Natural Science Forum / Earth Science / Oceanography / November 2006

"billy" <perkins236@hotmail.com> wrote in news:1162399229.659859.262720
@k70g2000cwa.googlegroups.com:

Good question.  As is often the case, this good simple question has a
complex answer.

There are three things going on here, and the answer lies in the
interactions between all three.  We need to look at the circulation in
the bay or inlet, the local circulation of the ocean outside the bay or
inlet, and the exchange of water between those two bodies of water.

There are a number of different things that can cause circulation in a
bay.  Let's think about just one of them in a simple, idealized sort of
way.  Let's say the wind is blowing in one direction at a constant
velocity for a long time.  The wind will push water toward the down wind
side of the bay making the water higher on that side.  However, the water
will not keep getting higher and higher. As soon as water gets elevated
above the free surface of the bay, a counter current, flowing in the
opposite direction to the wind, is created.  Since the wind driven
current is a surface current, the counter current happens at depth.  We
say that the wind stress driven surface current is balanced by a pressure
gradient driven current at depth.  This is one of several mechanisms that
can cause circulation in a body of water.

The take home lesson from this example is that events that cause currents
in one place or depth in the bay can cause circulation in other places or
depths.  Depending on the particular situation, the circulation pattern
in a bay or inlet may be simple or complex, relatively stable with time
or highly variable.

With similar examples and arguments, you can see that the same holds for
the circulation in the ocean generally and, in particular, immediately
outside of the bay.  This means that you were correct in your intuition
that when the tide is flooding it is probably not the same water that
came out during the previous ebb.

This is also true for the bay water, to a lesser degree, depending on the
amount of circulation inside the bay.  At least some of the water that
goes out in the ebb is likely different than the water that came in on
the previous flood.

Even in a theoretical bay with absolutely no circulation, there will
still be some mixing of ocean water and bay water caused by the tidal
currents, especially in narrow inlets or in channels.  The velocity of
the inrushing water causes turbulence as it flows past the relatively
stagnant bay water.  This turbulence causes mixing of the two water
masses at their interface.  So even in this case, some of the water will
be exchanged in the tidal cycle.

Putting all this together, the answer to your question is it depends on
the specific conditions of the particular bay or inlet.  Some of the
conditions include how big it is, the size of the tides, how much it
rains, is there river flow into the bay, how windy it is, how much
evaporation there is from the surface, how big the entrance is, what the
topography is like, and many other factors.

When we approach a question like yours, we tend to think in terms of a
mean (average) residence time of the water in the bay.  Some of the
molecules of water that enter the bay will be there only a very short
time.  Think about a water molecule in a drop of rain that lands very
near the entrance during a strong ebb tide.  Its residence time in the
bay could be a few minutes or less.  Now think about a raindrop that
falls far in a back bay corner where there is very little circulation.  
Its residence time could be measured in weeks, months, or even years.  By
calculating an average residence time, we have a number that can be used
to compare different bays and inlets in terms of their water exchange
with the ocean.

I hope this answered your question.