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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

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

Abstract

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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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RenéD. Garreaud and John M. Wallace

Abstract

Transient incursions of midlatitude air to the east of the Andes Mountains into subtropical and tropical latitudes are a distinctive feature of the synoptic climatology over South America. The mean synoptic-scale structure of these incursions is documented in this paper on the basis of a compositing analysis of NCEP–NCAR reanalyzed meteorological fields and satellite-based outgoing longwave radiation measurements. Although these incursions are a year-round phenomenon, with relatively modest seasonal changes in their structure, this analysis is focused on the austral summer, when they have their largest impact in the precipitation field.

The summertime incursions move equatorward at a mean speed of 10 m s−1 and retain their identities over intervals of about 5 days. The upper-level circulation is characterized by a midlatitude trough–ridge couplet that provides the quasigeostrophic forcing of the system. Before the onset of the incursions of midlatitude air, the approaching upper-level trough tends to deepen the semipermanent area of low pressure over the central plains of the continent (25°–10°S). The strong southward advection of warm, humid air creates favorable conditions for the development of deep convection over central and southern Argentina. During the next two days, a migratory cold anticyclone, steered by the ridge aloft, moves onto the southern plains of South America (40°–35°S), establishing a large-scale, southward-directed pressure gradient. Due to the blocking effect of the Andes, the induced low-level circulation assumes the form of an agesotrophic southerly flow that produces the equatorward advection of the cold air. In this stage, enhanced convection occurs in a well-defined band of the intensified low-level convergence at the leading edge of the cold surge. This synoptic-scale banded structure in the convective cloudiness also emerges as the dominant mode of the day-to-day variability of the deep convection, and the contribution of these systems to the summertime precipitation varies from ∼25% in the central Amazonia up to ∼50% over the subtropical plains of the continent. The equatorward incursions of cool, dry air (and their hydrostatically induced surface pressure anomalies) finally vanish as a result of strong surface heat fluxes at low latitudes.

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Rei Ueyama and John M. Wallace
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Roberta Quadrelli and John M. Wallace

Abstract

The principal patterns of variability of the extratropical Northern Hemisphere (NH) wintertime circulation are examined, based on 42 yr of data from the NCAR–NCEP reanalyses. The two-dimensional phase space defined by the two leading PCs of the monthly mean sea level pressure (SLP) field poleward of 20°N is used as a basis for surveying the structure of the geopotential height and surface air temperature (SAT) fields. Together these two patterns account for roughly half the variance of SLP on interannual time scales and longer, and virtually all the planetary-scale SLP trends over the 42-yr period of record. The leading EOF corresponds to the NH annular mode (NAM), and the second EOF resembles the Pacific–North America (PNA) pattern.

The leading EOF of the monthly mean geopotential height field at various levels throughout the troposphere and lower stratosphere is well represented by linear combinations of these two SLP patterns, as are the intraseasonal and interannual SLP fields, the NAM, the North Atlantic Oscillation (NAO), the PNA pattern, the pattern corresponding to the North Pacific index (NP), the cold ocean–warm land (COWL) pattern, the seasaw between the depths of the Aleutian and Icelandic lows (AIS), and the leading EOFs of lower-tropospheric temperature and midtropospheric wind. The combined influence of these patterns on temperature and rainfall and other variables can be represented in terms of compact vectorial plots.

Interesting differences emerge when the EOF analysis is performed separately on the intraseasonal and interannual components of the NH SLP field. The former patterns appear to be hemispherically trapped, whereas the latter appear to be reflections of global structures, with ENSO clearly dominating the structure of interannual EOF2.

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Yochanan Kushnir and John M. Wallace

Abstract

Low-frequency variability in wintertime 500 mb height is examined, with emphasis on its structure, geographical distribution, and frequency dependence. A 39-year record of 500 mb geopotential height fields from the NMC analyses is time filtered to partition the fluctuations into frequency bands corresponding to periods of 10–60 days, 60–180 days and > 180 days. Winter is defined as the six month period November through April. Variance, teleconnectivity, and anisotropy fields, and selected loading vectors derived from orthogonal and oblique rotations of the eigenvectors of the temporal correlation matrix for each band are shown and discussed.

