Browsing by Author "Lamont, Tarron"

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    Bio-optical investigation of phytoplankton production in the southern Benguela ecosystem
    (2011) Lamont, Tarron; Barlow, RG; Shillington, Frank
    This study aims to evaluate various existing models for the estimation of PP in the southern Benguela ecosystem, and to investigate the photophysiological responses of phytoplankton to changes in environmental conditions.
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    Building A Mean-state Of Oceanographic Properties (Temperature And Salinity) For The Kwazulu-natal Bight Using The Roms Model: A Contribution Towards Marine Protected Areas Analysis
    (2018) Malange, Mathabo Noxolo; Morris, Tamaryn; Collins, Charine; Lamont, Tarron; Ansorge, Isabel
    The KwaZulu-Natal Bight, located along the east coast of South Africa, is an important recruitment and nursery area for various marine species. In an effort to conserve a number of threatened species, two Marine Protected Areas (MPA) have been established in the Bight. The African Coelacanth Ecosystem Programme is conducting MPA analyses along the Bight through a series of biological and oceanographic studies and this study forms part of the oceanographic research component that will assist in the decision-making process of MPAs in the region. This study uses a 30-year, high-resolution, regional ROMS simulation to build a climatology representative of the mean-state of the Bight. The model is also used to investigate the seasonal and annual variability as well as the influence of the Agulhas Current on the shelf. The Bight was cooler and less saline than the surrounding waters and seasonal variation was limited to the upper 50 m of the water column. The depth of the Bight ranges from 50 m in the inner shelf to 100 m at the shelf edge in the central region of the Bight. In the northern and southern region of the Bight, the depth of the water column extends down to about 150 m at the shelf edge. In summer, surface temperatures were on average 4.8°C and 4.3°C warmer than in winter over the uThukela Banks and Aliwal Shoal respectively. Bottom temperatures at both MPAs had a mean seasonal variation of about 3°C. Salinity, a more conservative variable, showed little variability over the year throughout the water column except for at 50 m where lower salinities were observed in the winter months. Wavelet analysis showed that a strong annual (12 month) signal was dominant at the surface (10 m). Bottom temperatures displayed a weaker annual signal than the surface in addition to a slight semi-annual cycle. Further investigations indicated that the Agulhas Current influenced the Aliwal Shoal MPA more than the uThukela Banks MPA as they shared similar temperature values (at the surface and bottom) throughout the 30-year period. In contrast, the uThukela was cooler than the Agulhas Current by 0.5 to 1.5°C at the surface and 1 to 2.5°C at the bottom. These time series also enabled us to identify anomalous features such as the Natal Pulse that could have important implications for temperature-sensitive species in the area.
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    Comparative analysis of the seasonal sea surface temperature & wind stress in the four major eastern boundary current systems
    (2006) Lamont, Tarron; Shillington, Frank; Roy, Claude
    The coastal upwelling regions along northwest Africa (the Canary system), southwest Africa (the Benguela system), North America (the California system), and South America (the Peru-Chile system) were studied and compared on a seasonal timescale. A 17 -year NOAA Pathfinder Sea Surface Temperature (SST) dataset with a spatial resolution of 9km was used to describe the large-scale temporal and spatial variability of upwelling within the four regions. An upwelling index (OSST) was derived in order to add to the patterns of upwelling variability described from the sea surface temperature. The upwelling index was also used to describe the similarities and differences between the four upwelling regions. A 10-year ERS wind stress dataset with a spatial resolution of 10 x 10 was used to derive the offshore Ekman Transport in each of the regions. The offshore Ekman Transport was used to supplement the description of SST's in each system. Principal Component Analysis was used to investigate the variance structure of the anomalies of the sea surface temperature in each of the four regions. The results of the principal component analysis are interpreted in terms of the underlying physical dynamics.
