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John M. Wallace

Abstract

In large-scale wave disturbances in the lower stratosphere, the poleward and upward velocity components are positively correlated so that typical air trajectories, when projected onto the meridional plane, slope upward toward the pole. The slope of the air trajectories can be reconciled with the observed poleward, countergradient eddy heat flux at these levels if one takes into account the poleward acceleration of warm air in the wave troughs and the equatorward acceleration of cold air in the ridges. These temperature anomalies are produced by subsidence in the wave troughs and ascent in the ridges. The same processes are capable of producing poleward and downward eddy fluxes of potential vorticity, ozone, and other tracers whose values or concentrations increase rapidly with height.

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John M. Wallace

Since 1966, two types of wave motions have been discovered in the tropical stratosphere. These have been identified with the two gravest modes of a family of equatorial waves. These waves are characterized by downward phase propagation, which renders them important in the vertical transport of energy and zonal momentum. In the tropical lower troposphere there exists a separate class containing wave modes which do not propagate vertically, one of these being the familiar easterly wave.

The role of these two classes of waves in the tropical general circulation is discussed and the possible energy sources for the waves are enumerated.

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John M. Wallace

Abstract

Rawinsonde data from the western Pacific region for the summer of 1967 are expanded in terms of complex empirical orthogonal functions and the results are compared with those of previous investigations based on the same data set. The results and conclusions are consistent with those based on spectrum analysis and compositing, except in a few cases where it has been possible to resolve ambiguities in the earlier work. The new method has been particularly helpful in clarifying the relationship between the mixed Rossby-gravity waves of the upper troposphere and the synoptic-scale, westward propagating waves of the lower troposphere. The new results indicate that these two disturbance types are so strongly coupled that it is not possible to separate them.

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John M. Wallace

Abstract

No abstract avaiable.

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Yochanan Kushnir and John M. Wallace

Abstract

Two sets of 15-day numerical forecasts are performed with a general circulation model to examine aspects of the mutual interaction between high-frequency, baroclinic-wave variability and the low-frequency components of the atmospheric flow. A control run based on an initial field, arbitrarily chosen from the history tapes of a previous model integration and a forecast based on a time-filtered version of the same initial state are compared. The results indicate that the high-frequency variability of the flow in the latter forecast returns to normal amplitudes about one week after the initialization time, at which state it is only weakly correlated in space with the high-frequency component of the flow in the control run. The low-frequency components of the flow seems to behave differently depending on their zonal scale: Ultralong waves (wavenumber 1–3) are only weakly affected by the removal of the baroclinic activity from the initial conditions, while long waves (wavenumber 4–6) react to the removal of the baroclinic waves by drifting eastward faster than their counterparts in the control run.

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Xinhua Cheng and John M. Wallace

Abstract

Hierarchical cluster analysis based on the method of Ward is performed on the Northern Hemisphere wintertime 10-day low-pass-filtered 500-hPa height field, using the NMC operational analyses for the period 1946–85. Input data are gridded fields at 5-day intervals, a total of 702 maps, each with 445 grid points. The measure of similarity between maps is the squared height difference, averaged over all grid points; that is, the squared “distance” between the maps in multidimensional phase space. The closest two of the 702 maps are merged to form a cluster that, in subsequent calculations, replaces the maps from which it was formed. This procedure (modified slightly, to deal with the differing numbers of maps in the clusters) is repeated 701 times until all the maps have been merged to form a single cluster whose centroid corresponds to the climatological mean map. The two clusters involved in the final merger, the pair of smaller clusters that merged to form each of them, and so on, are represented in terms of a “family tree” that is traced back to the point where the clusters become too small to be of practical interest. The reproducibility of the larger clusters is compared by seeing how well various ones are replicated when the analysis is repeated on randomly chosen halves of the dataset in an ensemble of 50 runs.

The three most reproducible clusters, which together account for ∼⅓ of the 702 maps in the dataset, can be reconstructed remarkably well from linear combinations of the two leading EOFs of the covariance matrix. They are related to features of the probability density function (PDF) in a two-dimensional phase space defined by the expansion coefficients of these EOFs. One is marked by a closed anticyclone over the southern tip of Greenland, one by a ridge over the Gulf of Alaska, and one by a ridge over the Rockies. In comparison to other clusters of comparable size, their centroids are conspicuously far from the climatological mean map. Positive 500-hPa height anomalies in excess of 200 m are observed in association with the first two clusters, over regions of large variance and strong positive skewness of the 500-hPa height field. Occurrences of these two clusters have often been marked by extreme cold over parts of North America. Similar clusters are obtained when the analysis is performed on the Pacific/North American and Atlantic/European sectors of the hemisphere. The results are compared with those obtained in other studies, based on a variety of analysis techniques.

