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1.
Summary The west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm day−1 at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment, was carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between 14° N–16° N and near 19° N. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.  相似文献   

2.
Summary  Intra-seasonal fluctuations of summer convection over southern Africa are studied through principal components (PC) analysis. Pentad (5 day) satellite outgoing long-wave radiation (OLR) departures are used to characterise the space and time scales of terrestrial cloudiness in the domain 10–35° S, 10–40° E. Areas of intra-seasonal convective influence are analysed according to spatial pattern and corresponding temporal character. Eight distinct geographic domains are identified, four tropical and four sub-tropical. The three most significant modes occur over southern Tanzania, Namibia, and Zambia, and refer to pulsing of: the Indian NE monsoon, surface heating in the western desert, and the zonal ITCZ, respectively. Temporal characteristics vary widely but an underlying near-monthly rhythm is detected. The variety of modes suggests that convective weather systems respond to external forcing (wave trains) and internal dynamics, to produce intra-seasonal fluctuations over southern Africa. Received February 19, 1998 Revised July 10, 1998  相似文献   

3.
 Monthly sea surface temperature anomalies (SSTA) at near-global scale (60 °N–40 °S) and May to October rainfall amounts in West Africa (16 °N–5 °N; 16 °W–16 °E) are first used to investigate the seasonal and interannual evolutions of their relationship. It is shown that West African rainfall variability is associated with two types of oceanic changes: (1) a large-scale evolution involving the two largest SSTA leading eigenmodes (16% of the total variance with stronger loadings in the equatorial and southern oceans) related to the long-term (multiannual) component of rainfall variability mainly expressed in the Sudan–Sahel region; and (2) a regional and seasonally coupled evolution of the meridional thermal gradient in the tropical Atlantic due to the linear combination of the two largest SSTA modes in the Atlantic (11% with strong inverse loadings over the northern and southern tropics) which is associated with the interannual and quasi-decadal components of regional rainfall in West Africa. Linear regression and discriminant analyses provide evidence that the main July–September rainfall anomalies in Sudan–Sahel can be detected with rather good skills using the leading (April–June) or synchronous (July–September) values of the four main oceanic modes. In particular, the driest conditions over Sahel, more marked since the beginning of the 1970s, are specifically linked to the warm phases of the two global modes and to cold/warm anomalies in the northern/southern tropical Atlantic. Idealized but realistic SSTA patterns, obtained from some basic linear combinations of the four main oceanic modes appear sufficient to generate quickly (from mid-July to the end of August) significant West African rainfall anomalies in model experiments, consistent with the statistical results. The recent negative impact on West African rainfall exerted by the global oceanic forcing is primarily due to the generation of subsidence anomalies in the mid-troposphere over West Africa. When an idealized north to south SSTA gradient is added in the tropical Atlantic, strong north to south height gradients in the middle levels appear. These limit the northward excursion of the rainbelt in West Africa: the Sahelian area experiences drier conditions due to the additive effect (subsidence anomalies+latitudinal blocking) while over the Guinea regions wet conditions do not significantly increase, since the subsidence anomalies and the blocking effect act here in opposite ways. Received: 26 June 1997 / Accepted: 3 October 1997  相似文献   

