Abstract

Some aspects of the interannual and the diurnal variations of the convection over the tropical Africa and the Atlantic Ocean are derived using Meteosat data. The study is based on four summer months (June, July, August and September) of three years from 1983 to 1985, for regions of 2.5°×2.5° extending from 5°S to 25°N and from 50°E to 50°W. Using ECMWF (European Centre for Medium Range Weather Forecasts) analyses, mean cloud fields and interannual changes are interpreted in terms of dynamical forcing and feedback. Anomalies in the thermal wind at 700 mb between 1985 (relatively wet year) and the two other years are consistent with previous results based on more contrasted wet and dry years.

Defining the high cloudiness by a threshold in the infrared signal, the amplitude of the diurnal variation is maximum over land with larger values over regions of large mean fractional cloudiness corresponding generally to regions of highlands. The diurnal cycle of high clouds is generally not sinusoidal and the period of development is shorter than the period of dissipation. Over land the maximum high cloud coverage occurs between 1800 LST and midnight and the minimum between 0900 LST and noon. Over oceanic coastal region the maximum high cloudiness is maximum around local noon. The phase, however, becomes more noisy far from land areas.

The diurnal composite of the infrared histogram of selected regions gives additional information. A striking result is the general presence of a large concentration of midlevel cloud with tops typically near 500 mb. These clouds have a maximum development near sunrise and a minimum in the afternoon. This particular diurnal phase modifies their radiative forcing, giving an enhancement of their greenhouse effect at the expense of their albedo effect. Another striking result is the existence of a strong coherent diurnal cycle of the cloudiness over all oceanic convergence areas. This diurnal behavior of the cloudiness is basically the same over all ocean regions studied and is compatible with results obtained for regions of the tropical Pacific.

Abstract

Using 38 years of the ERA-Interim dataset, an objective tracking approach is used to analyze the origin, characteristics, and cyclogenesis efficiency (CE) of synoptic-scale vortices initiated over West Africa and the Atlantic Ocean. Vortices initiated over the ocean at a given pressure level often result from a vertical expansion of a “primary” vortex track initiated earlier over West Africa. Low-level (850 hPa) primary vortices are initiated mainly in July near the Hoggar Mountains (24°N, 5°E), while midlevel (700 hPa) primary vortices are initiated mainly in August–September near the Guinea Highlands (10°N, 10°W). The CE of all these vortices is about 10% in July and 30% in August. The average CE is, however, smaller for low-level “Hoggar” vortices because they peak in July when the cyclogenesis potential index of the Atlantic Ocean is weak. Seasonal and interannual modulations of the cyclogenesis is related more to this index than to the number of vortices crossing the West African coast. Cyclogenesis is nearly equally distributed between the coast and 60°W, but the part of the cyclogenesis due to vortices initiated over West Africa decreases from 80% near the coast to about 30% at 60°W. The most probable delay between the vortex vertical expansion and cyclogenesis is 2 days, but it can be up to 10 days. This analysis also confirms previous results, such as the larger CE for vortices extending at low levels over the continent at 10°N, or the delayed and therefore west-shifted cyclogenesis of low-level “Hoggar” vortices.

Abstract

Using Meteosat data and European Centre for Medium-range Weather Forecast (ECMWF) analyses, we examine easterly waves and their relation with the cloudiness over West Africa and the tropical Atlantic Ocean for three summers (June, July, August and September 1983-85). Spectral analysis of the low-level meridional wind in the 2.8–5.1 day band reveals maximum wave amplitude near the West African coast at 20°N. During August and September the wave amplitude is larger than during June and July and a secondary maximum appears around 7.5°N.

Composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ECMWF analyses. The phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions. This phase varies however grossly in four large land and ocean regions, centered at 7.5° and 17.5°N, respectively, for which we derive vertical cross sections of the wave modulation of the atmospheric state and of the vertical distribution of clouds for the summer of 1985.

