In 2023 and 2024, tropical South America experienced unprecedented, record-breaking droughts and anomalous surface air temperatures, which severely affected fire risk, biodiversity, and socioeconomic activities. Notably, 2024 witnessed an 85% increase in burned area in the Brazilian Amazon compared to 2023. In this context, a deeper understanding of fire responses to climate variability has become a critical research focus. Using regression analyses, this study investigates the influence of sea surface temperature anomalies in the Pacific, Atlantic, and Indian Oceans on rainfall variability and fire occurrence in the Amazon and Brazilian Cerrado regions during the dry seasons of 2023 and 2024. The results indicate that in 2023, the combined effect of El Niño, the tropical North Atlantic (TNA) warming and a positive Indian Ocean Dipole (IOD) intensified and prolonged drought conditions, significantly increasing fire activity into October and November in the northern and central Amazon. In 2024, simultaneous TNA and Indian Ocean Basin-Wide (IOBW) warming were the primary drivers of the drought, with peak fire activity occurring from June to September in the southwestern Amazon, including Bolivia, Paraguay, and central Brazil. Findings indicate that interactions among multiple climate variability modes not only intensify droughts but also alter the temporal and spatial distribution of fires, thereby impacting areas that have historically been less affected. These findings underscore the importance of accounting for climate variability when developing effective public policies for fire prevention and control in the region.

We investigate the components of radiation and energy balance and heat storage flux in an urban area of Central Amazonia, during the wet and dry seasons of 2022. Detailed radiation and turbulent energy fluxes measurements were conducted using a 30-meter micrometeorological tower. The analyses include an assessment of the energy balance closure, incorporating the urban canopy heat storage term. The main findings were: (i) A footprint analysis showed that during the wet season, the primary energy flux sources were from the impermeable surfaces, while in the dry season, in addition to impermeable surfaces, green areas also influenced the fluxes; (ii) Incoming shortwave radiation was significantly higher during the dry season; (iii) Albedo was higher in dry season compared to the wet season; (iv) Latent heat flux showed low sensitivity to seasonal variability, compared to sensible heat flux; (v) Energy balance closure significantly improved with the inclusion of urban canopy heat storage and soil heat flux, highlighting their critical roles in reducing energy imbalances. The measurements presented in this study are the first Eddy Covariance measurements for an urban region of the Amazon. These results are important for urban climate modeling in tropical regions, providing insights into the impacts of urbanization in the Amazon region.

ABSTRACTThe isolated/combined impacts of the positive Indian Ocean Dipole (pIOD) and El Niño (EN) events on precipitation in South America (SA) were investigated during austral winter and spring for the 1901–2012 period, considering both Atlantic Multidecadal Oscillation (AMO) phases. Under the warm phase (WAMO), EN events are well characterised in winter and, in spring, are accompanied by anomalous warming of the Tropical North Atlantic (TNA); thus, variations in the Walker circulation and the northward shifted Intertropical Convergence Zone (ITCZ) reduce precipitation over northern SA. In the cold AMO phase (CAMO), EN events are weak in winter. At the same time, an intense cooling in the equatorial North Atlantic, favoured by the CAMO, enhances moisture transport from the Amazon to central and southeastern Brazil, increasing precipitation in the South Atlantic Convergence Zone region. As EN develops in spring, the anticyclone off the east coast of SA associated with the Pacific‐South American (PSA) pattern decreases (increases) precipitation in central (southeastern) SA. The pIOD events predominantly occur during WAMO phase, when warming in the TNA is favoured by AMO. In winter, the moisture transport to northern SA is weakened, and the ITCZ remains northward shifted, inhibiting the precipitation over northeastern Brazil and southeastern Amazon. In spring, pIOD intensified ascending motions in equatorial Atlantic, increasing precipitation over northeastern Brazil. A wave train from the Indian Ocean strengthens the South Atlantic subtropical high, suppressing precipitation in central and eastern SA. EN‐pIOD events are well established in both AMO phases, though the sea surface temperature anomalies in the TNA depend on the AMO. During WAMO, reduced precipitation in western Amazon and northeastern SA is influenced by the Walker circulation, while in CAMO, TNA cooling enhances moisture transport from the Amazon into southeastern SA, where the PSA pattern and wave train from the Indian Ocean increase precipitation.