4.
Summary This paper characterizes Mesoscale Convective Systems (MCSs) during 2001 over Iberia and the Balearic Islands and their meteorological settings. Enhanced infrared Meteosat imagery has been used to detect their occurrence over the Western Mediterranean region between June and December 2001 according to satellite-defined criteria based on the MCS physical characteristics. Twelve MCSs have been identified. The results show that the occurrence of 2001 MCSs is limited to the August–October period, with September being the most active period. They tend to develop during the late afternoon or early night, with preferred eastern Iberian coast locations and eastward migrations. A cloud shield area of 50.000 km2 is rarely exceeded. When our results are compared with previous studies, it is possible to assert that though 2001 MCS activity was moderate, the convective season was substantially less prolonged than usual, with shorter MCS life cycles and higher average speeds. The average MCS precipitation rate was 3.3 mm·h−1 but a wide range of values varying from scarce precipitation to intense events of 130 mm·24 h−1 (6 September) were collected. The results suggest that, during 2001, MCS rainfall was the principal source of precipitation in the Mediterranean region during the convective season, but its impact varied according to the location. Synoptic analysis based on NCEP/NCAR reanalysis show that several common precursors could be identified over the Western Mediterranean Sea when the 2001 MCSs occurred: a low-level tongue of moist air and precipitable water (PW) exceeding 25 mm through the southern portion of the Western Mediterranean area, low-level zonal warm advection over 2 °C·24 h−1 towards eastern Iberia, a modest 1000–850 hPa equivalent potential temperature (θe) difference over 20 °C located close to the eastern Iberian coast, a mid level trough (sometimes a cut-off low) over Northern Africa or Southern Spain and high levels geostrophic vorticity advection exceeding 12·10−10 s−2 over eastern Iberia and Northern Africa. Finally, the results suggest that synoptic, orographic and a warm-air advection were the most relevant forcing mechanisms during 2001.  相似文献   

5.
Summary Using digitized IR images from the European satellite Meteosat, 153 squall lines (SLs) were observed over Western Africa during July, August and September 1986 and 87. The SL mean rainfall volume was computed using the daily rainfall amounts of more than 800 raingauges: 15 km3 in 1986 and 22.5 km3 in 1987. A mean amount of 15 mm was collected per rainy episode at a given station, however, as only about half of the stations situated under a SL experienced a rainy episode, the rainfall amount averaged along the total SL’s swath was 6.8 mm. With the help of the 8 daily Meteosat slots the SL’s area daily variation could be estimated: namely a minimum around noon and a maximum around midnight. Using the SLs displacements (east–west) one had access to the daily variation of the rainfall volume, a minimum in the morning and a maximum toward the end of the afternoon, as already found by several authors who used Meteosat images.  相似文献   

6.
 NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude 17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics. The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component. The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR reanalyses, especially for the 3–5-day regime over the Atlantic. Received: 28 May 1998 / Accepted: 2 May 1999  相似文献   

7.
Summary ?The LITFASS project (‘Lindenberg Inhomogeneous Terrain – Fluxes between Atmosphere and Surface: a Long-term Study’) of the Deutscher Wetterdienst (DWD, German Meteorological Service) aims to develop and to test a strategy for the determination and parameterisation of the area-averaged turbulent fluxes of heat, momentum, and water vapour over a heterogeneous land surface. These fluxes will be representative for an area of about 10 * 10 km2 (while the typical patch size is between 10−1 to 100 km2) corresponding to the size of a grid cell in the present operational numerical weather prediction model of the DWD. LITFASS consists of three components: – the development of a non-hydrostatic micro-α-scale model (the LITFASS local model – LLM) with a grid-size of about 100 * 100 m2, – experimental investigations of land surface – atmosphere exchange processes and boundary layer structure within a 20 * 20 km2 area around the Meteorological Observatory Lindenberg, – the assimilation of a data base as an interface between measurements and modelling activities. The overall project strategy was tested over a three-week period in June 1998 during the LITFASS-98 field experiment. This paper gives an overview on the LITFASS project, on the design and measurement program of the LITFASS-98 experiment, and on the weather conditions during the period of the experiment. Conclusions are formulated for the operational realisation of the LITFASS measurement concept and for future field experiments aimed at studying the land surface – atmosphere interaction in the Lindenberg area. Selected results from both experimental and modelling activities are presented in a series of companion papers completing this special issue of the journal. Received June 18, 2001; revised March 18, 2002; accepted April 2, 2002  相似文献   

8.
 This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall. It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific. This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations: anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level pressure over West Africa. Received: 14 April 1998 / Accepted: 24 December 1998  相似文献   

