Browsing by Author "Monteiro, Pedro"

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    A Comparison of Gas Exchange Models in the Estimation of CO2 Fluxes in the South Atlantic South of Africa for the Summer Season of 2008/2009
    (2014) Rainier, stephanie Megan; Monteiro, Pedro; Waldron, Howard
    There is a problem in the determination of air-sea CO2 fluxes because of the number of different relationships used in calculating gas transfer velocities. There is also a problem with the CO2 sink in the Southern Ocean being greatly underestimated. Data were collected underway using an autonomous pCO2 system during three separate relief cruises over the course of austral spring 2008 to austral autumn 2009 onboard the RV SA Agulhas in the South Atlantic Ocean. The wind speed product was extracted from QuikSCAT. Using the data we investigated the sensitivity of the five gas transfer velocity parameterisations to the uncertainty in the wind speed product of 2m.s-1. We found that the Stagnant Film Model was unresponsive. Liss and Merlivat's (1986) linear model for three wind regimes showed a gradual increase in sensitivity with wind speed. The quadratic relationship developed by Nightingale et al., (2000) also showed a steady increase in sensitivity with an increase in wind speed. Wanninkhof's (1992) quadratic relationship showed the greatest response at low wind speeds and then a continuing increase in response through medium to high wind regimes. The cubic relationship from Wanninkhof and McGillis (1999) showed the smallest response at low wind speeds but had the greatest response to the uncertainty in the wind speed product in medium and high wind regimes. We also calculated regional and seasonal averages of the CO2 flux with the five gas transfer velocities based on the different relationships between gas transfer velocity and wind speed. We found that there was a CO2 flux into the ocean ranging from 4mmol.m-2.day-1 to 12mmol.m-2.day-1 between 33.5 and 68°S, except during autumn between 45-50°S where there is a flux out of the ocean of 2mmol.m2 .day-1. Between 68-70°S the flux into the ocean strengthens to between 28mmol.m2 .day-1 and 52mmol.m-2.day-1. Gas transfer velocity is not dependant on wind speed alone, but currently it is the only variable that it measureable on a global scale. Further investigations are in place to measure gas transfer velocity in situ. The Southern Ocean is greatly undersampled spatially and temporally which leads to a lack of understanding 13 about possibly one of the most important CO2 sinks. New methods are being taken on to increase the sampling resolution.
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    A comparison of gas exchange models in the estimation of CO2 fluxes in the South Atlantic South of Africa for the summer season of 2008/2009
    (2011) Ranier, Stephanie Megan; Monteiro, Pedro; Waldron, Howard
    There is a problem in the determination of air-sea CO2 fluxes because of the number of different relationships used in calculating gas transfer velocities. There is also a problem with the CO2 sink in the Southern Ocean being greatly underestimated. Data were collected underway using an autonomous pCO2 system during three separate relief cruises over the course of austral spring 2008 to austral autumn 2009 onboard the RV SA Agulhas in the South Atlantic Ocean. The wind speed product was extracted from QuikSCAT. Using the data we investigated the sensitivity of the five gas transfer velocity parameterisations to the uncertainty in the wind speed product of 2m.s-1. We found that the Stagnant Film Model was unresponsive. Liss and Merlivat’s (1986) linear model for three wind regimes showed a gradual increase in sensitivity with wind speed. The quadratic relationship developed by Nightingale et al., (2000) also showed a steady increase in sensitivity with an increase in wind speed. Wanninkhof’s (1992) quadratic relationship showed the greatest response at low wind speeds and then a continuing increase in response through medium to high wind regimes. The cubic relationship from Wanninkhof and McGillis (1999) showed the smallest response at low wind speeds but had the greatest response to the uncertainty in the wind speed product in medium and high wind regimes. We also calculated regional and seasonal averages of the CO2 flux with the five gas transfer velocities based on the different relationships between gas transfer velocity and wind speed. We found that there was a CO2 flux into the ocean ranging from 4mmol.m-2.day-1 to 12mmol.m-2.day-1 between 33.5 and 68°S, except during autumn between 45-50°S where there is a flux out of the ocean of 2mmol.m- 2.day-1. Between 68-70°S the flux into the ocean strengthens to between 28mmol.m- 2.day-1 and 52mmol.m-2.day-1. Gas transfer velocity is not dependant on wind speed alone, but currently it is the only variable that it measureable on a global scale. Further investigations are in place to measure gas transfer velocity in situ.
