Atmospheric fronts in the lower troposphere often mark regions of significant weather and serve as an important conceptual tool that is frequently used and well understood by the public. This study uses an objective method to identify fronts in the ERA‐40 reanalysis and to compile a quantitative global climatology of their occurrence. The climatology confirms the mid‐latitude storm tracks as the regions of highest front frequency and identifies slow moving frontal boundaries in the subtropics corresponding to major climate features, such as the Mei‐Yu front. The climatology also shows fronts to be common features of the South Pacific convergence zone. In the northern hemisphere fronts shift poleward in summer. In the southern hemisphere this and other aspects of seasonal behavior are rather different.

[1] Atmospheric fronts are important for the day-to-day variability of weather in the midlatitudes, particularly during winter when extratropical storm-tracks are at their maximum intensity. Fronts are often associated with heavy rain, and strongly affect the local space-time distribution of rainfall. A recently developed objective front identification method that distinguishes between cold, warm and quasi-stationary fronts, is applied to reanalysis data and combined with a daily global gridded data set to investigate how precipitation around the globe is associated with atmospheric fronts. A large proportion (up to 90%) of rainfall in the major stormtrack regions is associated with fronts, particularly cold and warm fronts. Precipitation over the oceanic storm-tracks is mostly associated with cold fronts, while over the Northern Hemisphere continents precipitation is mainly associated with warm fronts. There are seasonal and regional variations in the proportion of precipitation associated with fronts.

Abstract. The AMMA (African Monsoon Multidisciplinary Analysis) program is dedicated to providing a better understanding of the West African monsoon and its influence on the physical, chemical and biological environment regionally and globally, as well as relating variability of this monsoon system to issues of health, water resources, food security and demography for West African nations. Within this framework, an intensive field campaign took place during the summer of 2006 to better document specific processes and weather systems at various key stages of this monsoon season. This campaign was embedded within a longer observation period that documented the annual cycle of surface and atmospheric conditions between 2005 and 2007. The present paper provides a large and regional scale overview of the 2006 summer monsoon season, that includes consideration of of the convective activity, mean atmospheric circuCorrespondence to: S. Janicot (serge.janicot@locean-ipsl.upmc.fr) lation and synoptic/intraseasonal weather systems, oceanic and land surface conditions, continental hydrology, dust concentration and ozone distribution. The 2006 African summer monsoon was a near-normal rainy season except for a large-scale rainfall excess north of 15 • N. This monsoon season was also characterized by a 10-day delayed onset compared to climatology, with convection becoming developed only after 10 July. This onset delay impacted the continental hydrology, soil moisture and vegetation dynamics as well as dust emission. More details of some less-well-known atmospheric features in the African monsoon at intraseasonal and synoptic scales are provided in order to promote future research in these areas.

The life cycle of an intense African easterly wave (AEW) over the African continent is examined using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, Meteosat satellite images, and synoptic observations. This system, the strongest AEW of 2000, can be tracked from central North Africa into the eastern Atlantic Ocean, where it is associated with the genesis of Hurricane Alberto. Synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity (PV), allowing exploration at the role of multiple scales in the evolution of this AEW. The authors’ analysis promotes the division of the AEW life cycle into three distinctive phases. (i) Initiation: The AEW development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in Sudan. This convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical North Africa. (ii) Baroclinic growth: A low-level warm anomaly, generated close to the initial convection, interacts with a midtropospheric strip of high PV that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. This interaction is reinforced by the generation of subsynoptic-scale PV anomalies by deep convection that is embedded within the baroclinic AEW structure. (iii) West coast development: Near the West African coast, the baroclinic structure weakens, but convection is maintained. The midtropospheric PV anomalies embedded within the AEW merge with one another and with PV anomalies that are generated by convection over topography ahead of the system. These mergers result in the production of a significant PV feature that leaves the West African coast and rapidly undergoes tropical cyclogenesis.

