J. M. Wallace: On the Arctic and Antarctic Oscillations

On the Arctic and Antarctic Oscillations
by J. M. Wallace
Date: July 17, 2000
Notes by: Eric DeWeaver and Michael Palmer


1. Introduction

2. Motivation

3. Perspectives on the Northern Hemisphere annular mode

4. AO and AAO signatures in atmospheric and surface variables

5. The AO's effect on blocking and extreme weather events

6. The AO and climate change

7. Concluding remarks


Note: All figures shown here can be downloaded as postscript files. To access a postscript file, click on the corresponding image.

1. Introduction

Both the Northern and Southern Hemispheres have leading modes of circulation variability with deep, barotropic, zonally symmetric structures. We refer to these modes as the Arctic and Antarctic Oscillations (AO/AAO), or more generically as annular modes. This talk gives a historical perspective on the modes, and documents their spatial patterns in temperature, sea level pressure (SLP), and zonal-mean zonal wind ([u]). While the AO is defined as a monthly-mean, extratropical, tropospheric pattern of variability, its influence extends well beyond these categories. As will be shown below, it has connections to extreme weather events and long-term climate trends, a distinctive signature in the tropics, and important connections to the stratosphere.

Dave Thompson, who has been the driving force behind much of the recent activity related to the AO and AAO, has put together an informative webpage on annular modes, which can be found at http://tao.atmos.washington.edu/data/annularmodes . Anyone wishing further information on annular modes or more detail on the material presented here should consult this webpage.

2. Motivation

Why do we like ring-like modes? There are several reasons that we might expect annular modes to figure prominently in the low-frequency behavior of the atmosphere. First, it is well known (e.g. Pedlosky 1987) that vorticity conservation leads to an upscale energy cascade in barotropic fluids, so that large-scale patterns evolve from small-scale isotropic turbulence. In the case of rotating fluids, Rhines (1975; see also Pedlosky 1987) showed that the upscale cascade leads to the development of strong zonally elongated zonal jets. Also, zonally oriented eddies propagate more slowly than meridionally oriented eddies of the same size, so zonal structures -- and particularly zonally symmetric patterns, which do not disperse as Rossby waves at all -- should be more prominent in the time mean.

Second, the studies by Lorenz (2000) and DeWeaver and Nigam (2000) show that [u] anomalies can generate anomalous eddies which provide further momentum to sustain the original [u] anomalies. This positive feedback means that annular anomalies are likely to persist longer than regional anomalies.

Third, numerical models provide evidence that annular modes are easily excited by external forcing. Byron Boville found that the circulation in the winter stratosphere in CCM0 was extremely sensitive to changes in the radiation code. He found that a zonal-eddy feedback was important in establishing the circulation changes, which had an annular structure extending from the stratosphere to the ground. Shindell et al. (1999) found similar circulation changes in the response of their model to anthropogenic greenhouse gas forcing. They characterized the response as an excitation of the positive phase of the AO.

3. Perspectives on the Northern Hemisphere annular mode

The beast that we refer to as the AO or the Northern Hemisphere annular mode (NAM) has been characterized in different ways by different researchers. The principal schools of thought are summarized in figure 1. One group of researchers, starting with Walker and Bliss (WB, 1932) and including van Loon and Rogers (1978) and Hurrell (1995), regarded the phenomenon as a regional climate variation and referred to it as the North Atlantic Oscillation (NAO). A parallel effort, including work by Rossby, Willett, and Namias, studied esssentially the same variability by looking at fluctuations of the zonal-mean circulation. They defined various zonal indices to identify this variability, which they regarded as fundamentally zonally symmetric or annular. Finally, a third group, including Kutzbach (1971), Trenberth (1981), and Wallace and Gutzler (1981), characterized the dominant mode of Northern Hemisphere variability using the leading EOF of SLP. Like these authors, we use the leading SLP EOF to define the AO (Thompson and Wallace 1998, 2000; Thompson et al. 2000).

Figure 1

The regional perspective of WB may be less a deliberate decision than a consequence of the limited data available at the time. The top panel in figure 2 shows the spatial pattern of the NAO from WB, while the bottom panel shows the spatial pattern that results when their NAO index is reconstructed using a modern data set (see Wallace 2000 for details). As you can see, the pattern takes on a much stronger circumpolar aspect when a more comprehensive data set is used. Given more complete data, Walker and Bliss might have called their pattern the AO rather than the NAO.

