A Note on Pacific Decadal Oscillation, El Nino Southern Oscillation, Atlantic Multidecadal Oscillation and the Intertropical Front in Sahel, Africa

Abstract

The advance and retreat of the Sahara Desert are repetitive in decadal time scale. This climate variability, unique in the world, results from the irregular motion of the Intertropical Front (ITF), a discontinuity originated from the convergence of two air masses over the African continent. It results from the convergence of a dry air mass flowing off the North African anticyclone and a southwestern warm and humid air mass that originates off the South Atlantic anticyclone. As it crosses the equator, this air stream is shifted rightwards, becoming southwesterly winds and transporting moisture into the continent. The equatorial thermal low associated with ITF over the continent is deeper and more intense than the Intertropical Convergence Zone (ITCZ) one and its thermodynamic properties differ remarkably, moisture concen tration in particular. The ITF annual latitudinal displacement is larger than that of the Atlantic ITCZ, reaching 20?N commonly during the North Hemisphere summer. In this manuscript one discuss about the possibility that Pacific De cadal Oscillation (PDO) is not a regular physical oscillation and its phases duration may not be predictable and be de pendent of large magnitude seismic events, which are rare and random. An important point, however, is that the Pacific sea surface temperature (SST) pattern, once established and recognized, may remain for a relative long (decades) period. Its SST pattern, due to its large area extension, modulates the global climate forcing the large scale general circulation features, ENSO in particular, to produce distinct regional impacts, depending on the signal and strength of the SST anomalies. This happened with the ITF in the Sahel and may possibly have occurred with other large scale atmospheric structures, such as the Asian Monsoon, convergence zones and subtropical anticyclones centers positioning.