Browsing by Author "Roberts, Mike"

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    Comparing in situ and satellite temperature data on the Agulhas Bank to understand changes in anchovy (Engroulis encrasicolus) distribution
    (2014) Schlegel, Robert William; Moloney, Coleen; Roberts, Mike; Van der Lingen, Carl D
    Satellite data have high spatial/ temporal resolution, extensive coverage and are easily accessible, making them a common part of many studies on the oceans. One such important study to use satellite data found a relationship between the cross-shelf SST difference on the Agulhas Bank and the relative distribution of anchovy (Engraulis encrasicolus) spawner biomass east of Cape Agulhas. However, other studies have shown that nearshore satellite data may not be as accurate as originally believed. Using the relationship observed in the aforementioned study as a test, I compared time series' from two types of in situ temperature data to satellite SST data. A combined CTD/CalVET (PISCTD) data set was used for in situ data on the coastal and offshore regions of the Agulhas bank whereas a data set with two UTRs was used for inshore in situ data. None of the data sets correlated significantly with each other, save the two UTRs. Both in situ data sets showed a negative relationship between the size of the cross-shelf temperature difference on the Agulhas bank and the increase in anchovy biomass there, whereas the satellite data showed a positive relationship. This was largely due to the pattern of decadal warming observed in the in situ data whereas the satellite data showed decadal cooling. Even though it was found that the difference in sampling methodology between the satellite and in situ data sets prevented them from being accurately compared, the difference in the annual and decadal patterns between these two types of data do support other findings showing discrepancies between remotely-sensed and in situ data for nearshore environments.
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    Driving mechanisms of the Port Alfred upwelling cell inshore of the Agulhas Current
    (2013) Malan, Neil; Ansorge, Isabelle Jane; Roberts, Mike
    The presence of a semi-permanent upwelling cell, with a surface expression more than 40 % of the time has previously been described inshore of the Agulhas Current in the vicinity of Port Alfred, South Africa. This study employs a combination of in-situ mooring data, hydrographic cruises and satellite remote sensing in order to investigate the nature and variability of this upwelling cell, as well as to investigate possible driving mechanisms. The study takes place over a period of 11 months. Special focus is given to the subsurface variability due to its possible implications for the greater Agulhas Bank environment. Upwelled water was found to be present on the shelf 85% of the time, highlighting the importance of subsurface variability in this area. The main timescales of variability were observed at 50-70 days, 8-12 days and 4-6 days. Upwelling was found to be maintained by continuous processes, driven by the interaction of the Agulhas Current with the changing bathymetry at Port Alfred. This upwelling is modulated by the effects of mesoscale features on the inshore edge of the current. While not a primary driver of upwelling, wind events were observed to have an effect on inshore bottom temperatures as well as the surface expression of the upwelling cell. A high degree of variability in was observed, with bottom temperatures at three mooring site fluctuating through a range of approximately 10° C. Future directions include further theoretical and idealised modeling studies to separate out the exact mechanisms of topographically driven, site-specific upwelling. The range of mesoscale interactions of the Agulhas Current with the shelf circulation also require further observational study.
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    Shelf edge upwelling off Northern Mozambique
    (2010) Malauene, Bernardino Sergio; Shillington, Frank; Moloney, Coleen; Roberts, Mike
    A combination of satellite and in situ data were used to infer the occurrence and temporalspatial variability of upwelling near Angoche. The data were derived from MODIS SST and Chl-a between 2003 and 2007, in situ moored underwater temperature recorder at a depth of 18 m for the period 2003ô2007 and two shipboard surveys providing CTD and XBT data in December 2008 and in August 2009 in the northern Mozambique region. The results con rm that shelf edge upwelling occurs along the northern coast of Mozambique near Angoche between 15 and 18oS, covering an area of approximately 68 000 km2. The upwelling signature was not strong at the surface. Two upwelling core regions were identi ed: (1) the shelf core region and (2) the slope core region. At the shelf core region upwelling was more persistent than at the slope core upwelling. The upwelling displayed seasonal variation between persistent downwelling (warm water) between AprilôJuly and intermittent upwelling (cool water) events between AugustôMarch. Generally the upwelling lasted for a period of about two months but, shorter periods between 8 and 30 days were also observed. The driving mechanism for the shelf edge upwelling o northern Mozambique was determined from satellite observations (NOAA/NCDC) of blended sea surface wind data, and by multi-satellite (AVISO) altimeter sea surface anomaly data between 2003 and 2007 and also during the two cruise surveys in December 2008 and August 2009. These data showed upwelling is in part wind-driven in response to the northôeasterly (NE) monsoon weak-wind velocities between AugustôMarch in the austral spring-summer. The intermittent nature of the upwelling season is also partly due to uctuations in wind direction during the NE monsoon winds. Eddies appear to play a limited role in driving the upwelling.
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    Wave energy distribution across the Agulhas Bank, a source of renewable energy for a seawater pumped storage scheme
    (2013) Francisco, Francisco Gemo Albino; Reason, Chris; Roberts, Mike