Figure 2

Rossby (1939) introduced the zonal index as a measure of the change in strength of the midlatitude westerlies. However, this idea was revised by Namias (1950), who proposed that index fluctuations were really changes in the meridional position of the zonal jet rather than its strength. In the high phase of the index, the jet is shifted towards higher latitude, the polar vortex is intensified, and cold air is walled off in the polar vortex. In the low phase, the vortex is weaker, the jet is shifted southward, and cold air outbreaks are more common. Lorenz (1951) considered the variability of Northern Hemisphere SLP, and showed that pressure tends to be out of phase between high and low latitudes. He suggested that an index made by averaging [u] at 55N (U55) would serve as a good measure of this pressure variability. A similar index, called the ``polar pressure deficit'', was constructed by Gates (1950), who took the average SLP from 45N to the north pole and subtracted it from the zonal-mean SLP at 45N.

Despite their differing regional and circumpolar perspectives, the patterns produced by NAO and zonal indices are quite similar. Figure 3 shows the correlation between SLP at each gridpoint and the two-point Portugal - Iceland NAO index (i.e., SLP in Iceland minus SLP in Portugal), Lorenz's U55 index, and the leading EOF of SLP. All three are highly correlated, and all show opposition between pressure over the polar cap and the subpolar belt.

Figure 3

It follows that the NAO and AO are synonyms: they are different names for the same variability, not different patterns of variability. The difference between the terms is in whether that variability is interpreted as a regional pattern controlled by Atlantic sector processes or as an annular mode whose strongest teleconnections lie in the Atlantic sector. The AO is also the embodiment of what earlier researchers have called the Northern Hemisphere ``index cycle''.

4. AO and AAO signatures in atmospheric and surface variables

To examine the signature of the AO in SLP, [u], and surface and midtropospheric temperature, we regress these fields against the standardized time series for the AO, formed by calculating the leading EOF of SLP from 20N to 90N. The regression of [u] and SLP against the AO time series is shown in figure 4. For this figure the AO time series is generated using all months of the year, but January, February, and March make the largest contributions to the patterns. Note that the Pacific SLP center is much more prominent here than in the correlation map in figure 3. The correlation is weak, but the North Pacific is a region of high variance, so the center shows up with greater amplitude in the regression map.

Figure 4

The [u] regression shows that when pressure is low over the pole and high in the subpolar belt, abnormally strong westerlies show up north of 45N. The westerly anomalies are accompanied by easterly anomalies which are centered on 35N but extend into the tropics at low levels, where they can be viewed as a strengthening of the trades. The trade strengthening occurs mostly in the Atlantic sector, but there is a small Pacific contribution as well. The tropical easterlies extend from the surface to the jet stream level, and westerly anomalies overly the easterlies at high levels in the deep tropics.

The Antarctic Oscillation is the dynamical twin of the AO, and it can be calculated as the leading EOF of SLP or 850mb height from 20S to the south pole. The patterns of [u] and SLP regressed against the AAO time series, shown in figure 5, have much in common with the AO patterns. Again we see a polar low surrounded by a high pressure belt, and a deep dipole pattern in [u]. By superimposing the AO and AAO plots, you can see the high level of agreement in the [u] patterns for the two hemispheres. The level of agreement is even higher if the AAO regression is done just for November and December, because in that season the AAO [u] anomalies extend strongly into the stratosphere.

Figure 5

An important aspect of AO variability is its connection to surface air temperature (SAT). Figure 6a shows the SAT pattern for the AO from the Comprehensive Ocean-Atmosphere Data Set (COADS), and figure 6b shows the corresponding land SAT from the Global Historical Climatology Network (GHCN). By superimposing the two you can see that the high phase of the AO (low pressure over the pole) is accompanied by cold temperatures over eastern Canada and Greenland, while warm conditions prevail over Siberia and the United States in the subpolar belt.

Figure 6a

Figure 6b

The tropospheric temperature anomalies of the AO and AAO extend well into the tropics, and even across the equator. Figure 7a shows the tropospheric temperatures from John Christy's MSU 2LT data set. As in the surface temperatures, cold temperatures over Greenland are surrounded by a warm ring. But the figure also shows that the cold pole is accompanied by a slight cooling of the global tropics (in 7a and 7b the colors saturate so that the weak tropical temperature anomalies are visible). The same banded structure is evident for the AAO: the tropics are cold when the pole is cold. Figure 7b shows the Pacific half of the temperature patterns.

