Intra-seasonal convection oscillations over southwestern Tanzania during the December to February season are examined using 15 yr (1979 to 1994) of pentad Outgoing Longwave Radiation (OLR) data and kinematic/thermodynamic parameters from ECMWF data. Most significant spectral energy for an area-averaged OLR index is concentrated in periods of 16 to 33 d. Time-longitude diagrams of filtered OLR, and zonal wind anomalies at 850 hPa averaged over the 7.5 to 10°S latitude band for the period November to May reveal eastward (62%), westward (11%) and quasi-stationary convective (27%) features. Phase speeds for transient cases are in the range of 2 to 8 m s -1 . In the eastward propagating case study, 1 to 15 January 1993, map sequences reveal that deep convection shifts northeastward as a southern mid-latitude trough couples with the ITCZ. Convective events over tropical east Africa are associated with an influx of northeasterly Indian monsoon flow followed by increased westerlies from the Guinea/Congo region. Eastward propagating OLR anomalies couple with zonal circulations, suggesting that a transient Walker cell emerges periodically from East Africa. KEY WORDS: Intra-seasonal climate · East AfricaResale or republication not permitted without written consent of the publisher
The synoptic-scale structure and evolution of wet spells over northern Tanzania are described. Pentad (5 d) ECMWF data for major convective events during November in the period 1986 to 1992 were averaged to form a sequential composite, and a seasonal reference mean is subtracted to produce anomalies. The composite results show that a NW-SE line of convergent moisture shifts southward from the NW Indian Ocean 2 pentads before the wet spell, while uplift occurs over the highlands to the north. A high-pressure cell intensifies in the SW Indian Ocean 1 pentad before the wet spell. During this time a low-level vorticity dipole is maintained in the west Indian Ocean, either side of the equator. The main flux of moisture is from the southern Indian Ocean. A southern subtropical, upper westerly trough amplifies during the onset phase. The results offer statistical guidance in medium-range weather forecasts that may assist agricultural management. Dynamical insights are also gained to improve knowledge of climate-weather teleconnections and tropicalsubtropical interactions.KEY WORDS: Intra-seasonal oscillations · Rainfall · Tanzania Resale or republication not permitted without written consent of the publisherClim Res 15: 239-248, 2000239-248, & Wang (1993, who found a prominent centre of action in the central Indian Ocean for 30 to 60 d convective variability. Rui & Wang (1990) documented the development and dynamic structure of tropical intraseasonal convection anomalies using pentad outgoing longwave radiation (OLR) and ECMWF-derived 200 and 850 hPa wind divergence. Murakami (1988), Anyamba (1992) and Matarira & Jury (1992) have discussed intra-seasonal variability in the African sector; however, the evolution of synoptic-scale forcing and its relationship to local convection have not been described.External precursors and regional controls of wet spells over tropical east Africa are documented here for the November onset event. Rainfall during this period contributes up to 40% of the annual total in northern Tanzania (Nyenzi 1988). Using principal component analysis, Ogallo (1989) found that the first principal component explained 70% of the total rainfall variance over tropical east Africa, with a bimodal distribution and widespread spatial loading. The short rains are associated with a shift of the ITCZ to south of the equator over the western Indian Ocean following dissipation over India (Asnani 1993). During the 'ITCZ onset event', which usually occurs in November, convection is concentrated over the plateau of northern Tanzania, and water resources, depleted during the preceding dry winter season, are replenished.Tanzanian wet spells are hypothesised to be related to moisture advection from the western Indian Ocean, enhanced upper westerly shear and in situ convection due to local instability and heating. To diagnose the mechanisms underlying intra-seasonal variability, ECMWF gridded meteorological field data for the period 1986 to 1992 were used to provide insight into the synoptic-scale process...
A statistical analysis of African rainfall and temperature, and surface wind components and sea level pressure over the tropical Atlantic and Indian Oceans is done in the period 1965 to 1995. The annual cycle is found to be the dominant mode, exhibiting a subtropical north-south dipole with a cycle period of 12 months. The second mode is equatorial with 6-month oscillations prevalent. Annual cycle variance is of the order of 40%, compared with ~ 10% for the interannual residual. The north-south alternation of winds over the adjacent oceans accounts for 92% of north-south rainfall oscillations over the African continent. The annual cycle varies coherently over periods of 3 to 8 years. Years of high (low) annual cycle correspond with La Nina (El Nino) and increased (decreased) inter-annual fluctuations of the African climate. A composite analysis shows that the South Atlantic Hadley cell and standing waves in the subtropical jet may connect annual and interannual signals.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.