9.
Summary ?We have analyzed daily rainfall trends throughout the second half of the 20th century in the western Mediterranean basin (Valencia Region, E of Spain). The area is characterized by high torrentiality, and during the second half of the 20th century some of the highest daily rainfall values in the Mediterranean basin have been recorded. In this area, mean annual rainfall varies between 500 and 300 mm and is overwhelmingly dependent on just a few days of rain. Daily maximum rainfall varies on average from 120 mm day−1 to 50 mm day−1, and represents a mean of 17% (coastland) to 9% (inland) of annual rainfall. The 10 days in each year with the heaviest rainfall (called “higher events”) provide over 50% of the annual rainfall and can reach more than 400 mm on average. We compared the annual rainfall trend and the trend of higher and minor events defined by percentiles, both in volume and variability. We, therefore, tested whether annual rainfall changes depend on the trend of the higher (rainfall) events. To overlap spatial distribution of trends (i.e.: positive, no significant and negative trends) we have used cross-tab analysis. The results confirm the hypothesis that annual rainfall changes depend on changes found in just a few rainy events. Furthermore, in spite of their negative trend, higher events have increased their contribution to annual rainfall. As a consequence, although torrential events may have diminished in magnitude, future scenarios seem to be controlled by a limited number of rainy events which will become more and more variable year on year. The high spatial density of data used in this work, (97 observatories per 24.000 km2, overall mean 1 observatory per 200 km2), suggests to us that extreme caution should be applied when analyzing regional and sub-regional changes in rainfall using GCM output, especially in areas of high torrentiality. Received August 1, 2002; revised November 11, 2002; accepted December 1, 2002 Published online May 19, 2003  相似文献   

10.
Summary During the summer season, typhoons form in the western north Pacific Ocean and travel westward towards China. Some recurve northward off the coast, whereas others continue in over land. These typhoons bring heavy rainfall to the Huai river basin in eastern central China. In August 1975, the remnant of typhoon Nina caused exceptionally heavy rainfall in the Hongru river basin, in the mountainous upper reaches of the Huai river. The rainfall lasted five days from 4 to 8 August. This type of nearly stationary typhoon can cause rainfall of large intensity for a long duration, and is suitable for maximization to derive probable maximum precipitation (PMP) estimates. The PMP is transformed into a probable maximum flood hydrograph that is subsequently used to design spillways etc. In this study the PMP values have been estimated using a hydrometeorological method involving depth-area-duration analysis, moisture maximization, and altitude adjustment for typhoon Nina, for 1, 2, and 3 days duration. Areal PMP values were obtained for the entire Hongru river catchment, as well as for the subcatchments upstream the dams at Banqiao (762 km2), Shimantan (230 km2), Boshan (580 km2), and Suyahu (4 498 km2). For point values, the PMP was estimated to 1 200 mm/day, 1 460 mm/2 days, and 1 910 mm/3 days at altitudes about 100 m, which agrees well with previous studies. Received February 21, 1997 Revised May 27, 1997  相似文献   

11.
Summary This paper is to promote a further understanding of the interdecadal mode of the South Pacific. With this focus, we will specifically aim at better understanding the difference between interannual and interdecadal SSTA modes over South Pacific. We define the difference of the normalization area-averaged SSTA in the southern extratropical Pacific (160° W–110° W, 40° S–25° S) and the south subpolar Pacific (150° W–110° W, 60° S–45° S) as the South Pacific interdecadal index (I spd). It is found that the interannual mode is more coherent than the interdecadal mode in the central and eastern tropical Pacific, and the interdecadal mode is significant only during boreal winter (DJF). The interdecadal variation of SSTA firstly occurring in the extratropic South Pacific propagates to the western boundary of the South Pacific, then moves northeast to cross the equator, and finally reaches the central tropic Pacific. It takes about 8 years to propagate from southeast subtropical Pacific to the north hemisphere. The previous studies have suggested the mechanism of waves in the subsurface in the South Pacific. Our study also highlights the Rossby waves play important roles in linkage between the extratropics-tropics South Pacific SSTA on interdecadal time scales. Moreover, the paper shows that the interdecadal variability originated in the extrotropic southeast Pacific is mainly induced by interannual variability in the tropic Pacific.  相似文献   

