http://hdl.handle.net/10204/934 2013-05-25T01:44:37Z 2013-05-25T01:44:37Z Mariotti, V Bopp, L Tagliabue, A Kageyama, M Swingedouw, D http://hdl.handle.net/10204/6720 2013-05-03T21:55:11Z 2012-10-01T00:00:00Z

Title: Marine productivity response to Heinrich events: a model-data comparison Authors: Mariotti, V; Bopp, L; Tagliabue, A; Kageyama, M; Swingedouw, D Abstract: Marine sediments records suggest large changes in marine productivity during glacial periods, with abrupt variations especially during the Heinrich events. Here, we study the response of marine biogeochemistry to such an event by using a biogeochemical model of the global ocean (PISCES) coupled to an ocean-atmosphere general circulation model (IPSL-CM4). We conduct a 400-yr-long transient simulation under glacial climate conditions with a freshwater forcing of 0.1 Sv applied to the North Atlantic to mimic a Heinrich event, alongside a glacial control simulation. To evaluate our numerical results, we have compiled the available marine productivity records covering Heinrich events. We find that simulated primary productivity and organic carbon export decrease globally (by 16% for both) during a Heinrich event, albeit with large regional variations. In our experiments, the North Atlantic displays a significant decrease, whereas the Southern Ocean shows an increase, in agreement with paleo-productivity reconstructions. In the Equatorial Pacific, the model simulates an increase in organic matter export production but decreased biogenic silica export. This antagonistic behaviour results from changes in relative uptake of carbon and silicic acid by diatoms. Reasonable agreement between model and data for the large-scale response to Heinrich events gives confidence in models used to predict future centennial changes in marine production. In addition, our model allows us to investigate the mechanisms behind the observed changes in the response to Heinrich events. Description: Copyright: 2012 European Geosciences Union. This is an Open Access journal. This journal authorizes the publication of the information herewith contained. Published in Climate of the Past, vol. 8(5), pp 1581-1598

2012-10-01T00:00:00Z Malherbe, J Engelbrecht, FA Landman, WA http://hdl.handle.net/10204/6658 2013-04-11T21:55:10Z 2012-12-01T00:00:00Z

Title: Projected changes in tropical cyclone climatology and landfall in the Southwest Indian Ocean region under enhanced anthropogenic forcing Authors: Malherbe, J; Engelbrecht, FA; Landman, WA Abstract: The conformal-cubic atmospheric model, a variable-resolution global model, is applied at high spatial resolution to perform simulations of present-day and future climate over southern Africa and over the Southwest Indian Ocean. The model is forced with the bias-corrected sea-surface temperatures and sea-ice of six coupled global climate models that contributed to Assessment Report 4 of the Intergovernmental Panel on Climate Change. All six simulations are for the period 1961–2100, under the A2 emission scenario. Projections for the latter part of the 21st century indicate a decrease in the occurrence of tropical cyclones over the Southwest Indian Ocean adjacent to southern Africa, as well as a northward shift in the preferred landfall position of these systems over the southern African subcontinent. A concurrent increase in January to March rainfall is projected for northern Mozambique and southern Tanzania, with decreases projected further south over semi-arid areas such as the Limpopo River Basin where these systems make an important contribution as main cause of widespread heavy rainfall. It is shown that the projected changes in tropical cyclone attributes and regional rainfall occur in relation to changes in larger scale atmospheric temperature, pressure and wind profiles of the southern African region and adjacent oceans. Description: Copyright: 2012 Springer-Verlag Berlin Heidelberg. This is an ABSTRACT ONLY. The definitive version is published in Climate Dynamics DOI 10.1007/s00382-012-1635-2

2012-12-01T00:00:00Z Engelbrecht, CJ Engelbrecht, FA Dyson, LL http://hdl.handle.net/10204/6651 2013-04-11T21:55:09Z 2012-01-01T00:00:00Z

Title: High-resolution model-projected changes in mid-tropospheric closed-lows and extreme rainfall events over southern Africa Authors: Engelbrecht, CJ; Engelbrecht, FA; Dyson, LL Abstract: Mid-tropospheric closed-lows (cold-core cut-off lows and warm-core tropical lows) are important rain producing weather systems for the southern Africa region. Over South Africa, most wide-spread flood events are caused by these systems. It is therefore important to explore the potential impact of anthropogenic forcing on the occurrence of closed-lows and extreme rainfall events over the region. Coupled global circulation models (CGCMs) can not be directly applied for this purpose because of their relatively low spatial resolution – some form of downscaling is required to adequately resolve these systems and the rainfall they cause. In this study, a variable-resolution atmospheric global circulation model is applied as a regional climate model to simulate closed-low characteristics over southern Africa under current and future forcings. The model is forced with greenhouse gas concentrations according to the A2 SRES scenario and with sea surface temperatures (SSTs) and sea-ice as specified by the CSIRO Mk3 CGCM. The model projects a general decrease in closed-low frequencies over the region, which occurs in association with a general strengthening of the subsiding branch of the Hadley cell. However, the climate-change signal shows variation in time and space and certain sub-regions are projected to experience an increase in closed-low frequencies during certain seasons. A general increase in extreme rainfall events is projected over southern Africa despite the projected decrease in closed-low frequencies. It is deduced that this increase in extreme rainfall events is driven by intense convective rainfall events occurring within more frequently forming tropical-temperate cloud bands. Over Mozambique, extreme rainfall events are projected to increase in association with more frequently occurring closed-lows. Description: Copyright: 2012 Royal Meteorological Society. This is an ABSTRACT ONLY. The definitive version is published in International Journal of Climatology, vol. 33, pp 173-187

2012-01-01T00:00:00Z Lück-Vogel, M O’Farrell, PJ Roberts, W http://hdl.handle.net/10204/6616 2013-03-25T21:55:26Z 2012-12-01T00:00:00Z

Title: Remote sensing based ecosystem state assessment in the Sandveld region, South Africa Authors: Lück-Vogel, M; O’Farrell, PJ; Roberts, W Abstract: We present a remote sensing based approach for assessing ecosystem state or intactness to inform land use management and conservation planning. Using segmented multispectral medium resolution satellite imagery, parameters related to the image objects’ spectral brightness and heterogeneity, and compactness are used to derive a scoring system of 0 to 10 for the ecosystem intactness, with 0 being completely degraded and 10 being pristine. Linked to the remote sensing approach we suggest a field validation approach that focuses on 10 ecosystem-relevant visually assessed parameters which, when combined, produce a score out of 10 as well. The approach was tested in the South African Sandveld region using a SPOT 5 image from 2009 and a Landsat 7 ETM+ image from 2011. Field assessments took place in 2011. Both image data sets returned consistent results suggesting an inter-sensor transferability of the approach. Inconsistencies between satellite and field scores occurred mainly on sites where crops were currently being grown and on fields where various stages of succession were underway, following abandonment. Masking out of those sites which are of little interest from an ecosystem state perspective would improve overall accuracies. For regions with vegetation types that differ significantly in cover and structure, a stratified approach is suggested to optimise the results per vegetation type. Outputs suggest that the approach with its standardised and robust results and its repeatability provides a suitable tool for long term monitoring of large regions with a degree of detail sufficiently high to allow for fine scale planning. Description: Copyright: 2012 Elsevier. This is the pre/post print version of the work. The definitive version is published in Ecological Indicators, pp 1-11.

2012-12-01T00:00:00Z