Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 20

The main two components of the African Monsoons are the West African Monsoon (WAM), which prevails during the Northern Hemisphere summer (June through September), and the East African Monsoon (EAM) with occurs during spring (March–May) and autumn (October–December). The combined influence of the Indo‐Pacific and the Atlantic Oceans drive the interannual and decadal monsoon variability over these regions.

The key feature of the WAM is low‐level SW flow from the Atlantic Ocean and the Intertropical Convergence Zone (ITCZ) north of the equator. The WAM is characterised by the migration of zonally banded rainfall from the Guinea coast to the Sahel (a 3860‐km arc‐like landmass immediately south of the Sahara Desert stretching east–west across the breadth of the African continent) and back again, resulting in two rainy seasons per year in the south and one in the north. The dynamics of the WAM circulation are linked to the existence of the African easterly jet, a mid‐tropospheric flow with peak amplitude of about 10–20 m s⁻¹ in August at latitude 15°N. This jet is a basic part of the momentum balance of the WAM circulation and has a meteorological impact on the patterns of rainfall in the Sahel. Unlike the Australian monsoon, over West Africa the monsoon trough moves much further inland, some 15° or more in latitude from the Guinea coast, and therefore the pressure gradients over much of the Sahel are directed from the humid south towards the dry north. There is a seasonal cycle of the WAM with two phases: the ‘pre‐onset’ and the ‘onset’ (Janicot et al. 2011). In the ‘pre‐onset’ phase, the average (10°W − 10°E) of the intertropical front (the confluence of monsoonal winds from the south and dry winds from the north) crosses 15°N on its northward migration, while the ITCZ is still in the south. The first rainy season along the Guinean coast occurs between mid‐April and the end of June. In the ‘onset’ phase during the summer monsoon season, the ITCZ moves from 5°N to 10°N bringing rain over the Sahel.

The formation of the Atlantic cold tongue is the dominant seasonal SST signal in the eastern equatorial Atlantic and influences the onset of the WAM (Caniaux et al. 2011). From March to mid‐June (‘onset phase’), the cold tongue results from the intensification of the southeast trade winds associated with the anticyclone system located off the island of St. Helena. Steering of surface winds by the basin shape of the eastern equatorial Atlantic imparts optimal wind stress for generating the maximum upwelling south of the equator. From mid‐June to August, wind speeds north of the equator increase due to the northward progression of the intensifying trade winds and due to significant surface heat flux gradients produced by the differential cooling between the Atlantic cold tongue and the tropical waters circulating in the Gulf of Guinea. Thus, there is a close link between the eastern equatorial Atlantic and the West African monsoon.

The East African monsoon system corresponds with the Greater Horn of Africa region, extending from Tanzania in the south to Yemen and Sudan in the north. During winter, when NE trade winds flow across the NW Indian Ocean and equatorial moisture moves over the Indian Ocean exhibiting strong westerly flows over the equatorial Indian Ocean, East African rainfall is limited to a few highland areas (Funk et al. 2016). During spring, the Indian monsoon circulation transitions, the trade winds over the NW Indian Ocean reverse, and East African moisture convergence supports the ‘long’ rains. In summer, the SW Somali Jet intensifies over East Africa. As subsidence forms along the westward flank of this jet, precipitation shuts down over eastern portions of East Africa. In autumn, the Jet subsides but easterly moisture supports rainfall in limited regions of the eastern Horn of Africa. Thus, Indian Ocean SSTs drive East African rainfall variability by altering the local Walker circulation (see Section 2.6). The influence of the Pacific Ocean appears to be minimal (Tierney et al. 2013). SST anomalies were the dominant forcing of the severe drought of the 1970s and 1980s (Biasutti 2019). However, ENSO showed a positive relationship with the East African ‘short’ rain during the 1949–2016 period. Only during a recent period (2000–2016) was a significant relationship found between ENSO and East African ‘long’ rain. The strengthened interannual relationship between ENSO and East African’ long’ rain is associated with an Indian Ocean Walker Cell in spring, which implies that their relationship could be affected by either multidecadal natural variability or anthropogenic forcing (Park et al. 2020).