The teleconnections between El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and Tropical North Atlantic warming (+TNA) play a critical role in characterizing extreme drought events in the Amazon Basin (AB). This study examines the seven most recent drought extreme events up to 2023, using seasonal composites of the sea surface temperature and atmospheric variables over a five-quarter period starting at the austral spring(−1) of the year preceding that when the lowest water level at Manaus port was recorded. Two distinct patterns emerge, driven by consecutive ENSO events with opposite phases, referred to as cyclic La Niña–El Niño and cyclic El Niño–La Niña drought events. For these events, IOD and ENSO modes are coupled in the same phase during the springs, with ENSO triggering and enhancing IOD, and the IOD, in turn, enhancing and sustaining ENSO through Walker circulation. This interaction can amplify extratropical Rossby waves in both hemispheres, originating from the equatorial Indian and Pacific Oceans. Notably, strong positive ENSO and IOD phases trigger or sustain the +TNA by weakening northeasterlies through a Pacific-North America wave train. The IOD, ENSO and +TNA, individually or combined, influence atmospheric circulation patterns over South America through Rossby waves and anomalous Walker and Hadley circulation patterns, causing dry periods in the AB marked by negative precipitation anomalies across specific regions or the entire AB, consequently modulating the water levels at Manaus. The strong coupling of these three tropical modes is crucial to leading multiyear drought events in the AB. This study underscores the potential for a robust climate forecasting system by monitoring the oceanic indices.

The precipitation trend patterns in South America (SA) are determined using trend empirical orthogonal function analysis for the 1951–2016 period. The associated large‐scale tropical and extratropical anomalous circulation patterns are also examined. The words “total” and “residual” refer to the monthly anomalies and monthly anomalies without the El Niño–Southern Oscillation (ENSO) effects, respectively. The total precipitation features a positive trend in southeastern SA (SESA, southern Brazil, Uruguay, most of eastern Argentina) and northern Chile, and a negative trend over central‐eastern Brazil and central Amazonia. The residual precipitation shows an increased positive trend over most of the coastal extension of northern SA and Colombia; a weak positive trend over southern Brazil, northeastern Argentina, and northern Chile; and a negative trend over central‐eastern SA and western Amazonia. The differences between the total and residual precipitation trend patterns in tropical SA is explained as responses to total and residual zonally asymmetric anomalous sea surface temperature (SST) patterns, respectively. The total SST pattern along the equatorial Pacific configures the Pacific Decadal Oscillation, which impacts ENSO variability and as response intensifies the Walker circulation. Without the ENSO, the Walker cell is mainly driven by the tropical Indian and Atlantic Oceans, which configure residual asymmetric anomalous warming. Furthermore, the warming in the equatorial Indian and eastern Pacific Oceans, in the presence of ENSO, induces a Rossby wave train‐type anomalous pattern that extends across the South Pacific into SA and modulates the atmospheric anomalous circulation over SESA. In this region, an anomalous anticyclonic accompanied by an intensified South American Low‐Level Jet induces a moisture transport to SESA. This anticyclone is also observed in the absence of ENSO but is weaker. The results suggest the importance of ocean warming in the western Pacific‐Indian in the modulation of extratropical teleconnections to SESA in the tropical ocean warming scenario.

The seasonal precipitation contrasts in South America (SA) associated with two types of multiyear El Nino-Southern Oscillation (ENSO) events - reintensified and persistent - during the period 1901-2012 were investigated. These multiyear events differ in the timing of the maximum anomalies in Sea Surface Temperature (SST) in the central tropical Pacific Ocean during the austral summer of the first year (Y1) relative to the second year (Y2). These SST differences drive or couple with other modes of climate variability in the adjacent oceans, modifying in different ways the Walker circulation. For El Nino (EN), intensification [persistence] starts in the winter [autumn] following Y1 summer, when the strengthened [weakened] northeast [east] trade winds couple with the Walker circulation leading to the strengthening of subsidence in Indonesia and the intensification [persistence] of warming in the central Pacific until Y2 summer. In response, precipitation anomalies in SA during Y1 seasons exhibit different [similar] positioning and are more intense compared to the Y2 seasons during reintensified [persistent] events. For both events, dry conditions in northern and northeastern SA are modulated by the position and intensity of the descending branch of the Walker circulation. The most severe dry conditions occur in the Y1 summer, but they are more intense and with larger coverage during persistent events when EN is more intense. Wet conditions show substantial spatial variability in central eastern and southern SA and are associated with changes in regional atmospheric circulation and Rossby wave trains. Considering the linearity of ENSO, in the sense that EN and La Nina (LN) have nearly opposite effects, our results are also valid for LN events, but with reversed sign of the above-described precipitation and atmospheric circulation anomaly patterns. So, the results can be valuable for climate modeling, prediction, and monitoring.

Gridded precipitation (PRP) data have been largely used in diagnostic studies on the climate variability in several time scales, as well as to validate model results. The three most used gauge-based PRP datasets are from the Global Precipitation Climatology Centre (GPCC), University of Delaware (UDEL), and Climate Research Unit (CRU). This paper evaluates the performance of these datasets in reproducing spatiotemporal PRP climatological features over the entire South America (SA) for the 1901–2015 period, aiming to identify the differences and similarities among the datasets as well as time intervals and areas with potential uncertainties involved with these datasets. Comparisons of the PRP annual means and variances between the 1901–2015 period and the non-overlapping 30-year subperiods of 1901–1930, 1931–1960, 1961–1990, and the 25-year subperiod of 1991–2015 for each dataset show varying means of the annual PRP over SA depending on the subperiod and dataset. Consistent patterns among datasets are found in most of southeastern SA and southeastern Brazil, where they evolved gradually from less to more rainy conditions from 1901–1930 to the 1991–2015 subperiod. All three datasets present limitations and uncertainties in regions with poor coverage of gauge stations, where the differences among datasets are more pronounced. In particular, the GPCC presents reduced PRP variability in an extensive area west of 50° W and north of 20° S during the 1901–1930 subperiod. In monthly time scale, PRP time series in two areas show differences among the datasets for periods before 1941, which are likely due to spurious or missing data: central Bolivia (CBO), and central Brazil (CBR). The GPCC has less monthly variability before 1940 than the other two datasets in these two areas, and UDEL presents reduced monthly variability before 1940 and spurious monthly values from May to September of the years from 1929 to 1941 in CBO. Thus, studies with these three datasets might lead to different results depending on the study domain and period of analysis, in particular for those including years before 1941. The results here might be relevant for future diagnostic and modelling studies on climate variability from interannual to multidecadal time scales.

Esta pesquisa teve como objetivo analisar os eventos extremos diários de precipitação (EEDP) ocorridos em Manaus, AM, e sua relação com eventos de El Niño-Oscilação Sul (ENOS). Foram utilizados dados diários de precipitação para o período de 1979–2019. O cálculo dos quantis foi empregado aos dados e os EEDP foram definidos com base no percentil de 90% das séries diárias de precipitação separadas sazonalmente. Variações interanuais e tendências de longo prazo na ocorrência de EEDP também foram avaliadas. O teste de Mann-Kendall e o Método de Sen foram utilizados para avaliar a tendência dos EEDP. Foram calculadas as correlações lineares com os padrões oceânicos e atmosféricos a partir das anomalias mensais de Temperatura da Superfície do Mar tropical global e do fluxo de umidade integrado verticalmente e de sua divergência. A frequência e intensidade dos EEDP apresentaram comportamento similar ao da precipitação total sazonal, com acentuada variabilidade interanual. Maiores ocorrências e frequência de EEDP ocorreram em Março–Maio. A intensidade dos EEDP não mostrou grande diferença entre as fases do ENOS quando se compara o intervalo inter-quartil. Quando observamos a mediana, os anos de La Niña (LN) modulam a intensidade dos EEDP durante suas fases de desenvolvimento, madura e final. Tendência de aumento de EEDP ocorreu no trimestre de Dezembro–Fevereiro. Foi observado que o Atlântico Tropical Norte é uma das principais fontes no transporte de umidade para a região. Este estudo mostrou que a variabilidade da precipitação sazonal e eventos extremos estão associadas às condições globais de TSM.