For land and oceanic regions around 7.5°N, the larger deep convective activity at and ahead of the wave trough is well related to the maximum low-level convergence and high-level (200 mb) divergence. At latitudes near 17.5°N over the Saharo-Sahelian region, the deep convection has a primary maximum ¼ wavelength east of the trough, and a secondary maximum cast of the ridge. At and ahead of the trough axis there is highly suppressed cloud condition over Saharo-Sahelian regions consistent with a strong shallow dry convection described by ECMWF analyses. For oceanic trade regions near 17.5°N, the cloudiness is maximum during the phase of maximum southerly wind. This study also shows that the wave modulation of analyzed temperature and moisture profiles is in reasonable agreement with previous results and the observed cloud modulation.

Abstract

Using METEOSAT data in the ISCCP B2 format, we study the mean radiation fields and their fluctuations during Northern Hemisphere summer (June, July, August) of 1983, 1984 and 1985, for regions of 5°×5° located from 50°N to 50°S and from 60°E to 60°W. The study is performed for the IR atmospheric window channel (10.5–12.5 μm) and the water vapor band (5.7–7.1 μm). The year-to-year differences of the mean fields delineate large regions of positive and negative anomalies with principally a zonal distribution. This suggests that interannual perturbations in the large-scale meridional circulation have a strong influence on the radiation field, principally by way of convective activity.

To study diurnal variations, we separate coherent diurnal variance obtained by compositing over the 3 months, and the total intradiurnal variance obtained by integration of the power spectra over periods lower than 1 day. In the IR window, the coherent diurnal variance, expressed as a percentage of the total intradiurnal variance, is stronger over subsidence areas, reaching values greater than 98% over desert regions due to surface temperature and values near 70% over ocean regions due to diurnal variations of stratiform cloudiness. Over ITCZ or midlatitude regions, this percentage is lower. In the WV band, the latitudinal distribution presents maximum values of this percentage between the equator and 10°N (>35%) with a progressive decrease up to 30° of latitude (<5%) in both hemispheres. The coherent diurnal variation is larger (up to 50%) over central and eastern Africa and related to convective activity over highlands.

Spectral analysis of interdiurnal fluctuations reveals a progressive shift of the dominant time scales from short time scales (1–2.5 day band) over convective zones to periods longer than 9.2 days over subsidence areas. Regional aspects are revealed by mapping the spectral variance in selected frequency bands as a percentage of the total interdiurnal variance. Over subsidence areas and many other regions where the IR signal depends strongly on surface temperature and lower atmospheric levels inaccessible to the WV channel, the strong coherence between the two channels suggests that the same time scales dominate over the entire vertical extent from low to middle troposphere.

Abstract

Using ERA-Interim global atmospheric reanalysis, an original tracking approach is developed to follow tropical low pressure systems from the early tropical depression (TD) stage up to possible intensification into developed tropical cyclones (TCs). The different TC stages are identified using the IBTrACS dataset. This approach detects many more TD initiations compared to IBTrACS alone and thus gives a more comprehensive dataset to study the cyclogenesis by considering separately TD initiations and the probability of intensification.

In the south Indian Ocean (SIO), the MJO modulation of the number of TCs is primarily due to the modulation of the number of TD initiations and secondarily to the probability of their intensification. The TD initiations are more probable at 55°, 75°, and 95°E and can be primarily attributed to the development of an unstable cyclonic meridional shear of the zonal wind at low levels. The reinforcement of this shear results from (i) a heat low, related to a precipitation anomaly, which triggers westerly winds equatorward of the initiation region and (ii) an easterly wind strengthening south of the initiation regions due either to a reinforcement of the subtropical high (for western and central SIO) or to a large-scale depression over the western Maritime Continent (for eastern SIO). Over the western and central SIO, the concomitance of precipitation and subtropical high anomalies at the origin of the shear reinforcement could be partly stochastic, giving a weaker relation with MJO and ENSO. Over the eastern SIO, the large-scale MJO (and ENSO) perturbation pattern alone can reinforce the shear, giving a larger modulation of the number of TD initiations.

