Impacts of warm and cold situations in the Mediterranean basins on the West African monsoon: observed connection patterns (1979–2006) and climate simulations

Abstract

Using both empirical and numerical ensemble approaches this study focuses on the Mediterranean/West African relationship in northern summer. Statistical analyses utilize skin temperature, sea surface temperature, in situ and satellite rainfall, outgoing longwave radiation (OLR) observations and reanalyzed data winds and specific humidity on isobaric surfaces. Numerical investigations are based on a large set of sensitivity experiments performed on four atmospheric general circulation models (AGCM): ARPEGE-Climat3, ECHAM4, LMDZ4 and UCLA7.3. Model outputs are compared to observations, discussed model by model and with an ensemble (multi-model) approach. As in previous studies the anomalous Mediterranean warm events are associated with specific impacts over the African monsoon region, i.e., a more intense monsoon, enhanced flux convergence and ascendances around the ITCZ, a strengthening of low level moisture advection and a more northward location of ascending motion in West Africa. The results show also new features (1) thermal variability observed in the two Mediterranean basins has unalike impacts, i.e. the western Mediterranean covaries with convection in Gulf of Guinea, while the eastern Mediterranean can be interpreted as Sahelian thermal-forcing; (2) although observations show symmetry between warming and cooling, modelling evidences only support the eastern warming influence; (3) anomalous East warm situations are associated with a more northward migration of the monsoon system accompanied by enhanced southwertely flow and weakened northeasterly climatological wind; (4) the multi-model response shows that anomalous East warm surface temperatures generate an enhancement of the overturning circulation in low and high levels, an increase in TEJ (Tropical Eeasterly Jet) and a decrease in AEJ (African Eeasterly Jet).

Recent changes in precipitation, ITCZ convection and northern tropical circulation over North Africa (1979–2007)

“…It is also known to exert a significant forcing on the African monsoon activity both on interannual and decadal scales, as reported in numerical experiments (Rowell, 2003;Fontaine et al, 2009).…”

“…Briefly the cold and warm simulations correspond to a thermal forcing defined through observed SST canonical correlation analyses and only added over the Mediterranean: the boundary conditions over all other seas and oceans (Atlantic, Indian, Pacific Oceans) were the climatology observed. SST conditions have then been prescribed with these anomaly patterns added to the climatology of observed SST (Fontaine et al, 2009). Figure 15(a) and (b) presents the zonal and meridional mean moisture fluxes averaged between 20°W and 30°E in the low-and mid-tropospheric layers.…”

Recent unprecedented warming and oligotrophy of the eastern Mediterranean Sea within the last millennium

“…The eastern Mediterranean region is a key region for tropical-extratropical interactions [Fontaine et al, 2010]. Sea surface temperatures and deep water formation in the eastern Mediterranean (EM) basin have an important role in preconditioning North Atlantic thermohaline circulation [Bethoux et al, 1999].…”

“…In summer, it lies under the dominant influence of the subtropical desert belt, an area where the net fluxes of heat and moisture to the atmosphere result in a warm and salty sea surface layer [Bethoux et al, 1999]. Its thermal variability initiates a global atmospheric teleconnection that influences meridional overturning circulation, deep convection, and precipitation over the Sudan-Sahel region [Fontaine et al, 2010]. The Mediterranean Sea is also considered a sensitive region for climate forcing because of its size and limited exchange with the Atlantic Ocean [Bethoux et al, 1999].…”

Can an Earth System Model simulate better climate change at mid-Holocene than an AOGCM? A comparison study of MIROC-ESM and MIROC3

“…The SST in each basin, such as the Mediterranean Sea, Indian Ocean, and Atlantic Ocean, affects the inter-annual variability of the monsoon, whereas the global pattern affects the decadal variability Mohino et al, 2011;Losada et al, 2010a,b). Fontaine et al (2010) suggested that anomalously high Mediterranean SST leads to a strong West African monsoon. On the other hand, Bader and Latif (2003) suggested that anomalously high Indian Ocean SST correlates to less Sahel precipitation.…”