Abstract
The study diagnoses the relative impacts of the four known tropical Indo-Pacific drivers, namely, El Niño Southern Oscillation (ENSO), ENSO Modoki, Indian Ocean Dipole (IOD), and Indian Ocean Basin-wide mode (IOBM) on African seasonal rainfall variability. The canonical El Niño and El Niño Modoki are in general associated with anomalous reduction (enhancement) of rainfall in southern (northern) hemispheric regions during March-May season. However, both the El Niño flavours anomalously reduce the northern hemispheric rainfall during June-September. Interestingly, during boreal spring and summer, in many regions, the Indian Ocean drivers have influences opposite to those from tropical Pacific drivers. On the other hand, during the October-December season, the canonical El Niño and/or positive IOD are associated with an anomalous enhancement of rainfall in the Eastern Africa, while the El Niño Modoki events are associated with an opposite impact. In addition to the Walker circulation changes, the Indo-Pacific drivers influence the African rainfall through modulating jet streams. During boreal summer, the El Niño Modoki and canonical El Niño (positive IOD) tend to weaken (strengthen) the tropical easterly jet, and result in strengthening (weakening) and southward shift of African easterly jet. This anomalously reduces (enhances) rainfall in the tropical north, including Sahelian Africa.
Highlights
The study diagnoses the relative impacts of the four known tropical Indo-Pacific drivers, namely, El Niño Southern Oscillation (ENSO), ENSO Modoki, Indian Ocean Dipole (IOD), and Indian Ocean Basinwide mode (IOBM) on African seasonal rainfall variability
This suggests that the IOD events could influence the African rainfall through modulating the intensity of jet streams. This mechanism is different from the well-known mechanisms such as the anomalous low level convergence patterns induced over east Africa as a direct response to the anomalous warming in the tropical west Indian Ocean during a positive IOD event[22,25,26]. To confirm that such an attributed change in zonal winds during the boreal summer is due to the forcing from Indo-Pacific events, we perform the following several sensitivity experiments using the LMDZ (Laboratoire de Meteorologie Dynamique and Z stands for Zoom) Atmospheric General Circulation Model (AGCM), (a) a control experiment with climatological Sea Surface Temperatures (SST), and experiments in which the AGCM is forced with SSTs representative of a typical (b) El Niño Modoki, (c) canonical El Niño and (d) positive IOD
The study suggests that the ENSO Modoki events significantly, and distinctly, affect the seasonal rainfall Africa
Summary
The study diagnoses the relative impacts of the four known tropical Indo-Pacific drivers, namely, El Niño Southern Oscillation (ENSO), ENSO Modoki, Indian Ocean Dipole (IOD), and Indian Ocean Basinwide mode (IOBM) on African seasonal rainfall variability. The El Niño Modoki and canonical El Niño (positive IOD) tend to weaken (strengthen) the tropical easterly jet, and result in strengthening (weakening) and southward shift of African easterly jet This anomalously reduces (enhances) rainfall in the tropical north, including Sahelian Africa. Given the interference between teleconnections of the various tropical Indo-Pacific drivers, this study is focused on assessing the relative impact of the known tropical Indo-Pacific drivers, namely ENSO Modoki, canonical ENSO, and IOD/IOBM (depending on the seasons they mainly prevail in), on African rainfall variability during different seasons, and in particular, to bring out the hitherto unexplored potential influence of ENSO Modoki on African rainfall. Canonical ENSO1,2,15 and IOD22,25 are reported to have strong influence over the tropical African region, including Sahel, which receive more than 80% of annual total rainfall during boreal summer
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