The Two Faces of the North Atlantic Oscillation
By John M. Wallace
1. Introduction
The atmosphere-ocean system has exhibited some rather pronounced trends during the
past few decades. Winters have been getting milder, particularly over northern Europe,
Russia, and central and western Canada. Global warming skeptics take delight in
pointing out that the strongest warming trends have been observed over the coldest
regions of the hemisphere. One notable exception to the trend toward global warming
has been Labrador and west Greenland, where winters have been growing more severe.
Barometric pressure has been dropping over the Arctic and the westerlies at subpolar
latitudes (~55°N) have intensified. Winter storms over the North Atlantic have been
getting more intense and wave crests have been getting higher. Winter precipitation over
Scotland and Norway has increased. Snow accumulations at higher elevations in Norway
have been so heavy that a number of the glaciers have begun to advance, even in the face
of the trend toward milder temperatures. Meanwhile, winter rainfall over southern
Europe has been decreasing, with major droughts during the early 1990's and again last
year. Winter temperatures in the lower stratosphere 20 kilometers above the earth’s
surface have been cooling over the Arctic polar cap region, with temperature drops of 5 C
or more, and there has been appreciable thinning of the stratospheric ozone layer
poleward of 40°N.
In this lecture I will argue that all the trends noted in the previous paragraph are all linked
to the same phenomenon and that equally pronounced changes over and around the
Antarctic are linked to its Southern Hemisphere analogue.
2. Background
The challenge of understanding climatic change is promoting a remarkable synthesis of
the earth sciences. A generation ago, when I entered the field, geology, meteorology and
oceanography were largely separate disciplines, and each of them touched on only
limited aspects of the climate problem. The study of climates of the past was largely the
province of geologists who, in collaboration with paleo- botanists and zoologists were
assembling an increasingly detailed picture of the record of past climate variations based
on proxy evidence. Some of them carefully scrutinized their proxy records of past climate
with hopes of finding evidence of orbital and other periodicities might lend some insight
into the underlying causes of climate variability, but beyond that, most of the scientists
working in this field weren't in a position to do much more than gather evidence and
speculate. They lacked the tools to perform a global synthesis of the proxy records and to
investigate the dynamical mechanisms that link climate variations in widely separated
parts of the world.
The meteorologists were developing the tools needed for investigating climate variability.
They were already having some success in using numerical models based on fundamental
physical principles to predict time evolution of the global wind field, but at that time the
skill of weather forecasts was limited to a few days. By performing extended runs with
the same numerical models, researchers were able to realistically simulate many of the
features of the present climate, a notable example being the transport of water vapor in
the global hydrologic cycle, as documented by the late Professor Jose Peixoto of this
university. Yet despite the promising developments in climate modeling, I think it is fair
to say that most meteorologists at that time tended to view climate as unchanging, and
climatology as a stagnant field. It was only a few of our peers who recognized the great
potential of numerical models for the study of climate variability.
A generation ago much of the research on the oceans was focused on regional
phenomena. The more global thinkers in the field were still struggling with fundamental
issues, like the factors that govern the strength of the western boundary currents, and the
rate of recirculation of water between the surface layer of the ocean and the deep abysses
below. Ocean general circulation models were just in their infancy. Yet already,
developments within the field of oceanography were helping to set the stage for a
renaissance in climate research. Marine sediment cores were providing a wealth of
detailed information on past climate variability. Leading chemical oceanographers of that
time were beginning to consider the carbon cycle and other biogeochemical cycles from a
global, interdisciplinary perspective. And pioneering efforts in coupled atmosphere-ocean
modeling were already on the drawing boards.
3. The El Nino Southern Oscillation phenomenon
1966, the year I entered the field also marks the publication date of the first of a series of
highly influential papers of Jacob Bjerknes [1] on what has come to be referred to as the
El Niño-Southern Oscillation (ENSO) phenomenon. ENSO provides what is perhaps the
clearest example of how the atmosphere and oceans, as a coupled system, are capable of
supporting a mode of space / time variability altogether different than the modes of
variability of the individual components of the system.
ENSO involves an atmosphere-ocean feedback loop, as shown in Fig. 1. During the
onset of a warm 'El Niño' event, weakening of the equatorial Pacific tradewinds leads to
a deepening of the thermocline on the eastern side of the basin and decreased upwelling,
both of which enable equatorial sea surface temperatures to warm toward the ambient
temperature of the tropical oceans. Raising the temperature of the surface waters imparts
additional buoyancy to the overlying air, enabling it to support deep cumulus convection.
