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David S. Gutzler

Abstract

Oceanic and atmospheric data from the tropical western Pacific are analyzed to describe decadal-scale trends during the last 20 years, and these low-frequency trends are compared with shorter-term Southern Oscillation-related variations. Regional indices of western and central equatorial Pacific SST exhibit significant upward trends in recent decades. The decadal variability in the tropical Pacific is large enough relative to interannual variability to significantly affect the interpretation of standardized SST anomaly indices used to monitor Southern Oscillation phenomena. Specific humidity in the tropical western Pacific boundary layer exhibits a statistically significant upward trend consistent with previously published results based on a shorter data record. The convective instability of the tropical troposphere is increasing, but two indices related to precipitation show no evidence of a trend. These trends cannot be explained as an aggregate of the effects of more frequent El Niño warm events in recent years because the tropical western Pacific response to El Niño includes negative (i.e., dry) boundary-layer humidity anomalies and decreased convective instability. On interannual timescales there seems to be a distinct separation between the processes affecting tropospheric temperature within and above the tropical western Pacific boundary layer.

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David S. Gutzler and Jagadish Shukla

Abstract

A 15-winter sample of daily gridded values of Northern Hemisphere 500 mb heights is examined for the existence of recurrent flow patterns (“analogs”). The analog search is repeated several times after degrees of freedom are successively removed from the data by spatial filtering, temporal averaging, and consideration of smaller sectors of the hemisphere. The root mean square difference (or “rms error”) between the most closely analogous maps, defined over the middle latitudes (30–70°N), is slightly greater than half the average error between randomly chosen maps, with an estimated rms error doubling time of nearly 8 days. If the analog search is conducted using only the longwave component (zonal wavenumbers 0–4) of each map, the rms error between the best analog pairs is reduced to less than half of the rms error between long-wave anomalies on randomly chosen maps, but the doubling time is also reduced to less than 7 days. If the analog search is further restricted to a limited region over North America or Europe, the rms error between the best analog pairs is less than 40% of the rms error (for the sme region) between randomly chosen maps, but the error doubling time is further reduced to 4–5 days. In all cases, the degradation of analog quality is so rapid that a forecasting scheme based on the analogs would fail to produce more skillful forecasts than simple persistence.

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David S. Gutzler and Roland A. Madden

Abstract

Seasonally varying spectral and cross-spectral calculations are carried out on multiyear time series of vertically and zonally averaged daily zonal wind fields to describe the seasonal cycle of the 40–50-day oscillation of atmospheric angular momentum. Intraseasonal variability (including 40–50-day fluctuations) of global momentum is largest in late boreal winter and smallest in boreal autumn; however, the 40–50-day spectral peak is most pronounced in boreal summer when lower-frequency intraseasonal variance is depressed. The 40–50-day spectral peak in global momentum is much less pronounced and apparently is restricted to a narrower frequency band, than corresponding peaks in zonal wind spectra from individual tropical rawinsonde stations. Contributions to global momentum fluctuations from three near-equal-area latitude bands (tropics, Northern Hemisphere, and Southern Hemisphere) are compared, confirming that intraseasonal momentum fluctuations are tropical in origin. The variance of extratropical momentum at this time scale is about an order of magnitude less than the tropical momentum variability. Coherent tropical–extratropical interactions are found principally in boreal winter, with the highest coherence between the tropics and Northern Hemisphere. The corresponding phase difference between tropical and Northern Hemisphere momentum is suggestive of poleward propagation of momentum out of the tropics.

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David S. Gutzler and Roland A. Madden

Abstract

Seasonal and geographical variations in tropical intraseasonal wind variance are described using bandpass filtered 850 and 150 mb wind time series derived from rawinsonde observations. Three bandpass filters, with central response periods of 31, 47, and 99 days, are applied to the daily time series. The intermediate filter is designed to isolate variance associated with the “40–50 day oscillation.” The spatial coherence of the bandpass filtered wind fluctuations is examined using complex eigenvector analysis.

Comparisons are made of u and v variance and large-scale structure of filtered wind anomalies for each season and frequency band, with emphasis on the u component. At stations across the western Pacific the 47-day filtered u 150 variance is nearly constant with season. The largest seasonal variability in 47-day filtered zonal wind variance is at 150 mb at stations along and to the north of the equator between Africa and Southeast Asia, and in the central Pacific. Compared to the u 150 variance over the western Pacific, the variance at these stations is much larger in the boreal winter and much smaller in the boreal summer. Large variance at 850 mb is found in each frequency band from the central Indian Ocean eastward to the dateline, with u 850 and u 150 fluctuating out-of-phase and the largest u 850 variance in the summer hemisphere. Eastward propagation of u 150 anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u 150 anomalies have a wavenumber 2 structure, consistent with the leading pattern of large-scale convection anomalies. From the dateline eastward across Africa the scale of the u 150 anomalies is broader, closer to a wavenumber 1 scale.

The results suggest that the 40–50 day oscillation in the global tropics has a “two-regime” character. Across the eastern Indian and western Pacific Oceans (the “convective regime”) the 40–50 day oscillation occurs year-round and its spatial structure indicates that it is closely coupled to convection. Elsewhere (the “dry regime”) the oscillation is clearly evident only in the upper troposphere and is subject to strong seasonal modulation.

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Maurice L. Blackmon, Ronald A. Madden, John M. Wallace, and David S. Gutzler

Abstract

Temporal (but nonseasonal) fluctuations in the geopotential height field exhibit large regional contrasts in vertical structure, as manifested in the geographical distributions of the correlation between 1000 and 500 mb height, and the ratio of the amplitudes of the fluctuations at those levels. This geographical variability is investigated in order to ascertain its seasonal, frequency and zonal wavenumber dependence and its relation to other indicators of vertical structure: statistics involving the 1000–500 mb thickness, and the structure of the dominant mode in an eigenvector analysis expansion of geopotential height in the vertical. Results are based on operational analyses by the United States National Meteorological Center over a 15-year period.

Particularly striking is the contrast between transient fluctuations over the eastern oceans, which exhibit a highly barotropic structure with strong vertical coherence in the geopotential height field and small temperature variability, and those over the interior of the continents, whose structure is much more baroclinic, with low or negative temporal correlations between 1000 and 500 mb height. Such contrasts show up clearly in station data; they are observed during both winter and summer, and for temporal frequencies ranging from synoptic to interannual time scales. They are largely a reflection of the vertical structure of planetary-scale fluctuations. There is also evidence of smaller scale regional contrasts in vertical structure, some of which appear to be associated with synoptic-scale disturbances.

On the basis of 1000 and 500 mb height data alone it is possible to represent, with a high degree of accuracy, the geographical distribution of the shape of the dominant eigenvector in the expansion of the vertical profile of geopotential height in transient disturbances.

The implications of these results on the design of observing networks and objective analysis procedures are discussed.

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