The mechanisms associated with the transitions of strong El Ni & ntilde;o (EN) events and their implications for the South American precipitation were investigated for the 1950-2023 period. Strong EN events exhibit cyclic or episodic characteristics in their transitions. Cyclic EN events are both preceded and followed by La Ni & ntilde;a (LN) conditions, whereas episodic EN events are preceded by neutral conditions, with a more uncertain transition following. For cyclic EN, tropical Pacific mechanisms initiates and peak warming in the eastern tropical Pacific from austral winter to early summer. In contrast, for episodic EN, coupled subtropical and tropical Pacific mechanisms, respectively, initiate and peak warming in the central tropical Pacific from autumn to late summer. The Pacific Decadal Oscillation (PDO) mean state modulates EN's decay stage. During the +PDO mean state, cyclones in the eastern subtropical Pacific of both hemispheres sustain the warming of episodic EN, whereas during the -PDO mean state, anticyclones in the eastern subtropical Pacific accelerate the decay of cyclic EN, favouring its transition to an LN. These mechanisms explain why episodic EN initiates earlier, peaks later, is more intense and decays more slowly than cyclic EN. During an episodic EN summer, the strengthened atmospheric circulation maintains the Atlantic Intertropical Convergence Zone (ITCZ) north of the equator, causing persistent negative precipitation anomalies in north-northeastern South America (SA) until the following winter, while positive precipitation anomalies in southeastern SA are driven by south-southeastward moisture transport from equatorial Atlantic. Conversely, during a cyclic EN summer, negative (positive) precipitation anomalies impact north-northwestern (southeastern) SA; however, the anomalous atmospheric circulation and precipitation in SA quickly return to normal conditions in the autumn, and positive precipitation anomalies appear in northern SA in the following winter. Understanding these mechanisms is crucial for predicting EN's future changes and, consequently, their potential socio-economic impacts globally.

Studies related to monitoring changes in frequency, intensity and duration of precipitation extremes are key to creating efficient climate change measures and forest conservation policies. This study describes new insights into rainfall precipitation extremes over the Amazon basin (AB) during the last four decades (1981-2021) from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPSv2). Here we analysed the trends of daily extreme precipitation indices proposed by the Expert Team on Climate Change Detection and Indices (ETCCDI) at the seasonal scale, using the trend-empirical orthogonal function (TEOF). Our results indicate that the frequency of precipitation extremes increased over Peruvian Amazonia and northeastern Brazilian Amazonia, and reduced in the centre of AB, mainly during the first seasons of the year: December-January-February (DJF) and March-April-May (MAM). The cooling trend over the eastern and central tropical Pacific and the warming trend over the tropical and western subtropical Pacific could associate with the increase in frequency of precipitation extremes in DJF. Furthermore, during June-July-August (JJA) and September-October-November (SON), rainfall intensity indices showed a decrease in Colombia and the Bolivian Amazon; in contrast, northern and southern Peru delivered an increased pattern. The trend pattern in the JJA and SON seasons could be associated with a warming trend over most of the North Atlantic and a cooling in the 40 degrees-60 degrees S band. Our results demonstrate that the precipitation extremes over the AB have spatially varying trends. These heterogeneous trends over the space might be take into account for robust adaptation policies over the countries that are parts of the AB, such as Bolivia, Brazil, Colombia, Ecuador, Guyana, Per & uacute;, Surinam and Venezuela. The trend-empirical orthogonal function (TEOF) approach allowed us to identify the spatiotemporal patterns of trends in extreme rainfall intensity and frequency indices over the Amazon basin and their differential relationship with trends in large-scale climate variability phenomena. image