The increased convective rainfall along the equator favors a further weakening of the
trades, which allows sea surface temperature to warm further--- a positive feedback. The
redistribution of the tropical rainfall also affects the upper air circulation at higher
latitudes, producing a complex array of temperature and rainfall anomalies at the earth's
surface, as illustrated in Fig. 2, in which the color shading shows the region of enhanced
rainfall, as indicated by infrared satellite imagery, and the contours show the anomalies in
the temperature of the troposphere, as inferred from satellite microwave imagery. During
winters of 'El Niño years' when the surface waters of the equatorial Pacific are
abnormally warm, temperatures over Alaska and western Canada tend to be warm as
well, as shown in Fig. 3 and regions of the Americas at subtropical latitudes tend to be
wetter than normal. El Niño years are also usually marked by drought over Northeast
Brazil, Central America, and parts of Africa, and a suppression of hurricanes over the
Caribbean Sea.
The equatorial Pacific sea surface temperature anomalies that occur in association with El
Niño events typically persist for about a year: much longer than the characteristic
'memory time' of either the winds or the equatorial ocean currents alone. The longer time
scale is thus attributable to the coupling between the atmosphere and the ocean. By virtue
of this extended memory, the coupled system is far more predictable than the atmosphere
alone. This extended predictability is being exploited to make forecasts of El Ninorelated climate anomalies out to several seasons in advance, using state-of-the-art coupled
atmosphere-ocean models. The principal beneficiaries of these forecasts are residents of
tropical countries, many of whom suffer drought during El Niño years, and residents of
the countries of North and South America. The effects of ENSO on European climate are
more subtle, but the forecasts are nonetheless of some value.
The ENSO paradigm has been instrumental in demonstrating the existence of a certain
degree of order in the year-to-year variations in temperature and rainfall in the tropics and
over some parts of the extratropics. It has also revealed important linkages between the
physical variables and the biosphere. For example, the deepening of the thermocline in
the eastern Pacific during the El Niño years results in a decreased availability of nutrients
to the euphotic zone, to the detriment of many species of marine life. But the increased
rainfall along the coastal drainage basins of Ecuador during these intervals provides
increased nutrients for shrimp farms the coastal estuaries and habitat for mosquitoes. For
the same reasons, sea birds languish while land birds thrive on the Galapagos Islands
during El Niño years. These ENSO-related variations in the biosphere are of
considerable interest in their own right, and they provide valuable clues as to how the
physical variables in the climate system (temperature, ocean currents, winds, etc.)
influence marine and terrestrial ecosystems, not only on the year-to-year time scale, but
potentially on much longer time scales as well.
Around 1976-77 the ENSO cycle underwent a discernible shift. Within the 30-year
interval prior to that time, cold phases tended to be slightly more intense and more
prevalent than the warm phases, whereas in the years since then, the opposite has been
the case. This 1976-77 'regime shift' is more strongly apparent in North Pacific climatic
time series and in various indicators of the state of the marine ecosystem [2]. There are
indications of an analogous shift, but in the opposite sense around 1946, and perhaps
another just within the past 2-3 years, coinciding with the extended cold phase of the
ENSO cycle that has persisted from 1998 until the last month or two. This interdecadalscale ENSO-related variability is also sometimes referred to as the Pacific Decadal
Oscillation. Whether it is a distinctive phenomenon in its own right, or merely a peculiar
feature of the ENSO cycle remains to be seen, but it's generated considerable interest in
the fisheries community because it appears to be strongly correlated with the decade to
decades ups and downs in salmon recruitment along the coasts of Alaska, British
Columbia and Washington.
Research on ENSO is continuing. Efforts are underway to improve the skill of ENSO
prediction, to extend the record of the ENSO cycle farther back into the past using proxy
records, to quantify the impacts of ENSO on physical climate variables, on marine and
terrestrial ecosystems, and on human activities, to understand the nature of ENSO-like
variability on decadal time scale, and to predict whether and how global warming might
impact ENSO. But even as research on ENSO continues, another phenomenon that may
ultimately prove to be equally important from a global perspective, and perhaps even
more important from a European perspective, has been attracting increasing attention-- a
phenomenon variously known (in different segments of the research community) as the
North Atlantic Oscillation, the Arctic Oscillation, and the Northern Hemisphere annular
mode.
4. Early history of the North Atlantic Oscillation
The observed tendency for winter temperatures over Greenland and Scandinavia to vary
out of phase (i.e., for one to be above normal while the other is below normal) can be
traced back 200 years or more [4]. The realization that this distinctive pattern of
temperature anomalies is linked to an equally distinctive north-south see-saw of sea-level
pressure anomalies between Iceland and a broad east-west belt centered on Portugal,
dates back roughly 100 years. The French meteorologist Tesserenc de Bort, the German
meteorologists Julius Hann and Felix Exner, and the British applied mathematician /
meteorologist Sir Gilbert Walker all contributed to its discovery. Figure 4 shows Exner's
rendition of the pressure and temperature patterns, as represented in a paper published in
1913 [5]. The contours represent correlations with a time series of winter sea-level
pressure over the Arctic. During winter months when the pressure over the Arctic is
above normal, the pressure over Portugal and much of temperate latitudes tends to be
below normal, as indicated by the belt of negative correlations centered over Portugal. In
a similar manner, when the pressure over the Arctic is below normal, the pressure over
Portugal tends to be above normal. Since the winds tend to follow the pressure contours
with low pressure to the left, it follows that below normal pressures over the Arctic favor
abnormally strong westerlies over the North Atlantic from Nova Scotia toward Scotland
and Scandinavia and vice versa, as illustrated in Figure 5. Hence, the pressure pattern
defined by Exner's correlations can equally well be characterized as a modulation in the
strength of the westerlies across the North Atlantic shipping lanes and, more generally, at
subpolar latitudes.