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    Downstream evolution of ocean properties and associated fluxes in the Greater Agulhas Current System: Ad hoc Argo experiments and modeling
    (2020) Morris, Tamaryn; Ansorge, Isabel; Hermes, Juliet; González, Borja Aguiar; Lamont, Tarron
    The evolution of cyclonic eddies across the Southern Mozambique Chanel and the downstream evolution of the Agulhas Current was investigated using Argo floats, in combination with output from ocean general circulation reanalysis models. Two dedicated experiments were undertaken in April and July 2013, whereby eight floats were deployed within two separate cyclonic eddies. Floats were set to either daily and five-daily profiling from 1000 db to the surface, with park depths ranging from 300 db to 1000 db. The two cyclonic eddies propagated southwestward across the Mozambique Channel from southwest Madagascar to the KwaZulu-Natal Bight, a distance of approximately 1300 km, in approximately 130 days at a mean speed of 0.13 m s−1 . Estimates indicate the April (July) eddy showed mean trapped depths of 595 ± 294 m (914 ± 107 m), volume transport of 13.4 ± 5.2 Sv (21.2 ± 9.1 Sv), heat flux of -0.07 ± 0.06 PW (-0.2 ± 0.09 PW) and freshwater flux of 0.04 ± 0.04 Sv (0.09 ± 0.05 Sv). These results highlight the role of Madagascar cyclonic eddies as transporters of cooled and freshened source waters into the Agulhas Current. During a third experiment, six floats were deployed in the Agulhas Current, and exited the current within 9 - 12 days at mean speeds of 0.51 – 0.76 m s−1 . An evolution of properties was shown from north to south for both Argo data and model output; for volume transport (16.76 – 38.18 Sv; 17.70 – 32.51 Sv), heat fluxes (0.85 – 1.79 PW; 0.99 – 1.91 PW) and salt fluxes (0.60 – 1.37 x 1012 kg s−1 ; 0.63 – 1.17 x 1012 kg s−1 ). This study illustrates the first near-real time survey of the Agulhas Current, and a potential method of quasi-synoptic surveys using Argo float technology. These experiments highlight alternative methods of studying regions of turbulence by altering the mission parameters of Argo floats. Increased observations of eddies and Western Boundary Currents are critical to our understanding of the global oceans and impacts on the earths climate. Even more so for the understudied Indian Ocean.
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    Interannual variability and long-term trends of surface hydrography around the Prince Edward Island Archipelago, Southern Ocean
    (2022) Toolsee, Tesha; Lamont, Tarron; Rouault, Mathieu
    The Prince Edward Islands (PEIs) Archipelago are situated in a prime location for the study of ecosystem response to intrinsic climate variability in the Southern Ocean and the impact of climate change. They are positioned in the Polar Frontal Zone, which is constrained by the subAntarctic Front and the Antarctic Polar Front, all of which are part of the strong, uninterrupted Antarctic Circumpolar Current (ACC). Due to its remoteness and challenging accessibility, there is a severe lack of data in the Southern Ocean and at the PEIs. The existing data are only available as single points observations or snapshots from past research cruises. This study thus makes use of 23 years (1993 – 2016) of satellite and reanalysis data to determine the annual/interannual and long-term variability of Sea Surface Temperature (SST), wind forcing and surface circulation at the PEIs and determine whether natural modes of climate variability like the El Niño Southern Oscillation (ENSO), Southern Annular Mode (SAM) or SemiAnnual Oscillation (SAO) were affecting these parameters. SST, wind speed, wind stress curl and the Ekman current did not express any long-term trend. A significant increasing but very small trend was only perceived in the geostrophic current and total surface current which was concluded to not be associated with the intensification of the ACC caused by a more positive SAM phase. The anomalies in SST showed striking interannual variability at a periodicity of 0.8, 2.8 and 7.5 years showing a similar pattern to that of ENSO with a periodicity of 1.5, 2.9 and 6 years. There has however been no relationship established between SST and any of the climate modes, but the Antarctic Circumpolar Wave (ACW), which is one of ENSO's teleconnection, could be responsible for the interannual changes seen in the SST anomalies. The anomalies in wind speed did not show any apparent periodicity and no relationship with ENSO. More so, while the impact of SAM and SAO has been seen on the westerly wind belt which governs the latitude of the PEIs, no correlation was established between the wind speed at the islands and SAM or SAO. The anomalies in wind stress curl presented no visible interannual variability but some sign of short-term variability. There was also no link 2 established between wind stress curl at the PEIs and any of the climate modes. Finally, a periodicity of 1.3 and 4 years was seen in the geostrophic current anomalies which also coincided with the pattern of ENSO but only showed minor correlation with ENSO. The ACW was deduced to perhaps also be responsible for the surface currents anomalies since the ACW is primarily propagated within the ACC. The trends perceived in the parameters considered for this study and the impact of climate modes on them appeared to be different to patterns which has been historically observed across the Southern Ocean. This further confirms the fact that the neighbouring oceanography and surface wind speed variability surrounding the PEIs differ from other regions of the Southern Ocean, most probably due to the frequent mesoscale instability such as eddies and frontal movement influencing the region. The impact of climate change on the PEIs ecosystem thus cannot be expected to be the same as the rest of the Southern Ocean.
