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Julia Nogués-Paegle

An informative and stimulating conference on phenomenological studies, data systems, analysis and assimilation techniques, and numerical prediction motivated by the Global Weather Experiment (GWE) took place during the AMS annual meeting in January 1986. This conference is summarized and includes thoughtful contributions provided by R. Fleming, program chairman; J. Brown, Jr.; J. Fein; R. Greenfield; D. Johnson; E. Kalnay; D. Sargeant; and J. Theon.

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Julia Nogues Paegle and Jan Paegle

Abstract

Frequency spectra of heights and geostrophic vorticities are computed for several points over the western continental United States and eastern Pacific. These spectra exhibit horizontal variations which appear to be, at least partly, attributable to the underlying topography. This conclusion is supported by a highly simplified, barotropic, mountain-flow model.

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Julia Nogues Paegle and Jan Paegle

Abstract

Observed perturbation kinetic and available energy are presented for a region about 3000 km on a side to study the horizontal homogeneity and general characteristics of geostrophic motions. Frequency spectral analysis is used to determine the dependence of these characteristics on time scales. For all time scales considered the perturbation energies display horizontal inhomogeneities, but these are less pronounced for shorter time scales. For time scales smaller than 4 days the spectra of horizontal kinetic and available potential energies decrease with increasing frequency, and approximately fit power laws with exponents between −2 to −3.5, depending on location. The frequency spectra for geostrophic vertical velocities are markedly different for different climatic locations. The frequency spectra are related to one-dimensional wavenumber spectra by introducing suitable transformation of variables. The results obtained for the higher end of these spectra are interpreted in terms of those predicted by Charney for quasi-geostrophic turbulence.

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Julia Nogues Paegle and Jan Paegle

Abstract

Time-dependent flow solutions for steady supergradient pressure patterns are presented for a variety of initial flow configurations. These solutions discriminate initial conditions and pressure patterns that produce stable and unstable flow evolutions. It is shown that steady-state divergent flows are realizable for commonly observed pressure patterns of the upper troposphere. In these cases, the steady state is approached on relatively short time scales. Solutions agree roughly with observed features of atmospheric flows and constitute a plausible explanation for strong upper level outflows.

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Julia Nogues-Paegle and Kingtse Mo

Abstract

The seasonal transition from spring to summer in the Northern Hemisphere and from fall to winter in the Southern Hemisphere is studied for 1979 using gridded datasets produced by the Goddard Laboratory for Atmospheres. Winds at 200, 500 and 850 mb are decomposed into their rotational and divergent components. The streamfunction (ψ), velocity potential (χ), and height fields (Z) are projected onto spherical harmonics to quantify the behavior of the lowest-order planetary modes. Comparison of the planetary scales with those at full resolution reveals that the low-order truncation represents over half of the total energy at 200 mb and that zonal deviations dominate the week-to-week time changes. The velocity potential spectrum is dominated by the wave component of largest meridional and zonal scale (χ1 1). This mode exhibits a clear eastward propagation, circling the globe in about six weeks. This propagation is discussed in connection with observed changes of outgoing longwave radiation and related to the 30–60 day oscillation, which exhibited a strong signal during the 1979 summer season. It is found that χ1 1, ψ2 1 and Z 4 1 propagate with similar phase speeds, linking the planetary scale divergent patterns with those of the rotational flow and height field. The streamfunction is found to contain more of the 30–60 oscillation signal than the height fields for global scales. Analyses of outgoing longwave radiation and velocity potential at 5°N at full resolution are compared with the zonal wind at 34°S. It is found that the divergence pulse and low values of outgoing longwave radiation are observed up to one week before zonal-wind acceleration. These accelerations peak when the divergence pulse is at about 160°E, with high values of the subtropical jet at similar longitudes. Removal of the seasonally averaged circulation from the weekly averages reveals a dominant baroclinic structure, with the 250- to 850-mb wind shear exceeding the vertically averaged wind over extensive areas, which extend for particular weeks to the polar caps. A dominant barotropic structure emerges when the basic sate is included. The seasonal transition is found to occur abruptly during the middle of the 12-week periods considered here, with pronounced rearrangements of global patterns due to excitation of planetary scales.

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Julia Nogues-Paegle and Roger Daley
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Julia Nogués-Paegle and Kingtse C. Mo

Abstract

Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.

This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.

In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.