12.
Summary ?Thirty years (1958–1987) of daily rainfall data for Kenya and north eastern Tanzania are analysed with the aim to characterize the interannual variability of the onset and cessation of the East African “long rains” (boreal spring). The leading principal component (PC1) depicts consistent rainfall variations over much of the region. Cumulative PC1 scores for each year serve to identify onset and cessation dates. The robustness of the dates derived from this method is demonstrated through the use of an independent sample of stations. Their spatial representativity is assessed by daily rainfall composites. Average onset occurs on March 25th, and cessation on May 21st. The interannual variability of the onset (standard deviation of 14.5 days) is larger than that of the withdrawal (10.3 days), but the onset is also spatially much more consistent. Mean dates and dates in selected anomalous years agree well with previous studies. The relationship between onset time-series and large-scale atmospheric and oceanic fields is analysed. On a monthly time-scale, interannual variations in “long rains” onset are associated with sea-surface temperature (SST) and sea-level pressure (SLP) patterns that have a different sign for the Atlantic and Indian Oceans. A warm South Atlantic and a cool Indian Ocean are associated with low and high SLP anomalies, respectively. These patterns are conducive to enhanced equatorial easterlies and surface divergence over East Africa. This maintains the meridional branch (north–south orientated) of the Intertropical Convergence Zone (ITCZ) further west, and the net result is a delayed onset of the “long rains”. Some of the South Atlantic features are already present during January–February, suggesting some potential for monitoring interannual variations in the wet season onset, based on SST and SLP patterns. Additional signals are found over Europe and the Mediterranean Sea in terms of the interaction between the Northern Hemisphere extratropics and equatorial eastern Africa. A surge in the mid-tropospheric northerlies at this time induces instability that may lead to an early onset event. Received July 3, 2002; revised November 28, 2002; accepted December 7, 2002 Published online March 17, 2003  相似文献   

13.
Summary By analyzing 12-year (1979–1990) 200 hPa wind data from National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, we demonstrate that the intraseasonal time scale (30–60 days) variability of the Tropical Easterly Jet (TEJ) reported in individual case studies occurs during most years. In the entrance region (east of ∼70° E), axis of the TEJ at 200 hPa is found along the near equatorial latitudes during monsoon onset/monsoon revivals and propagates northward as the monsoon advances over India. This axis is found along ∼5° N and ∼15° N during active monsoon and break monsoon conditions respectively. Examination of the European Centre for Medium Range Weather Forecasts reanalysis wind data also confirms the northward propagation of the TEJ on intraseasonal time scales. During the intraseasonal northward propagations, axis of the TEJ is found about 10°–15° latitudes south of the well-known intraseasonally northward propagating monsoon convective belts. Because of this 10°–15° displacement, axis of the TEJ arrives over a location about two weeks after the arrival of the monsoon convection. Systematic shifting of the locations by convection, low level monsoon flow and TEJ in a collective way during different phases of the monsoon suggests that they all may be related.  相似文献   

