Class Notes April 3

We started the lecture by talking about the difference between the Rossby
radius ofthe Atmosphere and that of the Ocean and how this sets the scale
for the heat transport.

The ocean is responsible for most of the heat transport in the Tropics,
mainly through Eckmann transport, but that takes place on a scale set by the
atmosphere. In the Extratropics is the atmosphere that does most of the heat
transport.

The East-West Asymmetries
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1) The Equatorial band

All along the equator we have upwelling, because of the trade winds and the
divergent Eckmann transport in opposite directions in the two hemispheres.
The thermocline is tilted east-west and that may be (or may not be) a
consequence of the trade winds that blow along the equator. Where the
thermocline is shallower, in the Eastern Pacific, the upwelling processes
are bringing cold, deep waters to the surface, while in the Western Pacific,
where the thermocline is deeper, the same upwelling processes are bringing
warm waters to the surface. This can explain the existence of the so-called
cold tongue on the eastern side and warm pool on the western side. The same
situation exists in the Atlantic Ocean, but here the cold tongue extends
over a much larger area than the warm pool. It is not the case for the
Indian Ocean, where we don t have so well-defined features as the cold
tongue or the warm pool. This might be due to the monsoons but maybe also
due to the influence of the warm waters brought in from the Pacific.

2) The Bjerkness feedback

There is a strong dynamical coupling between ocean and atmosphere. It is
difficult to decide what is the cause and what is the effect, which of the
two is driving the other one.  Consider the trade winds; they determine the
piling of the waters in the western part of the basins, the east-west
tilting of the thermocline and upwelling along the equator. This in turn (as
explained in the previous paragraph) increases the difference between SST-s
in the east and SST-s in the west. A specific structure of the convective
heating is then organized. The Walker circulation is increased and, if we
also take into account the boundary layer processes, this results in an
increased zonal wind stress. The east to west SST gradient will be further
increased and so on. The succession of all this processes and their
inter-dependence constitutes the Bjerkness positive feedback.

One can ask how does this feedback eventually break? To answer this question
we need to consider the role of the equatorial waves, in particular the
Rossby and the Kelvin waves.

Why is Europe warm in winter? (relative to eastern Canada for example)
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The warming of the European continent during winter can be due to three main
factors:  -The Gulf Stream; - The Rockies; - The different thermal inertia
of the land and ocean and winds blowing from west to east.

First, we analyzed the influence of the Gulf Stream. The main argument
supporting theidea that Europe might be warmer during winter because of all
the heat transported by the Gulf Stream is that the heat budget measurements
clearly show there is a much larger heat transport in the Atlantic Ocean as
compared to the Pacific Ocean. Still, the temperature in the eastern Pacific
(where we don t have much heat transport) is very similar to that in the
eastern Atlantic (where we do have an important heat transport).  And also,
as mentioned at the beginning of this summary, in midlatitudes is the
atmosphere that does most of the work in transporting the heat, and not the
ocean.  But, probably the most convincing evidence against the Gulf Stream
being the main reason for why is the European continent is warmer during the
winter season is the modeling experiments. The runs were done using an
Atmospheric General Circulation Model coupled with a Slab Ocean Model, and
we looked at the outputs from a control run and a run with no oceanic heat
flux in the North Atlantic. The differences between the two runs were very
small, meaning that the relative high temperatures in Europe during winter
are not due to the ocean currents but due to the atmosphere (the atmosphere
is transporting the heat through storms and stationary eddies).

Then we considered The Rockies. The modeling experiments show that the
east-westdifferences in temperature in and around the northern Atlantic
basin are, in part, due to the Rockies Mountains. 

We compared a control run with a no-mountains run, for the 500mb
geopotential height, sea surface temperature and eddy surface temperature
fields, to see the influence of the mountains on the general airflow. The
mountains determine a troughing of the circulation over the eastern part of
the North American continent and over the western part of the North Atlantic
Ocean. This means cold air advection on the eastern North America and warm
air advection on the European continent. So the Rockies do seem to determine
a warming over Europe but they cannot account for all of it. 

After analyzing the first two factors (the Gulf Stream and the Rockies) that
might be causing the warming of the European continent during winter, and
concluding that they are not the main contributors to this process, we can
then assume that the third factor, theland-ocean thermal contrasts, is the
one responsible for this.  The land and the ocean have different thermal
inertia and different capacities of storing summer insolation. This
determines diabatic-heating differences between the two elements. The
atmosphere off the northeastern American continent is heated up from the
ocean. This produces a pressure low over the continent and a cold air
advection over the eastern Canada and northeast US. Consider the same
reasoning, over the European continent we will have warm air advection. And
this can explain the cold season temperatures differences between Europe and
eastern Canada.

The conclusion to this full analysis was that, during winter, Europe is
warmer because of the cold air advection over eastern Canada by the
stationary flow, due to the different thermal inertia in the ocean and in
the atmosphere. The Rockies also have a small contribution to this process.