Browsing by Author "Jury, Mark R"

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    Composite structure of tropical cyclones in the SW Indian Ocean
    (1994) Parker, Bhawoodien Abbasmia; Jury, Mark R
    This work examines SW Indian Ocean tropical cyclone (TC) structure as a function of movement using ECMWF gridded data. Analyses of the mean summer climate for the period 1987­ 1992 shown that conditions conducive to the development ofTCs are present between the latitudes 5° and 15°S. Six westward moving TCs are selected to form the westward moving composite TC and four are used in the recurving TC composite from 3-days before the maximum surface intensity to 1­ day after. The westward moving composite was found to have a steady TC signature. Comparisons were made with TC Firinga which passed near Mauritius on 29 January 1989. The recurving TC composite was found to exhibit rapid growth with sharp parameters tendencies. Interaction with a sub­ tropical trough is seen in both TC composites. Upper level westerly winds create effective uplift which strengthens the recurving TC on the poleward side. Spectral analysis of a rainfall index compiled from Mauritius data demonstrates spectral peaks at the seasonal and intra-seasonal scale. Three peaks were identified. one within the 40-60 day Madden­ Julian Oscillation. one at 23 days and one with a 16 day oscillation. The 23 day oscillation was found to have a highest spectral power. The results of TC and rainfall analyses offer useful insights to the climate and weather of the SW Indian Ocean.
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    Intra-seasonal convection dynamics over Southwest and Northeast Tanzania : an observational study
    (1997) Mpeta, Emmanuel Jonathan; Jury, Mark R; Brundrit, Geoff B
    Intraseasonal convection oscillation over the northeastern and southwestern Tanzania during MAM and DJF seasons respectively are examined using December, 1979 to May, 1994 pentad (5-day mean) Outgoing Longwave Radiation (OLR) as an indicator of convective cloud distribution. Area-averaged OLR indices are derived for the two areas. Time series of OLR indices for MAM and DJF indicate large quasi-periodic OLR fluctuations in some years and small fluctuations in other years. Periodogram analyses results reveal that dominant periodogram values for the oscillations were different in different years over both areas. Dominant periodogram peaks with periods more than 6 pentads (30 days) occurred 40% of the time on the average. Based on the pentad OLR time series plots deep convection and their precursors are composited. The time evolution of composite OLR maps reveal that patterns of low OLR values (indicating deep connection) shift north-eastwards coupled with low OLR values associated with mid-latitude troughs and linked to the ITCZ. Composite of kinematic and thermodynamic parameters associated with deep conJection and precursors are composited.
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    Intra-seasonal oscillations of convection over southern Africa
    (1993) Levey, Kevin M; Jury, Mark R
    This work examines intra-seasonal rainfall variability over the summer rainfall region of South Africa. Spectral analysis of a precipitation minus evaporation index (P-E) compiled from 16 rainfall stations in the western Transvaal region of South Africa has shown that two dominant spectral peaks occur in the water budget or convection as identified by an outgoing longwave radiation (OLR) index and P-E index. A longer 40-60 day oscillation or Madden Julian Oscillation (MJO) and a shorter 20-30 day ISO are identified. The interaction between the Rossby waves of the mid-latitudes and the MJO of tropics appears to be the main cause of the 20-30 day ISO operating over southern Africa. Hovmoller analysis at 10° S and 40° S has shown that four modes exist in the propagation and oscillatory characteristics of various anomalies. Eastward and westward propagation is noted as well as standing wave patterns and "flip-flop" modes where anomalies alternate between positive and negative across the latitude band with time. The composite analysis revealed a tropically initiated signal of the 20-30 day ISO in the water vapour flux (WVF) anomaly field where strong easterly WVF vectors are noted between the equator and 10°S and between 40° E and 80° E. Strong convergence of water vapour occurs during the wet phase of the 20-30 day ISO over the south-western Indian Ocean and together with strong ridging of the South Atlantic Anticyclone (SAA) causes the south-easterly advection of moisture from the southern Mozambique Channel and over the warm Agulhas Current into South Africa. Post-wet phase anomaly composites do not show much similarity with pre-cursor anomaly composites. This implies that the 20-30 day ISO may be a half cycle of 40-60 days which is consistent with the MJO. However, it is felt that 20-30 day ISO's affecting the southern African region alternate in intensity within the period of the MJO.