Figure 7a

Figure 7b

Figure 8 summarizes the features of the AO in its high index phase, defined as the polarity in which the subpolar westerlies are anomalously strong. In this phase, a cold low sits over the pole, surrounded by a belt of enhanced westerlies at 55N, which is accompanied by a slackening of the westerlies around 35N. Warm high pressure conditions prevail between 35N and 55N, and cold anomalies occur in the tropics, accompanied by a strengthening of the trades. In addition, high level westerlies appear over the equator. The [u] dipole in the extratropics is accompanied by a mean meridional circulation which acts to decelerate the upper-level [u] anomalies through Coriolis torque. The anomalies are maintained against the torque by convergence of eddy momentum fluxes, represented by the thick arrows. At the surface, the mean meridional circulation acts in the opposite sense, maintaining the [u] anomalies against friction and enhancing the surface trades.

Figure 8

In addition to these features, low ozone values occur in the high latitude stratosphere during the high index phase. The low values can be understood as a consequence of a reduction in the strength of the stratospheric Lagrangian mean circulation. The Lagrangian mean circulation transports ozone from the tropical stratosphere to the pole, so any reduction of the circulation serves to deplete ozone at high latitudes.

The key features in figure 8 can be explained in terms of wave refraction. In midlatitudes, upward propagating Rossby waves are refracted more or less strongly towards the tropics, depending on the strength of the lower stratospheric polar vortex. In the low AO phase, the polar vortex is weak, so more waves are refracted into it. When these waves break they decelerate the vortex even more. In the high phase the strong vortex refracts more wave activity into the tropics, and the breaking of these waves transports momentum into the vortex, making it stronger. This feedback is illustrated schematically in figure 9, in which the thin arrows represent the Eliassen-Palm flux of planetary waves and the contours show the strength of the westerlies.

Figure 9

The thick arrows show the Lagrangian mean circulation in the stratosphere, also known as the Brewer-Dobson circulation. This circulation is always poleward in the Northern Hemisphere, driven by wave breaking and diabatic cooling in the polar vortex. In the high index phase of the AO, there is less wave breaking in the polar vortex and the Lagrangian mean circulation is weaker, while the opposite holds in the low index phase. Since the Brewer-Dobson circulation is responsible for bringing ozone into the polar stratosphere, the amount of ozone in the polar stratosphere depends on the strength of the circulation.

The schematic in figure 8 suggests that there will be coupling between the tropospheric AO and conditions in the lower stratosphere, and evidence for this coupling is presented by Baldwin and Dunkerton (1999). They form a multi-level AO pattern by taking the leading EOF of 90-day low-pass geopotential height north of 20N at five levels (1000, 300, 100, 30, and 10 hPa), and using regression against the EOF time series to obtain the AO pattern at all available levels from 1000 to 10hPa. The AO pattern at each level is then projected onto the filtered data for the level, and the projection coefficients are plotted in figure 10 as a function of time and height. In the figure, red (blue) represents above (below) average geopotential height in the polar cap. The figure shows a significant correlation between the troposphere and the stratosphere, with a tendency for signals to propagate downward. Thus the troposphere feels the influence of the stratosphere, although it's not clear whether this relationship is strong enough to be a useful forecasting tool.

Figure 10

Figure 11 documents the AO's effect on the motion of Arctic sea ice. The figure, adapted from Rigor et al. (2000), shows the movement of Arctic sea ice during low (top) and high (bottom) AO phases. The vectors come from an objective analysis of buoy data for the period 1979-98. In the low phase, there is a great deal of recirculation in the clockwise ``Beaufort gyre'', which enhances sea ice thickness by allowing the ice to remain in the cold central Arctic, growing thicker from year to year. Also, the clockwise circulation causes more rafting and piling up of the ice due to Ekman convergence, again making thicker ice floes. On the other hand, the high AO phase leads to a reduction in the recirculation and shorter ice residence times. Meanwhile, reduced advection from the Canadian side promotes opening of the ice on the Russian side, and there is an increase in the passage of ice through Fram Strait and out into the North Atlantic. Thus the AO can have a strong effect on Arctic sea ice thickness.

