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Divergent Circulation (divergent + circulation)
Selected AbstractsTrends in the boreal summer regional Hadley and Walker circulations as expressed in precipitation records from Asia and Africa during the latter half of the 20th centuryINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2008Hongxu Zhao Abstract West African summer rainfall, north China summer rainfall and a new climate proxy, snow accumulation from the Dasuopu ice core in the southern Himalaya, have all experienced decreasing trends during the latter half of the 20th century. In this paper, we investigate the existence of a common mechanism that explains these geographically dispersed trends during the boreal summer. In particular, we explore the hypothesis that these trends are related to changes in the regional Hadley and Walker circulations. We show that the divergent circulation in the NCEP reanalysis indicates the existence of trends in these circulations that are consistent with the observed changes in the precipitation records. In addition, the regressions of the divergent circulation in the NCEP reanalysis against these precipitation records indicate that a similar globally coherent signal is associated with the time series and their linear trends while the regressions against the de-trended residuals do not contain statistically significant large-scale signals. These similarities lead us to conclude that the decreasing trends in the three precipitation time series during the latter half of the 20th century are consistent with large-scale changes in the global overturning circulation during the boreal summer. Copyright © 2007 Royal Meteorological Society [source] Water vapour transport associated with tropical,temperate trough systems over southern Africa and the southwest Indian OceanINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2004Martin C. Todd Abstract Tropical,temperate trough (TTT) systems are the dominant rain-producing synoptic type over southern Africa. They represent an important mechanism of poleward transport of energy and momentum. This paper provides an analysis of water vapour transport in TTT systems. An objective sampling of TTT systems is conducted from analysis of daily satellite rainfall products. During the sampled TTT events, rain bands extend from tropical southern Africa near 10 °S to the midlatitudes of the southwest Indian Ocean. The divergent and non-divergent (streamfunction) components of vertically integrated water vapour flux associated with major TTT events are calculated, from National Centers for Environmental Prediction reanalysis data. During TTT events the streamfunction circulation dominates water vapour transport, although adjustments to the divergent component are relatively more important. The results indicate that TTT events facilitate a substantial water vapour flux from the tropics into the midlatitudes. Water vapour transport to the midlatitudes along the TTT axis is facilitated largely by changes to the streamfunction, associated with a strengthening and eastward displacement of the Indian Ocean high, and the advent of midlatitude transients. It is also shown that TTT systems are major regions of moisture convergence. The high vapour concentrations along the full extent of the TTT rain bands are maintained by the divergent circulation. The structure of these divergent vapour flux anomalies is suggestive of adjustments to the Walker circulation, involving strong anomalous divergence over the maritime continent/west Pacific and a weaker enhancement of the major divergence centre over the tropical Atlantic. Copyright © 2004 Royal Meteorological Society [source] Remote weather associated with South Pacific subtropical sea-level high propertiesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 7 2004Richard Grotjahn Abstract The subtropical highs in sea-level pressure (SLP) are little studied and incompletely understood. In recent years, three groups of theories, i.e. tropical divergent circulations, subtropical Rossby wavetrains, and midlatitude frontal cyclone interactions, have been proposed for remote maintenance of these highs. The latter is presented here as a remote forcing of these highs for the first time in the reviewed literature. The focus of the study is upon illuminating associations between these mechanisms and the South Pacific subtropical high in SLP (SP high). Precipitation, outgoing longwave radiation, velocity potential, and divergent winds are used as proxy markers for the remote forcing mechanisms. The tools used include composites, one-point correlations, autocorrelations, cross-correlations, and cross-spectra. Observational evidence, in monthly and daily data, is examined that appears to support each mechanism. Associations seen in monthly data are better understood in daily data at various lags. Convection over Amazonia, coordinated with suppressed convection in the western tropical Pacific, leads to enhanced SLP on the tropical side of the high. Midlatitude weather systems are the strongest influence upon the maximum SLP and the SLP on the higher latitude side of the high. The western side is associated with both middle-and lower-latitude phenomena, such as the South Pacific convergence zone. Various properties of the high have a strong period around 45 days. Associations to the Madden,Julian oscillation and El Niño,southern oscillation are explored and are strong only for the tropical side of the SP high. Copyright © 2004 Royal Meteorological Society [source] Dynamics of jet streaks in a stratified quasi-geostrophic atmosphere: Steady-state representationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 600 2004Philip Cunningham Abstract The structure and dynamics of jet streaks in the extratropical upper troposphere are examined in the context of a continuously stratified quasi-geostrophic (QG) framework. It is hypothesized that jet streaks may result from the superposition of monopolar or dipolar vortices of mesoscale dimensions with the enhanced potential-vorticity gradients constituting the tropopause. Based on this hypothesis, steady-state monopolar and dipolar vortices in a uniform zonal background flow on an f -plane are investigated for their applicability as idealized dynamical representations of jet streaks. The representations of jet streaks satisfy the nonlinear governing equations of the continuously stratified QG framework: the monopolar vortex is specified in terms of axisymmetric distributions of QG potential vorticity in the interior of the domain and perturbation potential temperature on upper (tropopause) and lower (surface) boundaries, whereas the dipolar vortex is adapted from a closed-form analytical solution for the geostrophic stream function. Through the incorporation of vertical structure and divergent circulations, these representations of jet streaks extend those presented previously by the authors using a non-divergent barotropic model. It is shown that these vortex representations display characteristic signatures similar to those observed in atmospheric jet streaks. In particular, both the monopole and the dipole exhibit an ageostrophic wind directed towards lower geopotential height in the entrance region of the streak and towards higher geopotential height in the exit region. For the monopole, this ageostrophic wind is entirely rotational and there is no vertical motion. For the dipole, the rotational part of the ageostrophic wind dominates the divergent part; the latter is associated with a four-cell pattern of vertical velocity similar to that described in conceptual models of straight jet streaks. For both the monopole and the dipole, the jet streak is induced by the vortex structure such that the wind speed maximum translates at the same speed as the individual vortices; this translation speed is slower than the maximum wind speed in the core of the speed maximum, consistent with observations of jet streaks. It is proposed that the above representations provide a formal theoretical foundation for the conceptual models of jet streaks prevalent in the literature; these conceptual models typically are based on heuristic kinematic or parcel arguments and not on consistent solutions to a physically plausible set of equations. The representations also provide a foundation upon which to explore the unsteady behaviour of jet streaks in terms of the superposition of monopolar and dipolar vortices with non-uniform zonal background flows. Copyright © 2004 Royal Meteorological Society. [source] | ||||||||||