The corresponding surface air temperature correlation pattern indicates that high pressure
over the Arctic is conducive to above normal temperatures in Greenland and above
normal temperatures over northern Europe and vice versa. Exner reasoned that the
temperature and pressure patterns are related by way of the winds: e.g., abnormally
strong westerlies that prevail when Arctic pressure is below normal carry an abundance
of warm air off the Atlantic into northern Europe causing temperatures to be above
normal there. The influence of the winds upon the temperatures is clearly apparent in Fig.
5: when the westerlies are strong the western limit of subfreezing temperatures, indicated
by the edge of the bluish shading, lies near the border between Germany and Poland,
whereas when the westerlies are weak subfreezing conditions extend all the way into
France.
It was Sir Gilbert Walker who coined the term 'North Atlantic Oscillation' in a paper
published in 1924 [6]. Walker also coined the term, 'Southern Oscillation', which is the
SO in ENSO, and the less enduring term 'North Pacific Oscillation' describing a suite of
statistical relationships in the Pacific sector. By attaching names to suites of statistical
relationships, Walker elevated them to the status of phenomena, at least in the minds of
those who read his papers and took them seriously. In the case of the Southern
Oscillation, his intuition has proven correct: the statistical relationships embodied in that
name have proven to be the signature of the response of the global atmosphere to a
distinctive mode of atmosphere-ocean interaction in the equatorial Pacific. But it was not
until the work of Bjerknes and his successors, nearly half a century after Walker coined
that term, that proof was forthcoming. Prior to that time, believing in the Southern
Oscillation required something of a leap of faith. In an exchange of correspondence some
years later, Walker was challenged to defend his belief. When one of his contemporaries
gently chided him about the lack of a plausible mechanism that would explain the global
pattern of correlations that he identified with the Southern Oscillation, he replied: "I
think the relationships of world weather are so complex that our only chance of
explaining them is to accumulate the facts empirically... there is the strong presumption
that when we have the data of the pressure and the temperature at [altitudes of] 10 and 20
km, we shall find a number of new relationships that are of vital importance" [8]. Walker
was absolutely right: upper air data proved to be instrumental in understanding the
ENSO phenomenon. His intuition regarding the North Atlantic Oscillation has also
withstood the test of time.
Beginning in the late 1930's Carl Gustaf Rossby (regarded by many as the father of
dynamical meteorology) and collaborators at Massachusetts Institute of Technology
promoted the concept of an 'index cycle', which they hoped would prove to be a useful
tool in forecasting weather on the 1-2 week time scale [9]. This notion was based, not on
the prior work cited in previous paragraphs (which they were apparently unaware of at
the time) but upon the fact that (1) observations that the strength of the prevailing
westerlies in middle latitudes fluctuates from one week to the next and (2) their
simplified analysis of the governing equations suggested that these relatively slow
fluctuations should be reflected in the configuration of planetary waves, which
determines the longitudes where temperatures are above or below the longitudinally
averaged values. For example, one week, when the westerlies are abnormally strong,
Europe might lie within a cold trough in the planetary wave pattern, whereas a week
later, when the westerlies are weaker, it would lie within a warm ridge. The 'zonal index'
was originally defined as a measure of the strength of the westerlies that encircle the
North Pole at latitudes ~45°N.