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    Intra-seasonal oscillatory modes of the southern Africa summer circulation
    (1995) Makarau, Amos; Makarau, Amos; Jury, Mark R
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    Modulation of South African summer rainfall by global climatic processes
    (1993) Pathack, Beenay M R; Jury, Mark R
    Global climatic processes which control the interannual variability of summer rainfall over South Africa are studied. Monthly and seasonal rainfall variations are analysed with respect to fluctuations in sea surface temperature (SST), outgoing longwave radiation (OLR) and tropospheric winds. OLR is used as a proxy for convective intensity and for the identification of areas of sympathy and opposition to convection over South Africa. Wind data (and derived parameters) are employed to explore large- scale tropical dynamical structures. Plausible explanations are offered for the observed associations. A change in sign of the correlation structure from the October/November rainfall regime to the December through March regime is indicative of a shift from downstream advective processes (Atlantic side) to a teleconnection-type of behaviour (Indian Ocean side). Rainfall variations during the late summer months show significant (and negative) links with SST fluctuations within the equatorial/tropical Pacific and Indian Ocean areas and are consistent with results obtained in analyses with respect to OLR fluctuations. December OLR in the Western Equatorial Indian Ocean is associated with a large portion of the variance in late summer rainfall, and points to a possible relation with the evolution of the Indian monsoon. The positive association implies that reduced cloudiness off the eastern coast of equatorial Africa in the spring precedes above normal mid- and late- summer rainfall over South Africa. Vertical mass overturnings are investigated through the velocity potential and derived parameters (the Zonal Circulation and Meridional Circulation Indices). The results suggest that the vertical tropospheric cells are among the important associated components which modulate climate across southern Africa, and that broad scale flows have an impact upon regional circulation cells. Evaluation of the vertical circulations with respect to wet and dry composites reveals that the Walker-type cell which connects a branch over the Indian Ocean gradually forms after November and reaches peak development in February. A slight increase of SST in the Central Equatorial Indian Ocean (CEI) modifies the Walker cell anomaly leading to below normal summer rainfall over South Africa. Additional thermodynamic inputs in the CEI region are conducive to deeper convection, hence elevated outflow signatures are observed in the velocity potential and related fields. It is conjectured that the teleconnections between South Africa, the CEI and the remote Pacific Ocean regulate the depth of moisture influx and convergence over South Africa. Based on the results of this study, it is believed that empirical models could be designed for long-range prediction of summer rainfall anomalies over the central interior of South Africa.
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    Monsoon surges, tropical cyclones and extreme rainfall events in NW Madagascar
    (1995) Nassor, Abdallah; Jury, Mark R
    Rainfall variability in NW Madagascar and the kinematics and thermodynamics of flood-producing weather systems affecting the region are examined. Daily rainfall in the austral summer are used to select 16 cases for the composite analysis of flood events in the period 1987-1992. In addition two cases of flood events are studied individually. The first flood event occurred in 1991 and brought420 mm of rainfall in 24 hours at Maintirano during the passage of tropical cyclone (TC) Cynthia, an eastward moving TC from the Mozambique Channel. The second flood event occurred in 1992and brought 347 mm of rainfall in 24 hours at Mahajanga due to the passage of TC Bryna, a westward moving TC from the Indian Ocean. Inter-annual rainfall variability is correlated with the indices: TC day frequency, QBO and SOI, and a useful value is found with the first index but not with the two latter global indices. An intraseasonal analysis indicates that wet spells have a duration of 15 to 20 days. Daily spectra analysisof rainfall reveals the complexity of interplay between the convective forcing features. Cycles of 10-20 days are contributed by easterly waves, while 40 day cycles are produced by monsoon surges. The composite analysis identifies the following features: an eastward shift of active convection from Mozambique across Madagascar in conjunction with a surge of the NW monsoon and weakening of the easterlies in the lower troposphere. These factors promote TC genesis. Other supporting influences include the development of an anticyclonic circulation in the upper level over the island with north westward outflow. This often coincides with an east phase QBO, an upper level eastward moving mid-latitude trough and a persistent moisture flux convergence over the island throughout TC passage. The topography of Madagascar plays an important role in the intensification of the flood producing weather system by enhancing the cloud vortex in the NW region of the island.
