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Predictability of NDVI in semi-arid African regions 总被引：1，自引：1，他引：0
N. Martiny N. Philippon Y. Richard P. Camberlin C. Reason 《Theoretical and Applied Climatology》2010,100(3-4):467-484
In semi-arid Africa, rainfall variability is an important issue for ecosystems and agricultural activities. However, due to its discrete nature in time and space, rainfall is difficult to measure, quantify, and predict. In the dry tropics, a good proxy for rainfall is vegetation activity since this parameter is well correlated with rainfall variations. In this study, over 20 years of Normalized Difference Vegetative Index (NDVI) data from the Advanced Very High Resolution Radiometers are used. The goal is to assess the skill of linear statistical models in estimating regional NDVI interannual variability based on ocean and atmospheric fields (but not rainfall) and then to hindcast it with a 1- to 2-month lead-time. Three semi-arid areas of ～150 000 km2 located in Western, Southern, and Eastern Tropical Africa are considered for this purpose. The predictors are: the Niño3.4 sea surface temperature index, the main modes of National Center for Environmental Prediction (NCEP) surface temperature variability in a window centered over Africa, and regional-scale indices based on NCEP surface temperatures and atmospheric variables (relative humidity, geopotential heights, and winds). The regional indices, which are physically and statistically robust, are generally asynchronous with the NDVI predictand. The statistical models, based on linear multiple regressions, give significant results, and the correlation between observed and cross-validated NDVI is 0.67 in Southern Africa, 0.76 for the long rains and 0.83 for the short rains in Eastern Africa, and 0.88 in Western Africa. The results have implications for (1) better understanding the role of El Niño/Southern Oscillation in semi-arid Africa, and (2) highlight the importance of regional climate processes for vegetation growth at these scales, notably the role played by the Mediterranean Sea and its influence on the West African monsoon. The predictability of NDVI over these African regions is discussed. 相似文献
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C. J. C. Reason M. Rouault J.-L. Melice D. Jagadheesha 《Meteorology and Atmospheric Physics》2002,80(1-4):19-29
Summary The Southwestern Cape (SWC) region of South Africa is characterized by winter rainfall mainly via cold fronts and by substantial
interannual variability. Evidence is presented that interannual variability in SWC winter rainfall is related to sea-surface
temperature (SST) and sea-ice anomalies in the central South Atlantic and adjoining Southern Ocean and to large scale ocean–atmosphere
interaction in this region. During wet winters, the jet is strengthened just upstream of the SWC and significant cyclonic
anomalies extend from the SW Atlantic over the region. SST tends to be anomalously warm (cool) in the SW Atlantic and SE Atlantic
(central South Atlantic) and sea-ice extent increased in the central South Atlantic sector of the Southern Ocean. These patterns
favor increased cyclogenesis upstream, a more northward track of midlatitude depressions, local intensification near the SWC
and enhanced rainfall. Roughly the reverse patterns occur during dry winters. Some preliminary results from atmospheric GCM
experiments are presented which help support these findings.
