Sedimentary evolution and effects of structural controls on the development of the Zambezi mixed turbidite-contourite system (Mozambique channel, southwest Indian Ocean) since the Oligocene

Abstract 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. On the annual mean, the recirculation in the southwest Indian Ocean subgyre is by far the dominant contribution to the volume transport of the Agulhas Current in the model. On average the recirculation also contributes the largest amount of heat, although the difference between the three sources is not as great as that seen in the volume flux since the water in the Mozambique Channel may be warmer than in the recirculation. Local winds seem to be the dominant forcing mechanism of variability in this recirculation, although it is also partly in phase with the zonally averaged wind stress curl over the subtropical Indian Ocean. A strong relationship was found between the transport of the recirculation and that of the Agulhas Current, particularly on interannual time scales. Consistent with observations, the model flow through the Mozambique Channel is dominated by eddies, with a strong annual cycle, lagging the South Equatorial Current by 1 month and a weaker semiannual cycle. The southern limb of the East Madagascar Current also shows an annual and semiannual variation in transport at 20°S, partly in phase with the local winds. South of about 24°S, the East Madagascar Current breaks up into eddies. An investigation into the sensitivity of the flows in the source regions and in the retroflection index to a 2° southward shift in the mean winds was conducted. In the model run with the shifted winds, the transport strengthened in the recirculation subgyre together with increased mesoscale activity as well as reduced leakage into the southeast Atlantic Ocean when the winds were shifted south by 2°.

Download Full-text

Circulation pattern controls of wet days and dry days in Free State, South Africa

Meteorology and Atmospheric Physics ◽

10.1007/s00703-021-00822-0 ◽

2021 ◽

Author(s):

Chibuike Chiedozie Ibebuchi

Keyword(s):

South Africa ◽

Indian Ocean ◽

Atmospheric Circulation ◽

Southern Africa ◽

Austral Summer ◽

Free State ◽

Austral Spring ◽

Study Region ◽

Southwest Indian Ocean ◽

South Indian

AbstractAtmospheric circulation is a vital process in the transport of heat, moisture, and pollutants around the globe. The variability of rainfall depends to some extent on the atmospheric circulation. This paper investigates synoptic situations in southern Africa that can be associated with wet days and dry days in Free State, South Africa, in addition to the underlying dynamics. Principal component analysis was applied to the T-mode matrix (variable is time series and observation is grid points at which the field was observed) of daily mean sea level pressure field from 1979 to 2018 in classifying the circulation patterns in southern Africa. 18 circulation types (CTs) were classified in the study region. From the linkage of the CTs to the observed rainfall data, from 11 stations in Free State, it was found that dominant austral winter and late austral autumn CTs have a higher probability of being associated with dry days in Free State. Dominant austral summer and late austral spring CTs were found to have a higher probability of being associated with wet days in Free State. Cyclonic/anti-cyclonic activity over the southwest Indian Ocean, explained to a good extent, the inter-seasonal variability of rainfall in Free State. The synoptic state associated with a stronger anti-cyclonic circulation at the western branch of the South Indian Ocean high-pressure, during austral summer, leading to enhanced low-level moisture transport by southeast winds was found to have the highest probability of being associated with above-average rainfall in most regions in Free State. On the other hand, the synoptic state associated with enhanced transport of cold dry air, by the extratropical westerlies, was found to have the highest probability of being associated with (winter) dryness in Free State.

Download Full-text

Variability in the Mozambique Channel Trough and Impacts on Southeast African Rainfall

Journal of Climate ◽

10.1175/jcli-d-19-0267.1 ◽

2020 ◽

Vol 33 (2) ◽

pp. 749-765 ◽

Cited By ~ 3

Author(s):

Rondrotiana Barimalala ◽

Ross C. Blamey ◽

Fabien Desbiolles ◽

Chris J. C. Reason

Keyword(s):

Indian Ocean ◽

Austral Summer ◽

Western Indian Ocean ◽

Strong Relationship ◽

Ocean Basin ◽

Moist Convection ◽

South Indian Ocean ◽

Mozambique Channel ◽

Mascarene High ◽

South Indian

AbstractThe Mozambique Channel trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene high (MH) is located farthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross-equatorial northeasterly monsoon in the tropical western Indian Ocean, which curves as northwesterlies toward northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the south Indian Ocean. Variability of the MCT shows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical–extratropical cloud bands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern African mainland. Most years with a weak MCT are associated with strong positive south Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.

Download Full-text