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Roland A. Madden and Peter Speth

Abstract

The average structure of westward traveling disturbances that contribute to relative maxima found in space-time spectra from 13–32 days at northern latitudes is determined for each season. A compositing method used employs a minimum of space and time filtering in order to avoid biasing the results. The average latitudinal structure is “global” in that it is discernible in the Southern Hemisphere during December–February and September–November. It is primarily confined to northern latitudes during March–August. In all seasons the disturbance is out of phase between northern high latitudes and subtropical and tropical latitudes. The longitudinal structure is primarily zonal wavenumber 1 in all seasons. Further work is suggested to confirm the structures determined here and to learn if they reflect the superposition of a number of occasionally excited Rossby normal modes.

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Peter Speth and Roland A. Madden

Abstract

A space-time spectral analysis of a long time series of observed geopotential heights for each season at several levels and latitudes of the Northern Hemisphere was performed as part of a continuing investigation of large-scale traveling waves. The data set that is analyzed consists of the first six zonal wavenumbers. A discussion emphasizes westward traveling wave 1 with periods near 16 and 5 days which we argue are consistent with external Rossby warm. An additional outstanding feature is an eastward propagating wave 6 which may result from baroclinic instability.

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Peter Knippertz, Andreas H. Fink, Andreas Reiner, and Peter Speth

Abstract

In contrast to the winter rain-dominated region along the Atlantic and Mediterranean coasts in northwest Africa, the semiarid to arid southern foothills of the Atlas Mountains receive significant contributions to their annual rainfall amounts from rainy episodes in late summer/early autumn. Three such cases (September 1988, September 1990, August–September 1999) are studied with respect to the sources and the vertical and horizontal transports of moisture, as well as local factors for precipitation generation. Besides station reports of precipitation, the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses and Meteosat water vapor images are considered.

All three cases presented reveal similar tropical–extratropical interactions. Convective cloud clusters or squall lines over tropical West Africa and the adjacent tropical Atlantic Ocean, several of them associated with low-level African easterly waves, could be identified as midlevel moisture source regions by the use of trajectory analysis. The moisture is transported northward to the east of an mid- to upper-level subtropical trough, which extends anomalously deep into the Tropics. Most of the transport occurs above the dry Saharan planetary boundary layer. The moisture converges at midlevels (700–400 hPa) over northwestern Africa underneath a strong upper-level divergence center at the inflection point of the trough. The dynamically forced ascent in connection with orographic lifting at the Atlas Mountains in the southerly flow and surface heating over the elevated terrain triggers convective rainfalls, which occur preferably close to and downwind of the mountain chain.

The three cases differ with respect to the synoptic evolution of the upper-level subtropical trough and the paths of the moisture export from the Tropics. At the end of the episode in September 1988, the tropical air over northwest Africa is displaced by polar air connected with some heavy rainfall events. The presented cases are compared to studies of tropical plumes and Soudano–Saharan depressions.

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Dayton G. Vincent, Andreas Fink, Jon M. Schrage, and Peter Speth

Abstract

Using 20 yr of outgoing longwave radiation observations, the complex behavior of the higher- (6–25-day) and lower- (25–70-day) frequency bands of tropical intraseasonal convective oscillations is investigated. Emphasis is given to the mean annual cycle and interannual variability of both bands and to the interaction between the two bands. The focus with regard to the interannual variability within each band is on the warm and cold events associated with the El Niño–Southern Oscillation (ENSO) cycle. The study encompasses the tropical and subtropical Indian and Pacific Oceans (including Australasia).

The strongest intraseasonal signals are, for the most part, aligned with the intertropical convergence zone (ITCZ) and South Pacific convergence zone. In some cases, the 6–25-day signal is not collocated with the Madden–Julian oscillation (MJO) signal and/or occurs remotely from the ITCZ. In these cases, the higher-frequency intraseasonal convective perturbations are associated with phenomena independent from the MJO, such as easterly waves, monsoon depressions, typhoons, or circulations involved in tropical–extratropical interactions. Over the equatorial eastern Indian Ocean, strong activity in both bands persists throughout the year, but the bands are found to be anticorrelated, regardless of the ENSO phase.

The effect of ENSO timescales is further examined by looking at December–February anomalies for five El Niño and two La Niña events during this 20-yr sample. A well-defined response of the two bands is restricted to the northwestern and central Pacific. Over the northwestern Pacific Ocean, the two bands complement one another with suppressed (enhanced) convection occurring during El Niño (La Niña) events. Both bands also complement each other over the equatorial central Pacific but are out-of-phase with those in the western Pacific on ENSO timescales. In contrast, over the Australian monsoon region and the eastern Indian Ocean, neither band shows a uniform response in terms of anomalous activity when the latest five ENSO warm events, 1977–78, 1982–83, 1986–87, 1991–92, and 1992–93, are considered.

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Dayton G. Vincent, Thomas Sperling, Andreas Fink, Stefan Zube, and Peter Speth

Abstract

The intraseasonal (40–50 day) oscillation in convection over the tropical Southern Hemisphere (0°–15°S) is examined using two years of ECMWF analyses. The initial period investigated was 1 May 1984–30 April 1986. This diagnosis revealed that the oscillation was essentially absent in the Southern Hemisphere during the winter months. Therefore, the paper focuses on two subperiods, 1 November 1984–30 April 1985 (Year 1) and 1 September 1985–15 April 1996 (Year 2), when the oscillation could be detected. Although several variables were examined, the velocity potential at 200 hPa (χ2) and outgoing longwave radiation (OLR) were found to be the best indicators of the oscillatory convective activity; consequently, these variables are the only ones presented. One of the unique features of this study is that the data were not temporally filtered, except for removing the time mean and linear trend, until after it was established that statistically significant peaks occurred on the intraseasonal time scale. This was an important step in this case because the dominant spectral peaks for the oscillation in each year were considerably different. In Year 1 the significant intraseasonal period was between 50 and 67 days, while in Year 2 it was centered near 33 days. Based on this, a recursive bandpass filter of 40–80 days was applied to Year 1 and 27–44 days to Year 2. If the data was temporally filtered at the onset (e.g., 30–60 day band pass), the proper conclusions may not have been reached.

For the most part, the findings agree with those of previous investigators. The oscillation propagated eastward, and its convective activity in both years was more intense over the Indian Ocean-Indonesia-western Pacific region than elsewhere. Furthermore, the χ2-wave could be followed continually around the globe, but the convection (OLR) associated with the oscillation was weak and difficult to track over much of the Western Hemisphere. The primary difference between the two years, besides the period of oscillation, was that the correlation between χ2 and OLR was much greater in Year 1.

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