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
Questioning the Myth
After completing my Ph.D. at Columbia University in New York City, I
took a temporary postdoctoral position at the University of
Washington in Seattle, where I should have immediately realized that
something was wrong with the Gulf Stream-European climate story.
Seattle and British Columbia, just to the north, I discovered, have
a winter climate with which I was very familiar—mild and damp,
quite unlike the very cold conditions that prevail on the Asian side
of the Pacific Ocean. This contrast exists despite the fact that the
circulation of currents in the Pacific Ocean is very different from
the situation in the Atlantic.
The analogue of the Gulf Stream in the Pacific Ocean is the
Kuroshio Current, which flows north along the coast of
Asia until it shoots off into the interior of the Pacific Ocean east
of Japan. From there, it heads due east (unlike the Gulf Stream,
which heads northeast) toward Oregon and California. As such, there
is almost no heat carried northward into the Pacific Ocean at the
latitudes of Washington and British Columbia. Hence oceanic heat
transport cannot be creating the vast difference in winter climate
between the Pacific Northwest and similar latitudes in eastern
Asia—say, chilly Vladivostok.
Strangely, experiencing a Seattle winter firsthand was not enough to
make me question the myth. However, in Seattle I did become good
friends with David S. Battisti, a professor of atmospheric sciences
at the University of Washington. Battisti is one of those great
scientists who, with relish and an air of mischief, loves to
question conventional wisdom. Over the years he and I have enjoyed
many a long evening indulging our shared passions for Italian
cooking and wine while talking about climate research. During one of
those conversations, sometime in 2000 as I recall, he brought up
that he wanted to test the Gulf Stream-European climate idea. It was
perfect timing, because just then I had been conducting a series of
experiments with a numerical climate model, ones designed to examine
the role the ocean plays in determining the global and regional
features of the Earth's climate. So Battisti and I went to work.
First we had to consider the range of possibilities. If oceanic heat
transport does not create the differences in regional climate across
the North Atlantic (or North Pacific), what does? An obvious
alternative explanation is that standard of high school geography
education: Because the heat capacity of water is so much greater
than that of rock or soil, the ocean warms more slowly in summer
than does land. For the same reason, it cools more slowly in winter.
That effect alone means that the seasonal cycle of sea-surface
temperature is considerably less than that of land surfaces at the
same latitude, which is why summers near the sea are cooler and
winters are warmer than at equivalent sites located inland.
The effect of differing heat capacities is augmented by the fact
that the Sun's heat is stored within a larger mass in the ocean than
on land. The heat reservoir is bigger because, as the Sun's rays are
absorbed in the upper several meters of the ocean, the wind mixes
that water downward so that, in the end, solar energy heats several
tens of meters of water. On land, the absorbed heat of the Sun can
only diffuse downward and does not reach deeper than a meter or two
during a season. The greater density of soil and rock (which ranges
up to three times that of water) cannot make up for this difference
in volume of material that the Sun heats and for the difference in
heat capacity of water compared with soil or rock.
Because sea-surface temperatures vary less through the seasonal
cycle than do land-surface temperatures, any place where the wind
blows from off the ocean will have relatively mild winters and cool
summers. Both the British Isles and the Pacific Northwest enjoy such
"maritime" climates. Central Asia, the northern Great
Plains and Canadian Prairies are classic examples of
"continental" climates, which do not benefit from this
moderating effect and thus experience bitterly cold winters and
blazingly hot summers. The northeastern United States and eastern
Canada fall somewhere in between. But because they are under the
influence of prevailing winds that blow from west to east, their
climate is considerably more continental than maritime.
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