Re: [Phys-L] Earth as a "heat engine"

[Francois Primeau <fprimeau@uci.edu>]

On Wed, Oct 9, 2019 at 9:06 AM John Denker via Phys-l <
phys-l@mail.phys-l.org> wrote:

> On 10/9/19 8:28 AM, Francois Primeau wrote:
>
> > the ocean is heated and cooled at
> > the surface, which is roughly a surface of constant gravitational
> potential
> > energy,
>
> OK.....
>
> > and therefore does little to directly drive ocean currents.
>
> I wouldn't have said that.  Thermohaline circulation in the
> oceans is a big deal.  In particular, you have to account for
> the fact that there is more heating at some latitudes and less
> at others -- leading to verrrry large-scale convection:
 https://en.wikipedia.org/wiki/Thermohaline_circulation

> Rather, the ocean currents are driven mechanically by the wind stress at
> > the surface.
>
> That's quite an overstatement.  *Some* currents (e.g. the
> Gulf Stream) are primarily wind-driven ... but others are
> not.

Yes it is an overstatement. :-) But maybe not as big as one might think.
Especially if we take an energetics point of view and focus on large-scale
ocean currents.  For example,  the maintenance of the large-scale
overturning circulation (i.e. the thermohaline circulation) depends
critically on the input of energy from the winds.  The cold deep waters
formed by convective instability in the winter, when surface waters become
more dense than the waters below, must be returned to the surface to close
the overturning cell.  To a large extent this happen in the Southern Ocean
where the winds drive a surface divergence that pulls cold waters up to the
surface where they can be warmed by surface heat fluxes. In the process
there is a large  transformation of (wind) kinetic energy into
gravitational potential energy stored in the ocean in the form of
horizontal density gradients.  Another way in which the overturning cell is
closed is by the downward mixing of heat by turbulent eddies. The source of
energy that maintains the turbulence can be traced back to the input by the
winds and by tides, not by the heating and cooling at the surface through
some kind of heat engine.  Of course  there is more to ocean currents than
just energetics and I agree that the latitudinal imbalances of heating and
cooling are very important for the thermohaline circulation.   My point is
simply that the heating and cooling of the surface ocean is not the source
of energy for the meridional overturning circulation the way the heating
and cooling at the bottom of the atmosphere is the source of energy driving
the overturning circulation of the atmosphere.


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