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    Seasonal and intra-seasonal dynamics and precursors of rainfall over northern Tanzania
    (1995) Kabanda, Tibangayuka Abbas; Jury, Mark R
    The structure and variability of inter-annual and intra-seasonal circulation anomalies and their precursors are investigated over northern Tanzania. Area rainfall departures are computed in the period 1960-1990 and wet and dry years are identified. Northern Tanzania experiences bimodal rains centred on the seasons: March to May (Masika) and October to December (Vuli). Station inter-correlations in Masika are of order +0.4, whereas in Vuli correlations are of order+0.7. Using composite sequences of gridded sea surface temperatures (SST),outgoing longwave radiation (OLR) and winds in the lower- and upper troposphere, the- precursors of flood and drought are assessed. Composites are averaged for the specific season and for periods 2, 4 and 6 months prior; and the historical mean is subtracted to produce anomalies. In this way, evolution of extreme rainfall scenarios is tracked for predictive purposes. The patterns analysed show that when positive SST anomalies persist in the Indian Ocean, sympathetic negative OLR values and local rainfall enhancement are observed. Using pentad (5 day mean) rainfall time series for the area, wet spells are analysed for structure and evolution. ECMWF data for the peak in Vuli rainfall each year from 1986 to 1991 are averaged to form a single composite. The historical mean is subtracted from the composite to produce anomalies for the wet spell and the preceeding pentads.
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    Southern African climate anomalies, summer rainfall and the Angola low
    (1999) Mulenga, Henry Mubanga; Jury, Mark R; Brundrit, Geoff B
    Anomalous climatic conditions have contributed to poverty, wlnerability and unemployment, which are major concerns of many southern African governments. Western countries continue to give food aid during drought periods and are looking for new and effective ways of supporting national food security plans (Walker, 1989a). Food security is a very difficult problem in Africa. A number of agricultural national programs have been planned to alleviate the problem but crop yields and living standards continue to deteriorate in many African countries. Unfavorable macro-economic conditions, debt repayments, civil war, political instabilities and mismanagement of resources make the situation more complex. It may be considered that anomalous climatic events (droughts or floods) are important factors, which contribute to acute food shortage. Seasonal rainfall forecasts are an important management tool for donor countries as well as local farmers. The failure to utilize forecasts based on sound scientific knowledge would negate attempts at achieving food security (W orId Meteorological Organization, 1996). It is for this reason that rainfall is an important input parameter in attaining self-sufficiency in food. Rainfall varies in space and time over southern Africa (preston-Whyte and Tyson, 1988; Walker 1989b; Mason, 1992; Rocha, 1992; Makarau, 1995). Further investigations of year-to-year variability of southern African summer rainfall are required in order to understand mechanisms and make accurate seasonal forecasts. Therefore, research work in climate should have high priority in African countries. The need to understand and predict the interannual variations of the atmosphere and the oceans has resulted in formation of global programs like the Tropical Ocean Global Atmosphere (TOGA) and World Climate Research Program (Climate Variability and Predictability, CLIVAR, 1995 and 1998). 2 Determination of the dynamics of droughts and floods continues to be a major problem. Correct prediction of extreme events such as droughts, floods, cold and warm spells involves knowing the mechanisms as well as the local and remote forcings. Several mechanisms have been suggested but more empirical research is required to understand and predict climate variability of southern African on intra-seasonal and inter-annual time scales. This study focuses on inter-annual and intra-seasonal variability of southern African climate with the view of identifying climatic regional and local features that influence summer rainfall and its fluctuations over southern Africa as whole. In order to achieve this, determination and refining of rainfall-SST anomaly relationships is carried out. Teleconnection patterns and coherent structure of interannual variability are revealed. Atmospheric mean meteorological features over Southern Africa and adjacent oceans have direct impact on summer rainfall. Thus determination of mean features using up-graded new and longer data sets provides a basis for investigating inter-annual and intraseasonal variability. This study presents mean characteristics of peak summer (December, January and February) based on 14 years ECMWF gridded data set. This study is motivated by a desire to understand climatic controls of interannual summer rainfall variability over southern African. The intensity of summer rainfall is modulated on synoptic, intra-seasonal, interannual and decadal time scales.