A Amazônia tem enfrentado incêndios florestais com emissões de poluentes para a atmosfera, impactando na qualidade do ar e na saúde humana. Manaus se destaca como uma importante área de estudo para avaliar o impacto das queimadas na qualidade do ar. A regulamentação brasileira recomenda o limiar máximo de 25 µg/m3 (em 24h) para o Material Particulado fino (MP2.5), para que o ar seja considerado de boa qualidade. Assim, o objetivo deste trabalho foi estudar a variabilidade temporal do MP2.5 na atmosfera de Manaus, durante o período de 2003 a 2021, a fim de verificar se os níveis de poluição do ar estavam dentro dos limites recomendados. Os dados foram avaliados utilizando análises de diagramas de caixa, considerando o período total do estudo como referência, para avaliar dois períodos de distanciamento social para o combate a COVID-19. Os resultados mostram que o MP2.5 apresenta um ciclo anual bem definido, com concentrações menores do que 25µg/m^3 no período chuvoso. No período seco, as concentrações de MP2.5 atingem valores superiores ao máximo recomendado. Durante isolamento social foi possível observar uma diminuição na concentração média mensal, que pode estar associada à diminuição das atividades industriais e a redução na mobilidade urbana

The present study examines the effects of the central Atlantic Ni & ntilde;o (CAN) and eastern Atlantic Ni & ntilde;o (EAN) events on the seasonal precipitation in South America (SA) during the 1951-2020 period. For the CAN during the summer and autumn, an interhemispheric sea surface temperature (SST) dipole mode induces an anomalous thermally direct circulation in the 10 degrees N-10 degrees S band and is the main factor causing precipitation anomaly patterns with a dipole structure between northern (negative) and northeastern (positive) SA. For winter and spring, the SST pattern featuring a South Atlantic dipole induces meridional and zonal anomalous circulations, which are the mechanisms causing positive precipitation anomalies in tropical SA to the north of 20 degrees S. In contrast, for the EAN, the precipitation anomaly patterns show large areas with anomalous dryness, particularly during summer, autumn, and spring. For summer and autumn, the east-west SST anomaly gradient in the equatorial Atlantic and the associated sea level pressure (SLP) anomalies induce equatorial westerlies and a regional Walker cell with descending motions in most tropical SA, where large areas with anomalous dryness are noted. During spring, a northward SST gradient in the tropical Atlantic induces a meridional cell with descending motions in the 0 degrees-10 degrees S band; meanwhile, the westward SST gradient in the equatorial Atlantic and tropical South Atlantic induces a zonal circulation with descending motions over northeastern and eastern Brazil. These descending motions extend the anomalous dryness over a large area. For the EAN events, the east-west SST gradient and the associated east-west circulation in the South American/Atlantic region are crucial elements to modulate precipitation variability in SA. Therefore, the CAN and EAN events induce distinct precipitation anomaly patterns in SA due to distinct associated regional circulation patterns. The results presented here have not been discussed before and might have relevant implications for climate monitoring and modelling studies. Central Atlantic Ni & ntilde;o (CAN) and eastern Atlantic Ni & ntilde;o (EAN) events have distinct effects on the precipitation in South America (SA). The CAN events contribute to the increase of seasonal precipitation in extensive areas of tropical SA. On the other hand, the EAN events contribute to drying out large areas of this continent. This contrast is particularly noticeable in spring.image

This study examines the Interdecadal Pacific Oscillation (IPO) modulation of the El Niño–Southern Oscillation (ENSO) teleconnections in its decaying stages with the tropical ocean by focusing on the Indian Ocean Basin‐Wide (IOBW) mode and the precipitation over South America (SA) in the 1901–2012 period. Composite analyses revealed that the ENSO teleconnections are IPO‐modulated due to the differential ENSO decaying speed, which is slower during the positive than negative IPO phase, for both El Niño (EN) and La Niña (LN) cases. Negative precipitation anomalies related to EN persist over northeastern SA until austral winter for the positive IPO phase (POS IPO), while significant opposite sign anomalies occur in this region for the negative IPO phase (NEG IPO). These results are associated with the Walker circulation's reversal during NEG IPO which is, in turn, accompanied by negative IOBW. During the POS IPO, the positive IOBW causes upward movements over there and, by continuity, downward movements over SA. In the NEG IPO, for LN events the wave train originating in the north of Australia propagates toward subtropical SA, which, coupled with the surface circulation, causes dryness in this region. In addition, the rapid decay of LN in the NEG IPO, followed by the emergence of EN, caused changes in the Walker circulation, such that enhanced upward movements occurred over the Pacific and SA region, and downward movements over the Indian Ocean until austral winter. In turn, the slower decay of the LN in the POS IPO maintains strong subsidence over the central Pacific and weak upward motions over western SA. So, the EN (LN) and positive (negative) IOBW during the POS (NEG) IPO prolong the scarcity of precipitation over equatorial (subtropical) SA. Persistent dry periods over these regions during the ENSO decaying stage might have important implications for the seasonal forecasts.

Differences in the seasonal distribution of precipitation over South America (SA) associated with single‐year (SY) and multiyear (MY) El Niño (EN) events were analysed based on reanalysis data for the 1901–2012 period. The results suggest that Pacific Sea Surface Temperature (SST) anomalies associated with MY EN events interact with the tropical Atlantic and Indian Oceans SST from austral fall to spring, after the event's first year, affecting SA precipitation distribution in subsequent seasons. Compared to SY EN events, which reproduce well the north–south dipolar precipitation anomaly pattern over SA, the MY EN events show differences in the intensity and positioning of precipitation anomalies. Precipitation decreases over northern SA during all seasons are observed for SY and MY EN events except for differences in magnitudes. Variations in the position and longitudinal extension of the downward motions of Walker circulation in response to these events explain these differences. Over southern and southeastern SA, differences in anomaly positioning are more evident. The positive precipitation anomalies over these regions in the austral spring and summer are weakened and southward shifted during the MY EN in relation to those during SY EN events. These variations are associated with the Rossby‐wave train pattern path that depends on the EN and season. Consequently, the associated local atmospheric circulation patterns also depend on the season. The intense (weak) South American low‐level jet (SALLJ) for the first year of MY EN contrasts with the weak or inexistence (intense) SALLJ for the SY EN during summer (springer). Complementary to previous studies, the results indicate that the differences in the intensity and duration of the SY and MY EN events contribute to changes in the EN‐related atmospheric teleconnection pattern that impacts SA precipitation. The results can be useful for climate prediction and monitoring purposes.

The effect of multiyear La Nina (LN) events on precipitation in South America (SA) was assessed considering 10 persistent LN events over two successive years, referred to as Y1 and Y2 for the 1901-2012 period. Y1 spans from the austral winter of the first year to autumn of the second year, and Y2 spans from the austral winter of the second year to autumn of the third year. Comparisons were performed season by season of the Y1 and Y2. Composites revealed that the teleconnections related to a multiyear LN event during its Y1 and Y2 years, responsible for distinct seasonal precipitation anomaly patterns in SA, were associated with different tropical ocean conditions. In spring, the negative sea surface temperature (SST) dipole in the Indian Ocean during the Y2 was not observed during Y1. Different LN-related SST anomaly patterns in the tropical Atlantic between Y1 and Y2 occurred in the other seasons. Over northern/northeastern SA, the positive precipitation anomalies became weaker (stronger) during austral summer and autumn (winter and spring) of the Y2 than Y1 and were associated with changes in the Walker cells. During austral spring and summer, southeastern presented drier conditions during the Y2 than Y1. In the spring of Y2, two Rossby wave trains, one associated with the LN-related anomalous cooling in the equatorial Pacific and another triggered by the upper-level anticyclone in the tropical Indian Ocean, characterized the circulation pattern over SA which explains the difference in precipitation anomalies between the Y2 and Y1. These drier (wetter) conditions during austral spring and summer (winter and spring), particularly over southern and southeastern Brazil (Colombia) in the Y2, might have more severe effects on the regional hydrological cycle than those in the Y1. The results here indicate that accurate prediction of LN duration is crucial in a climate-monitoring context.

