FEATURE ARTICLE
The Source of Europe's Mild Climate
The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth
Richard Seager
A Model of Contrasts
Battisti and I naturally wondered whether we could explain the
difference in winter conditions between Europe and eastern North
America as simply the difference between a maritime climate and a
more continental one. To find the answer, he and I used two climate
models, ones that normally serve for studies of natural climate
variability or for assessments of future climate change. As in all
such models, Earth's atmosphere is represented on a
three-dimensional grid (latitude, longitude and pressure level in
the vertical). For each grid point, the computer solves the relevant
equations for the winds, temperature, specific humidity, fluxes of
solar and terrestrial radiation and so forth while keeping track of
the precipitation and energy fluxes at Earth's surface. The packing
of the grid points was sufficiently dense so that we could
accurately capture the endless progression of storm systems, which
transport vast quantities of heat and moisture poleward. As with the
computer models used to forecast the weather (which are basically
the same as climate models), the computer code we used calculated
conditions forward in time until, for these experiments, a
statistical steady state was achieved. To get a representative
picture of overall climate, we averaged together many years of
simulated weather.
The joy of such numerical models is that you can make radical
changes to a virtual Earth's climate system with nothing more than a
click of the mouse. To assess the importance of the heat transported
by ocean currents such as the Gulf Stream, we compared the results
of two versions of these climate models. The first versions were the
standard ones, which compute sea-surface temperature after
accounting for the heat moved by ocean currents, the absorption of
the Sun's rays, and the exchange of heat between the ocean and the
atmosphere. In the second versions, the computer code accounted for
solar warming and the relevant surface heat exchanges but did not
allow the model ocean to transport heat horizontally.
What we found in these tests was that, south of northern Norway, the
difference in winter temperature across the North Atlantic was
always the same, whether or not we let the ocean move heat around.
This result would suggest that oceanic heat transport does not
matter at all to the difference between the winter climates of
western Europe and eastern North America! We concluded that the
temperature difference must, as we had speculated before, be caused
by other processes, most likely the seasonal absorption and release
of heat by the ocean and the moderating effect this process has on
maritime climates downwind.
Our revised view of things did not, however, mean that heat
transport in the ocean does not influence climate. The ocean indeed
absorbs more heat from the Sun near the equator than it loses back
to the atmosphere (primarily by evaporation). And oceanic currents
indeed move the excess heat poleward before releasing it to the
atmosphere in the middle latitudes. Consequently, removal of the
oceanic heat transport globally in our modeling exercise warmed the
equator and cooled everywhere else. The climates produced by the
models deprived of oceanic heat transport were colder in the
subpolar North Atlantic by as much as 8 degrees in some places. The
cooling over land areas was more modest, typically less than 3
degrees. These temperature changes, large as they are, are not
terribly dramatic compared with the much larger temperature contrast
across the North Atlantic Ocean.
Why doesn't the ocean exert a greater influence on North Atlantic
climate? According to scientists' best estimates, the ocean and
atmosphere move about an equal amount of heat in the deep tropics.
But at mid-latitudes, the atmosphere carries several times more
heat. Thus, if one considers the region north of, say, 35 degrees
North, the atmosphere is much more effective than the ocean in
warming winter climates. Also, the winter release of the heat
absorbed during the summer is several times greater than the amount
of heat that the ocean transports from low to high latitudes in a
year. Hence it is the combined effect of atmospheric heat transport
and seasonal heat storage and release that keep the winters outside
the tropics warmer than they otherwise would be—by several
tens of degrees.
Although these numbers are instructive, they are not directly
relevant to understanding the warming of Europe. For that, one needs
to consider some details of geography. The Gulf Stream and
associated current systems in the North Atlantic focus heat (and
lose it to the atmosphere) in two clearly defined areas. One is
immediately to the east of the United States, where the warm Gulf
Stream flows north after leaving the Gulf of Mexico and rounding the
tip of Florida. During winter, the prevailing winds blow frigid, dry
air off the North American continent and across the Gulf Stream.
Because of the large difference in moisture and temperature content
between air and sea, the heat lost from the ocean through
evaporation and direct heat transfer is immense—a few hundred
watts per square meter. Much of this heat is picked up by storms in
the atmosphere and carried over the eastern United States and
Canada, effectively mitigating what would otherwise be a cold
continental climate.
Where else does the Gulf Stream deposit its heat? After departing
the American coast, the Gulf Stream heads northeast and turns into
what is called the North Atlantic Drift and, farther downstream, the
Norwegian Current. After spawning many Atlantic storms, it loses
most of the remainder of its heat in the Nordic seas. There the heat
can effectively be moved eastward by the prevailing winds to warm
northwest Europe. Thus the transport of heat taking place in the
North Atlantic warms both sides of the ocean and
by roughly the same amount, a few degrees. This leaves the much
larger, 15-to-20-degree difference in winter temperatures to be
explained by other processes.
One subtle but important effect stems from a fundamental principle
in physics: the conservation of angular momentum. In meteorology,
this principle translates to a rule that atmospheric flow must
closely conserve the total angular momentum of a column of air. The
angular momentum of the air contains two components: one arising
from the rotation of the Earth (which meteorologists call the
"planetary component") and another from the curvature of
the fluid flow itself. The planetary component, which in the
Northern Hemisphere is directed counterclockwise, is at a maximum at
the pole and zero at the equator.
The conservation of angular momentum, it turns out, causes the
mountains of North America to contribute substantially to the
dramatic difference in temperatures across the Atlantic. To fathom
why, you must first understand that the troposphere (the
lower part of the atmosphere, where weather takes place) is bounded
at the top by the tropopause, a region of stability where
temperature increases with height and which acts somewhat like a
lid. Thus when air flows over a mountain range—say, the
Rockies—it gets compressed vertically and, as a consequence,
tends to spread out horizontally. When a spinning ice skater does as
much, by spreading his arms, the conservation of angular momentum
slows his spin. An atmospheric column going up a mountain
behaves in a similar way and swerves to the south to gain some
clockwise spin, which offsets part of the counterclockwise planetary
component of its spin.
On the far side of the Rockies, the reverse happens: The air begins
to stretch vertically and contract horizontally, becoming most
contracted in the horizontal when it reaches the Atlantic. And as
with an ice skater pulling in his arms, conservation of angular
momentum demands that the air gain counterclockwise spin. It does so
by swerving to its left. But having moved to the south after
crossing the mountains, it is now at a latitude where the planetary
component of its angular moment is less than it was originally. To
balance this reduction in angular momentum, the air acquires more
counterclockwise spin by curving back around to the north. This
first southward and then northward deflection creates a waviness in
the generally west-to-east flow of air across North America and far
downwind to the east.
Such waves are of massive scale. The southward flow takes place over
all of central and eastern North America, bringing Arctic air south
and dramatically cooling winters on the East Coast. The return
northward flow occurs over the eastern Atlantic Ocean and western
Europe, bringing mild subtropical air north and pleasantly warming
winters on the far side of ocean.
Topographically forced atmospheric waves contribute significantly to
the large difference in winter temperature across the Atlantic. When
Battisti and I removed mountains from our climate models, the
temperature difference was cut in half. Our conclusion was that the
large difference in winter temperature between western Europe and
eastern North America was caused about equally by the contrast
between the maritime climate on one side and the continental climate
on the other, and by the large-scale waviness set up by air flow
over the Rocky Mountains.
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