Decadal Rainfall Dipole Oscillation over Southern Africa Modulated by Variation of Austral Summer Land–Sea Contrast along the East Coast of Africa

Abstract

A rainfall dipole mode characterized by negative correlation between subtropical southern Africa and equatorial eastern Africa is identified in instrumental observation data in the recent 100 years. The dipole mode shows a pronounced oscillation signal at a time scale of about 18 years. This study investigates the underlying dynamical mechanisms responsible for this dipole pattern.It is found that the southern African rainfall dipole index is highly correlated to the land-sea contrast along the east coast of Africa. When the land-sea thermal contrast strengthens, the easterly flow toward the continent becomes stronger. The stronger easterly flow, via its response to east coast topography and surface heating, leads to a low pressure circulation anomaly over land south of the maximum easterly flow anomalies and thus causes more rainfall in the south.On a decadal time scale, an ENSO-like SST pattern acts to modulate this land-sea contrast and the consequent rainfall dipole. During a ''wet in the south and dry in the north'' dipole, there are warm SSTs over the central Indian Ocean and cold SSTs over the western Indian Ocean. The cold SSTs over the western Indian Ocean further enhance the land-sea contrast during austral summer. Moreover, these cold western Indian Ocean SSTs also play an important role in regulating land temperature, thereby suppressing clouds and warming the land via increased shortwave radiation over the less-cloudy land. This cloud-SST coupling acts to further strengthen the land-sea contrast.

The African Humid Period, rapid climate change events, the timing of human colonization, and megafaunal extinctions in Madagascar during the Holocene: Evidence from a 2m Anjohibe Cave stalagmite

“…8, Panels A and C; also Burrough and Thomas, 2013). In fact, measured data show a rainfall dipole with the equatorial east Africa and subtropical southeast Africa poles negatively correlated (Zhang et al, 2015). Importantly, our data from Anjohibe Cave fit with the east African pattern and are opposite to conditions in southeast Africa.…”

“…The rainfall dipole between east Africa/northwest Madagascar and southeast Africa/southcentral Madagascar has been confirmed by instrumental data (e.g. Jury et al, 1995;Mason and Jury, 1997;Mason, 2001;FEWS NET, 2014;Neukom et al, 2014;Zhang et al, 2015).…”

Holocene hydrologic and vegetation developments in the Orange River catchment (South Africa) and their controls

“…Badewien et al, 2015; Chevalier and Chase, 2015; Collins et al, 2014; Hahn et al, 2016; Partridge, 1997; Zhao et al, 2016). First, increased local insolation leads to stronger atmospheric convection due to enhancement of the atmospheric low pressure cell during austral summer (Zhang et al, 2015). Second, higher insolation leads to a stronger thermal land–sea gradient resulting into a stronger easterly flow and thus to more precipitation in South Africa (Zhang et al, 2015).…”

“…First, increased local insolation leads to stronger atmospheric convection due to enhancement of the atmospheric low pressure cell during austral summer (Zhang et al, 2015). Second, higher insolation leads to a stronger thermal land–sea gradient resulting into a stronger easterly flow and thus to more precipitation in South Africa (Zhang et al, 2015). Chevalier and Chase (2015) suggested that the timing of the maximum humidity shifts from early to mid-late Holocene towards the south due to direct insolation forcing.…”

Rainfall and dry spell occurrence in Ghana: trends and seasonal predictions with a dynamical and a statistical model

“…West African rainfall is highly variable on interannual and decadal time scales and is highly correlated with sea surface temperature (SST) (Zhang et al 2015). Globally, dry conditions over the Sahel and wet conditions over Guinea are associated with positive El Niño-Southern Oscillation (ENSO) SST anomalies of the eastern tropical Pacific, with positive SST anomalies of the Southern Hemisphere Atlantic and with negative anomalies of the Northern Hemisphere Atlantic (the Atlantic dipole) and positive SST anomalies of the tropical Indian Ocean (Folland et al 1986;Janicot et al 1998;Rowell 2001;Matthews 2004).…”