Received November 9, 2001 Revised December 28, 2001 相似文献
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Summary This study investigates the circulation anomalies associated with the intraseasonal evolution of wet and dry years over western Tanzania (29–37° E, 11.5–4.75° S) and how the onset and withdrawal of the rainy season as well as its wet spell characteristics are modified. It is found that for wet years, the rains begin earlier and end later, with strong wet spells occurring during the season, and there tend to be a greater number of moderate wet spells (although not necessarily more intense wet spells) than in dry years. In dry years, late onset and early cessation of the rainy season occur, often with an extended dry spell soon after the onset, and there tend to be a greater number of dry spells within the season. Large negative outgoing long wave radiation (OLR) anomaly values tend to be located between 20° and 40° E with anomalous westerly flow at 850 hPa occurring across the continent from 10° E to the tropical western Indian Ocean during wet spells in the anomalously wet seasons. Anomalously dry seasons are characterised by large positive OLR anomalies over 30–50° E as well as easterly anomalies at 850 hPa and westerly anomalies at 200 hPa. Eastward propagating intraseasonal anomalies are slower during the wet years implying that the convection remains over Tanzania longer. On the intraseasonal scale, Hovmoeller analyses of OLR and 850 and 200 hPa zonal wind indicate that convection over western Tanzania may be associated with a flux of moisture from the tropical southeast Atlantic and Congo basin followed by weak easterlies from the tropical western Indian Ocean.On interannual scales, wet (dry) years are characterized over the Indian Ocean by weaker (stronger) equatorial westerlies and weaker (stronger) trades that lead to less (more) export of equatorial moisture away from East Africa and increased (decreased) low-level moisture flux convergence over southern Tanzania, respectively. These anomalies arise from an anticyclonic (cyclonic) anomaly over the tropical western Indian Ocean during wet (dry) austral summers that may be related to cool (warm) SST anomalies there. Large scale modulation of the Indian Ocean Walker cell is also evident in both cases, but particularly for the dry years.Current affiliation: Tanzania Meteorological Agency, P.O. Box 3056, Dar es Salaam, Tanzania 相似文献
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M. R. Ramesh Kumar Syam Sankar Chris Reason 《Theoretical and Applied Climatology》2012,110(1-2):209-227
Inter-annual variability in the formation of the mini warm pool [sea-surface temperature (SST)>30°C] over the south eastern Arabian Sea (SEAS) and its role in the formation of the monsoon onset vortex (MOV) has been examined using two independent SST data sets. The role of SST, convection, integrated columnar water vapour and the low-level jet in the setting up of the monsoon onset over Kerala (MOK) is examined. It is found that the MOV which forms over the SEAS region upsets the delicate balance between convection, buildup of moisture and strengthening and deepening of the westerlies over the SEAS that is needed for the setting up of the MOK. Thus, the formation over the SEAS of an MOV is not necessarily conducive for MOK. Furthermore, it is shown that a mini warm pool over the southeastern Arabian Sea is not a sufficient condition for the formation of an MOV because such a warm pool is present over this region during most of the years, but an MOV does not necessarily form over there. 相似文献
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Syam Sankar M. R. Ramesh Kumar Chris Reason 《Theoretical and Applied Climatology》2011,103(3-4):359-374
Interannual variations of the monsoon onset over Kerala (MOK) have been studied using data from over 60?years (1948?C2009) of NCEP/NCAR reanalysis and outgoing long-wave radiation. The sea surface temperature fields over the North Indian Ocean associated with the MOK have been examined in association with El Nino and Indian Ocean Dipole (IOD) events which originate in the Pacific and Indian Ocean, respectively. An analysis of the tropical convective maximum showed significant differences in its strength and location during the El Nino, IOD, early, normal, and delayed MOK composites. Further, we also looked into the role of the convective systems formed over the Arabian Sea and Bay of Bengal on MOK. The most significant features during early (delayed) MOK years is the abnormal persistence of westerlies (easterlies) several days prior to MOK and enhanced (suppressed) deep convection over the southeastern Arabian Sea and the southern Bay of Bengal. Moisture builds up over peninsular India several pentads prior to MOK during La Nina, negative IOD, and concurrent La Nina and negative IOD years as compared to the El Nino, positive IOD, and concurrent El Nino and positive IOD years, indicating its significant role on MOK. The monsoon Hadley cell and Walker circulations are weaker (stronger) during a delayed (early) MOK. Further, the sea surface temperature anomalies in the western Pacific are negative (positive) during delayed (early) MOK. 相似文献
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Abstract Coastally trapped stratus events involve the alongshore propagation of a low‐level mesoscale ridge of high pressure, and generally occur only during summer in British Columbia. During these events, a body of cool dense air overlain by stratus cloud propagates northwards in the marine layer. This situation results because Coriolis effects trap the dense air against the coastal mountains and an inversion layer prevents vertical leakage of energy. Detailed analysis of a late summer event and comparison of the observations with available theory are presented. It is shown that the theoretical speeds for the event agree reasonably with observations except near major topographic variability. The theoretical trapping scale (internal Rossby radius) computed from the sounding data matches the observed width of the coastal stratus seen on satellite imagery. It is found that the properties of the event may be accounted for by the propagation in the marine layer of a coastally trapped intrusion with steep leading edge or shock. Attention is drawn to the role that the complex topography of Vancouver Island plays in influencing the propagation of the event, and some suggestions for forecasting these phenomena are made. 相似文献