By the late 1940's Rossby had lost interest in the index cycle, but in 1951 two younger
members of the same department published papers that seemed to imply that this concept
still held promise. In one of the papers [10] Jerome Namias proposed a redefinition of the
index cycle: instead of monitoring the strength of the westerlies as in the earlier papers,
Namias argued that it would be more informative to characterize the hemispheric
circulation in terms of the mean latitude of the westerlies. Based on his experience as a
synoptic meteorologist, he reasoned that when the westerlies are poleward of their mean
position, cold, Arctic air masses tend to be confined within the polar cap region, and
temperatures in midlatitudes tend to be above normal. Conversely, when the westerlies
are displaced equatorward of their mean position, incursions of cold air masses into
middle latitudes are more severe and more frequent. As a measure of the mean latitude of
the westerlies Namias used an index based on the difference between the strength of the
westerlies along 55 N and 35 N. He used the term 'high index' as characterizing the state
of the hemispheric circulation when the westerlies are north of their mean position, and
'low index' as characterizing the state when they are south of their mean position. The
other paper [11] was one of the first publications of a new professor by the name of
Edward Lorenz: a statistical analysis of the correlations between the pressures and winds
at various latitudes. Lorenz's study was reminiscent of Exner's study nearly 40 years
earlier, but for one important distinction: the pressure data were averaged around latitude
circles before the correlations were calculated. But despite this difference in the
methodology, Lorenz's results were remarkably similar to those of Exner, which he
wasn't aware of. In particular, Lorenz confirmed that sea level pressure over the polar
cap region tends to be negatively correlated with pressure along 45 N (the latitude of
Lisbon). And consistent with the more subjective impression of Namias, he reported that
this favored pattern in his statistical analysis is well represented by an index consisting of
the strength of the winds averaged around 55 N.
In retrospect, it is clear that the North Atlantic Oscillation (NAO) and the index cycle are
different ways of describing the elephant pictured in Fig. 6. Namias's 'high index' and
'low index' states have subsequently been used by many authors used as a way of
characterizing the contrasting states of the NAO, where 'high index' refers to below
normal sea-level pressure over Iceland and enhanced westerlies across the North Atlantic,
along 55 N. Consistent with Namias's findings, the high index state is indeed associated
with a easterly wind anomalies along 35 N. In the Atlantic sector to the southwest of
Portugal it is manifested as a strengthening of the tradewinds [Fig. 7].
5. The Renaissance
Later in the 1950's the index cycle fell out of favor for lack of evidence of simultaneous
week-to-week variations in weather in different sectors of the Northern Hemisphere.
When I was in graduate school at MIT during the 1960's it was barely mentioned. The
North Atlantic Oscillation fared somewhat better. In a paper published in 1978, Harry
van Loon and his student Jeffery Rogers drew attention to its strong influence on winter
temperatures over North America, as shown in Fig. 8 [12]. Another of van Loon's former
students, James Hurrell documented its strong influence on precipitation over Europe
(Fig. 9) [13]. Over Portugal and much of southern Europe most of the wintertime rainfall
occurs when the NAO is in its low index state. Over Scotland and Scandinavia the
situation is just the opposite: it is the high index state, characterized by strong westerlies
off the North Atlantic that accounts for most of the rainfall.
Over the years, several of us have also shown, by means of a statistical analysis technique
known as empirical orthogonal function (EOF) analysis, that the elephant pictured in Fig.
6 is also the most pronounced mode of variability of the Northern Hemisphere sea-level
pressure field; i.e., it accounts for more of the month- to-month variability of sea-level
pressure averaged over the hemisphere than any other single pattern, including the one
associated with ENSO [14].
Starting with Hurrell's paper in 1995 there has been a meteoric rise in the number of
research papers devoted to the NAO and its various pseudonyms, which I believe is
attributable to recent discoveries relating to: (1) the strong resemblance between the NAO
and the principal mode of variability in the Southern Hemisphere (2) the NAO-like
signature in observed climate trends over the past few decades and, (3) dynamical
linkages between the NAO and the wintertime polar night jet in the stratosphere. These
discoveries occurred almost simultaneously, so I've chosen to present them in thematic,
rather than chronological order.
5.1 The NAO's austral twin
Meteorologists have long been aware that the circulation of the Southern Hemisphere
exhibits much greater circular symmetry about the pole than that of the Northern
Hemisphere. The greater symmetry is attributable to preponderance of oceans
equatorward of Antarctica, which prevents the buildup of cold, continental air masses
analogous to those that cover Siberia and Canada during the winter months. Like the
mean circulation, the principal mode of month-to-month variability tends to be circularly
symmetric or annular. Moreover its structure exhibits a remarkable resemblance to the
'index cycle' described by Namias and Lorenz, with a 'seesaw' in sea-level pressure
between the polar cap region and temperate latitudes [Fig. 10] and a seesaw in the
strength of the westerlies between latitude belts centered near 35 and 55 S [15]. A strong
resemblance is also apparent in the temperature field.
Hence, the NAO has an austral twin. And because this twin exists in an environment
devoid of strong land-sea heating contrasts, it is living proof that the existence of the
NAO transcends the peculiarities of the Northern Hemisphere land-sea distribution. In
trying to describe the significance of this realization, I have likened have it to discovering
that the newspaper reporter whom one has known for years by the name 'Clark Kent', is
really Superman. In several of our papers, my former graduate student, David Thompson
and I have gone so far as to argue that the name 'North Atlantic Oscillation' is a
misnomer; i.e., that from the point of view of the governing dynamics, a more global
name like 'Arctic Oscillation (AO)' or 'Northern Hemisphere annular mode' would be
more appropriate for describing Exner and Walker's pattern than the name 'North Atlantic
Oscillation (NAO)', which singles out a particular longitudinal sector of the hemisphere
[16].