Abstract

Numerous low-level vortices are initiated downwind of the Hoggar Mountains and progress towards the Atlantic coast on the northern path of African Easterly Waves (AEWs). These vortices occur mostly in July and August and more specifically when the northern position of the Saharan heat low (SHL) generates stronger and vertically expanded easterly winds over Hoggar mountains. At synoptic time-scales, a composite analysis reveals that vortex initiation and westward motion are also statistically triggered by a reinforcement of these easterly winds by a wide and persistent high-pressure anomaly developing around the Strait of Gibraltar and by a weak wave trough approaching from the east. The vortices are generated in the lee of the Hoggar, about 1000 km west of this approaching trough, and intensify rapidly. The evolution of the vortex perturbation is afterward comparable with the known evolution of the AEWs of the northern path and suggest a growth due to dry barotropic and baroclinic processes induced in particular by the strong cyclonic shear between the reinforced easterly winds and the monsoon flow. These results show that vortex genesis promoted by changes in orographic forcing due to the strengthening of easterly winds over Hoggar mountains is a source of intensification of the northern path of AEWs in July and August. These results also provide a possible mechanism to explain the role of the SHL and of particular mid-latitude intraseasonal disturbances on the intensity of these waves.

Abstract

Spectral analysis of the outgoing longwave radiation (OLR) time series over equatorial Africa reveals large oscillations of the convection with periods of between 3 and 6 days. In March and April, when the intertropical convergence zone (ITCZ) migrates northward and crosses equatorial Africa, this periodic behavior is most pronounced with a marked peak at 5–6 days. Robust horizontal and vertical patterns, consistent with a convectively coupled Kelvin wave, can be extracted by a simple composite technique based only on the phase of the convective oscillations over equatorial Africa. The composite reveals differences between continental and adjacent oceanic regions. Over the continent, the stronger oscillation of the convection is associated with larger temperature and moisture anomalies near the surface, suggesting an influence of diabatic processes on the amplitude of the perturbations. Some convective events over equatorial Africa are triggered by waves propagating eastward over the equatorial Atlantic. However, this cannot explain the robust periodic behavior observed over equatorial Africa because the convective variability over the Amazon basin and the equatorial Atlantic have different spectral characteristics with no marked peak at 5–6 days in March and April.

The mesoscale convective systems embedded in these synoptic disturbances are studied using satellite brightness temperature at higher spatial (0.5°) and temporal (3 h) resolution than the OLR (respectively, 2.5° and daily average). The diurnal and the wave modulations of occurrence, size, and life cycle of the mesoscale convective systems are inspected. These systems are generated preferentially over the western slopes of the Rift Valley highlands. They propagate west-southwestward over the Congo basin where they reach their maximum size. The 5–6-day perturbations do not modify the diurnal triggering of convective systems notably, but the perturbations do modify their development into larger organized convection, especially over the Congo basin. The implication of these results for understanding the physical source of these 5–6-day perturbations is discussed.

Abstract

This paper presents a method, the local mode analysis (LMA), that makes it possible to extract the most persistent oscillations present in the time evolution of an atmospheric field. This method is particularly suitable to analyze intermittent tropospheric oscillations related to dynamic or thermodynamic instabilities such as the intraseasonal oscillation (ISO). These intermittent oscillations generally exhibit various spatial structures that succeed one another in time and that are difficult to isolate in a simple and comprehensive manner using conventional approaches such as empirical orthogonal functions or composite analyses. The main objective of the LMA approach is to identify the different structures of a given oscillation in order to better understand its physical origin and to test the applicability of different theoretical hypotheses. The LMA also makes it possible to test the representativity of a mean structure in regard to actual modes that succeed one another in time.