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Ming Bao and John M. Wallace

Abstract

Clusters in the Northern Hemisphere wintertime, 10-day low-pass-filtered 500-hPa height field are identified using the method of self-organizing maps (SOMs). Results are based on 1) a 57-winter record of ERA and 2) a 93-winter record of the NOAA Twentieth-Century Reanalysis (20CR). The clusters derived from SOMs appear to be more robust and more linearly independent than their counterparts derived from Ward’s method, and clusters with comparable numbers of member days are more distinctive in terms of the standardized Euclidean distances of their centroids from the centroid of the dataset. The reproducible SOM clusters in the hemispheric domain are 1) the negative polarity of the North Atlantic Oscillation (NAO), 2) a pattern suggestive of Alaska blocking with a downstream wave train extending over North America and the North Atlantic, 3) an enhancement of the climatological-mean stationary wave pattern in the Western Hemisphere that projects positively upon the Pacific–North America (PNA) pattern, and 4) a pattern that projects upon the negative polarity of the PNA pattern. The first three patterns have important impacts on the wintertime climate in North America and Europe. In particular, they are helpful in interpreting prevailing flow patterns during the exceptional winters of 1930–31, 2009–10, and 2013–14. Because of the very limited number of independent samples in a single winter, the number of days per winter in which the circulation resides within individual clusters varies erratically from winter to winter, rendering attribution difficult.

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Zhifang Fang and John M. Wallace

Abstract

Northern Hemisphere sea ice concentration, 500-hPa height, sea level pressure, and 1000–500-hPa thickness at 7-day intervals are examined for the period 1972–1989, with emphasis on the winter season. The temporal variability of sea ice concentration is largest along the climatological mean ice edge where its frequency distribution is strongly bimodal with ice-free and ice-covered conditions being observed much more frequently than partial ice cover. These results confirm impressions, based on visual inspection of satellite imagery, that most of the variability in these regions is associated with the advance and retreat of the ice edge.

Relationships between large-scale patterns of atmospheric variability and sea ice variability are investigated, making use of singular value decomposition of the temporal covariance matrix. The analysis is conducted separately for the Atlantic and Pacific sectors. In agreement with earlier studies based upon monthly mean data on sea ice concentration, the strongest sea ice pattern is comprised of a dipole with opposing centers of action in the Davis Straits/Labrador Sea region and the Greenland and Barents seas. Its temporal variability is strongly coupled to the atmospheric North Atlantic oscillation (NAO). The relationship between the two patterns is strongest with the atmosphere leading the ocean by two weeks. An analogous dipole pattern is observed in the Pacific sector, with opposing centers of action in the Bering Sea and the Sea of Okhotsk, which is related to a distinctive pattern of atmospheric circulation anomalies in the Pacific sector. One polarity of the NAO and its Pacific counterpart is associated with blocking episodes, during which the influence of the atmosphere is strong enough to temporarily hall the climatological mean advance of the ice edge in some regions and substantially accelerate it in others.

The relationships between the fields is indicative of local forcing of sea ice in most regions, with wind stress and thermodynamic fluxes at the air-sea interface both contributing. A possible exception is the Greenland Sea, where it may be necessary to invoke some form of remote forcing in order to explain the observed changes on the interannual time scale.

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Hisashi Nakamura and John M. Wallace

Abstract

Synoptic behavior of individual baroclinic eddies in the course of their interactions with amplifying blocking anticyclones is examined, based upon a 30-year record of the tropospheric circulation in the Northern Hemisphere winter. High-pass-filtered as well as unfiltered fields of geopotential height and potential vorticity were composited relative to the onset of the five different types of blocking patterns. Before the compositing, the entire sequence of the fields was slightly shifted in time and space in such a way that the strongest baroclinic eddy, during the onset of each blocking event, occupies a prescribed position in the upstream storm track when it reaches its maximum intensity. Since this shifting considerably reduces the cancellation between the individual high-frequency migratory eddies, this type of compositing can present a more synoptically oriented view of those interactions than conventional compositing.

In our composite results, one or two pairs of cyclonic and anticyclonic eddies associated with baroclinic waves appear to interact with the growing blocking anticyclone within its onset period that lasts about a week. These eddies become less baroclinic as they approach the block. Each anticyclonic eddy seems to advect low potential vorticity air from lower latitudes, which becomes entrained into the block. The stronger anticyclonic, migratory eddies at the tropopause level, in association with short-wave ridges, undergo significant distortion, as the eddies interact with the preexisting ridge and finally merge with it. In contrast, during the onset of a blocking ridge located much closer to the jet-stream axis, the migratory synoptic-scale eddies tend to travel around the ridge without rapid decay or strong meridional stretching, while yielding the poleward flux of westerly momentum that acts to reinforce that amplifying ridge.

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Rei Ueyama and John M. Wallace
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