five BGMs better resolved observations of the air-sea CO₂ flux seasonal cycle in the SAZ, but was generally out phase with observations in the polar zone. In this study two setups of the ocean biogeochemical model NEMO PISCES was used to investigate the characteristics of the air-sea CO₂ flux seasonal cycle in the Southern Ocean in the period 1993- 2006. The study focused on two aspects i.e. (i) the sensitivity of air-sea CO₂ flux seasonal cycle to model resolution: comparing the ORCA2-LIM-PISCES (2° x 2° cos Ø) and PERIANT05 (NEMO-PISCES) (0.5° x 0.5° cos Ø) model configurations relative to climatological mean observations for the year 2000 (Takahashi et al., 2009) , and (ii) the sensitivity of air-sea CO₂ flux seasonal cycle to zonal boundary definition: comparing the air-sea CO₂ flux seasonal cycle and annual fluxes for three different boundaries i.e. Lenton 2013 RECCAP boundaries (44°S – 58°S and south of 58°S), geographic boundaries (40°S -50°S and south of 50°S) and dynamic boundaries (Sub-Antarctic Zone and Antarctic Zone, defined using climatological frontal positions). The seasonal cycle of the air-sea CO₂ flux in ORCA2 was found to be out of phase and overestimated the CO₂ flux compared to observations in almost all the sub-regions considered. The use of dynamic boundaries was found not to improve resolving observations seasonal cycle of air-sea CO₂ flux in both ORCA2 and PERIANT05. Boundary definition was found to affect the magnitude of ORCA2 annual air-sea CO₂ fluxes surface area based, where sub-regions of larger surface area gave larger annual CO₂ uptake and vice versa. This was mainly because ORCA2 air-sea CO₂ fluxes were found to show a general CO₂ in-gassing bias and spatially uniform in most parts of the SO and hence integration over a larger surface area gave larger annual fluxes. On the contrary PERIANT05 air-sea CO₂ fluxes spatial variability was not uniform in most parts of the SO however influenced by regional processes and hence annual fluxes were found not surface area based. The poor spatial representation and seasonal cycle sensitivity of ORCA2 air-sea CO₂ fluxes was found to be primarily due to lack or weak winter CO₂ entrainment and biological CO₂ draw down during the summer season. PERIANT05 on the contrary showed the effect of winter CO₂ entrainment, however maintains lack of or weak biological CO₂ draw down in the seasonal cycle. PERIANT05 was also found to show major weakness in the spatial representation of air-sea CO₂ fluxes north of the polar front with relative to T09 observations.
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    Understanding modelled sea-air CO2 flux biases in the Southern Ocean through the seasonal cycle
    (2018) Mongwe, Ndunisani Precious; Monteiro, Pedro M S; Vichi, Marcello
    The Southern Ocean forms a vital component of the earth system as a sink of CO2 and heat, taking over 40% of the annual oceanic CO2 uptake (75% of global heat uptake), slowing down the accumulation of CO2 in the atmosphere and thus the rate of climate change. However, recent studies based on the Coupled Model Intercomparison Project version 5 (CMIP5) Earth System Models (ESMs) show that CMIP5 ESMs disagree on the phasing of the seasonal cycle of the CO2 flux (FCO2) and compare poorly with available observation estimates in the Southern Ocean. Notwithstanding these differences, the seasonal cycle is a dominant mode of CO2 variability in the Southern Ocean, and hence this is an important bias. Previous studies suggest that these biases of FCO2 in ESMs might be a significant limitation to the long-term simulation of CO2 characteristics in the Southern Ocean. Consequently, this study has three primary objectives: first, to develop a process-based diagnostic method to analyze and isolate key biases and their underlaying mechanisms in the model-observations seasonal cycle of FCO2 differences for forced ocean models and ESMs. Second, to use this framework to examine sources of biases responsible for the limited skill of CMIP5 models in simulating the seasonal cycle of FCO2 with respect to observed estimates. Thirdly, to investigate how these present-day biases in the seasonality and drivers of CO2 in CMIP5 ESMs affect modelled longterm changes in the mechanisms of CO2 uptake in the Southern Ocean. In the first part of the dissertation, an objective diagnostic framework was established to analyze model-observation biases in the seasonal scale of FCO2 using the NEMO PISCES ORCA2LP model output, and Takahashi et al. (2009) observed estimates. The diagnostic framework focuses on examining the relative contributions of the competing drivers (SST and DIC) and related processes (solubility, biological and mixing) to instantaneous monthly changes in surface pCO2 (and FCO2) at the seasonal scale. In the second part of the dissertation, this approach is applied to 10 CMIP5 models in the Southern Ocean, to investigate the mechanistic basis for the seasonal cycle of FCO2 biases. It was found that FCO2 biases in CMIP5 models can be grouped into two main categories, i.e. group-SST and group-DIC. Group-SST models are characterized by an exaggeration of the seasonal rates of change of Sea Surface Temperature (SST) in autumn and spring during the cooling and warming peaks, respectively. These faster-than-observed rates of change of SST tip the control of the seasonal cycle of pCO2 and FCO2 towards SST and result in divergence between the observed and modelled seasonal cycles, particularly in the Sub-Antarctic Zone. While almost all analyzed models show these SST-driven biases, 3 out of 10 (namely NorESM1-ME, HadGEM2-ES and MPI-ESM, collectively the group-DIC models) compensate the solubility bias because of their exaggerated primary production, such that biologically-driven DIC changes become the regulators of the seasonal cycle of FCO2. It was also found that despite significant differences in the spatial characteristics of the mean annual fluxes, CMIP5 models show a zonal homogeneity in the seasonal cycle of FCO2 at the basin-scale in contrast to observed estimates. In the final third of the dissertation, using five CMIP5 ESMs from the RCP8.5 scenario, it was found that CMIP5 models present climate biases in the seasonality and drivers of FCO2 are fundamental to how models simulate long-term changes in the mechanisms of CO2 uptake in the Southern Ocean. Although all five analyzed models show an increased annual mean CO2 uptake by the end of the century, they show significant differences in the mechanisms. The present-day temperature biased models (group-SST) generally maintain the dominance of the temperature driver in the seasonal variability of FCO2 to end of the century. But show enhanced CO2 uptake due to increased anthropogenic atmospheric CO2 and decreased surface CO2 buffering capacity but they display a weak to null role of biological activity in the increased CO2 sink. On the other hand, the increased CO2 uptake at the end of the century in group-DIC models is explained increased biological driven CO2 uptake in spring, linked to increased Revelle factor and solubility driven CO2 uptake in winter. Increased Revelle factor at the end of the century enhance pCO2 changes for even smaller DIC changes.