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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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