A Change in the Weather

 1
 2
 3
 4
 5
 6

We've seen a dramatic change in weather here over the past several weeks:  lower humidity, cooling sea and air temperatures, and the end of the rain.  But what's happened?  What's the cause of this significant change in the weather?

First, four basic concepts, upon which everything else is based:

Hot air rises.  

The sun unevenly heats the surface of the planet.

As the season's change (from the perspective of the northern hemisphere), the zenith of the sun shifts to the south in the autumn, then northward in the spring. 

The Earth is revolving on a titled axis at the speed of about a 1000 mph (about 1600 kph).  A consequence of this is the Coriolis Effect, what we experience when passing a ball to a friend on a merry-go-round.  The path of the ball, like the motion of the wind across the surface of the Earth, bends in the direction of the rotation.  (See http://www.baesi.org/TRG/coriolis/CORIOLIS.MOV for a nice illustration of the merry-go-round example.)

Now, let's fit these elements together (if you're a visual kinda person, check out the animations listed below):

Warm air rises above the surface of Africa, creating convection currents, centered at a latitude corresponding to the zenith of the sun.  Naturally, as the seasons change, and the zenith of the sun shifts, so too does the relative position of the current.  This convection current is called the Hadley Cell, and the point at which warm air (from the north and south) converges is called the Intertropical Convergence Zone (ITCZ).  Fancy, eh?  Where's that Trivial Pursuit!

Images 1-3 illustrate the ITCZ and Hadley Cell.  

The ITCZ is a weather factory, and the source of the African monsoon, which has shifted to the south, leaving us drier weather (as shown in picture 3).  The ITCZ is also the source of many of the cyclonic storms which march across the Atlantic Ocean toward the Caribbean, pushed by the trade winds -- which bend to the right, with the Coriolis Effect.  

Check out the following COOL  animations of seasonal fluctuations in the ITCZ and the Hadley Cell:

http://daphne.palomar.edu/pdeen/Animations/23_WeatherPat.swf

http://www.eoearth.org/image/Solar_influence_global_circuation.gif

http://cimss.ssec.wisc.edu/satmet/modules/stewards/global/cmoll

http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp29/animations/ch29/global_wind_circulation.swf

http://people.cas.sc.edu/carbone/modules/mods4car/africa-itcz/index.html

Why are the sea temperatures dropping?

First, the northwest coast of Africa is under the influence of the Canary Current, illustrated above in images 4 & 5.  The current flows along the African coast from north to south between 30 degree N and 10 degrees S latitude.

This time of year, the prevailing winds are off-shore.  As a consequence, warmer, nutrient-depleted surface waters are pushed seaward, and replaced by deeper, colder, nutrient-rich waters.  The presence of these nutrients is the result of marine snow, or the sinking and deposition of organic matter (plankton guts).  Upwelling brings this material to the surface where it becomes pizza for phytoplankton, which bloom with delight.  Such areas sustain relatively good fisheries, with the ecosystem replenished at its base with lots of grass, or it were, or as the ecologist might say, the primary production of the overlying waters strongly influences benthos standing stock and productivity rate.  

As a result of seasonal upwelling, the Senegalese coast is one of the most productive maritime fisheries in the world.

For several cool animations illustrating the Canary Current and upwelling, see:

http://www.mpl.ird.fr/suds-en-ligne/ecosys/ang_ecosys/upwelling/canaries.htm (note the animations on the right of the page)

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2405/es2405page01.cfm?chapter_no=visualization

http://www.whoi.edu/oceanus/viewVideoGallery.do?gallery=true&clid=2418

Interestingly, we experience a strong seasonal fluctuation in upwelling and sea temperature, varying up to 15 degrees Centigrade, as warm surface currents from the south introduce tropical-like conditions in the summer.  (You'll need a full wetsuit for winter snorkeling.)  Mauretania does not experience this fluctuation;  the upwelling there is permanent and continuous.  

There you have it.  Now go and impress your mom and dad with how smart you are!  And if they shoo you away, have pity on them;  not everyone can be as passionate about this stuff as you and I!