Juliet Hermes | University of Cape Town (original) (raw)
Address: Cape Town, Western Cape, South Africa
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Deep Sea Research Part II: Topical Studies in Oceanography, 2003
Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean ar... more Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both mesoand longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean-atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about 0:84 PW (standard deviation 0:13 PW) of heat flows west into the South Atlantic across 201E (longitude of Cape Agulhas, the southernmost point of Africa), with just over 1 PW (standard deviation 0:4 PW) flowing north into the South Atlantic across 351S: The seasonal variations in this transport are about 10% at 351S in the South Atlantic and around 20% through 201E; the model value of 0:66 PW for summer (standard deviation ranging from 0:13 PW in January to 0:17 PW in March) appears consistent with respective estimates of 0.51 and 0:60 PW derived from two WOCE summer cruises southwest of Cape Town to 451S in 1990 and 1993. Volume transports of the Agulhas Current section through 351S in the SW Indian Ocean range from 58 to 59 Sv in summer/autumn to 64-65 Sv in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate. r Fig. 3. The monthly mean volume transport (Sv) (bold line) along 201E between 45-351S together with the maximum (upper line) and minimum (lower line) values and one standard deviation above and below the mean (light lines) (a). Panel (b) shows a time series of this transport for the years 31-35 of the integration. C.J.C. Reason et al. / Deep-Sea Research II ]
An eddy-permitting, regional ocean model has been used to examine the variability in the source r... more An eddy-permitting, regional ocean model has been used to examine the variability in the source regions of the Agulhas Current on a range of time scales. These source regions are the East Madagascar Current, the flow through the Mozambique Channel, and the recirculation of the southwest Indian Ocean. The effect of variability in these source regions on the interocean leakage at the Agulhas retroflection south of Africa has been quantified using a retroflection index.
Variability in sea-surface temperature (SST) and winds in the Angola Benguela frontal zone (ABFZ)... more Variability in sea-surface temperature (SST) and winds in the Angola Benguela frontal zone (ABFZ) in the tropical south-east Atlantic Ocean has previously been shown to be important for regional fisheries and for seasonal rainfall anomalies over Angola/Namibia in austral summer and coastal West Africa in boreal summer. This study investigates intraseasonal variability in winds and SST over this region using QuikSCAT and tropical rainfall measuring mission (TRMM) satellite data for 1999-2004. Wavelet analyses reveal periods of relatively strong power in the 20-30 or 30-64 day frequency bands throughout the record but that there is substantial interannual variability in the occurrence of these intraseasonal oscillations. The implications of this variability for seasonal rainfall anomalies during the main rainy seasons in southern Africa (austral summer) and coastal West Africa (boreal summer) are discussed.
A global ocean model (ORCA2) forced with 50 yr of NCEP-NCAR reanalysis winds and heat fluxes has ... more A global ocean model (ORCA2) forced with 50 yr of NCEP-NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics.
Deep Sea Research Part II: Topical Studies in Oceanography, 2003
Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean ar... more Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both mesoand longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean-atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about 0:84 PW (standard deviation 0:13 PW) of heat flows west into the South Atlantic across 201E (longitude of Cape Agulhas, the southernmost point of Africa), with just over 1 PW (standard deviation 0:4 PW) flowing north into the South Atlantic across 351S: The seasonal variations in this transport are about 10% at 351S in the South Atlantic and around 20% through 201E; the model value of 0:66 PW for summer (standard deviation ranging from 0:13 PW in January to 0:17 PW in March) appears consistent with respective estimates of 0.51 and 0:60 PW derived from two WOCE summer cruises southwest of Cape Town to 451S in 1990 and 1993. Volume transports of the Agulhas Current section through 351S in the SW Indian Ocean range from 58 to 59 Sv in summer/autumn to 64-65 Sv in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate. r Fig. 3. The monthly mean volume transport (Sv) (bold line) along 201E between 45-351S together with the maximum (upper line) and minimum (lower line) values and one standard deviation above and below the mean (light lines) (a). Panel (b) shows a time series of this transport for the years 31-35 of the integration. C.J.C. Reason et al. / Deep-Sea Research II ]
An eddy-permitting, regional ocean model has been used to examine the variability in the source r... more An eddy-permitting, regional ocean model has been used to examine the variability in the source regions of the Agulhas Current on a range of time scales. These source regions are the East Madagascar Current, the flow through the Mozambique Channel, and the recirculation of the southwest Indian Ocean. The effect of variability in these source regions on the interocean leakage at the Agulhas retroflection south of Africa has been quantified using a retroflection index.
Variability in sea-surface temperature (SST) and winds in the Angola Benguela frontal zone (ABFZ)... more Variability in sea-surface temperature (SST) and winds in the Angola Benguela frontal zone (ABFZ) in the tropical south-east Atlantic Ocean has previously been shown to be important for regional fisheries and for seasonal rainfall anomalies over Angola/Namibia in austral summer and coastal West Africa in boreal summer. This study investigates intraseasonal variability in winds and SST over this region using QuikSCAT and tropical rainfall measuring mission (TRMM) satellite data for 1999-2004. Wavelet analyses reveal periods of relatively strong power in the 20-30 or 30-64 day frequency bands throughout the record but that there is substantial interannual variability in the occurrence of these intraseasonal oscillations. The implications of this variability for seasonal rainfall anomalies during the main rainy seasons in southern Africa (austral summer) and coastal West Africa (boreal summer) are discussed.
A global ocean model (ORCA2) forced with 50 yr of NCEP-NCAR reanalysis winds and heat fluxes has ... more A global ocean model (ORCA2) forced with 50 yr of NCEP-NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics.