Extreme Rainy Research Articles

Multimodel comparison and assessment of short to medium range precipitation and temperature forecasts over India: Implications towards forecasting of meteorological indices in India

AbstractA comprehensive assessment of the forecast skill of various meteorological indices over the Indian region from different numerical weather prediction (NWP) models is lacking in the literature. In this study the performance of four NWP models, namely Global Ensemble Forecast System (GEFSv12), European Center for Medium Range Forecasting (ECMWF), Climate Forecast System (CFSv2) and Indian Institute of Tropical Meteorology (IITM) towards forecasting of precipitation, temperature and associated meteorological indices, is evaluated at short to medium timescales across the Indian region. Further, the effect of ocean atmospheric (OA) oscillations on the precipitation/temperature forecast skill from the different NWP models is also assessed. Results show that the NWP models are better in predicting the meteorological indices than the quantitative forecasts. The ECMWF model was found to be the best for PCP forecasting with CFSv2 performing poorly. For temperature the GEFSv12 model performance was the lowest, compared to rest of the models. The models show poor skill in forecasting monsoon season precipitation compared to non‐monsoon and the temperature forecasts from the NWP models are particularly poor for the northern basins. Skilful temperature forecasts are observed in the Northwestern, Indo‐Gangetic and Central basins for the CFSv2 and ECMWF models. The forecast skill of precipitation indices are higher in the northwestern, central and Indo‐Gangetic basins compared to the rest. The skill of precipitation indices, namely rainy days, extreme rainy days and consecutive wet days, is higher during the monsoon seasons while the prediction skill of consecutive dry days is higher during the non‐monsoon season. OA analysis revealed that the ENSO phases have dominant effect on the forecast skill of the precipitation only. The temperature and meteorological indices forecasts are not affected significantly by the OA phases. The outcomes of this study have implications towards irrigation scheduling and water resources management decision making in India.

Oceanic SACZ produces an abnormally wet 2021/2022 rainy season in South America

The oceanic South Atlantic Convergence Zone (SACZ) has played a major role during South America’s 2021/2022 summer extreme rainy season, being responsible for more than 90% of the precipitation in some regions of Southeast Brazil and in some regions of the Southwestern Atlantic Ocean (SWA). The summer of 2021/2022 was unique and rare and considered an abnormally humid season as verified by official Brazilian Institutes. First, the unusual number of cases of SACZ episodes (seven), was the highest recorded in the last decade. Second, all the cases that occurred were oceanic SACZ that assumed characteristics of an Atmospheric River and produced an excessively anomalous amount of precipitation during this period. Excess precipitation along with the regions located in mountainous and very uneven relief, which by orographic effects favors high precipitation volumes, were responsible for amplifying the observed impacts, such as landslides and floods that caused several losses to society. We also showed the main effects of coupling and interaction between the waters of the surface layer of the SWA and the atmosphere. Our learning from this study ends with the unprecedented results of how the marine atmospheric boundary layer (MABL) is locally modulated by the sea surface temperature (SST) that lies just below it. Until the present moment, we emphasize that this important mechanism has not been widely highlighted in the literature, showing that even though the ocean is colder than before oceanic SACZ is established, it is still warmer than the overlying air, thus, the ocean continues to be an active source of heat and moisture for the atmosphere and enhances the MABL instability process.

An investigation into trends in frequency and proportion of different durations of various types of extreme precipitation in Iran

AbstractClimate change can be traced in any given region by searching for changes in climatic elements' characteristics. Extreme precipitation, for example, is a characteristic of precipitation whose changes, especially in recent years, can be attributed to global warming and climate change. This study aimed to investigate possible changes in extreme precipitation in Iran by analysing the frequency of extreme rainy days duration. To this end, gridded precipitation datasets from the hydrological years 1971–1972 to 2015–2016 were used, and days associated with extreme precipitation were extracted by using thresholds of 90th, 95th, and 99th percentiles (as heavy, very heavy, and extremely heavy precipitation, respectively). Latitude and longitude showed the most significant correlation with precipitation characteristics in the spatial distribution during the study period. In addition, heavy (HP), very heavy, and extremely heavy precipitation experienced durations of 1–8, 1–7, and 1–4 days, respectively. At all extremes, the 1‐day duration covered more areas and had more repetition, while the longer duration had the lowest tempo–spatial repetition. According to the results of the present study, the one‐day duration of HP contributed more significantly to the annual precipitation. The proportion of this type of precipitation has been more significant in the southern and central regions of the country. Furthermore, it was revealed that the frequency of durations of short‐term HP increased during the study period, whereas the duration of long‐term HP declined.

Village Complaints Application System Based On Android Webview

Many residents in the village of Entalsewu, Buduran District, Sidoarjo Regency, often complain about the situation in the village. Complainants do not know where to go to convey their complaints in the village environment. For example, roads that are damaged and have potholes due to frequent vehicles and extreme rainy weather along the roads in Entalsewu village and also the dead and dim public street lighting (PJU) have not been repaired so that road users experience accidents at night, many residents' houses flooded water that enters the house. Therefore, to overcome this problem, the author designed a village complaint application system based on android webview with a case study of village development. Which aims to make it easier for the public to make complaints in the village of Entalsewu, Buduran District, Sidoarjo Regency. This android application can be used by the public to make complaints online. This application can also make it easier for the village government to respond to community complaints. In this study, the author uses the Waterfall method as a process or flow of research results.

Multi-criteria coastal environmental vulnerability assessment using analytic hierarchy process based uncertainty analysis integrated into GIS

Changes in environmental conditions influence vulnerability due to interacting stresses and pressures across the nations and regions. Coastal resources are under severe stress due to climate change, growing trade and commerce, and the human population depends on them. The coastal vulnerability to changing climatic variables has created a major concern at regional, national and global scales. The present model study assessed the coastal vulnerability of the densely populated districts in South India, which are prone to extreme climatic events at a higher frequency. The seven crucial influencing variables that have been selected for the study were sea-level rise, coastal elevation, coastal slope, extreme rainy days, historical shoreline change, tidal range, and geomorphology. The identified variables were ranked by relative importance and linked by weightage using analytical hierarchy process-based uncertainty analysis. Mapped and reclassified variables have been integrated to derive the overall vulnerability using geospatial techniques. The study shows that the coast has experienced high vulnerability to SLR impact, extreme rainfall, geomorphology, and elevation; medium vulnerability to the shoreline change and least vulnerable to coastal slope and tidal range. Of the coastal regions studied, 29% and 14.3% had high vulnerability; 70.5% and 85.7% had medium vulnerability in the two selected densely populated districts (Kancheepuram and Tiruvallur District). Applying geospatial techniques to assess the environmental vulnerability resulted in reliable and informative maps which will serve as a model to determine the critical coastal regions to plan for the conservation and adaptation measures.

Cause of an extreme warm and rainy winter inShanghaiin 2019

AbstractAn extreme warm winter (ranked first during the past 30‐years) occurred in Shanghai in 2019, accompanied with an extreme rainy winter (ranked thirdly in the same period). An observational diagnosis shows that the extreme warm and rainy winter arose from southerly anomalies associated with an anticyclone east of Shanghai. The cause of the anticyclone was attributed to V‐shaped upper‐tropospheric Rossby wave activity fluxes, originated from the North Atlantic and tropical Indian Ocean. Numerical model experiments indicate that the anomalous heat source in the tropical Indian Ocean played a dominant role (~65%) in causing the local anomalous circulation, while the heating in North Atlantic also played a role (~35%). A further analysis of the past 30 year data revealed that an extreme warm winter in the past did not coincide with an extreme wet winter. While the warm winter composite shows a large‐scale anticyclone anomaly over Central and East Asia, the rainy winter composite exhibits a circulation dipole pattern with an anomalous cyclone (anticyclone) west (east) of Shanghai. Numerical experiments confirm that the former was forced by the combined effect of precipitation anomalies over the North Atlantic and tropical Indian Ocean (IO) and Maritime Continent (MC) through Rossby wave energy dispersion, whereas the latter was caused by a dipole heating pattern over the tropical IO/MC sector.

Determinants of zooplankton functional dissimilarity during years of El Niño and La Niña in floodplain shallow lakes

In floodplain lakes, precipitation events shape the environmental heterogeneity and the occurrence of species and functional traits. However, the importance of each biotic and abiotic factor on structuring communities might depend on the local environmental conditions and the identity of such communities. We investigated through Permutational analyses of variance and functional β-diversity components, the effects of neutral and extreme events of precipitation on environmental heterogeneity and distribution of zooplankton functional traits in floodplain lakes. We also examined through distance-based redundancy analysis and fourth-corner analysis, how different variables (physical–chemical, food availability, predation, and temporal dynamics) structure the distribution of zooplankton functional traits and which traits were related to these variables. We used data from six lakes of the Upper Parana River floodplain over 19 years. Environmental heterogeneity and β-diversity were higher during El Nino and lower during La Nina (extreme rainy and extremely dry season, respectively). Physical–chemical were the variables that most explained zooplankton β-diversity during all seasons and predation during both extreme seasons (La Nina and El Nino). We observed a set of multiple biotic and abiotic factors causing complex effects on zooplankton functional diversity. The homogenization of traits—observed during extreme dry seasons—can lead to losses in ecosystem functions and services if these events continue to be more frequent and intense. We reaffirm the importance of natural flood caused by the rainy seasons to maintain high biotic and abiotic heterogeneity and ecosystem processes.

Continuous grazing disrupts desert grass-soil seed bank composition under variable rainfall

Long-term field studies help unveil mechanisms of grass soil seed bank (SSB) persistence and resilience. We wonder whether grazing is a disruptive force that changes grass SSB composition and size, while heavy rainfall is a positive bottom-up force that restores the grass SSB in grazed areas of the Monte desert, Argentina. We used core sampling to study the grass SSB in cattle-free (17 year) and grazed (10 year) grasslands. The SSB size in cattle-free sites usually responded in a continuous positive fashion to rainfall, and the proportion of palatable grasses in the SSB was similar among years. Most grass species formed a transient SSB, but the presence of a stand of perennial plants would prevent their local extinction. In contrast, the SSB size in the grazed habitats showed a threshold response to rainfall (i.e. it did not increase under low-moderate rainfall), the seeds of palatable grasses were always scarce, and the proportion of seeds of less palatable grasses increased under extreme rainy conditions. Seed production from grasses that grew tillers from axillary buds during a drought was zero, suggesting that the bank of plants could have a role in the replenishment of the grass SSB only in mesic years. Within the time span assessed here, continuous heavy grazing together with scarce as well as heavy rainfall were disruptive forces that reduced the number and proportion of seeds from palatable grasses in the SSB, which are also the species preferred by seed-eating animals.

O efeito do relevo nas chuvas na porção central do Estado de São Paulo em anos padrão extremos

A precipitação é um elemento particularmente importante e variável no tempo e espaço, cuja ocorrência de eventos extremos tem potencial de ocasionar impactos significativos à população. Nessa perspectiva, a orografia é um fator que pode contribuir nesse processo desencadeado pela atmosfera, apresentando influências espaciais por meio das particularidades regionais de cada superfície topográfica e geomorfológica. Em vista disso, o presente estudo optou por investigar uma região que envolve 16 municípios localizados na porção central do Estado de São Paulo, inseridos numa área de transição geomorfológica, de relevância ecológica e palco de sucessivas ocorrências de eventos extremos de chuva. Nessa área de estudo, foram empreendidos esforços para definir anos-padrão representativos da dinâmica atmosférica habitual e excepcional da precipitação, realizou-se uma análise espacial das chuvas e selecionou-se os dois anos mais representativos do padrão extremo chuvoso e seco (1983 e 2014, respectivamente) para uma análise detalhada do ritmo climático, buscando avaliar a participação e repercussão do relevo na distribuição das chuvas na região observada. Dessa maneira, verificou-se que o relevo apresentou uma significativa influência nas precipitações, com destaque para chuvas mais intensas em morfoestruturas de serras e cuestas existentes na região, fator que possivelmente continuará influindo na distribuição e intensidade das chuvas sobre a área estudada.

Modelling tools for quantitative evaluations on the Versilia coastal aquifer system (Tuscany, Italy) in terms of groundwater components and possible effects of climate extreme events

The Versilia coastal plain hosts an important and strategic aquifer for water supply. Like all coastal aquifers, it is particularly vulnerable to the saltwater intrusion, which can be amplified not only by fresh water over-exploitation, but also by the effects of climate change, including the increase of extreme events. For an optimal management of this precious resource and for its protection both in quantitative and qualitative terms, an adequate knowledge of the aquifer system is necessary through the development of conceptual and mathematical hydrogeological models. The conceptual hydrogeological model was defined on the base of an integrated multidisciplinary approach with the elaboration of stratigraphic, hydrogeological and geochemical-isotopic data. Subsequently, groundwater flow mathematical models were created using the ModFlow code and Groundwater Vistas like graphical interface, subsequently transferred to the Freewat open platform. The models enabled acquiring further knowledge about this aquifer system and to identify and, where possible to quantify, the main processes and groundwater components involved, including the seawater ingression. An important groundwater component, both in terms of water quantity and quality,resulted widespread in the fan of the Versilia River and mainly fed by the river itself in the foothill zone. Although this component seems to be able to guarantee relative protection against marine ingression, in the summer season some piezometric depressions tied to groundwater exploitation tend to expand and move towards the coast, thus favouring the seawater intrusion process. These issues can be amplified by the extreme rainy events that frequently occur in the Apuan Alps region. The huge quantity of water that quickly flows by the river up to the sea during extreme events represents a lack of feeding respect to the aquifer, and consequently the mitigation role of the fan component towards seawater intrusion can be significantly weakened. Thanks to the water budget achieved by numerical model and considering real extreme events recently occurred in the Apuan-Versilian region it was possible to make considerations about possible effects these climate regimes on the aquifer system. As outcomes, we concluded that extreme events as those occurred in the area in the past, and awaited more frequently in the future, represent a concrete threat for the coastal aquifer system that over next decades could suffer more and more seawater intrusion. Given the reliance of local human activities on groundwater, far-sighted actions of water management (e.g. managed aquifer recharge) are recommended for mitigating such as climate effects.

The El Niño event of 2015–2016: climate anomalies and their impact on groundwater resources in East and Southern Africa

Abstract. The impact of climate variability on groundwater storage has received limited attention despite widespread dependence on groundwater as a resource for drinking water, agriculture and industry. Here, we assess the climate anomalies that occurred over Southern Africa (SA) and East Africa, south of the Equator (EASE), during the major El Niño event of 2015–2016, and their associated impacts on groundwater storage, across scales, through analysis of in situ groundwater piezometry and Gravity Recovery and Climate Experiment (GRACE) satellite data. At the continental scale, the El Niño of 2015–2016 was associated with a pronounced dipole of opposing rainfall anomalies over EASE and Southern Africa, north–south of ∼12∘ S, a characteristic pattern of the El Niño–Southern Oscillation (ENSO). Over Southern Africa the most intense drought event in the historical record occurred, based on an analysis of the cross-scale areal intensity of surface water balance anomalies (as represented by the standardised precipitation evapotranspiration index – SPEI), with an estimated return period of at least 200 years and a best estimate of 260 years. Climate risks are changing, and we estimate that anthropogenic warming only (ignoring changes to other climate variables, e.g. precipitation) has approximately doubled the risk of such an extreme SPEI drought event. These surface water balance deficits suppressed groundwater recharge, leading to a substantial groundwater storage decline indicated by both GRACE satellite and piezometric data in the Limpopo basin. Conversely, over EASE during the 2015–2016 El Niño event, anomalously wet conditions were observed with an estimated return period of ∼10 years, likely moderated by the absence of a strongly positive Indian Ocean zonal mode phase. The strong but not extreme rainy season increased groundwater storage, as shown by satellite GRACE data and rising groundwater levels observed at a site in central Tanzania. We note substantial uncertainties in separating groundwater from total water storage in GRACE data and show that consistency between GRACE and piezometric estimates of groundwater storage is apparent when spatial averaging scales are comparable. These results have implications for sustainable and climate-resilient groundwater resource management, including the potential for adaptive strategies, such as managed aquifer recharge during episodic recharge events.