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Julia Nogues-Paegle and Kingtse C. Mo

Abstract

The effect of tropical latent heat release in accelerations of the Southern Hemisphere subtropical jet is discussed based on a case study for 6–8 August 1979 and general circulation model simulations. This jet is a main feature of the winter time circulation extending over Australia and the western Pacific Ocean. An intensification of this jet was observed for 8 August, with peak values of 93 m s−1. Satellite pictures and daily precipitation amounts revealed that the 6–8 August period was characterized by extensive rainfall in the tropics, with precipitation diminishing from that day on. Level III-b data for the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory for these dates was projected into the normal modes of a primitive equation model linearized about a basic state at rest. The analysis shows that the jet accelerations were due both to external and internal Rossby modes, with somewhat stronger contributions from the external mode. An enhancement of the local meridional circulation is due to these extensive precipitation areas which project mostly in internal gravity modes with maximum values upstream of the jet accelerations.

Conjecture that the jet stream is reacting in this short time scale to tropical latent heat release is tested with the Goddard Laboratory for the Atmospheres fourth order general circulation model. A 15 day integration starting on 1 August 1979 is used as the control case. This run produced peak values of 82 m s−1 for the zonal wind at 200 mb during August 8 centered over Australia at about 26°S in conjunction with increased divergent circulations at about 6°N about 50° longitude upstream from the jet maximum. A simulation was started on 4 August and run for ten days suppressing tropical Pacific heating from 90°E through 120°W. Normal mode contributions were also obtained for the control and “no tropical heating” experiment. The case study and GCM simulations suggest the following time scale of response of subtropical latitudes to tropical latent heat release: divergent circulations which project mostly into inertia-gravity waves react to changes in latent heat release in 1–2 days, its impact in subtropical latitudes is felt in 2–4 days; and becomes fully established after about 6 days.

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Julia E. Nogues-Paegle and Zhao Zhen

Abstract

The upper-level circulation of the Southern Hemisphere winter is characterized by two distinct zonal wind maxima: a subtropical jet found in the vicinity of Australia and the western Pacific Ocean, and a polar jet which maximizes in the Indian Ocean in the 45°–55°S latitudinal belt. This paper describes the global characteristics of the atmosphere for cases with strong subtropical jets during the 1979 Northern Hemisphere summer. Such cases are shown to co-exist with episodes of strong release of latent heat in the Northern Hemisphere at similar longitudes. Gridded analyses of the Global Weather Experiment produced by the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration are used to obtain composites of atmospheric motions prior to the onset and during active episodes of the Asian Monsoon. Projections of these motions into the normal modes of a linearized primitive equation model about a basic state at rest are presented to isolate observed global flow characteristics. Relative contributions from internal and external modes are shown as well as those from gravity, Rossby and equatorially trapped modes. Results indicate that active hemispheric interactions take place in the longitudes of the Southern Hemisphere subtropical jet and are accomplished by motions possessing the horizontal structure of internal gravity modes. The subtropical and polar jets have very different vertical structures, projecting mostly in internal and external modes, respectively. Accelerations of the subtropical jet occur due to changes both in the internal and external Rossby modes, the latter contributing most to these accelerations.

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Kingtse C. Mo and Julia Nogues Paegle

Abstract

Precursory signals in sea surface temperature anomalies (SSTAs) associated with summer precipitation over the Southwest (SW) United States are examined using data from 1900 to 1996. Two patterns of SSTAs are found to influence summer and winter precipitation evolution. One pattern shows positive anomalies extending from the central Pacific to the west coast of the Americas during winter. Warm SSTAs over the equatorial central Pacific in winter are associated with wet conditions in the SW and dry conditions over the Pacific Northwest. The following summer, SSTAs in the central Pacific diminish, but positive SSTAs remain over the tropical eastern Pacific and in the vicinity of Central America. Warm SSTs are conducive to enhanced rising motion and convection. A compensating downward branch is found over the SW favoring dry conditions over that area. Negative SSTAs have the opposite effect. This mode is similar to the SSTA pattern discussed by Higgins et al. When this mode is active, a wet (dry) SW winter is followed by a dry (wet) summer.

In addition to SSTAs over the central and eastern Pacific, another controlling factor is the SSTA over the North Pacific. Negative (positive) SSTAs over the North Pacific persist from winter to summer and favor positive (negative) rainfall anomalies over the SW. When this mode is active, a wet (dry) SW persists from winter to summer.

The authors conclude that SW rainfall evolution from winter to the following summer is modulated by both SSTAs over the equatorial Pacific and over the North Pacific. SSTAs in the equatorial Pacific alone are not sufficient to explain the observed summer rainfall variability over the southwest United States.

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