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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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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 D. Horel
and
John M. Wallace

Abstract

No abstract available.

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John D. Horel
and
John M. Wallace

Abstract

Atmospheric phenomena associated with the Southern Oscillation are examined, with emphasis on vertical structure and teleconnections to middle latitudes. This paper is specifically concerned with the interannual variability of seasonal means for the Northern Hemisphere winter during the period 1951–78. Among the variables considered are sea surface temperature in the equatorial Pacific, precipitation at selected equatorial Pacific stations, a “Southern Oscillation Index” of sea level pressure, 200 mb height and tropospheric mean temperature at stations throughout the tropics, and Northern Hemisphere geopotential height fields. Selected statistics derived from surface data also are examined for the period 1910–45. Results are presented in the form of time series and correlation statistics for the variables listed above.

Results concerning the relationships between sea surface temperature, sea level pressure and rainfall are consistent with the major conclusions of previous studies by J. Bjerknes and others. Fluctuations in mean tropospheric temperature and 200 mb height are shown to vary simultaneously with equatorial Pacific sea surface temperature fluctuations, not only in the Pacific sector, but at stations throughout the tropics. The zonally symmetric component of these 200 mb height fluctuations is considerably larger than the Southern Oscillation in 1000 mb height, and the corresponding fluctuations in the mean temperature of the tropical troposphere are on the order of nearly 1 K.

The correlations between the tropical time series and Northern Hemisphere geopotential height fields exhibit well-defined teleconnection patterns. Warm episodes in equatorial Pacific sea surface temperature tend to be accompanied by below-normal heights in the North Pacific and the south–eastern United States and above-normal heights over western Canada.

Recent theoretical work by Opsteegh and Van den Dool (1980), Hoskins and Karoly (1981) and Webster (1981) on Rossby wave propagation on a sphere provides a basis for understanding the teleconnection in terms of the distribution of sea surface temperature and rainfall in the equatorial Pacific. The theory successfully explains several characteristics of the observed teleconnection patterns, including their horizontal scale and shape, their vertical structure and their seasonal dependence.

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Ioana M. Dima
and
John M. Wallace

Abstract

The three-dimensional structure of the annual-mean equatorial planetary waves in the 40-yr ECMWF Re-Analysis (ERA-40) is documented. The features in the free atmosphere are predominantly equatorially symmetric, driven by east–west heating gradients. The geopotential height and wind perturbations are strongest at or just below the 150-hPa level. Below the level of maximum amplitude, the circulations in the waves are thermally direct with latent heat release in deep convective clouds and radiative cooling in the intervening cloud-free regions. Within the overlying capping layer, the wave-related circulations are thermally indirect, with rising of the coldest air and sinking of air that is less cold. At the cold point, just above the 100-hPa (17 km) level, the ERA-40 annual-mean vertical velocity in the equatorial belt ranges up to 3 mm s−1 over the equatorial western Pacific during the boreal winter, implying diabatic heating rates of up to 3°C day−1, an order of magnitude larger than typical clear-sky values. Strong heating is consistent with evidence of widespread thin and subvisible cirrus cloud layers over this region. It is hypothesized that the air mass as a whole is rising (as opposed to just the air in the updrafts of convective clouds) and that this plume of ascending air spreads out horizontally at or just above the cold point, ventilating and lifting the entire lower stratosphere.

El Niño years are characterized by anomalously weak equatorial planetary waves in the Indo-Pacific sector and slightly enhanced waves over the Atlantic sector and cold years of the El Niño–Southern Oscillation (ENSO) cycle by the opposite conditions. Equatorial Pacific sea surface temperature is as well correlated with the strength of the equatorial planetary waves in the upper troposphere over the Indo-Pacific sector as it is with the conventional Southern Oscillation index based on sea level pressure.

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Ioana M. Dima
and
John M. Wallace

Abstract

The annual march of the climatological mean meridional circulations (MMCs) in the NCEP–NCAR reanalyses is dominated by two components of roughly comparable mean-squared amplitude: 1) a seasonally invariant pair of “Hadley cells” with rising motion centered near and just to the north of the equator and subsidence in the subtropics, and 2) a seasonally reversing, sinusoidally varying “solsticial” cell with ascent in the outer Tropics of the summer hemisphere and subsidence in the outer Tropics of the winter hemisphere. The meridional structure and seasonal evolution of the solsticial cell are suggestive of a close association with the monsoons. These results are consistent with previous analyses of the mean meridional circulation based on radiosonde data.

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Gregory M. Ostermeier
and
John M. Wallace

Abstract

The North Atlantic Oscillation–Northern Hemisphere annular mode (NAO–NAM) has exhibited a trend over recent decades toward high index values. It has been argued that this trend is unprecedented in the historical record and that it may be attributable to anthropogenic forcing. This study compares and contrasts wintertime trends during the recent period of rising NAO–NAM indices with trends earlier in the century when the indices declined.

This analysis finds that the spatial patterns of the trends in sea level pressure (SLP) and surface air temperature (SAT) are markedly different in the two periods. The prior SLP trends were large only in the Atlantic sector, whereas the recent trends of the last few decades were more hemispheric in extent. As a result, trends in a more localized NAO index are roughly comparable for those two intervals, but the recent upward trend in the hemispheric NAM index is more prominent than the earlier downward trend.

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James A. Renwick
and
John M. Wallace

Abstract

Technique for identification of well-predicted spatial patterns in numerical weather prediction model output are outlined and applied to a 14-winter set of Northern Hemisphere 500-mb geopotential height analyses and 1–10-day forecasts produced by the ECMWF operational model. Three approaches are investigated: canonical correlation analysis (CCA), singular value decomposition analysis, and predictable component analysis, the products of which are related to the optimization of forecast-analysis correlation, covariance, and rms error, respectively. In confirmation of earlier results, the most predictable anomaly pattern identified by all three methods is found to be similar to the leading empirical orthogonal function of the analyzed 500-mb height anomaly field, which is dominated by the Pacific-North American pattern. The time series of forecast and verifying analysis projections onto the leading pattern have temporal correlations of at least 0.75 at all forecast intervals out to 10 days and greater than 0.85 for 5-day averages of 6–10-day forecasts and analyses. The leading pattern displays strong temporal persistence and is prominent on the interannual timescale. CCA is found to be the most desirable technique for identification of such patterns.

When CCA is applied to the first seven winters' data (as a dependent sample), the amplitude of the leading pattern is well predicted in either polarity and the skill of the full forecast field is shown to increase as the amplitude of the leading pattern increases, regardless of the polarity. However, when the analyzed and predicted fields from the second seven winters of the dataset (an independent sample) are projected onto the patterns derived from the first seven winters, the skill of the full forecast field does not appear to be well related to the amplitude of the leading predictable pattern. Slight decreases in rms error were achieved by statistically correcting the independent data, but only at the expense of a considerable damping of forecast amplitude. It is concluded that continuing model improvements make such approaches to skill prediction and statistical correction of little value in an operational setting.

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