Polar Ice Sheets May Have Unexpected Effects on El Niños,
Ocean Circulation
(While early climate models foresaw fewer El Niños,
with the ocean warming and the differential of sea surface
temperatures between eastern and western Pacific declining,
the unusually warm 80s and 90s have witnessed the strongest
El Niños of this century. The discussion below from
a telephone interview with Dr. Terry Hughes of the University
of Maine at Orono may offer one explanation.)
Recent studies of El Niños suggest they may be triggered
by the climatic conditions associated with the West Antarctic
Ice Sheet (WAIS), a demonstration of the extent of polar region
influence, Dr. Terry Hughes of the University of Maine reported
in a September interview. Bursts of icebergs from the ice streams
that enter Pine Island Bay in the Amundsen Sea sector of West
Antarctica are carried by the Humboldt Current up the west coast
of South America chilling surface waters off Peru. (Ice streams
are fast currents of ice that discharge most of the Antarctic
ice into the sea, and are analogous to rivers that drain continents.)
This is contrary to the typical ENSO event, when the usual upwelling
of cold water is damped as the ocean becomes warmer. In the more
common El Niños, the air above the upwelling also becomes
warmer, causing a change in the pressure gradient which normally
drives the air flow of the Pacific westerlies. There are shifts
in where heavy rains fall in the western Pacific, and storms blow
up along the Pacific Coast of North America. But what the new
research suggests is that the Humboldt Current links events in
West Antarctica to cooling and warming the water off Peru. The
instabilities associated with the West Antarctic Ice Sheet are
therefore involved in regulating the frequency of El Niños.
The West Antarctic Ice Sheet is itself not very well balanced,
Hughes continued. A low pressure system off the WAIS sweeps back
and forth in a meteorological pattern that is very unstable, and
that may trigger El Niño. With changes in the WAIS, any
minor perturbations have an unpredictable influence. Because the
WAIS is grounded below sea level, and therefore not very steady,
it is like a foot on a banana peel. The repercussions of this
instability reach way beyond the West Antarctic. When John Mercer
first proposed the precarious stance of the WAIS in 1968, most
people felt it was a locally confined problem with consequences
of low probability and not something to worry too much about.
The more we learn about what is happening in West Antarctica,
the more we have to hedge on that question. The instability there
is not confined to the state of the ice sheets nor just a question
of sea level rise, but a matter affecting the entire global climate,
just as El Niño affects climate globally.
In a similar process in the Northern Hemisphere, icebergs are
flowing from Greenland into the Greenland and Labrador Seas where
North Atlantic Deep Water (NADW) forms. Instabilities in the Greenland
Ice sheet can cause iceberg outbursts that slow or stop NADW production.
This can shift global climate to its ice age mode- abruptly. This
puts us on notice, said Dr. Hughes, that we do have to pay close
attention to the stability of the ice sheets and make a determined
effort to understand them.
In Greenland, fresh water from melting icebergs moves toward
NADW production sites in the east and the Labrador Sea on the
west. The strength of the ocean circulation which transports massive
amounts of heat around the planet is very sensitive to small density
changes in this water, depending on the degree of saltiness and
the input of less dense fresh water. The ocean circulation is
basically a gigantic overturning motion, and the introduction
of more fresh water would slow the overturning but not in a simple
linear fashion.
A commentary by Stefan Rahmsdorf of the Potsdam Institute for
Climate Impact Research in an August issue of Nature notes, “Little
happens at first, as the circulation continually removes the freshwater
and replaces it with more salty water from the south. But there
is a well-defined critical threshold ... beyond which the thermohaline
circulation cannot cope with additional fresh water, and breaks
down.” Sediment and ice core records “strongly suggest
that the ... circulation has broken down or at least changed drastically
after pulses of fresh water entered the Atlantic and that this
caused cold spells lasting for hundreds of years.... Global warming
is expected to warm the surface waters and increase precipitation
in high northern latitudes, both of which will reduce water density
and move the Atlantic closer to the threshold.” This is a
timely reminder, Rahmsdorf concludes, “that swift action
is needed to reduce the risk of unwelcome climatic surprises.”
The warming and cooling of the equatorial Atlantic is tied to
what is happening in the North Atlantic, Hughes commented, with
a very rapid response time of weeks or months.