Whether the baby should be renamed at this point is debatable, but is becoming
increasingly clear that this pattern plays a role in climate variability throughout most of
the Northern Hemisphere, as envisioned in Namias's articulation of the 'index cycle'
paradigm. This pattern, whatever we choose to call it, does, indeed, orchestrate the
occurrence of cold air outbreaks and winter storms not only in Europe, but also
throughout most of the Northern Hemisphere [7]. These influences weren't apparent to
meteorologists back in the 1950's because they didn't have access to the analysis tools or
the extended historical records of past data that we have the benefit of today.
5.2 AO/NAO-related climate trends
James Hurrell’s 1995 paper was the first the first to point out that the pronounced trends
in wintertime climate since the late bear a striking resemblance to the patterns that we see
in the month-to-month variations associated with the AO/NAO. For example, the pattern
of falling pressures over the Arctic and rising over temperate latitudes resembles the
AO/NAO signature [Fig. 11]. The same is true of surface air temperature. Changing
wind patterns account for roughly half the observed temperature rises over the interior of
Eurasia. [17]. The AO/NAO signature is also clearly apparent in the trends in winter
precipitation.
If these trends are, indeed, related to systematic changes in the atmospheric circulation
that resemble the month-to-month changes associated with the AO/NAO, then instead of
thinking of them as being comprised of many separate elements as described above, it is
more meaningful to characterize them as manifestations of a shift in this pattern toward
its high index polarity, as shown in Fig. 12. Putting all the eggs in one basket, so to
speak, greatly simplifies the problem of attribution, since in this case we are justified in
focusing on the single question of what is causing the shift in this pattern. But before we
consider this question, let's look at the other factors that are contributing to the resurgence
of interest in this phenomenon.
5.3 The stratospheric connection
Another important development of the past decade is the growing appreciation of the
strong linkage between the AO/NAO and the so called 'polar night jet ', a major feature of
the circulation of the lower stratosphere. The polar night jet encircles the Arctic polar
cap region during wintertime, the season of perpetual darkness, and it blows from west to
east. It's a direct result of the strong heating contrast between the dark polar cap region
and the sunlit lower latitudes. Under certain conditions, natural or human induced
changes in the strength of the polar night jet can impact climate at the earth's surface. Let
me cite a few examples.
Meteorologists have noticed that major volcanic eruptions like that of Mt. Pinatubo back
in 1991 tend to be followed by relatively warm winters over much of middle latitudes.
The temperature pattern and the associated wind and pressure patterns bear a strong
resemblance to the distinctive signature of the AO in its 'high index' polarity: sea level
pressure is below normal over the Arctic, the westerlies across the North Atlantic and,
more generally along 55°N are abnormally strong, etc [18]. The apparent source of these
anomalies is a global cloud sulfate aerosols formed by the injection of sulfur dioxide into
the lower stratosphere. The aerosol particles absorb incoming solar radiation, warming
the lower stratosphere. The notable exception is the polar night region where there is no
sunlight to absorb. The differential heating between the sunlit and dark regions of the
global stratosphere strengthens the equator-to-pole temperature gradient which, in turn,
strengthens the polar night jet. The strengthening the polar night jet, in turn, initiates a
kind of 'chain reaction' that leads to a strengthening the westerlies along 55°N all the way
down to the earth's surface, as shown schematically in Fig. 13 [19].
The AO is also impacted by midwinter breakdowns of the polar night jet. In contrast to
the southern hemisphere, polar night jet, which is almost perfectly circular and remains
strong throughout the winter, the northern hemisphere jet is continually being perturbed
and distorted by upward propagating planetary-waves, driven by the thermal contrast
between the cold continents and the warm oceans and by the deflection of the westerlies
by the Rockies and the Himalayas. From time to time the planetary-waves over high
latitudes become so strong that the ring of cold air enclosed by the jet splits jet into pieces
that drift away from the pole and mix with the surrounding air. These breakdowns of the
jet are first evident in the upper stratosphere near the 50 km level, and they propagate
downward to the base of the stratosphere around 10 km above the earth's surface over the
course of a week or two. Once a major breakdown occurs, it takes radiative cooling over
the polar cap regions a month or two to restore the jet to its former intensity. During this
extended recovery period, the AO has been shown to exhibit a distinct preference for its
'low-index' polarity with above normal sea-level pressure over the Arctic [20]. These
conditions have been shown to favor wintry weather conditions in northern Europe, Asia
and North America [21].
