Senegal Dust, Dry Air the possible collapse of the rainy season

During the month of June when I and several students were in Senegal to install new equipment and to examine new data from a ceilometer that was put in last year, we noticed that Saharan dust was being observed in the lowest 500 meters (0-1500 feet).  This was a strange thing given that dust is often found between 1000-3000 meters or some 3000-10000 feet above the ground.  So as the picture for June shows above green represents dust aerosols and the are trapped below what is called the marine boundary layer.   Unfortunately this means that the dust cannot be dispersed but has remained in place.  But since we had never seen this before for Senegal we were skeptical.

There were days that the dust was apparent such as June 18 and 19th when there large amounts of dust flowing over Senegal.  However, this dust was above the layer that we continued to see day after day.  On these days we could see how the Sun was obscured by all of the dust in the air.

 

However, in the layer with the dust, we also noticed that the relative humidity was something line 1% between 600-1200 feet above the ground.  Initially we figured that there was something wrong with the instruments for measuring relative humidity (radiosondes).  However after looking at this situation more closely we now know that it is not an instrument problem.  As an example here is the relative humidity from the week of July 17 and it is very clear that there is a layer of very dry air over Dakar every day during this week. 


So you say so what to all of this???  Is it really that serious.  The answer is Yes.  Why?

1.)  The dust is unhealthy to breath in.  The particles are so small that they get trapped in your lungs.  Hence you feel really sick.  If you have asthma -- you have real problems breathing and given the state of the health care system in Senegal and the access to resources many people will get sick.  The dust most likely carries Mengintis which can lead to death if not treated.   Early indications are that Meningtis rates have increased across the entire country. 

2.)  The very dry layer with the dust creates a very stable layer which tends to disrupt rainfall.   Thunderstorms and other forms of organized rain prefers the opposite situation, with unstable air not no dry air.  There have been two heavy rain events thus far but most of the organized systems have fallen apart as it travels across Senegal.  Tomorrow begins the month of August with the highest rain amounts normally.  However, I believe that below normal rainfall is a strong possibility unless this pattern stops. 

Both reasons put millions of people in and around Dakar along with the other coastal cities at risk.  The early analysis suggest that the dusty air is coming from Western Sahara.  The dry air is probably coming from two sources: the Subtropical Atlantic and sinking over Dakar and from the Western Sahara.  

So how long has this been going on?  Since May..  For nearly 3 months.  When will it end??? No one knows.

Even thought a tropical storm is possible in the next day impacting the Caribbean, this pattern is not conductive to tropical storms forming over the Eastern Tropical Atlantic.  They will have to wait until they are away from the dry dusty air. 

I will continue to update.   As of now, there has been no change in status.  The dust and dry air remains in place this week.