MadSci Network: Earth Sciences

Re: Is there underwater weather in the oceans?

Date: Thu Nov 2 14:05:48 2006
Posted By: Michael O''Donnell, Post-doc/Fellow, Marine Science Institute, UC Santa Barbara
Area of science: Earth Sciences
ID: 1161911015.Es

Hi Ellen,

That is a great question, and it actually raises two issues.

First, yes, the ocean exhibits many of the same "weather" patterns as does
the atmosphere (although it doesn't really snow much... and no lightning).
These patterns are driven by the same forces that drive atmospheric
weather, but they happen over different time scales. This being true,
however, oceanic "weather" patterns will look much different from what you
see in the atmosphere.

To start, you need to have a decent understanding of how weather works in
the atmosphere. A starting point is ,
and The fundamental thing to understand
is that atmospheric weather is driven by differences in density between
masses of air. Usually, this is due to temperature differences; for
instance, air over warm parts of the ocean gets less dense, causing it to
rise... this leads to pressure differences between warm air masses and cold
air masses. The "fronts" that you see on a weather map are areas where warm
and cold air masses meet. Because different air masses are at different
pressures, air will flow from areas of high pressure to areas of low
pressure. Because the earth is rotating, the Coriolis effect will change
the direction of air movement (turning it to the right in the northern
hemisphere, to the left in the southern).

These basic processes driving weather are fundamental to fluid mechanics.
As such, exactly the same processes happen in the fluid that makes up the
oceans. Water of different temperatures will have different densities, for
instance, cold water near the poles sinks, driving bottom currents. Another
big driver of water moving around in the oceans is atmospheric winds
blowing over the surface... in this way, the ocean and the atmosphere are
tightly connected; remember, the temperature of the ocean water helps to
drive the winds. Large eddies of ocean currents look very much like
hurricanes blowing through the atmosphere.

Some important differences, though, have to do with the response times of
the air vs. the ocean. Ocean water is almost 1000 times as dense as air, so
it takes much longer to respond to changes in pressure. It also has a much
higher heat capacity, so it doesn't change temperature rapidly.
Additionally, water absorbs most of the sunlight that hits it, so you don't
get the same situation as in the air, where the sun warms different
surfaces to different temperatures, driving local changes in air
temperature and therefore winds. All of these factors mean that weather
events happen much more slowly in the ocean, and last much longer. A giant
ocean eddy can last months to years, while a massive weather event like a
hurricane might last only a few weeks. Winds in the atmosphere can blow at
1000s of meter/sec (100s of mph), while the Gulf Stream, the fastest ocean
current, moves about 2 m/sec (about 5 mph).

Just as atmospheric weather conditions bring rain to plants, oceanic
weather brings nutrients to the organisms that live there (see : )

Finally, your question asks about whether tsunamis are a type of weather
event. Actually, tsunamis are very different from any of the things I've
mentioned. Tsunamis are caused when an external event triggers a massive
change in pressure over a very local scale. Wikipedia has a good page on
what causes a tsunami ( ). Basically,
it is an instantaneous event which displaces an enormous mass of water
(thereby putting a huge amount of energy into the ocean). This is much
different from weather events which are driven by small temperature and
density differences within the air or ocean. The only atmospheric phenomena
that would be similar would be a bomb going off, an airplane breaking the
sound barrier, or a volcano erupting... maybe a Who concert would
qualify... none of these events are a part of normal weather patterns any
more than tsunamis are a component of general ocean weather.

I hope this helps.

Moose O'Donnell

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