In this research, we explored rainfall variability in the Sogamoso River Basin (SRB), its relationship with multiple scales of variability associated with El Niño–Southern Oscillation (ENSO), and the implications for rainfall prolongation during multiyear La Niña events. First, we examined time-frequency rainfall variations in the SRB based on the standardized precipitation index (SPI) from 1982 to 2019, using wavelet transform and principal component analysis (PCA). In addition, we applied wavelet analysis to investigate the links at different time scales between ENSO and the main mode of rainfall variability in the SRB. Finally, we explored the role that each scale of variability played in the prolongation and intensity of rainfall in the SRB during the 1998–2000 and 2010–2012 multiyear La Niña events. The results of the wavelet analyses revealed significant ENSO relationships affecting SRB rainfall at three different scales: quasi-biennial (2–3-years) between 1994 and 2002, as well as from 2008 to 2015; interannual (5–7 years) from 1995 to 2011; and quasi-decadal (9–12 years) from 1994 to 2012. This indicates that multiyear events are a consequence of the interaction of several scales of variability rather than a unique scale. During the 1998–2000 event, El Niño conditions were observed during the first half of 1998; subsequently, a cooling of the central and eastern tropical Pacific (western tropical Pacific) on the quasi-biennial (interannual) scale was observed during 1999; in 2000, only La Niña conditions were observed on the interannual scale. Therefore, during this event, the quasi-biennial (interannual) scale promoted wet conditions in the Caribbean, the Andes, and the Colombian Pacific from June–August (JJA) 1998 to JJA 1999 (during 1999–2000). During the 2010–2012 La Niña event, the interbasin sea surface temperature gradient between the tropical Pacific and tropical North Atlantic contributed to strengthening (weakening) of the Choco jet (Caribbean low-level jet) on the quasi-biennial scale during 2010, and the interannual scale prolonged its intensification (weakening) during 2011–2012, acting to extend the rainy periods over most of the Colombian territory. Variations on quasi-decadal scales were modulated by the Pacific decadal oscillation (PDO), resulting in a further intensification of the 2010–2012 La Niña event, which developed under conditions of the cold PDO (CPDO) phase, whereas the 1998–2000 La Niña occurred during the transition from warm (WPDO, 1977–1998) to cold (CPDO, 2001–2015) conditions. These results indicate that the interaction of quasi-biennial to quasi-decadal scales of variability could play a differential role in the configuration and prolongation of rainfall events in the SRB.

This work proposes an adaptation of the method developed in previous papers to determine the onset and demise of the rainy season (ONR and DER) dates in the areas of the South American monsoon system (SAMS) based on the pentad antisymmetric outgoing long‐wave radiation (AOLR). In those papers, the sign change of the mean AOLR in the central Amazon Basin (CAM) and western central Brazil (WCB) from positive to negative defined the ONR, from negative to positive, the DER dates. Since the monsoon convection presents a northwest–southeast oriented progression, the antisymmetric area to the WCB was selected subjectively. Thus, here we propose to use the Ward hierarchical clustering method to select areas in the SAMS and in the northern tropical America (NTA) for the regionalized AOLR calculation. The significant (at the 95% confidence level) negative correlations with the largest magnitude among the clusters in the SAMS and NTA and the outgoing long‐wave radiation (OLR) and precipitation annual cycles in each group define the pairs to calculate the AOLR. Then, the AOLR time series is calculated and subjected to a 5‐pentad running mean filter. This method keeps the climatological features of the convection annual cycle such that the closer (farther) the pair is to the equator the longer (shorter) the rainy season. The ONR and DER dates found with this new method are remarkably close to those found previously. Therefore, the new method proposed here highlights regional aspects of rainy season and can easily be automatized for its routine application at the operational climate monitoring centres, for instance at INPE. This is the most important advantage of the method and might be relevant to the SAMS rainy season monitoring.