The polar night jet and the AO both appear to be impacted by variations in the winds that
circle the globe in the equatorial stratosphere, which fluctuate back and forth between
easterly and westerly with remarkably regularity. The mean period of these equatorial
wind fluctuations is a little longer than two years: hence meteorologists refer to them as
the quasi-biennial oscillation or QBO [22]. It has been observed that breakdowns of the
stratospheric polar night jet tend to occur more frequently when the quasi-biennial
oscillation is in its easterly phase. The remarkable regularity of the QBO confers a
certain degree of predictability out to a year in advance that appears to be roughly
comparable, in terms of skill and economic value, with the predictability associated with
El Niño, as documented in Fig. 13. The QBO could conceivably prove to be the more
important predictor for European climate, which is more strongly impacted by the
AO/NAO than by El Niño.
Coincident with the trend in the AO toward its high index polarity over the past 30 years,
the winter stratosphere has been getting colder and the polar night jet has been getting
stronger and less easily pushed around by perturbations in the planetary waves. (In this
sense it's becoming more like its counterpart in the southern hemisphere). Midwinter
breakdowns haven't been occurring as frequently and the jets in both northern and
southern hemispheres have been persisting later into spring [23]. Increasingly
frequently, winter temperatures in the lower stratosphere over the northern hemisphere
polar cap region have been dropping below –80 C, the threshold for the formation of
polar stratospheric clouds, which serve as a medium for some of the chemical reactions
that lead to the catalytic destruction of ozone by freon and other industrial
chlorofluorocarbons. Quite likely in response to this trend, there have been indications,
within the past few years, of an incipient Northern Hemisphere 'ozone hole' .
The cooling of the polar cap and the strengthening of the polar night jet at stratospheric
levels over the past few decades is consistent with the observed trend toward the 'high
index' polarity of the AO (or NAO) down at the earth's surface, and it could even
conceivably be responsible for it . A recent series of numerical experiments by Drew
Shindell and colleagues at NASA’s Goddard Institute for Space Studies [24] suggest that
human activities might be responsible for the trends at both levels. In their model, as in
the real atmosphere, increasing concentrations of greenhouse gas molecules warm the
lower atmosphere by trapping outgoing infrared radiation, but they cool the stratosphere
by enabling it to radiate energy to space more effectively. As greenhouse gas
concentrations increase in their model simulation, the resulting cooling at stratospheric
levels tends to be particularly strong during wintertime over the polar cap region. The
trend toward colder winters over the polar cap is accompanied by a strengthening of the
polar night jet, which favors the 'high index' polarity of the AO (or NAO) down at the
earth's surface, with mild wet winters over northern Europe and dry conditions over
Portugal.
These experiments are suggestive, but not definitive at this point. They have yet to be
verified with other models and the mechanisms responsible for the various linkages have
yet to be fully elucidated. Plausible ideas have been put forward to explain the linkages,
but at this point, not all scientists are agreed that human activity is responsible for the
observed trend in the AO/NAO or even that the the trend is due to processes operating in
the stratosphere. For example, James Hurrell and Martin Hoerling have suggested, based
on their own simulations with a climate model, that the observed trends in both the
AO/NAO and in winter temperature and winds in the lower stratosphere could be
occurring in response to subtle shifts in sea surface temperature and rainfall patterns in
the tropics [25].
6. Impacts of variations in the annular modes in the Arctic and Antarctic
The AO/NAO and its austral twin are playing a role in shaping current research on the
polar regions on paleoclimate, and on terrestrial and marine ecosystems. Here are a few
illustrations.
The AO exerts a strong influence upon the pattern of surface winds over the Arctic and
the winds, in turn, control the movement of the sea ice. The prevailing winds drag the
sea ice around the pole in a clockwise sense. When the AO is in its high index state (as it
was during much of the 1990's) this clockwise gyre weakens, allowing a larger fraction of
the ice to exit the Arctic Ocean via the Fram Strait, rather than recirculating in the gyre,
as shown in Fig. 14. Under the same 'high index' conditions, the wintertime winds over
the coast of Russia have a stronger offshore component, dragging the thick, multi-year
ice away from the coast, promoting the formation of much thinner 'first year' ice which
melts during summer. Both effects, which are observable on a year-by-year basis, have
evidently contributed to the observed decline in ice thickness over the Arctic, and to the
increased incidence of open water during summer during the 1990's relative to the
previous decade. Figure 15 shows that the area along the Russian coast that has been
experiencing a trend toward open water during summer during the past 20-years is
virtually identical to the area of reduced sea ice typically observed in summers following
winters in which
Meanwhile, Antarctic climate has exhibited some strong trends that appear to be related
to the AO/NAO's austral twin, the 'Antarctic Oscillation'. In contrast to the situation over
most of the globe, surface air temperatures of over the Antarctic continent have not risen
significantly over the past few decades and Antarctic pack ice has not been retreating.