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    Interaction of the antarctic circumpolar current with topography: impacts on the Southern Ocean eddy dynamics
    (2013) Kobo, Nomkwezane Sanny; Reason, Chris; Monteiro, Pedro; Herbette, Steven
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    Intra-seasonal variability of Southern Ocean primary production: the role of storms and mesoscale turbulence
    (2016) Nicholson, Sarah-Anne; Lévy, Marina; Monteiro, Pedro; Swart, Sebastiaan; Vichi, Marcello
    The Southern Ocean is one of the stormiest places on earth; here strong mid-latitude storms frequently traverse large distances of this ocean. Underlying these passing storms, the Southern Ocean is characterized by having some of the highest eddy kinetic energy ever measured (eddies occupying the meso to sub-mesoscale). The presence of the passage of intense storms and meso to sub-mesoscale eddy variability has the potential to strongly impact the intraseasonal variability of the upper ocean environment where phytoplankton live. Yet, exactly how phytoplankton growth rates and its variability are impacted by the dominance of such features is not clear. Herein, lies the problem addressed by the core of this thesis, which seeks to advance the understanding of intra-seasonal variability of Southern Ocean primary production. The drivers of this intra-seasonal variability have been explored from two points of view: the local-scale and the remote-scale perspectives, with a suite of physicalbiogeochemical (NEMO-PISCES) numerical models of varying complexity. At the local-scale, these model experiments have suggested that intra-seasonal stormlinked physical supplies of dissolved iron (DFe) during the summer played a considerably more active and influential role in explaining the sustained summer productivity in the surface waters of the Southern Ocean than what was thought previously. This was through two important insights: 1. Storm-eddy interactions may strongly enhance the magnitude and extent of upperocean vertical mixing in both the surface mixed layer as traditionally understood as well as in the subsurface ocean. These two mixing regimes have different dynamics but act in concert to amplify the DFe fluxes to the surface ocean. 2. Storm initiated inertial motions may, through interaction with eddies, greatly reinforce w and thus, enhance the vertical advection of DFe to the surface ocean, an effect that may last several days after the storm. At the local-scale, such storm-eddy dynamics may greatly increase the intra-seasonal variability of primary production, a step towards helping to explain why this variability is so strong in large regions of the Southern Ocean. At the remote-scale, the cumulative impact of these short-term storm-eddy interactions have unexpected implications in respect of the larger-scale mean flow and its influence on the effectiveness of intra-seasonal forcing of DFe fluxes. This counter intuitive feedback is a reduced strength of the intra-seasonal variability in primary production despite what was shown at the local-scale. Moreover, the addition of storms intensified the main clockwise cell of the meridional overturning circulation particularly the downward branch thus, reducing DFe inventory from the upper-ocean. Such an impact could potentially be enhanced with increasing storm intensities as suggested by climate projections. Understanding these remote-scale and local-scale responses of primary productivity to storms and their interaction with the underlying ocean mesoscale turbulence may be key to better understanding the sensitivities of the carbon cycle to short-term variability and long-term trends in atmospheric forcing.
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    Modelling Benguela niños using the regional oceanic modeling system (ROMS)
    (2007) Queiroz, Eurico Tiago Justino; Shillington, Frank; Monteiro, Pedro
    This study is framed by three questions: firstly, could the Regional Oceanic Modelling System (ROMS) reproduce the seasonal cycle of the equatorial Atlantic? Secondly, what is the nature of the link between remote forcing in the western equatorial Atlantic and Benguela Niños/Niñas? Thirdly, what is the impact of these events on the equatorial Atlantic Ocean SST and circulation patterns? The results obtained suggest that the model is very sensitive to different wind stress forcing, particularly in respect of the impact on the mixed layer characteristics. As a result the equatorial upwelling is overestimated in both temporal and spatial scales.