Figure 11

The GHCN precipitation anomalies associated with the AO are shown in figure 12. Wetter conditions prevail throughout most of the Arctic, while drier conditions occur in southern Europe. Of particular interest to those of us at the University of Washington are the wetter conditions along the Pacific coast from Oregon to Alaska.

Figure 12

5. The AO's effect on blocking and extreme weather events

Blocking has a lot to do with the severity of winter weather in the Northern Hemisphere, and the AO has a strong ability to control blocking. To quantify this control, we define a blocking event as a week or more of excess pressure in the midtroposphere together with an anticyclone at the surface. By this definition, figure 13 shows that blocking occurs preferentially during the low AO phase in Alaska, the North Atlantic, and Russia. In the North Atlantic, there is no blocking at all in the high AO phase. A strong control in the North Atlantic is to be expected, given the strength of the AO signal there, but even in Alaska there is a two to one preference for blocking in the low AO phase.

Figure 13

In fact, the AO influences all indicators of severe weather to some extent, as can be seen from the statistics compiled in the next two figures. Figure 14 gives a sample of regions which have a much higher incidence of cold days during the low index. For instance, minimum temperatures less than -15C in Yakima, Washington are much more likely on low index days than on high index days, a fact which may be of some interest to the fruit growers there. Figure 15 shows a preference for the low AO phase in blocking days, cold surges, cold temperatures, frozen precipitation, and strong winds and waves in a variety of locales throughout the northern extratropics.

Figure 14

Figure 15

6. The AO and climate change

In addition to its importance for monthly-mean variability, blocking, and severe weather events, the AO has played a substantial role in the climate trends of recent decades. To show this we first plot the AO time series from the 1860s to the present in figure 16. The top curve shows a time series for the AO based on its signature in the SAT, while the bottom curve is the AO based on SLP data. In the top curve the SAT comes from the Jones et al. (1999) data set, and the backward reconstruction is largely determined by the Siberian SAT data. In the bottom curve, the blue color shows a reconstruction from the 1870s to 1932 using WB's NAO index (a combination of SLP and SAT at several stations). In both panels it is clear that AO has been on the rise since the 1960s, although there appears to be a decadal signal superimposed on the upward trend (the decadal signal is not correlated with the sunspot cycle). Since the AO is the leading mode of climate variability in the Northern Hemisphere, the climatic consequences of this trend are clearly of interest. The suite of figures that follows documents these consequences in a variety of important indicators of climate and atmospheric circulation.

Figure 16

The 30-year JFM trend in SLP is shown in the top panel of figure 17, while the bottom panel shows the AO's contribution to that trend. In the 30 years from 1968 to 1997, SLP over the Arctic has dropped 6 to 10 mb, while SLP over the North Atlantic and western European sectors has risen. Comparison of the top and bottom panels reveals that the AO makes an impressive contribution to these changes. The only place where the total trend differs from the AO pattern substantially is in the North Pacific, where ENSO-like decadal variability is the dominant player (e.g. Trenberth and Hurrell 1994, Zhang et al. 1997).

Figure 17

Likewise, the 30-year wintertime trend in zonal-mean circulation is strongly influenced by the AO. Figure 18 shows that the JFM [u] trend, with changes of up to 9m/s in stratospheric wind speed, is virtually identical to the AO [u] pattern. A similar result holds for the AAO in the Southern Hemisphere. The 30-year [u] trend in the Southern Hemisphere is plotted in figure 19 for the months of November and December, the months when the AAO [u] anomalies extend into the stratosphere. The [u] trend strongly resembles the AAO anomalies, although there are some uncertainties regarding the quality of the Southern Hemisphere data for the earlier years of the record.

Figure 18

Figure 19

The AO also makes a substantial contribution to [T], as shown in figure 20. However, the polar stratospheric temperature has decreased dramatically, and the AO contribution to the trend does not account for all of this decrease. The radiative effect of ozone depletion must also be involved.

Figure 20

Figures 21 and 22 display the SAT trend and SAT regressed against the AO index, respectively. The AO contributes to the warming of Siberia and the cooling of Greenland and the Middle East. Overall, the AO accounts for over 30% of the JFM warming of the Northern Hemisphere continents (more details can be found in Thompson et al. 2000).