Yet over the Antarctic Peninsula, temperatures have been rising, ice caps have been
melting, and penguin colonies have been relocating farther south. David Thompson and
Susan Solomon have shown that the sharply contrasting temperature trends over the
continent and the peninsula fit the pattern of the month-to-month variations associated
with the Antarctic Oscillation which, like its counterpart in the northern hemisphere, has
been residing in its 'high index' state a larger fraction of the time during recent decades.
The high index state favors stronger westerlies around the periphery of Antarctica,
sweeping relatively mild, marine air over the peninsula, while making it more difficult
for marine air masses to penetrate into the interior of the continent [27].
7. The annular modes and paleoclimate
The AO/NAO and its austral twin are evidently responsible for some of the large regional
climate trends of the late 20th century. Could they have played an analogous role in the
larger climate swings that have occurred on time scales of centuries, millennia and
longer? There are several intriguing possibilities.
The correspondence the coldest interval of the Little Ice Age and the "Maunder
Minimum" of the sunspot cycle during the 17th Century has fueled speculation that the
relative absence of solar ultraviolet emissions during this extended period might have
been responsible for the harsh climate during this period. This idea was initially greeted
with skepticism. Meteorologists readily acknowledge that the sun's plays a critical role in
many processes in the stratosphere, but it constitutes a negligible fraction of the solar
radiation penetrates into the lower atmosphere. Hence a decrease in solar ultraviolet
emission, in and of itself, would not make climate noticeably cooler at the earth's surface.
Drew Shindell and collaborators [28] argue that a weakening of the stratospheric
circulation that occurred in response to decreased ultraviolet radiation could have
affected climate at the earth's surface through its dynamical influence on the AO. Tree
ring data and other proxy evidence examined in their paper suggests that climatic
conditions observed during the Maunder Minimum, with harsh winters over Northern
Europe bear a marked similarity to the 'low index' polarity of the AO/NAO, consistent
with the results of their numerical experiments.
The AO and its austral twin has also been mentioned in connection with the much larger
temperature swings that have occurred in time scales of tens of thousands of years in
response to subtle cyclic variations in the earth's orbit. Alex Hall [29] has found that the
spatial patterns of sea-level pressure differences between glacial and interglacial climates,
as simulated by a climate model, bear a strong resemblance to these 'annular' or 'ring like'
modes. On even longer time scales, it's tempting to speculate that the absence of cold
winter weather at subarctic latitudes during the Cretaceous period 70 million years ago
might have been the reflection of an ultra high
index state of the AO.
But lest I overstate the case, it should be emphasized that not all the large climatic swings
of the distant (or not so distant) past bear the earmarks of the AO/NAO. For example, the
rapid temperature rise over Northern Europe during decade of the 1920's, which is
sometimes attributed to the trend toward the 'high index' polarity of the NAO, was
accompanied by warming of comparable magnitude over west Greenland. If the NAO
had been primarily responsible for the warming over Europe during this period, West
Greenland should have cooled, rather than warmed.
8. Impacts on ecosystems
The impacts of the AO/NAO on marine and terrestrial ecosystems are extensive.
Impacted species include shrimp, squid, lobsters, cod, salmon, seals, caribou, musk ox,
moose lynx, seabirds, songbirds, wheat, grapes, olives, a variety of flowering plants and
trees [30]. Interactions between climate scientists and ecologists is benefiting both
disciplines. Ecologists can test various hypotheses correlating their year by year time
series of species abundance and growth rates, nesting dates, flowering dates, etc. and
various sites with indices of the AO/NAO. Climate scientists, in turn can use ecological
data such as tree ring thicknesses and blooming dates as proxies for climate data and
thereby extend their historical indices farther backward into the past.
9. Concluding remarks
Some instructive parallels and contrasts can be drawn between the AO/NAO and ENSO.
ENSO is inherently an equatorial phenomenon, but it impacts winter climate over large
areas of the extratropics. In contrast, the AO/NAO and its austral twin exhibit largest
amplitudes over high latitudes, but their signatures extend all the way across the tropics,
into the subtropics of the opposing hemisphere. ENSO and the the AO/NAO both vary
on time scales much longer than the chaotic waves and vortices that dominate the
circulation of the lower atmosphere and consequently are to some degree predictable well
beyond the 1-2 week limit of conventional weather forecasts. Although some important
details remain unresolved, I think it's fair to say that the basic mechanism of the ENSO
cycle is relatively well understood. In contrast, fundamental issues concerning the nature
of the AO/NAO are still under debate: e.g., is it inherently sectoral or annular? what
gives it its distinctive shape? Is it more than just a statistical entity?
In the case of ENSO, the alternation between warm and cold states (El Niño vs. La Niña)
has been of primary interest, whereas the AO/NAO has been of primary interest in
relation to global climate change. It remains to be seen whether the pronounced trend
toward the high index state, which began in the late 1960's is indicative of a secular trend
that will continue into the future, or whether it is merely one rather dramatic realization
of random variations from decade to decade. Only time will tell.