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    The structure of the upper ocean, atmosphere and heat fluxes - Tropical Indian Ocean
    (1998) Majodina, Mark; Jury, Mark R; Lutjeharms, Johann R E; Rouault, Mathieu
    Sea surface temperatures in the tropical Indian Ocean have been shown to be inversely related to South African summer rainfall, making advanced predictions of this rainfall practicable. Such predictability has enormous potential economic benefits. However, these predictions have been purely statistical; very little is currently known about the marine-atmospheric processes in the Indian Ocean tropics. To address this lack of information, the structure of the upper ocean and lower atmosphere as well as the surface heat fluxes in the tropical Indian Ocean have been investigated. This was done by a special measurement programme on a research cruise in the region. Global gridded meteorological data have been used to complement the shipboard observations. Heat fluxes have been computed from the cruise observations and related to the main atmospheric patterns at the time. These patterns were identified from principal components analysis. Air-sea interaction could thus be estimated over the full tropical Indian Ocean. It is found that the thermocline depth is linked to the cyclonic ocean current shear and to the overlying distribution of wind stress curl. The meridional advection of air into the central Indian Ocean region is shown to modulate the characteristics of the atmospheric boundary layer in the tropical Indian Ocean. The maximum turbulent heat and moisture transports to the atmosphere are found near cyclonic atmospheric disturbances. These are the first reliable observations of the heat and moisture fluxes in that part of the tropical Indian Ocean implicated in South African rainfall. It is clear from this investigation that the synoptic atmospheric systems and the meridional flow of air are critical to enhanced atmospheric convection in the region.
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    Wet and dry troughs over Southern Africa during early summer
    (1992) Barclay, Jennifer Jayne; Jury, Mark R; Washington, Richard
    The synoptic scale structure of troughs transiting southern Africa in October and November is examined. Cases are chosen on the basis of an upper trough being present over southern Africa, and a minimum horizontal and vertical temperature change. Wet and dry troughs are differentiated by the extent and amount of interior rainfall produced. Once selected, a spatial and temporal framework was used on surface, upper-level synoptic maps and radiosonde sections. Individual and composite time-height and spatial sections are analysed for anomalies of temperature, geopotentials, kinematic, vorticity and divergence fields, dewpoint, dewpoint depression, mixing ratio, dry and total static energy and, equivalent potential temperature. European Centre for Medium Range Forecast (ECMWF) maps of vertical motion are analysed and ECMWF data were exclusively used in the wet and dry case study. The essential features of wet troughs include a large amplitude upper westerly wave with a diffluent and northward displaced sub-tropical jet stream, slow movement, westward tilted trough in the vertical and a negative - positive dipole where a high is located south of the low pressure system. In comparison dry troughs are characterised by a small amplitude upper wave, rapid movement, no tilted trough in the vertical, and a stationary high-pressure system over the western interior. Radiosonde moisture variables, circulation anomalies and ECMWF fields of moisture flux give evidence in the wet cases of inflow from the north-east, in conjunction with a ridging anticyclone south of the continent. In dry cases the trajectory of flow is north-westerly and the supply of moisture is limited. In the ECMWF composite maps of vertical motion, lift is weak over the interior in dry cases consistent with a gentle slope in the divergence profile. For wet cases upward motions are intense and widespread over the interior consistent with a steep slope in the divergence profile, and compensated by descending motions over the adjacent oceans along 30° S band. Precipitation in wet events is a combination of dynamical forcing, prefrontal moisture and unstable lower troposphere. In dry events, precursor moist inflow is limited, weak instability and, a gentle slope in the divergence/convergence fields are not conducive to sustain lift.