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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 concentration 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 Decadal Oscillation (PDO) is not a regular physical oscillation and its phases duration may not be predictable and be dependent 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.
Highlights
Human activity planning, in particular agriculture and energy generation, requires climate outlook with sufficient leading time, 5 to 10 years ahead, so its results can be beneficial for society
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 of distinct thermodynamics characteristics, moisture concentration in particular—that is the large-scale mechanism responsible for the rainfall over the Sahel
Rainfall composites for El Niño and La Niña events were made for each Pacific Decadal Oscillation (PDO) phase, a total of four composites
Summary
In particular agriculture and energy generation, requires climate outlook with sufficient leading time, 5 to 10 years ahead, so its results can be beneficial for society This need is highly apparent in the Sahel, a latitudinal land strip of about 300 to 400 km wide, subjected to north-south Sahara Desert border displacement, with consequent social and economical disturbances. The ITF, on the other hand, results from the convergence of a dry air mass flowing off the North African anticyclone and a southwestern warm and humid air mass which originates off the South Atlantic anticyclone. As it crosses the equator, this air stream is shifted rightwards, becom-. The PDO cold phase is characterized by negative sea surface temperature (SST) anomalies extending throughout the Tropical Pacific and along the coast of North and South America and positive SST anomalies in the extratropics, whereas the warm phase has opposite SST pattern (Figure 1)
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