Figure 21

Figure 22

Precipitation trends and AO-related precipitation anomalies are shown in figures 23 and 24. In both figures there is drying in southern Europe and the Sahel. A tendency for wetter conditions is also found in both plots extending inland from the Pacific coast of Alaska and throughout much of central Asia. The disagreement over China is partly due to the influence of decadal ENSO-like variability.

Figure 23

Figure 24

A table of the AO contribution to various climate indicators is presented in figure 25, listing contributions to SAT, SLP, rainfall in Norway and Spain, column ozone, and MSU4 lower-stratospheric temperature. In all cases, the AO contribution is substantial.

Figure 25

As discussed above (figures 8 and 9), the AO influences wintertime stratospheric ozone concentrations by modulating the Lagrangian mean stratospheric circulation. Thus one would expect the upward trend in the AO to lead to some ozone depletion, and this expectation is confirmed in figure 26. The top panel gives the column ozone trend in Dobson units for March 1979-1993, and the bottom panel gives the AO contribution to the trend. March is the relevant month because the sun rises over the Arctic in March. Arctic Ozone observations are not available during the polar night, and photochemical ozone depletion occurs primarily in March. The AO contribution bears a strong spatial resemblance to the total trend, and accounts for about half of its amplitude and much of its spatial structure.

Figure 26

Changes in Arctic sea ice movement also have a strong component congruent with the AO, as can be seen from figures 27 and 28. Figure 27 shows ice movement regressed on the AO, while figure 28 shows the difference in ice movement between the periods 1989-1998 and 1979-1988, taken from Rigor et al. (2000). The figures show that the AO is linked to a reduction in the strength of the Beaufort gyre recirculation (see figure 11). Also, the difference vectors have an component outward through Fram Strait, and this component increases with the AO. The AO is thus implicated in the thinning of the Arctic sea ice reported by Rothrock et al. (1999).

Figure 27

Figure 28

Since the high frequency variability of the AO is much greater than its decadal variability, it is appropriate to think of the change in the AO as a preference for the positive phase rather than a steady increase over time. A measure of this preference is given in figure 29, which shows the number of days with positive or negative AO anomalies exceeding one standard deviation for the decades 1958-67 and 1988-97. In the earlier period, low index AO days exceeded high index AO days by a factor of two in JFM, while in the more recent period high index days exceeded low index days by a factor of six. Surprisingly, the change in preference is also evident in summer (JJA), with low days exceeding high days by a factor of two for the earlier period, and the opposite ratio in the later period.

Figure 29

Finally, we consider the influence of the AO on the breakdown of the Northern Hemisphere polar vortex at the end of the winter season. In the positive AO phase the vortex tends to be stronger, so a positive AO trend in late winter implies an extension of the stratospheric winter season. The delay of the spring breakdown was demonstrated by Zhou et al. (2000), who have kindly contributed figure 30. In this figure, the fraction of the world covered by the polar vortex is plotted for the 650K, 550K, 450K, and 400K (approximately 20, 40, 80, and 120mb) isentropes over the course of the seasonal cycle. At all four levels, it can be seen that the breakup of the polar vortex happened later in 1992-98 (red curve) than in 1980-85 (black curve) or 1986-91 (blue curve). According to these curves, the winter season in the stratosphere has been extended by about two weeks.

Figure 30

7. Concluding remarks

The figures presented here highlight the differences between the AO and NAO nomenclature. To be sure, most of the statistical relationships are strongest in the North Atlantic sector. But it may be more useful to think of the dynamics in terms of zonal-mean cross sections, such as in figure 9, which depicts the interplay of the zonal-mean flow and the planetary wave fluxes. Also, the AO has a dynamical twin in the Southern Hemisphere, the AAO. The AO terminology highlights the analogous nature of the two structures, while the NAO terminology would suggest that we look for a South Atlantic Oscillation, or perhaps a North Pacific Oscillation. It must also be emphasized that although the AO has its strongest centers in the North Atlantic, its influence is at least hemispheric in scope, as can be seen from the broad tropical cooling in the MSU 2LT regressions and the enhancement of the trade winds. Furthermore, the AO's effect on blocking is felt throughout the hemisphere. Finally, the zonal-mean perspective is helpful for thinking about interactions between the stratosphere and the troposphere.

Annular modes have figured prominently in climate change, in SAT and in the winds, temperatures, and ozone concentrations aloft. An important challenge facing us now is to incorporate these AO-related changes into our thinking about human influences on climate.


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