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    Investigating the local circulation of the southeast Cape Basin
    (2018) Carr, Matthew David; Lamont, Tarron; Ansorge, Isabelle Jane
    Located off the west coast of southern Africa, the southeast Cape Basin is characterised by a unique combination of strong coastal upwelling and vigorous offshore mesoscale variability. The juxtaposition of offshore mesoscale variability and coastal upwelling results in a complex and dynamic environment. In this study a combination of in situ Acoustic Doppler Current Profiler (ADCP) data and satellite observations were used to identify, describe and characterise the features driving the local circulation within the southeast Cape Basin. The ADCP data was obtained from the South Atlantic MOC Basin-wide Array (SAMBA), which included four deep sea moorings located along ~34.5°S on the 1000, 2000, 3000 and 4500m isobaths respectively. There was a distinct difference in the circulation observed at the mooring located on the shelf edge (1000m) and the moorings further offshore (2000m, 3000m and 45000m). The offshore circulation, observed by the moorings located on the 2000, 3000, 4500m isobaths, were driven by large mesoscale eddies, both cyclonic and anticyclonic, originating at the Agulhas retroflection and within the Cape Basin itself. The mesoscale eddies induced high speed baroclinic transport events which impacted the upper water column to a depth of at least 400m. The in situ observations were used to show the precise characteristics of two cyclonic shelf eddies and one anticyclonic eddy through the upper water column (~50 to 500m). The analysis of these features was important as the physical characteristics of both the cyclonic shelf eddies and anticyclonic eddies through depth are not well known. Considering the short period of observations (18th September 2014 to 1st December 2015), the analysis was not used to assume the prevailing physical characteristics of cyclonic shelf eddies and anticyclonic eddies. Instead the analysis showed the potential for future long term studies to use the sustained in situ observations from the SAMBA mooring array and similar analysis to define the precise characteristics of mesoscale eddies through depth. This will greatly improve the understanding of how these features influence the interocean exchange between the Indian and Atlantic Oceans. The circulation at the shelf edge, observed by the mooring located on the 1000m isobath, was shown to be driven by a combination of offshore mesoscale eddies, the position of the upwelling front and warm filaments formed at the reflection of the Agulhas Current. Identifying the influence of both the offshore mesoscale eddies and coastal upwelling at the shelf edge showed connectivity between the upwelling system and the offshore mesoscale variability. An example of the connectivity between the offshore mesoscale variability and the upwelling front was identified and presented. A dipole formed by two mesoscale eddies was observed to induced cross shelf transport advecting productive shelf waters offshore. The in situ observations were used to calculate the amount water transported from the shelf to the open ocean during this cross shelf transport event. The filament representing the cross shelf transport event was calculated to have a total volume of ~2 x 10¹² m³ with a volume transport of ~1Sv suggesting the event would have had a substantial impact on the local biology. The detailed analysis and quantification of the cross shelf transport event aimed to improve the current understanding of how mesoscale features interact with the upwelling system. In situ observations of cross shelf transport are rare, therefore the quantification of the amount of transported in the cross shelf transport event can serve as a baseline for future studies attempting to assess the impact of comparable cross shelf transport events on the local biology.
