Re: why does surface ocean-water temperatures vary less

Hi Maria,
This is a great question, because it gets at two of the most important
things affecting Earth's climate: heat capacity and ocean circulation. 

"Heat capacity" is a term you may have heard--if something has a high
heat capacity, that means it is very hard to change its temperature.
Water has one of the highest heat capacities of any common substance on
Earth.  That means you have to heat it a long time to raise its 
temperature, or cool it for a long time to lower its temperature. It is 
why water takes so long to boil, and why a warm bath stays warm as long 
as it does.  What it means for the ocean is that it takes a whole summer
 of warming by the sun to raise the temperature a few degrees, and a 
whole winter of cooling by the cold winter air to bring the temperature 
back down.  This is extremely important for climate: imagine a world where
the temperature changed by 100 or more degrees whenever the sun went down
or came up! The Earth receives from the sun and loses to space huge amounts
of heat, every day and night, and from season to season.  But the average
temperature doesn't change that much, because Earth's surface is mostly 
water.  

The high heat capacity of water is also important because ocean water 
circulates in ocean currents.  Always remember that sea water in one spot
probably was somewhere else not too long ago.  For example, the water off
Connecticut in winter may have come from Florida via the Gulf Stream 
current, in as little as two weeks!  Since water is so hard to heat or 
cool, after only two weeks that water off Connecticut will still have some
of its warmth from the Caribbean sun, even in winter.  Likewise, the water
off California doesn't get too warm in the summer because cold water from 
Alaska is always coming southward in the California current.  If it weren't
for this kind of heat transfer by the oceans, the Arctic and Antarctic 
could be much colder than they are now, and the Tropics could be a great 
deal warmer.  

Hope this answers your question!
Dan Goldner (goldner@mit.edu)
oceanography student at MIT & Woods Hole Oceanographic Inst.