14.
Summary Monthly rainfall data for 135 stations for periods varying from 25 to 125 years are utilised to investigate the rainfall climatology over the southeast Asian monsoon regime. Monthly rainfall patterns for the regions north of equator show that maximum rainfall along the west coasts occurs during the summer monsoon period, while the maximum along the east coasts is observed during the northeast monsoon period. Over the Indonesian region (south of the equator) maximum rainfall is observed west of 125 °E during northern winter and east of 125 °E during northern summer. The spatial relationships of the seasonal rainfall (June to September) with the large scale parameters – the Subtropical Ridge (STR) position over the Indian and the west Pacific regions, the Darwin Pressure Tendency (DPT) and the Northern Hemisphere Surface Temperature (NHST) – reveal that within the Asian monsoon regime, not only are there any regions which are in-phase with Indian monsoon rainfall, but there are also regions which are out-of-phase. The spatial patterns of correlation coefficients with all the parameters are similar, with in-phase relationships occurring over the Indian region, some inland regions of Thailand, central parts of Brunei and the Indonesian region lying between 120° to 140 °E. However, northwest Philippines and some southern parts of Kampuchea and Vietnam show an out-of-phase relationship. Even the first Empirical Orthogonal Function of seasonal rainfall shows similar spatial configuration, suggesting that the spatial correlation patterns depict the most dominant mode of interannual rainfall variability. The influence of STR and DPT (NHST) penetrates (does not penetrate) upto the equatorial regions. Possible dynamic causes leading to the observed correlation structure are also discussed. Received October 10, 1996 Revised February 25, 1997  相似文献   

15.
Predictability of NDVI in semi-arid African regions   总被引：1，自引：1，他引：0  
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   

16.
Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.  相似文献   

17.
Summary.  Mesoscale convective precipitation systems in the Alpine region are studied by analyzing radar and rain gauge data. The data from weather radars in Austria, France, Germany, and Switzerland are combined into a composite. Availability of radar data restricts the study mainly to the northern part of the Alpine region. Mesoscale convective systems (MCS) occur often in this region and are comparable to large systems observed in the USA. Seven precipitation events lasting one to six days from the years 1992–1996 are examined in detail. They all moved west to east and showed no diurnal preference in formation or dissipation. They reach sizes of 2 − 6 · 104 km2. MCS with leading-line trailing-stratiform structure tended to be larger and more intense. A 25-year set of rain gauge data indicates that a giant MCS (covering more than 4 · 104 km2 with more than 30 mm/day) occurs every 6 years in the northern Alpine region. MCS occur more frequently in the southern Alpine region. Received February 25, 1999/Revised June 29, 1999  相似文献   

18.
Summary  The interannual variability of the Indian summer monsoon (June–September) rainfall is examined in relation to the stratospheric zonal wind and temperature fluctuations at three stations, widely spaced apart. The data analyzed are for Balboa, Ascension and Singapore, equatorial stations using recent period (1964–1994) data, at each of the 10, 30 and 50 hPa levels. The 10 hPa zonal wind for Balboa and Ascension during January and the 30 hPa zonal wind for Balboa during April are found to be positively correlated with the subsequent Indian summer monsoon rainfall, whereas the temperature at 10 hPa for Ascension during May is negatively correlated with Indian summer monsoon rainfall. The relationship with stratospheric temperatures appears to be the best, and is found to be stable over the period of analysis. Stratospheric temperature is also significantly correlated with the summer monsoon rainfall over a large and coherent region, in the north-west of India. Thus, the 10 hPa temperature for Ascension in May appears to be useful for forecasting summer monsoon rainfall for not only the whole of India, but also for a smaller region lying to the north-west of India. Received July 30, 1999 Revised March 17, 2000  相似文献   

19.
Summary  The design and operation of hydro-structures for flood control and water conservation bring a need for improved characterization of precipitation patterns. A 73 000 km2 study area in East Central China is situated in the East Asian monsoon region and experiences a strong seasonality in the rainfall regime. The characteristics of daily rainfall from 230 gauges during 1967–1986 were investigated for four periods in the summer monsoon season using empirical orthogonal function analysis (EOF) and extended empirical orthogonal function analysis (EEOF). The EOF analysis showed that for all four periods most of the variance was explained by an elongated spatial rainfall pattern. The pattern varied in direction, from roughly west-east to southwest-northeast, in the different periods. The zonally oriented patterns were interpreted as being caused by the stationary Mei-Yu front and the southwest-northeast patterns interpreted as cold fronts in cyclones that were developing over the study area. The latitude of the rain belt described by the first mode moved slightly northward with the advance of the East Asian monsoon from the first period, 9–22 June, to the third period, 23 July–5 August, and then withdrew southward again in accordance with the known seasonal movement of the Mei-Yu front. The EEOF analysis was used to show the development of the rainfall area over sequences of three days. During all four periods rainfall intensified on the second day, compared to the first and third days. During the first and last periods, 9–22 June and 5 August–30 September, respectively, there appeared to be little movement in the rainfall. During the second and third periods, the patterns were interpreted as a cold front in a developing cyclone. The results show the connection between the temporal variation in rainfall intensity and the temporal succession of spatial patterns over three day periods and should be used in the construction of design rainfalls for the study area. Received February 10, 1998 Revised June 23, 1998  相似文献   

