Why and how do scientists
study climate change in the Arctic? What are the Arctic climate
indices?
Some
important research concepts used by scientists to study climate
change
Nick Bond,
Jim Overland and Nancy Soreide
NOAA / Pacific
Marine Environmental Laboratory
Why is the Arctic important?
Dramatic changes have been occurring in the Arctic during the past
decade. These changes include unusual melting of glaciers, sea ice,
and permafrost, and shifts in patterns of rain and snow fall, freshwater
runoff, and forest/tundra growth. The consequences include disrupted
wildlife migration patterns, altered fish stocks, modified agricultural
zones, and increased forest fires. These changes have impacted the
lives of Native residents who depend on the environment for a continuation
of their traditional subsistence lifestyle, and may also have significant
impacts on the oil industry, tourism, and shipping routes. The US Arctic Research Commission (1999) stated "change in the Arctic
may play a substantial role in climate change throughout the globe",
and moreover, that "global change, particularly climate change may
have its most pronounced effects in the Arctic." Conditions in the
Arctic are very different from those at lower latitudes on the globe,
and "the
Arctic remains one of the least explored, studied and understood
places on earth."
What is climate?
Climate is the long-term average weather. The typical weather (e.g.,
temperature, rain and snowfall, wind) on any given day tends to
be most controlled by the cycle of the seasons from Spring through
Summer, Autumn and Winter. Other factors, with longer time scales,
can cause systematic changes to the climate. Increasing attention
is being devoted to understanding and predicting these factors.
A notable example is El Niño. While El Niño involves
only the tropical Pacific directly, it has a substantial impact
over much of the globe, and consideration of El Niño/La Niña
is an important tool for seasonal forecasts. Additional, longer
time-scale climate mechanisms, associated with Arctic climate change
have been identified and also have impacts around the world. Scientists
studying climate change must consider all of the many factors that
affect the conditions we experience every day. These interactions
are complicated, and scientists try to understand them by using
a framework based on climate indices.
What are some climate mechanisms and oscillations?
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References
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The most familiar climate mechanism or oscillation is the changing
seasons, which vary more or less regularly every year from warm
summer to cool winter. The next most familiar climate oscillation
is probably El Niño, whose intensity is indexed by the Southern
Oscillation Index (SOI), which is the difference in air pressure
between Tahiti and Darwin, Australia. This index is easy to measure
accurately, we have historical records of it, and it is an indicator
of the starting time and strength of an El Niño, which occurs
irregularly every two to seven years.
Several important climate mechanisms contribute to events occurring
at high latitudes and have impacts throughout the globe. These mechanisms
have longer time scales than the seasons or the El Niño-Southern
Oscillation, which means that they do not occur as frequently (every
ten to thirty years). Examples are the Pacific Decadal Oscillation
and the Arctic Oscillation, whose consequences include phenomena
such as increases or decreases in the severity of winter weather,
frequency or severity of winter storms, volume of river runoff,
and stocks of various important fishes.
What are the time scales associated with climate mechanisms
and modes?
The different modes or mechanisms of the climate system act on
different time scales. Some of these mechanisms occur on short time
scales, such as the changing of the seasons, which occurs regularly
every year, or the El Niño-Southern Oscillation, which occurs
irregularly every two to seven years. Others, such as the Arctic
Oscillation and the Pacific Decadal Oscillation, are important on
time scales of ten to forty years. Variations of thousands of years
are associated with orbital cycles of the planet, which appear to
be related to past ice ages. The different frequencies of these
variations harmonize, in a manner of speaking, to bring about the
symphony that is our climate.
It can be difficult to determine exactly when a climate cycle is
changing. For example, although everyone is familiar with seasonal
variations (spring warming, the hot days of summer, the cooling
in autumn and the colder weather in winter), it is difficult to
say exactly which day a season begins. Although we have designated
June 21, the solstice, as the first day of summer, the warming typical
of summer occurs slowly, over some period of time, and warm summer-like
days may occur as early as April, or as late as July. It is not
so easy to say that summer weather has started on any specific day,
or exactly how many days any given summer lasted. This holds true
for other climate cycles as well.
Many factors affect the conditions we experience every day. As
we have seen, these include the familiar day-to-day weather and
the changing seasons, but they also include mechanisms with longer
time scales, such as El Niño, and the longer time-scale climate
mechanisms have been identified as indices and associated with Arctic
climate change.
What are climate indices?
Scientists use climate indices in their attempt to characterize
and understand the various climate mechanisms that culminate in
our daily weather. Much in the way the Dow Jones Industrial Average,
which is based on the stock prices of 30 companies, is used to represent
the fluctuations in the stock market as a whole, climate indices
are used to represent the essential elements of climate. Climate
indices are generally identified or devised with the twin objectives
of simplicity and completeness, and each typically represents the
status and timing of the climate factor they represent. By their
very nature, indices are simple, and combine many details into an
generalized, overall description of the atmosphere or ocean which
can be used to characterize the factors which impact the global
climate system. Because the climate indices are generally determined
from measurements made in a localized area, they can have impacts
in other areas around the globe, through processes sometimes called
teleconnections.
