Impact of a greener West Africa on the Mediterranean climate
Marina Baldi, IBIMET - CNR, Rome, Italy; and G. A. Dalu and M. Segnalini
On the global scale, the total incoming solar energy is almost perfectly balanced by the energy radiated by the Earth towards outer space. However, even if the imbalance between the shortwave solar radiation and outgoing longwave radiation is small on a global scale, it can bring significant changes of the climate trend. On a time scale of 10000 years or longer, this trend were mainly due to the natural internal processes or of extraterrestrial origin; while, on a time scale of a millennia or of a century, man may have had a significant role, specifically on the regional climate variability in regions we have targeted. At this space scale, the variability results from rather complicated nonlinear feedbacks between many scales. In the last ten thousand years there have been substantial land surface changes in northern hemisphere. In fact 8000 thousand years ago glaciers covered the land as far south as 45-40 degree north. While, 5000 thousand years ago, North Africa was vegetated and enjoyed a reasonable humid climate, and the Middle East was fertile and cultivated, presently, North Africa and the Middle East are in large part deserts. In the last millennia the regional climate has changed from cold to warm several times, favouring the blooming or dooming important civilizations in regions of the Northern hemisphere. Several clues suggest a decline of solar activity during the Maunder Minimum (about 1645-1715), amounting to a radiative forcing of somewhere between -0.5 and -1.5 W/m**2. Decline and subsequent rise of solar activity to its present level may have contributed to the Little Ice Age and to the warming thereafter. Solar forcing since 1850 has been tentatively estimated at between +0.1 and +0.5 W/m**2. These causes and their synergies, like those between changes in the orbital parameters of the Earth and changes in the interaction between atmosphere, vegetation and the oceans have been certainly the major actors in the greening of Sahara in the middle Holocene, mainly due to a strong vegetation - precipitation feedback and the abrupt desertification of North Africa around 5500 years before present. It is well known that the main centres of the low wavenumber atmospheric planetary wave activity are in the North Pacific – North America region and in the North Atlantic – Eurasian region. The phase and the amplitude of these planetary wave pattern have a paramount impact on the storm track position upstream of the North America and of the Eurasian continents, and, therefore, on the distribution of the temperature and precipitations in these areas. The atmospheric perturbations are sensitive to the meridional distribution of the zonally averaged atmospheric flow, but on a regional scale, the patterns of these perturbations are very sensitive to the non-zonal component associated to low wavenumber planetary waves. Using a low resolution model of the atmospheric dynamics on a sphere, we examine the impact of a greener North Africa on the deformation of the large scale westerly flow over the North Atlantic and the Mediterranean region, and on the large scale easterly flow over North Africa. Specifically we investigated on the relative position and intensity of the North Atlantic anticyclone and on the anomaly of the position and intensity of the Libyan high induced by a more northerly penetration of the West Africa monsoon.
Session 5, Detection and Attribution of Regional Climate Change II
Wednesday, 17 January 2007, 4:00 PM-5:30 PM, 214D
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