Peter Speth

Some potential forcing mechanisms of the year-to-year variability of the tropical convection and its intraseasonal (25–70-day) variability

ABSTRACTThree potential forcing mechanisms for the convective signal of the Madden–Julian Oscillation (MJO) are investigated.Outgoing longwave radiation (OLR) is used to indicate convection, and data from 1974 to 1994 are subjected to a 25–70-daybandpass filter to represent the MJO. The focus of the paper is on interannual variability and the three mechanisms diagnosedare: tropical sea-surface temperature (SST); the phase of El Nin˜o–Southern Oscillation (ENSO); and atmospheric precipitablewater (W). The respective roles of these potential forcings are also considered for the year-to-year variability of the seasonalanomalies of mean convection.Observed seasonal anomalies of both mean convection and MJO activity over the equatorial Pacific from 160 x0e E to thecoast of South America are highly correlated to collocated anomalies of SST and the phase of ENSO. During El Nin˜o wintersthe eastward-propagating convective MJO events penetrate farther than normal into the central Pacific before curvingpoleward into the Southern Hemisphere. Over the maritime continent, moist convection is significantly suppressed during ElNin˜o. A corresponding signal in the MJO activity is confined to the Philippine Sea and North Australia during boreal winter,and to the Bay of Bengal in autumn. In contrast, there is no uniform MJO response over the eastern Indian Ocean when thelatest five ENSO warm events (1976–1977, 1982–1983, 1986–1987, 1991–1992 and 1993–1994) are considered.On a seasonal time-scale, W over the Indian and Pacific Oceans is closely related to convection anomalies, as well as tothe variance of the high-frequency convective perturbations. In contrast, convective activity of the MJO shows no relation tovariations in the amount of W. It is concluded that the interannual variability of the convective MJO signal over the IndianOcean and western Pacific Warm Pool is largely determined by the frequency and duration of ‘weak-signal’ episodes ratherthan by modulations of its amplitude. # 1997 Royal Meteorological Society. Int. J. Climatol., Vol. 17, 1513–1534 (1997)(No. of Figures: 12. No. of Tables: 2. No. of References: 66)

Three Late Summer/Early Autumn Cases of Tropical–Extratropical Interactions Causing Precipitation in Northwest Africa

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 traject...

High- and Low-Frequency Intraseasonal Variance of OLR on Annual and ENSO Timescales

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 Nino–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 independ...

Meteorological influence on upwelling off Northwest Africa

The seasonal variation of coastal upwelling areas off Northwest Africa is described by using five day non-overlapping mean values of sea surface temperature with a grid resolution by degree square. Between 20°N and 25°N upwelling occurs during the whole year, especially pronounced during spring and autumn; upwelling can be found south of these latitudes during late winter and spring while north of 25°N it prevails during summer and autumn with a peak in 30°N. Although this is true for the seven year period 1969 to 1976, within single years strong differences appear. From objective analysis differences of sea level pressure normal to the coastline were established, which are a measure for the wind component parallel to the coast. The relationship between wind and upwelling is most pronounced south of 20°N. The short-period fluctuations of differences of pressure and sea surface temperature show a phase difference of 2.5 days, i.e. with a delay of 2.5 days the ocean answers with single upwelling events to the driving force of the atmosphere (strong northerly winds).

Mean State and Wave Disturbances during Phases I, II, and III of GATE Based on ERA-40

Abstract Using ECMWFs second-generation reanalysis, ERA-40, the large-scale mean state and synoptic-scale features associated with African easterly wave disturbances (AEWs) are examined over West Africa and the adjacent eastern Atlantic Ocean during the three 21-day observing periods of the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) in 1974 (Phase I, 26 June–16 July; Phase II, 28 July–17 August; Phase III, 30 August–19 September). Results are partitioned into four geographical boxes, in order to highlight differences among the AEW vortices as they propagate westward along two tracks (northern and southern) over West Africa (land) and the adjacent eastern Atlantic Ocean (water). This marks the first time that a detailed diagnosis of the northerly track AEWs has been conducted. Results are also compared to previous GATE studies and a 30-yr climatology is extracted from ERA-40. In general, the subjectively analyzed wind fields presented in earlier studies compare favorabl...

Intraseasonal Oscillation of Convective Activity in the Tropical Southern Hemisphere: May 1984-April 1986

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. ...

Investigations on coastal upwelling off Northwest Africa and Portugal with empirical orthogonal functions

Earlier investigations on the influence of the atmospheric circulation on upwelling off Northwest Africa are expanded up to the coast off Portugal. In this area, upwelling can be found south of 40° N during summer and autumn; between 40° N and 43° N the upwelling period decreases with increasing latitude. The yearly amplitude of differences of sea level pressure normal to the coast — which represents the synoptic scale coastparallel wind component — and of sea surface temperatures is largest in 38° N, decreasing to the north. From empirical orthogonal functions it is suggested that off Portugal local winds induce a greater amount of upwelling than the winds off Northwest Africa.

Empirical orthogonal functions of sea level pressure and sea surface temperatures for the upwelling area off Northwest Africa

The influence of the atmospheric circulation on sea surface temperatures off Northwest Africa is described with the aid of empirical orthogonal functions. For the period 1973 through 1976 differences of sea level pressure normal to the coastline-which are a measure for the coastparallel and more local winds—and differences of sea surface temperatures by subtracting mid-ocean temperatures from coastal averages are investigated. The first three modes are sufficient to explain 95% of the total, variance and it is possible to differentiate between the three upwelling regions (during the whole year upwelling is present between 20°N and 25°N, during winter south of 20°N and during summer north of 25°N). Furthermore the influence of the large scale weather situation is examined by considering fields of sea level pressure and of differences of sea surface temperatures. The result is, that sea surface temperatures along the entire coast change more or less simultaneously. Seven functions are necessary to describe more than 90% of the total variance.

Introduction: The IMPETUS method

Regional climate models that take land use and land cover changes into account indicate a general decrease in rainfall and prominent surface heating in sub-Saharan Africa until 2050 (see sect. II-3.2). The high population growth predicted in this period is expected to cause rapid land use changes and to strongly influence water availability and demand. In this context, the IMPETUS research project was not intended to offer a prescription of rigid options for sustainable management of the hydrological cycle. Rather, it was intended to support the decisionmaking process within project countries. For this purpose, tools have been developed that allow for the comparison and balancing of different options with the aid of Spatial Decision Support Systems (SDSSs), Information Systems (ISs), and Monitoring Tools (MTs). The intention is to help decision makers by providing them with efficient and user-friendly tools for analyzing and managing decisionmaking problems and underlying phenomena. Target areas for this research were the Oueme basin in Benin and the Drâa catchment in Morocco. Target years were chosen as 2025 for Benin and 2020 for Morocco based on pre-existing, long-term government strategy papers.