Two idealized baroclinic wave-6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic-scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave-theoretic viewpoints, using the 'saturation-propagation-saturation' (SPS) picture of wave-6 life-cycle behaviour. The two life cycles, denoted by LC1 and LC2, are higher-resolution versions of the Simmons-Hoskins 'basic' and 'anomalous' cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees, two extreme types of behaviour here designated 'anticyclonic' and 'cyclonic', and epitomized by strongly contrasting upper-air trough behaviour. 'Anticyclonic' behaviour dominates the late stages of LCl and is characterized by backward-tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary-scale mid-stratospheric 'Rossby-wave breaking'. 'Cyclonic' behaviour dominates LC2 and is characterized by forward-tilted, broadening troughs wrapping themselves up cyclonically and poleward, producing major cut-off cyclones in high latitudes. These morphologies are visualized by upper-air maps of potential temperature on the nominal tropopause, defined as a constant-potential-vorticity surface. Some atmospheric mid-latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases.The SPS picture is re-examined, using Eliassen-Palm and refractive-index cross-sections. It is shown, in particular, by reference to a wave-activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large-amplitude counterpart of the nonlinear reflection scenario of Rossby-wave critical-layer theory. The late stages of LCl, by contrast, look more akin to a nonlinear critical-layer absorption scenario. LC2 exhibits a region of largely undular PV contours adjacent to a region of irreversibly deformed PV contours. In the latter region PV rearrangement, and hence absorption of Rossbywave activity, has largely ceased. This accounts for the more persistent EKE in the LC2 case.

International audienceAfrican Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve our knowledge and understanding of the West African monsoon (WAM) and its variability with an emphasis on daily-to-interannual time scales. AMMA is motivated by an interest in fundamental scientific issues and by the societal need for improved prediction of the WAM and its impacts on West African nations. Recognizing the societal need to develop strategies that reduce the socioeconomic impacts of the variability of the WAM, AMMA will facilitate the multidisciplinary research required to provide improved predictions of the WAM and its impacts. This will be achieved and coordinated through the following five international working groups: i) West African monsoon and global climate, ii) water cycle, iii) surface–atmosphere feedbacks, iv) prediction of climate impacts, and v) high-impact weather prediction and predictability. AMMA promotes the international coordination of ongoing activities, basic research, and a multiyear field campaign over West Africa and the tropical Atlantic. AMMA is developing close partnerships between those involved in basic research of the WAM, operational forecasting, and decision making, and is establishing blended training and education activities for Africans

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.

Automatic tracking of vorticity centers in European Centre for Medium-Range Weather Forecasts analyses has been used to develop a 20-yr climatology of African easterly wave activity. The tracking statistics at 600 and 850 mb confirm the complicated easterly wave structures present over the African continent. The rainy zone equatorward of 15ЊN is dominated by 600-mb activity, and the much drier Saharan region poleward of 15ЊN is more dominated by 850-mb activity. Over the Atlantic Ocean there is just one storm track with the 600-and 850-mb wave activity collocated. Based on growth/decay and genesis statistics, it appears that the 850-mb waves poleward of 15ЊN over land generally do not get involved with the equatorward storm track over the ocean. Instead, there appears to be significant development of 850-mb activity at the West African coast in the rainy zone around (10ЊN, 10ЊW), which, it is proposed, is associated with latent heat release.Based on the tracking statistics, it has been shown that there is marked interannual variability in African easterly wave (AEW) activity. It is especially marked at the 850-mb level at the West African coast between about 10Њ and 15ЊN, where the coefficient of variation is 0.29. For the period between 1985 and 1998, a notable positive correlation is seen between this AEW activity and Atlantic tropical cyclone activity. This correlation is particularly strong for the postreanalysis period between 1994 and 1998. This result suggests that Atlantic tropical cyclone activity may be influenced by the number of AEWs leaving the West African coast, which have significant low-level amplitudes, and not simply by the total number of AEWs.