Conceptually, it is useful to partition the three-dimensional tropical circulation into meridional and zonal components, namely, the Hadley and Walker circulations. The averaging involved in their definitions can introduce ambiguities. These problems can be circumvented by first partitioning the total vertical mass flux into components associated with overturning in the meridional and zonal directions, respectively, called here the local Hadley and local Walker circulations. Defining the local Hadley and local Walker circulations this way ensures the pair of two-dimensional overturning circulations can be added to give the original three-dimensional circulation, even when the averages are taken over limited domains. The method is applied to the vertical motion from the ERA-Interim reanalysis for the period 1979 to 2009. One important result is that the local Hadley circulation responds much more strongly to ENSO than the local Walker circulation, even though the local Walker circulation in the central Pacific weakens during El Niño years and strengthens and widens during La Niña years.

African easterly waves (AEWs) are identified in numerical model analyses using an objective technique based on the 700-hPa streamfunction field. This method has been developed to (i) reduce the amount of manual data interpretation, (ii) reduce the likelihood of unrelated phenomena being identified as AEWs, and (iii) facilitate completely objective comparisons between AEWs with different structures on multiple scales, in order to describe their variability. Results show this method performs well when compared to methods of AEW identification used in previous studies. The objective technique is used to analyze all AEWs that originated over tropical North Africa during July–September (JAS) 2004. Results indicate that the “average” AEW in this period bears a close resemblance to composite structures from previous research. However, there is marked variability in the characteristics and ultimate fate of AEWs. Most AEWs of JAS 2004 are first identified east of the Greenwich meridian and develop as they move westward. Mature structures over the African continent varied, ranging from isolated potential vorticity maxima confined equatorward of the objectively defined African easterly jet to broad cross-jet structures symptomatic of both baroclinic and barotropic growth. As many as 80% of the cases fell into the second category. After leaving the West African coast, 45% of the AEWs in JAS 2004 were associated with tropical cyclogenesis in either the Atlantic or Pacific Ocean basins.

An objective method for the identification of the intertropical convergence zone (ITCZ) in gridded numerical weather prediction datasets is presented. This technique uses layer- and time-averaged winds in the lower troposphere to automatically detect the location of the ITCZ and is designed for use with datasets including operational forecasts and climate model output. The method is used to create a climatology of ITCZ properties from the Interim ECMWF Re-Analysis (ERA-Interim) dataset for the period 1979–2009 to serve as an indicator of the technique's ability and a benchmark for future comparisons. The automatically generated objective climatology closely matches the results from subjective studies, showing a seasonal cycle in which the oceanic ITCZ migrates meridionally and the land-based ITCZ features are predominantly summertime phenomena. Composites based on the phase of the El Niño–Southern Oscillation index show a major shift in the mean position and changes in intensity of the ITCZ in all ocean basins as the index varies. Under La Niña conditions, the ITCZ intensifies over the Maritime Continent and eastern Pacific, where the ITCZ weakens over the central and equatorial eastern Pacific. An analysis of changes in the ITCZ and its divergence during the period 1979–2009 indicates that the mean position of the ITCZ shifts southward in the western Pacific and a broad global intensification of the convergence into ITCZ regions. The relationship between tropical cyclogenesis and the ITCZ is also examined, finding that more than 50% of all tropical cyclones form within 600 km of the ITCZ.

The underlying large-scale dynamical processes responsible for the development of heat waves in Victoria, southeastern Australia, in summer are presented here. Heat waves are defined as periods of at least three days and two nights for which daily maximum and minimum temperatures exceed the 90th percentile for a particular location and month, using a station daily temperature dataset. Composites of upper-level potential vorticity anomalies from the Interim ECMWF Re-Analysis (ERA-Interim) reveal that heat waves in southeastern Australia are associated with propagating Rossby waves, which grow in amplitude and eventually overturn. The process of overturning generates an upper-level anticyclone over southern Australia and an upper-level trough to the northeast, with maximum amplitudes near the tropopause. The northerly flow associated with the circulation around the surface anticyclone advects hot air from the continental interior over the southeast of Australia, leading to extreme surface temperatures. Composite rainfall shows that precipitation is enhanced in the vicinity of the upper-level trough over northeastern Australia, consistent with adiabatically forced vertical motion, destabilization of the atmosphere, and modified moisture fluxes. Heat waves in the southeast are frequently accompanied by heavy rainfall over the northeast of the continent and adjacent ocean.