Acknowledgements
To be added
References
1. J. Bjerknes, Tellus, 18, 820 (1966). J. Bjerknes, Mon. Wea. Rev., 97, 163 (1969). J.
Bjerknes. J. Phys. Oceanogr., 2, 212 (1972)
2. N. J. Mantua, S. R. Hare, Y. Zhang, J. M. Wallace, and R. C. Francis, Bull. Amer.
Meteorol. Soc., 78, 1069 (1997)
4. H. van Loon, J. C. Rogers, Mon. Wea. Rev., 106, 296 (1978)
5. F. M. Exner, Sitzungberichte der Mathematische-Naturwissenschaftlichen Klasse der
Akad. Wissenschaften, 122 Abteilung 2a, Zweiter Halbband, 6(10)1165 (1913)
6. G. T. Walker, Mem. Ind. Meteorol. Dept., 25, 275 (1924)
7. D. W. J. Thompson, J. M. Wallace, Science, 293, 85 (2001)
8. R. B. Montgomery, Mon. Wea. Rev., 39 (Suppl.) 1 (1940)
9. C. -G. Rossby, J. Mar. Res., 3, 38 (1939)
10. J. Namias, J. Meteorol., 3, 130 (1950)
11. E. N. Lorenz, J. Meteorol., 8, 52 (1951)
12 D. W. J. Thompson, J. M. Wallace, J. Climate, 13, 1000 (2000) in tradewind figure
13. later van Loon paper
14. J. E. Kutzbach, Mon. Wea. Rev., 98, 708 (1970). K. E. Trenberth, Mon. Wea. Rev.,
109, 1169 (1981). J. M. Wallace and D. S. Gutzler, Mon. Wea. Rev., 109, 784, TW98
15. D. Gong and S. Wang, Geophys. Res. Lett., 26, 459
16. D. W. J Thompson,., J. M. Wallace, Geophys.Res, Lett., 25, 1297 (1998). R. A.
Kerr, Science, 284, 241 (1999). J. M. Wallace, Quart. J. R. Meteorol. Soc., 126, 791
(2000)
17. D. W. J. Thompson, J. M. Wallace, and G. C. Hegerl, J. Climate, 13, 1018 (2000)
M. P. Baldwin and T. J. Dunkerton, Science, 294, 591 (2001).
18. P. M. Kelly, P.D. Jones, J. Pengqun, Int. J. Climatol. 16. 537 (1966)
19. K. Kodera., J. Geophys. Res., 99, 1273, (1994). A. Robock and J. Mao, Geophys.
Res. Lett., 12, 2405 (1992).
20. M. P. Baldwin and T. J. Dunkerton, Science, 294, 591 (2001).
21. Thompson D. W. J., M. Baldwin, J. M. Wallace, J. Climate (2001)
22. R. J. Reed, W.J. Campbell, L.A. Rasmussen and R.G. Rogers, J. Geophys. Res., 66,
813 (1966)
23. D. L.. Hartmann., J. M. Wallace, V. Limpasuvan, D. W. J. Thompson, and J. R.
Holton, Proc. Nat. Acad. Sci., 97, 1412 (2000, J. M. Wallace and D. W. J. Thompson,
Physics Today, 2, 28 (2002)
24 D. T. Shindell, R. Miller, G. Schmidt, L. Pandolfo, Nature, 399, 452 (1999)
25 M P. Hoerling and J. W. Hurrell, Science, 292, 90 (2001)
26. I. G. Rigor, J. M. Wallace and R. L. Colony, J. Climate, in press (2002) from
caption of Fig.
27. D. W. J., Thompson and S. Solomon, Science, in press (2002)
28. D. T. Shindell, G.A. Schmidt, M.E. Mann, D. Rind and A. Waple, Science, 294
(2001)
29. A. Hall, Amer. Geophys. Union Fall Mtg., A22B-10 (2001)
30. M. C. Forchammer and E. Post, Trends in Ecology and Evolution 15, 286.
Figure captions
Fig. 1 ENSO Feedback loop 15
Fig. 2 Hemispheric impacts of the 1982-83 and 1997-98 El Nino events. 16
Fig. 3 ENSO SAT pattern 17
Fig. 4 Exner's SLP map 27
Fig. 5 European maps from Science paper 28
Fig. 6 Elephant 43
Fig. 7 tradewinds 37
Fig. 8 AO and SAT N. American perspective 40
Fig. 9 AO and precip: European perspective 41
Fig. 10 Austral twin SLP 45
Fig. 11 SLP; trend and AO from Science article 50
Fig. 12 AO time series (updated)
53
Fig. 13 AO schematic 59
Fig. 14. Sea ice motion
ams
68
Fig. 15 summer sea ice concentation
ams
69