Climate change impact projections at the catchment scale in Tunisia using the multi-model ensemble mean approach

In this work, we developed a mean projection for climate change and assessed its impact on some hydro-meteorological indicators relevant to climatic condition, precipitation extremes magnitude and frequency for the Siliana catchment in Tunisia based on an ensemble of seven combinations of global circulation models (GCMs) and regional climate models (RCMs) derived from the EU-FP6 ENSEMBLES project. We performed quantile-based mapping (QM) bias correction technique of climate model projection using local observations. Because there is no warranty that the best climate model based on its performances in reproducing historic climate will be superior to other models in simulating future climate, we used the multi-model ensemble (MME) mean approach to derive a mean projection as the best guess for climate change projection for the Siliana catchment. We also quantified the uncertainty of the MME in the projected change in the selected indicators by comparing their values in the reference period (1981–2010) to these in the future period (2041–2070). Results reveal that the Siliana catchment will be prone to drier and warmer climate in the future with less rainy days for each month. The uncertainty associated with the MME projection suggests that no clear general tendency for extreme rainy days in the future is expected. These findings highlight the need to consider an ensemble of multi-climate models with an uncertainty framework if reliable climate change impact study is sought at the catchment scale.

Modelling erosion on a daily basis, an adaptation of the MMF approach

Effect of soil erosion causing negative impact on ecosystem services and food security is well known. On the other hand there can be yearly variation of total precipitation received in an area, with the presence of extreme rains. To assess annual erosion rates various empirical models have been extensively used in all the climatic regions. While these models are simple to operate and do not require lot of input data, the effect of extreme rain is not taken into account. Although physically based models are available to simulate erosion processes including particle detachment, transportation and deposition of sediments during a storm they are not applicable for assessing annual soil loss rates. Moreover storm event data may not be available everywhere prohibiting their extensive use. In this paper we describe a method by adapting the revised MMF model to assess erosion on daily basis so that the effects of extreme rains are taken into account. We couple it to a simple surface soil moisture balance and include estimation of daily vegetation cover changes for calculating rain interception and for estimating effective rain. The runoff fraction is based on the available daily storage of the effective hydrological depth. Unlike the original MMF model which accumulates annual runoff, the daily model accumulates daily runoff from upstream contributing area in a predefined flow network according to steepest slope direction. Annual soil loss is calculated by adding daily erosion rates. We compare the obtained results with that obtained from applying the revised MMF model in two locations: (i) in sub-humid tropics in central Thailand which is affected by deforestation and land cover changes resulting in excessive soil losses, and (ii) in semi-arid environment in Morocco which is affected by severe gully formation. Since the model results are daily estimates it is possible to see the effects of exceptional rain. In Morocco, the effects of extreme rains are clearly shown which were absent in the results obtained by using the annual model. The results also show that erosion rates can be moderate when rainfall pattern is normal without extreme rains in a year although total rain may be similar. This is clearly shown in the erosion assessment in Thailand for the years 2005 (1390 mm) and 2006 (1366 mm, and with the presence of extreme rainy days).

Verification of Soil Moisture Simulating Accuracy on Dry-Land Winter Wheat and Spring Maize Field by EPIC Model on the Loess Plateau of China

EPIC model is an effective tool to simulate soil water which is the key factor to influence the crop production on the Loess Plateau of China. So there is a practical meaning to evaluate the simulation results of soil water. In Loess Plateau amount of rainfall varies greatly throughout the year. So it’s annually and monthly distribution has a great significance for the development of crop production and recovery of soil moisture in different layers. In this study, the accuracy of simulated monthly soil moisture was assessed, based on measured monthly soil moisture (0-2m soil layer) data from 1987 to 1996 at Changwu Agricultural Station. Results showed that RRMSE (Relative Root Mean Square Error) between simulated and measured soil moisture (0-2m soil layer) in winter wheat field and spring maize field was 2.8% and -0.2% respectively, the value for RMSE (Root Mean Square Error) was 0.023m/m and 0.015m/m respectively. The accuracy of simulated soil moisture influenced by the precipitation amount of simulated years were lower in extreme rainfall years (extreme rainy years and extreme drought years) than in other rainfall years. Therefore, the modified EPIC model predicted well soil water in different soil layers and provided basis for the EPIC users to research the law of soil moisture changes in arid cereal land at Changwu arid-plateau. If reasonable crop database, soil database and meteorology database are built up into the EPIC model?the accuracy of simulated soil moisture will be increased.