The Andes mountain range divides Colombia into various climatic regions over the country, as the Andean, Caribbean, Pacific, Amazon, and Orinoco regions. Given this scenario, knowing the current change in total precipitation and their extremes values are relevant. In this study, the main goal is to assess the spatio-temporal trends of heavy and intense rainfall at a seasonal scale during the last 38 years (1981–2018) using the trend empirical orthogonal function (TEOF). An increase in maximum precipitation during five consecutive days (RX5day), Simple daily intensity index (SDII), and the number of days with precipitation above 20 mm (R20mm) and 30 mm (R30mm) during December–February and March–May was observed in most of the Colombian territory, except for the Amazon region for RX5day. A decrease in total rainfall in June–August was observed in the Andean, the Caribbean, and southern Pacific regions, while, in the northern Pacific, it increased, consistent with the trend patterns of RX5day, SDII, and R20mm. During September–November, there was a reduction in rainfall in the Amazon region and the South Pacific, and an increase in RX5day, SDII, R20mm, and R30mm in the Andean, the Caribbean, and North Pacific regions. The TEOF showed more pronounced spatial trend patterns than those obtained with the traditional Mann–Kendall test. The findings offer a better understanding of the climate extremes impacts in tropical latitudes and help planners to implement measures against the future effects of climate change.

In this study, we show the complexity associated with the recent land cover changes by elucidating the paths of 30 years of changes in the Upper Paran ' a River Basin (UPRB), a region severely impacted by agricultural activity, one of the areas with the highest density in the production of hydroelectricity, biofuels and food in the world. In this sense, a post-classification comparison approach based on Landsat images was used to identify detailed 'from-to' paths behind those land cover changes. The most expressive changes were the expansion of Cropland and Forest areas and the reduction in savannas, with a net change of 17.9%, 4.1%, and-16.9% of the UPRB area, respectively. Cropland areas showed an expressive increase between 1985 and 2015, rising from 249,439 km2 (27.7%) to 412,909 km2 (45.9%). Forest areas increased from 149,389 km2 to 185,839 km2 in the period. Notably, for this class, an intense spatial dynamic of losses (7.5%) and gains (11.6%) took place between 1985 and 2015. This behavior is related to the disappearance of native vegetation fragments in some sub-basins, as well as to afforestation, reforestation, and/or forest restoration in others. The Cerrado (a typical tropical savanna in South America), the most impacted natural biome of the Basin, decreased from 21.9% of the UPRB in 1985 (196,746 km2) to only about 5% of the whole UPRB area in 2015. Grassland areas, mostly used for livestock, decreased from 271,827 km2 (30.2%) to 229,007 km2 (25.5%). This net decrease was associated with a reduction of 160,830 km2 (17.8%) and the appearance of 118,010 km2 (13.2%) in new areas, previously occupied by tropical savannas in 1985. In conclusion, economic factors were the main drivers for land cover changes, especially agriculture and livestock activities, besides forestry and hydroelectric energy production. In addition, Grassland areas that predominated on the left banks of the UPRB in 1985 retreated with the advance of Cropland areas, mainly due to the expansion of sugarcane for ethanol production, a biofuel widely used in Brazil. In turn, pasture areas migrated to the right bank and occupied a significant part of the Cerrado. Finally, our results demonstrate that the transition dynamics among land cover classes can involve complex political-economical mechanisms that are not always captured by remote sensing.

Previous studies have shown that the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) have combined effects on the precipitation (PRP) variability over South America. The combined impacts have been assessed considering four mean states as the averages of the variable anomalies during sub-periods overlapping time intervals of the PDO and AMO phases. Since these sub-periods include years under El Niño-Southern Oscillation (ENSO) extremes, the extent to which these years’ occurrence affects the averaged anomaly patterns during different mean states is investigated. The analyses are done for the PRP and surface air temperature (SAT) during the austral winter (June to August) and summer (December to February) of the 1901–2014 period using a composite technique. The nonlinear ENSO response in each mean state for a variable corresponds to the sum of the anomaly composites of the El Niño and La Niña events. In each mean state, the nonlinear PRP and SAT anomalies are not negligible and show similar patterns of the corresponding mean state, with larger magnitudes. For both seasons and all mean states, these similarities are more pronounced for SAT than for PRP. Thus, the ENSO variability affects the mean state’s PRP and SAT anomaly patterns in different ways. As far as we know, analyses of the nonlinear ENSO response of the South American climate during distinct mean states were not performed before. Our results also indicate that the ENSO variability should be considered in the studies of the low-frequency modes and their effects on the mean state over South America. The results presented could be relevant for climate monitoring and modeling studies.