20.
Summary Teleconnections between the seasonal rainfall anomalies of March through May (“long-rains”) over eastern Africa (Uganda, Kenya and Tanzania) and the lower equatorial stratospheric (30-mb) zonal winds for the 32-year period 1964–1995 are examined using statistical methods. The analysis is based on the application of the simple correlation method and QBO/rainfall composite analysis. A statistical study of spatial correlation patterns is made in an effort to understand the climatic associations between the equatorial stratospheric zonal wind and regional rainfall at the interannual scale. The aim of this analysis is to establish whether this global signal can be employed as predictor variable in the long-range forecasts. The study is part of an ongoing investigation, which aims at designing a comprehensive and objective, multi-variate-forecast system of seasonal rainfall over eastern Africa. The correlation parameters include simultaneous (zero lag), and the non-zero lag correlations. The statistical significance of the correlation coefficient [r] is tested based on the Monte Carlo t-statistical method, and the standard correlation tables. Our results indicate significant positive simultaneous and non-zero lag correlations between rainfall over parts of East Africa and lower equatorial stratospheric zonal wind during the months of March–May and June–August. Significantly high correlations are concentrated over the western regions of eastern Africa with peak values of (+ 0.8) observed over these areas. These associations have been observed to be more prominent during lag than in the simultaneous correlations. Strong month to month lag coherence is observed after June prior to the onset of the March to May seasonal rainfall and persists for more than 4 months. Correlation indices for the eight homogeneous rainfall regions over eastern Africa which are derived from our Empirical Orthogonal Function/Cluster analysis shows a clear annual cycle with significant relationships between QBO and seasonal rainfall occurring during boreal summer (June–August). The season with the weakest relationship is December–February. It is however, noted that although the coherence between QBO-Index and rainfall during the long-rains is significantly high, there are some wet/dry years for which the relationship between the long rains and the lower equatorial zonal wind are not significant (for example in 1966, 1973 and 1983). These years have been associated with strong and prolonged ENSO events. Preliminary comparison of the QBO-Index and the newly found Indian Ocean dipole mode index (DMI) indicates that the two climate variables may be significantly related. Of the six high dipole mode events in the Indian Ocean that were observed in 1961, 1967, 1972, 1982, 1994 and 1997, all except 1967 coincided with the easterly phase of the QBO-Index and below normal rainfall over western highlands of eastern Africa. Contingency analyses indicate 60 percent likelihood for the occurrence of above normal rainfall during the westerly phase of the QBO and 63 percent likelihood of below normal rainfall during the east phase of the QBO. Our correlation analysis results indicate that about 36 percent of the variability of the long-rains season over eastern Africa are associated with the QBO-Index. Our results further show that the tendency of the lower equatorial stratosphe ric zonal wind prior to the season is a good indicator of the performance of the long rains of eastern Africa. A positive OND minus JJA QBO trend is a good indicator for the non-occurrence of drought over eastern Africa. Similarly, a negative trend is a good indicator for the non-occurrence of high rainfall over the region. The identified characteristics and domain of influence of the QBO signal in different regions of East Africa suggests that this global oscillator may offer useful input to objective multi-variate rainfall prediction models for eastern Africa. Received June 4, 1999 Revised November 25, 1999  相似文献   

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