The LMA is applied to the National Oceanic and Atmosphere Administration–Advanced Very High Resolution Radiometer outgoing longwave radiation time series in order to study the variability of the convective perturbation at the intraseasonal timescale (30–60 days). The LMA depicts the most intense and persistent modes of the ISO very well and shows the strong variability of the spatial organization of the convective perturbation at this timescale. Results exhibit interannual and seasonal variations of the mean period and amplitude of the ISO with a tendency to have less persistent modes and smaller periods of the oscillation during El Niño years and during summer. The maximum perturbation of the convection by the ISO is not located on the equator but rather around 10°–15° in the summer hemisphere. Several persistent modes exhibit neither the phase opposition between the Indian and Pacific Oceans nor the eastward equatorial propagation that characterize the average mode of Northern Hemisphere winter. Inspecting the ensemble of ISO modes, this eastward propagation of the convective perturbation is well defined only over the Indian Ocean. The convective perturbation over the Maritime Continent is basically stationary, and the eastward propagation over the Pacific Ocean appears only for the strongest convective perturbations.

Abstract

During periods of light surface wind, a warm stable layer forms at the ocean surface with a maximum sea surface temperature (SST) in the early afternoon. The diurnal SST amplitude (DSA) associated with these diurnal warm layers (DWLs) can reach several degrees and impact the tropical climate variability. This paper first presents an approach to building a daily time series of the DSA over the tropics between 1979 and 2002. The DSA is computed over 2.5° of latitude–longitude regions using a simple DWL model forced by hourly-interpolated surface radiative and turbulent fluxes given by the 40-yr ECMWF Re-Analysis (ERA-40). One advantage of this approach is the homogeneity of the results given by the relative homogeneity of ERA-40. The approach is validated at the global scale using empirical DWL models reported in the literature and the Surface Velocity Program (SVP) drifters of the Marine Environmental Data Service (MEDS). For the SVP dataset, a new technique is introduced to derive the diurnal variation of the temperature from raw measurements.

This DWL time series is used to analyze the potential role of DWLs in the variability of the tropical climate. The perturbation of the surface fluxes by DWLs can give a cooling of the ocean mixed layer as large as 2.5 K yr⁻¹ in some tropical regions. On a daily basis, this flux perturbation is often above 10 W m⁻² and sometimes exceeds 50 W m⁻². DWLs can be organized on regions up to a few thousand kilometers and can persist for more than 5 days. It is shown that strong DWLs develop above the equatorial Indian Ocean during the suppressed phase of the intraseasonal oscillation (ISO). DWLs may trigger large-scale convective events and favor the eastward propagation of the ISO convective perturbation during boreal winter. This study also suggests that the simple approach presented here may be used as a DWL parameterization for atmospheric general circulation models.

Abstract

For the determination of the earth radiation budget (ERB) from space, the reflected solar flux and the emitted longwave (LW: 4–100 μm) fluxes are estimated using radiance measurements made in shortwave (SW: 0.2–4.5 μm) and “total” (TW: 0.2–100 μm) channels. An accurate (1%) calibration and cross-calibration of these two channels is required for the determination of the ERB and for daylight determination of the LW radiance based on differences between the TW and the SW radiances. This paper presents an approach to calibrate the SW channel by using a cross-calibration between the SW channel and the SW part of the TW channel. This approach is applied and validated using data from the Scanner for Radiation Budget (ScaRaB) experiment.

The principle of this cross-calibration is to estimate the LW radiance from the infrared window (IR: 10.5–12.5 μm) radiance measurements over deep convective cloudiness in the Tropics. This estimate makes it possible to subtract the LW signal from the TW radiance measurement during daylight and thus to compare directly the SW radiances measured by the SW and the TW channels. An IR channel is already implemented on ScaRaB and on the Cloud and the Earth’s Radiant Energy System (CERES) instruments. Using ScaRaB data, it is shown that it is possible to estimate the LW radiance over deep convective cloudiness in the Tropics from IR radiance measurement with accuracy better than 1 W m⁻² sr⁻¹. The method applied to ScaRaB measurements gives calibration and cross-calibration parameters with accuracy better than 1%.