What are some Arctic Climate Indices?
A variety of separate modes seem to be responsible for fluctuations
in the Arctic climate. These modes, as summarized through their
individual indices, generally vary on decadal time scales. This
presents a problem, because it means that we have not experienced
enough cycles during the period of reliable observations, which
goes back roughly 50 to 100 years, to fully understand their causes
and effects. Nevertheless, we have accumulated enough evidence to
indicate that variations in these modes are accompanied by systematic
changes in the weather, with wide-ranging ramifications. The most
important of the Arctic modes, at least as identified to date, are
summarized below.
The Arctic Oscillation (AO) and the North
Atlantic Oscillation (NAO)
The Arctic Oscillation (AO) appears to be the cause for much of
the recent changes that have occurred in the Arctic. Its effects
are not restricted just to the Arctic; it also represents an important
source of variability for the Northern Hemisphere as a whole. The
AO has been described as "a
seesaw pattern in which atmospheric pressure at polar and middle
latitudes fluctuates between positive and negative phases. The negative
phase brings higher-than-normal pressure over the polar region and
lower-than-normal pressure at about 45 degrees north latitude. The
positive phase brings the opposite conditions, steering ocean storms
farther north and bringing wetter weather to Alaska, Scotland and
Scandinavia and drier conditions to areas such as California, Spain
and the Middle East."
The AO appears related to a well-known mode of variability for
the North Atlantic called the North Atlantic Oscillation (NAO).
The NAO has been recognized for decades and has been considered
"the dominant mode of
winter climate variability in the North Atlantic region ranging
from central North America to Europe and much into Northern Asia.
The NAO is a large scale see-saw in atmospheric mass between the
subtropical high and the polar low. The corresponding index varies
from year to year, but also exhibits a tendency to remain in one
phase for intervals lasting several years." The positive phase
of the NAO is associated with more frequent and intense storms in
the North Atlantic Ocean, warmer and wetter winters in Europe, and
cooler, drier winters in Greenland and northern Canada.
Some controversy exists concerning the relationship between
the AO and the NAO, namely whether they represent fundamentally
the same mode, or whether the distinctions between them are meaningful.
At any rate, a systematically positive sense to both indices since
the end of the 1980s has coincided with relatively warm conditions
in the Arctic and net melting of the Arctic ice pack.
The Pacific Decadal Oscillation (PDO) and
the North Pacific Index (NPI)
The Pacific side of the Arctic is also significantly influenced
by an inter-related pair of modes, the Pacific
Decadal Oscillation (PDO) and the North
Pacific Index (NPI). The PDO
is based on the pattern of SST in the North Pacific while the NPI
is based on sea level pressure. The positive phase of the PDO is
associated with warm ocean temperature along western North America
and with generally prosperous fisheries in Alaska and poor fisheries
along the west coast of the continental US, especially with regards
to salmon. The North Pacific Index provides a measure of the intensity
of the mean wintertime Aleutian Low pressure cell. An alternative
measure of the latter is provided by the Aleutian
Low Pressure Index (ALPI).
The PDO is strongly correlated to the NPI/ALPI through air-sea
interactions in the North Pacific. The effects of abnormal atmospheric
conditions over the North Pacific affect both the currents and temperature
of the ocean, which in turn, may feedback on the atmosphere. The
ultimate result of variations in these modes is tangible effects
on wintertime conditions in the Bering Sea, Alaska and western Canada.
A notable shift in these modes occurred in the late 1970s, bringing
about a rapid change from relatively cold winters in western North
America in the early 1970s to relatively warm, benign winters in
the late 1970s and 1980s. There is evidence, based on sea surface
temperature, that the PDO changed to a negative or neutral phase
in the late 1990s, bringing colder coastal waters once again to
the U.S. North Pacific coast.
Summary
Climate mechanisms in the Northern Hemisphere and the Arctic are
very active research topics, and our understanding of their causes
and effects is far from complete. The importance of this wide-ranging
research activity is very well stated by Dr. Nate Mantua, a researcher
at the University of Washington, as he speaks about the PDO: "Even
in the absence of a theoretical understanding, PDO climate information
improves season-to-season and year-to-year climate forecasts for
North America because of its strong tendency for multi-season and
multi-year persistence. From a societal impacts perspective, recognition
of PDO is important because it shows that 'normal' climate conditions
can vary over time periods comparable to the length of a human's
lifetime."
References
The
AO's impact on northern climate (University of Washington)
The AO (JISAO
at the University of Washington)
The NAO (Lamont)
NCEP
teleconnections page
PDO (University of Washington)
Compare
PDO/ENSO (University of washington)
NPI
(National Center for Atmospheric Research)
Climate-Ocean
Regime Indices (Fisheries Oceanography) from Canada's Fisheries
and Oceans
SOI
graph (University of Washington)
PDO
(University of Washington)
Pressure
difference between the Azores and Iceland (University of East Anglia)
The
Arctic Ocean Response to the North Atlantic Oscillation (AMS)
Pacific
Decadal Oscillation explanation (NASA/JPL)
Climate Change
Indicators (Taiga Net)
Bering Sea Climate and
Ecosystem
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