Extended Abstract (552K)P5.4
The Hydrological onset and withdrawal index (HOWI) for the West Africa Monsoon
Giovanni A. Dalu, IBIMET - CNR, Rome, Italy; and M. Gaetani, F. Meneguzzo, A. Crisci, G. Maracchi, F. Guarnieri, and V. Capecchi
Using the NCEP/NCAR reanalysis and the rainfall data of the Global Precipitation Climatology Project (GPCP), we study the behavior of the West African monsoon (WAM). Specifically, we focus in the region of West Africa confined between 10°W and 10°E, subdivided in four areas: gulf of Guinea 0°-5°N, coast of Guinea 5°-10°N, south Sahel 10°-15°N, and north Sahel 15°-20°N. Below 10°N the rainfall is bimodal, with a principal maximum in early summer, a relative minimum in mid-summer, and with a secondary maximum in late summer. Above 10°N, the rainfall has a single maximum in mid-summer. This behavior is related to the northward shift of the ITCZ (Inter-Tropical Convergence Zone) from 5° to 10°N at the end of June, which withdraws back in the second half of the summer. Therefore, there is one monsoon above 10°N, and a double monsoon below 10°N. The ITCZ is 3°-5° wide during WAM, reaching its largest width in full monsoonal season, defined as the period of 70% rainfall of the seasonal total. In the Guinea region, the rain originates directly in the ITCZ, while in the Sahel the rain is generated by barotropic-baroclinic instabilities between the ITCZ and the inter-tropical front (ITF). The ITF is generated by the north-easterly Harmattan dry wind, which has turned antcyclonically around the Libyan high. The ITF stays always 3°-5°N of the ITCZ. The barotropic-baroclinic instabilities north of the ITF are weak. The rainfall in the West African monsoon region is well correlated with the vertically integrated moisture transport (q*U), VIMT, and, therefore, with the hydrological index , HOWI, where q is the specific humidity and U is the wind vector. With X we denote the meridional component of the VIMT. The HOWI index has been computed for the following regions: the entire WAM region 5°-20°N, the Sahel region10°-20°N, and the north Sahel region 15°-20°N. In each of these regions the monsoonal rain substantially increases when the HOWI crosses the zero line, which occurs 2-3 weeks before the main monsoonal season. The HOWI falls below zero at the end of the main monsoonal season. The withdraw is twice faster than the onset of WAM. WAM has a marked daily pulsation, with a strong northward polarization of the odograph. A wavelet analysis shows a quasi-period at 5 days, at 15 days, and at 45 days. The 5-7 days pulsation is due to the baroclinic instabilities associated with easterly waves. We find that rainfall over the Sahel, i.e. the inland penetration of WAM, is anti-correlated with the Libyan high, and that the rainfall abundance, i.e. the intensity of the West Africa monsoon, is positively correlated with the North Atlantic anti-cyclone, and anti-correlated with South Atlantic anti-cyclone.
Poster Session 5, Poster Session: Seasonal to Interannual Variability
Thursday, 13 January 2005, 9:45 AM-11:00 AM
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