Late Holocene Storm Records from Lower Reaches of Narmada Valley, Western India

Abstract Storms from the Arabian Sea are the most significant meteorological feature in western India that brings extreme rainy days together with catastrophic flooding. The present study reports two such palaeo-storm horizons at 1.16 m and 3.2 m above the present day water level in the Narmada channel, 56 km inland based on sedimentology and foraminiferal records. Both the horizons show similar sediment facies and foraminiferal assemblage. The present findings instigate to look for such new sites and build palaeo-storm records for western India.

Investigação do Modo Sul em dados de precipitação no período de 1982 a 2006 no estado do Rio Grande do Sul

Este trabalho apresenta o estudo de um modo de variabilidade que influencia a precipitação no Sul do Brasil e é chamado Modo Sul de precipitação. Será mostrado que a ocorrência de máximos (e mínimos) do Modo Sul de precipitação pode estar relacionada à ocorrência de eventos extremos no Rio Grande do Sul, como vendavais, enchentes, granizo e estiagens Utilizando a análise de componentes principais em dados de precipitação diária filtrados na banda 10-50 dias, são encontrados campos espaciais e temporais que representam a máxima variância de determinadas variabilidades, e neles são detectados Modos de Variabilidade de precipitação. Desta maneira, foi possível determinar este modo de variabilidade, que aparece bem configurado na região do Rio Grande do Sul. A série de componentes principais foi usada para a escolha desses eventos. O Modo Sul foi calculado para o período de 01/03/1982 à 31/05/2006. Foi possível identificar que os eventos extremos chuvosos ocorreram em maior número do que os eventos extremos secos. Na análise decadal verificou-se um aumento no número de eventos, quando comparada às décadas de 80, 90 e 2000. Vale ressaltar que o número de eventos da década de 2000 em relação à década de 80 (proporcionalmente) foi o mais expressivo.

Statistical Prediction of Wet and Dry Periods in theComahue Region (Argentina)

General features of rainy season with excess or deficits are analyzed using standardized precipitation index (SPI) in Limay and Neuquen River basins. Results indicate that most of dry and wet periods persist less than three months in both basins. Furthermore, an increase of rainfall variability over time is observed in the Limay river basin but it is not detected in the Neuquen river basin. There is a tendency for wet (dry) periods to take place in El Ni?o (La Ni?a) years in both basins. Rainfall in both basins, have an important annual cycle with its maximum in winter. In addition, possible causes of extreme rainy seasons over the Limay River Basin are detailed. The main result is that the behavior of low level precipitation systems displacing over the Pacific Ocean in April influences the general hydric situation during the whole rainy season. In order to establish the existence of previous circulation patterns associated with interannual SPI variability, the composite fields of wet and dry years are compared. The result is that rainfall is related to El Ni?o- Southern Oscillation (ENSO) phenomenon and circulation over the Pacific Ocean. The prediction scheme, using multiple linear regressions, showed that 46% of the SPI variance can be explained by this model. The scheme was validated by using a cross-validation method, and significant correlations are detected between observed and forecast SPI. A polynomial model is used and it little improved the linear one, explaining the 49% of the SPI variance. The analysis shows that circulation indicators are useful to predict winter rainfall behavior.

Study of the precipitation evolution in Catalonia using a mesoscale model (1971–2000)

Abstract. In this work the MM5 mesoscale model is used in order to analyse the temporal evolution of the precipitation for the period 1971–2000 in Catalonia (NE Iberian Peninsula). Three one-way nested domains with 135, 45 and 15 km horizontal grid resolution and 23 vertical levels have been used. The simulation is performed nesting MM5 into the ERA40 reanalyses. Dynamical nudging is applied to the first domain. However, nudging is not applied in the second and third domains. In order to assess the performance of the developed methodology (main spatio-temporal precipitation characteristics), the results obtained in each simulation are compared with those obtained from ERA40 and observational data. The results show a climatologically reliable distribution of the simulated precipitation spatial patterns for annual, semi-annual, spring and summer precipitation compared to those obtained from 1100 rain gauges covering the whole study area. For winter and autumn the goodness of the results is much lower. Furthermore, the results for 15-km outputs are better than the 45-km ones. The simulations also reproduce well the evolution of annual anomalies for Catalonia and the probability density function (PDF) of monthly mean precipitation. They also improve the precipitation outputs from ERA40, which present an important negative trend and a drier PDF for the period 1971–2000. On the other hand, extreme values are not well reproduced by the simulation. Despite this fact, hydric extremes derived from extreme values (i.e. extreme rainy days and flood records) are well captured by the model.