The variability in all frequency bands exhibits substantial anistropy, with meridionally elongated features arranged as zonally oriented wave trains prevailing over the continents and zonally elongated features organized in the form of north–south oriented dipole patterns prevailing over the oceanic sectors of the hemisphere. The wave trains are most pronounced in the 10–60 day variability, while the dipoles are most pronounced at lower frequencies. Eastward energy dispersion is apparent in the wave trains, but there is no evidence of phase propagation.

Most of the “teleconnection patterns” identified in previous studies appear among the more prominent loading vectors. However, in most cases the loading vectors occur in pairs, in which the two patterns are in spatial quadrature with one another and account for comparable fractions of the hemispherically integrated variance. It is argued that such patterns should be interpreted as basis functions that can be linearly combined to form a continuum of anisotropic structures. Evidence of the existence of discrete “modal structures” is found only in the interannual (> 180-day period) variability, where two patterns stand out clearly above the background continuum: the Pacific–North American (PNA) pattern and the North Atlantic Oscillation (NAO). These patterns leave clear imprints upon the climatological mean variance of the 500 mb height field and the anisotropy tensor of the 500 mb wine field. The western Atlantic (WA) pattern stands out somewhat above the background continuum in the month-to-month (60–180 day period) variability.

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Hisashi Nakamura and John M. Wallace

Abstract

Departures of lowpass filtered 500 mb height and sea-level pressure anomalies from a Gaussian distribution have been examined, based on Northern Hemisphere operational analyses for 30 winters. At each gridpoint we evaluated several indices that are measures of the differences between the amplitudes and frequencies of the persistent anomalies with positive and negative polarities, and differences in the temporal variability observed within extended periods of positive and negative anomalies. The spatial patterns for these indices are all quite similar, and they resemble the distribution of the moment coefficient of skewness documented by White. Positive skewness is observed to the north of the stormtrack latitudes and the negative to the south of them. The skewness pattern in the sea-level pressure field is weaker, particularly at high latitudes, and it exhibits a bias toward negative skewness. Anomalies with amplitudes larger than two standard deviations, which occur about 5% of the time, account for most of the skewness. A smaller number of “extreme events” with amplitudes larger than three standard deviations account for about half of the negative skewness at the lower latitudes.

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Clara Deser and John M. Wallace

Abstract

Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Niño events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented.

SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Niño events are characterized by strong positive SST anomalies along the coasts of Ecuador and Peru and along the equator eastward of 130°W, and by an equatorward expansion and intensification of the Inter Tropical Convergence Zone (ITCZ) over the eastern Pacific. Basin-wide warm episodes exhibit positive SST anomalies along the equator eastward of 170°E, a southward expansion and intensification of the ITCZ, and an eastward shift and strengthening of the Indonesian convective zone. The movements of the precipitation zones are in good agreement with anomalous large scale surface wind convergence, Meridional wind anomalies dominate the anomalous surface convergence throughout the tropical Pacific.

Surface winds are consistent with the sea level pressure distribution, with down-gradient flow near the equator, and with Ekman balance in the subtropics. A center of below normal sea level pressure over the equatorial eastern Pacific, distinct from the negative pressure anomalies over the subtropical southeast Pacific, is observed during basin-wide warm episodes. This equatorial feature is highly correlated with local SST and appears to be a boundary layer phenomenon.

There is a net increase in deep convection over the tropical Pacific during warm episodes. Enhanced convection in the ITCZ during warm years is not accompanied by a net increase in surface wind convergence. A comparison between precipitation and surface wind convergence suggests that moisture convergence extends through a deeper layer in the equatorial western Pacific than in the ITCZ over the eastern Pacific.

The contrasting distributions of surface relative humidity, total cloudiness and air-sea temperature difference over the eastern tropical Pacific during basin-wide warm and cold episodes are described in the context of boundary layer processes.

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