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    Long-term climate variability at the Prince Edward Islands in the Southern Ocean
    (2021) Shangheta, Anna Liisa Penelao Tulimevava; Lamont, Tarron; Ansorge, Isabel; Rouault, Mathieu
    A warming Southern Ocean (SO), due to climate change and global warming, has many implications on the sub-Antarctic Islands in the SO. Due to the distance away from continental land these islands experience an oceanic climate, making them the perfect sentinels to climate change in this sector of the Southern Ocean. Studies have proposed that climate changes reported at the Prince Edward Islands (PEIs) correspond in time to a southward shift of the Antarctic Circumpolar Current (ACC) particularly the Subantarctic Front (SAF). While other studies have shown distinctive trends in ocean and atmospheric parameters such as sea surface temperature (SST), air temperature, sunshine, rainfall, air sea level pressure and wind speed and direction from the 1950s to the early 2000s, the aim of this study is to update those studies to a more recent time with updated time series. Among the changes recorded is an increase in SST and air temperature, which is a strong indication of the changing local and global climate. Using linear regression, this study showed that the rates of increase from 1949 to 2018 of the SST (0.022°C/year), minimum (0.0072°C/year) and maximum air temperatures (0.016°C/year) are smaller than estimated in previous studies. The increasing trend in SST and air temperature reported by previous papers has actually stopped since the 2000s, which reduces the formerly reported trend (0.028°C/year). Although the in-situ measured SST data had gaps, a good correlation with in-situ SST and large scale satellite derived Reynolds SST help to corroborate the covariation between SST, in-situ SST and air temperature giving weight to the hypothesis of a reversal of the positive temperature trends reported by others. The change in decadal variability a decrease in air pressure of 4 hPa since the late 1990s to late 2000s, which coincided with a decrease in minimum and maximum air temperatures of 1°C over the same period; decrease in westerly wind and an increase in the northerly component of the wind, which would explain the decrease of inshore sea surface temperature a while thereafter. This study further corroborates previous findings of a continued decrease in rainfall, while the sunshine has largely remained the same. The seasonal cycle of the air pressure is significantly associated with that of rainfall, showing that the bimodal high air pressure signature resulting from the Semi-annual Oscillation (SAO) is associated with a decrease in rainfall. The Southern Annual Mode (SAM) was significantly yet weakly correlated with the SST (0.24), rainfall (-0.25) and air pressure (0.16), indicating that it does have an impact at the PEIs but not as strong as previously speculated. The El Niño Southern Oscillation (ENSO) has very weak and insignificant relationships with the parameters examined except for a weak relationship with in-situ SST, sunshine and air pressure. These new insights, especially at the decadal timescale, could further our insight on how subAntarctic islands have responded to climatic changes.
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    Marine heatwaves and warm events in the Cape Peninsula upwelling cell, Southern Benguela
    (2023) Petzer, Kirstin Robyn; Lamont, Tarron; Rouault, Mathieu
    Due to global warming, Marine Heatwaves (MHWs) are considered to be one of the emerging threats to marine ecosystems globally. MHWs are prolonged periods of extreme warm Sea Surface Temperature (SST) anomalies which can cause severe ecological impacts by decreasing biodiversity, negatively affecting cold water species and increasing ocean stratification. Using the Cape Point CSIR half-hourly in situ SST, CCI, REMSS, ERA5 wind time series, over 17-years the occurrence from January 2003 to March 2020, duration and maximum SST values as well as the influence of the wind on the formation and end on marine heatwaves and warm events (WEs) at a single location in the Cape Peninsula Cell, in the Southern Benguela, was examined. The MHW events were identified using Hobday et al. (2016), when the SST exceeds the climatological 90th percentile for at least five days. The WE events, defined similarly to a MHW but the SST must exceed the climatological 90th percentile for at least three days, are also studied due to the high variability of the Southern Benguela. In the half-hourly CSIR time series 14 MHWs and 21 WEs occurred over the 17 years. The average duration is between 7 to 8 days but the longest events occurred during periods of decreased upwelling but the highest maximum SSTs occur during the periods of upwelling dominance. The daily CSIR, CCI and REMSS time series all identified double the number of MHWs and WEs events than the half-hourly time series, raising the concern of applying the Hobday et al. (2016) definition to sub-daily time series and the ability of satellites to be used for MHW identification in the Southern Benguela close to the coast. The dominant wind at the formation of MHWs and WEs is a north-westerly wind, indicating the main driver of events at the CSIR Cape Point mooring is the movement of warm water masses to the mooring location. The dominant wind direction at the end of the MHWs and WEs is a south-easterly wind indicating that coastal upwelling limits the duration of warm water events at the Cape Point mooring. Marine heatwaves are expected to worsen globally with climate change by lasting longer with high temperature increases but the projected increase in southeasterly winds could further limit the duration of MHWs in the Southern Benguela upwelling system.