Dynamics of climatic hazards in northeast Brazil

AbstractDeparture characteristics of the large‐scale circulation are studied in relation to extreme drought and flood years in northeastern Brazil identified from a collective of long‐term rainfall stations and series of river discharge. Ship observations during 1911–72 compiled with a one degree square resolution and extending between 30°N and 30°S from the African coast to the eastern Pacific form a major observational basis.The rainy season of northeastern Brazil is narrowly centred around March/April and is related to the southernmost seasonal migration of a lower‐tropospheric confluence axis over the adjacent eastern tropical Atlantic. the ‘Sěcas’ of Ceará province are characterized by an equatorward expansion of the South Atlantic, and a poleward retraction of the North Atlantic, high ‐ associated with a northward displacement of the enclosed near‐equatorial trough of low pressure. Concurrently, the zonally oriented bands of maximum cloudiness and precipitation frequency stay farther north, the North Atlantic trades weaken, and the South Atlantic trades become stronger than in the long‐term mean. the sea surface temperature pattern during a deficient rainy season in northeast Brazil is characterized by positive departures in a broad band across the North Atlantic and in the eastern Pacific, and anomalously cold waters in most of the South and equatorial Atlantic. During abundant rainy seasons in northeast Brazil, departure patterns are approximately inverse to those typical of drought years.A strong negative linkage exists between northeast Brazil rainfall and sea surface temperature along the Ecuador/Peru coast. This seems to be caused by inverse long‐term pressure variations over the eastern South Pacific and South Atlantic oceans, which in turn may be part of large‐scale mass adjustments of the Southern Oscillation type.Development of the equatorward expansions of the North and South Atlantic highs, pre‐season precipitation in northeast Brazil and the Guayanas, and location of the cloudiness and precipitation belts over the adjacent western tropical Atlantic, are among the more promising predictors for seasonal foreshadowing of extreme rainy season behaviour in northeast Brazil.

Variations in Low-Latitude Circulation and Extreme Climatic Events in the Tropical Americas

Departure characteristics of the general circulation in the tropical Atlantic and eastern Pacific are studied in relation to extreme climatic events identified from collectives of long-term rainfall stations and other hydrometeorological parameters, with emphasis to the Central American-Caribbean region (CARIB). Ship observations during 1911–72 compiled with a 1° square latitude-longitude resolution and extending between 30°N–30°S from the African coast to the eastern Pacific form a major observational basis. The quality of the rainy season in CARIB has a large negative correlation with annual precipitation in the U.S. Central Great Plains, and with rainfall and sea surface temperature along the Ecuadorian/Peruvian littoral; in addition there are weak negative correlations with northeastern Brazil rainfall and with discharge and water level in northern tributaries of the Amazon, and a strong positive correlation with hydrometeorological events in Subsaharan Africa. Northeastern Brazil rainfall is strongly negatively correlated with sea surface temperature along the Eucador/Peru coast, but not with rainfall. During extreme dry years in CARIB the North Atlantic high expands equatorward, meridional pressure gradients steepen, and the trades are stronger, albeit in a somewhat more southward location; at the same time, the ITCZ over the eastern Pacific stays farther south and the South Atlantic high contracts on its equatorward side. For extreme wet years, the reverse departure patterns from the 1911–70 mean maps are characteristic. Sea surface temperature anomalies reflect a response to variations in the subtropical highs and major ocean currents: advection of cold waters in the eastern part of the oceans is favored by equatorward expansion of the subtropical high in the respective hemisphere; the wintertime Gulf Stream system has a distinct signature concomitant with departures in the equatorial and South Atlantic; and in the equatorial eastern Pacific departures reverse from the winter preceding toward the height of an extreme rainy season, warm waters in July/August being characteristic for drought in CARIB. In view of the strong spatial correlations, departure patterns constructed from stratification according to extreme events in CARIB are expected to have more general validity. Anomalous rainy season conditions are signaled in advance by large-scale departure patterns in January/February, thus offering the prospect of foreshadowing extreme rainy season behavior from the setup of low-latitude circulation during the preceding northern winter.