The ITCZ and Rainfall Patterns on the Kenya Coast
Article 3/3: An overview of the Kenya Coast's climatic model
As described in the previous article - the earth orbits the sun with it's axis tilted at an angle of about 23.4 degrees in relation to the Plane of the Ecliptic. The resulting effect of the sun's changing position in the sky causing the subsolar point to move between the tropics, and across the equator twice a year causing a change in the season's in the northern and southern hemisphere, and more locally in East Africa, the changing of the monsoon winds, and the alternating wet and dry seasons.
This article looks at the Kenya Coast's weather pattern in more detail and outlines a generalised climate model for this region.
The equatorial region's weather patterns are dominated by the Hadley Cell, an atmospheric circulation pattern occurring between the tropics that produce the easterly trade winds.
In the Hadley cell, air rises near the equator, flows out towards the poles at high altitude, descends back to the Earth's surface at the outer edges of the tropics, and completes the cycle by flowing back towards the equator near the earth's surface.
The flow of air is generated by solar energy, being strongest at the subsolar point. The higher levels of solar energy due to the closer proximity of the sun to the earth's surface at the subsolar point heats the air, causing it to rise. This rising air creates an area of low pressure in the zone around the subsolar point. As this warm air reaches thetropopause (the layer of air between the lower troposphere and higher stratosphere roughly 18km high) it flows outwards towards the poles at this high altitude. The air is sufficiently cooled once it reaches the tropic lines and then descends causing an area of high pressure at the subtropics. On nearing the eath's surface - the air flows back towards the area of low pressure at the equator, replacing the rising air - completing the cycle of the Hadley Cell.
The reason why these winds generally have a slightly easterly bearing and are not simply northerly and southerly as you might expect given the air circulation from the Hadley Cell is due to the fact that the earth spins towards the east - the effect of the earth's spin on the atmosphere is know as the Coriolis Effect which turns winds towards the west, and is the reason why the trade winds are always easterly.
The area of low pressure and converging winds is known as theIntertropical Convergence Zone (ITCZ). The ITCZ and subsolar point are therefore closely linked, and the ITCZ follows the subsolar point roughly about a month and a half later.
As we have seen the ITCZ is associated with rising air. When air rises it begins to cool as it expands with the lower pressure exerted on it, as it cools, it condenses which results in the formation of clouds and in turn, rainfall. On the Kenya Coast - we have illustrated that the subsolar point passes over around the time of the equinoxes (and switch in monsoon winds) - March 21/22 and Sept 22/23. 4 to 6 weeks after the equinoxes and subsolar point, and when the ITCZ is passing over the region is when the Kenya Coast experiences it's peak rainy seasons in April/May, and Oct/Nov.