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    Spatial and Seasonal Variations of the Island Mass Effect at the Sub-Antarctic Prince Edward Islands Archipelago
    (2022-04-29) Lamont, Tarron; Toolsee, Tesha
    At the sub-Antarctic Prince Edward Islands (PEIs) in the Southern Ocean, the Island Mass Effect (IME) plays an important role in maintaining an ecosystem able to support diverse biological communities; however, limited in situ sampling has severely constrained our understanding of it. As such, our study used satellite chlorophyll a (chla) to provide the first detailed characterisation of the spatial extent and seasonal variability of the IME at the PEIs. Seasonal surface chla variations were remarkable, with localised increases observed from mid-austral spring to the end of autumn (October to May). In contrast, during June to September, there were no distinguishable differences between chla at the PEIs and that further afield. Seasonal chla changes were significantly correlated with higher light levels, warmer waters, and shallow upper mixed layer depths reflecting enhanced water column stability during summer and autumn, with the opposite pattern in winter and spring. The IME extended northeast of the islands and remained spatially distinct from elevated chla around the northern branch of the sub-Antarctic Front and the southern branch of the Antarctic Polar Front. From December to February, the IME was spatially connected to the island shelf. In contrast, during March–May and in October, higher chla was observed only to the northeast, some distance away from the islands, suggesting a delayed IME, which has not previously been observed at the PEIs. The clear association of this higher chla with the weak mean geostrophic circulation northeast of the islands suggested retention and accumulation of nutrients and phytoplankton biomass, which was likely aided by wind-driven northeastward transport of water from the shelf. Climatological mean chla to the northeast was generally higher than that on the PEI shelf, and further research is required to determine the importance of this region to ecosystem functioning at the islands.
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    Understanding variability across the Crossroad transect from 3 years (2013 to 2015) of hydrographic data
    (2018) Sejeng, Manare C; Ansorge, Isabel; Lamont, Tarron; Maes, Christophe
    The southwest continental shelf of Africa is characterized by a strong western boundary current with three interdependent components, namely the Agulhas Current, Agulhas Retroflection and Agulhas Return Current. This system plays a key role in setting oceanic conditions south of Africa. The Crossroad transect intersects both the Agulhas Current and Agulhas Return Current; a monitoring line established in 2013 to sample both the currents and determine inter-ocean fluxes, as well as the influence of the Agulhas Current on the Agulhas Bank shelf. The objective of the study was to examine both mesoscale and submesoscale features that influence the dynamic and variant nature of the Agulhas system. In this study we make use of Ship board Acoustic Doppler Current Profiler (SADCP), Conductivity Temperature and Depth (CTD), Thermosalinograph (TSG) and satellite Sea Surface Height data as main observations for analysis. The study also examines both the spatial and temporal characteristics of water properties across the Crossroad transect. The fundamental findings of the study include the abundance of both the mesoscale and submesoscale features observed in the Agulhas system, which are often overlooked. In addition, a noticeable variability in current measurements was observed, where velocity ranging from 2 to 2.5 m/s represented the Agulhas Current and 1.4 to 1.7 m/s, Agulhas Return Current. The position of the Agulhas Current and Agulhas Return Current displayed variation from 2013 to 2015, with the Agulhas Return Current exhibiting a meandering pattern in 2014 along the transect. Furthermore, an intrusion of cool (8 to 13 °C), lower salinity (34.8 to 35 psu) South Indian Central Water masses were also observed along the Agulhas Bank. The ability to combine altimetry and in situ data also contributed to the analysis of the results. Therefore, given the inherent advantage of satellite and in situ measurements, an overview of the variability across the Crossroad transect was determined.