Search

Copyright of Environment & Urbanization is the property of Sage Publications Inc. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Recent severe droughts, extreme floods, and increasing differences between seasonal high and low flows on the Amazon River may represent a twenty-first-century increase in the amplitude of the hydrologic cycle over the Amazon Basin. These precipitation and streamflow changes may have arisen from natural ocean-atmospheric variability, deforestation within the drainage basin of the Amazon River, or anthropogenic climate change. Tree-ring reconstructions of wetseason precipitation extremes, substantiated with historical accounts of climate and river levels on the Amazon River and in northeast Brazil found in the Brazilian Digital Library, indicate that the recent river-level extremes on the Amazon may have been equaled or possibly exceeded during the preinstrumental nineteenth century. The "Forgotten Drought" of 1865 was the lowest wet-season rainfall total reconstructed with tree-rings in the eastern Amazon from 1790 to 2016 and appears to have been one of the lowest stream levels observed on the Amazon River during the historical era according to firsthand descriptions by Louis Agassiz, his Brazilian colleague Joao Martins da Silva Coutinho, and others. Heavy rains and flooding are described during most of the tree-ring-reconstructed wet extremes, including the complete inundation of "First Street" in Santarem, Brazil, in 1859 and the overtopping of the Bittencourt Bridge in Manaus, Brazil, in 1892. These extremes in the tree-ring estimates and historical observations indicate that recent high and low flow anomalies on the Amazon River may not have exceeded the natural variability of precipitation and streamflow during the nineteenth century. [ABSTRACT FROM AUTHOR]

Understanding the hydroclimate representations of precipitation δ18O (δ18Op) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall and δ18Op, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of such δ18Op dipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to the δ18Op depletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to the δ18Op enrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship between δ18Op and rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennial δ18Op change in the central-western Amazon, while δ18Op does not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when using δ18Op as a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennial δ18Op dipole response and pose a challenge in reconciling the discrepancy. Significance Statement We want to comprehensively understand the hydroclimate footprints of δ18Op and the mechanisms of the millennial variability of δ18Op over tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennial δ18Op change in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it. [ABSTRACT FROM AUTHOR]

Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitation δ18O (δ18Op) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20–11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reasonably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA’s millennial hydroclimate variability. The spatial pattern of both precipitation and δ18Op show a northwest–southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteristics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model–proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon. Significance Statement We want to reconcile the controversy on whether there is a coherent or heterogeneous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mechanisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/δ18Op and monsoon intensity change. We highlight a heterogeneous dipole response in precipitation and δ18Op on millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydroclimate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central-west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future. [ABSTRACT FROM AUTHOR]

The land–atmosphere interactions play an important role in modulating climate variability at different spatial and temporal scales. In South America, two recognized hot spots of soil moisture–atmosphere coupling are located in southeastern South America (SESA) and eastern Brazil. Soil moisture variability may not only alter the climate locally but may also have nonlocal impacts through changes in the regional circulation. Here we explore how these two local coupling hot spots interact with each other, how soil moisture variability modulates the regional circulation, and what is the consequent nonlocal impact on precipitation. To this end, we analyze numerical experiments, performed with a regional climate model for the period October–March of 1983–2012, that allow us to isolate the influence of the soil moisture interannual variability on the regional climate. When the soil moisture–atmosphere interaction is enabled, we find a nonlocal coupling mechanism that links both hot spots at different temporal scales, favoring precipitation in eastern Brazil to the detriment of the precipitation in SESA through shifts in the regional circulation, when compared with a simulation with constrained soil moisture–atmosphere interaction. In northeastern Argentina, a subregion of SESA located at the exit of the South American low-level jet, it was found that the amount of nighttime precipitation is modulated by the proposed nonlocal coupling mechanism. A better understanding of the variability of precipitation due to the influence of land–atmosphere interaction processes may contribute to improving the predictability of precipitation and the interpretation of climate projections. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper examines the climate variability modes in the South American/Atlantic sector accompanying dry and wet years over north-eastern Brazil ( NEB) in which the tropical Pacific and Atlantic oceanic-atmospheric conditions usually associated, respectively, with dryness and wetness are absent. The analyses are for several variables and take into account the warm and cold Atlantic Multidecadal Oscillation ( AMO) phases ( WAMO and CAMO). Four cases are analysed: DRY-WAMO, DRY-CAMO, WET-WAMO and WET-CAMO. The main difference in the sea surface temperature ( SST) anomaly patterns in the tropics between the AMO phases responsible for the differences in the precipitation anomaly patterns is the differential longitude positioning of the warming or cooling of the surface waters in the equatorial Atlantic. In consequence, the anomalous Atlantic Hadley and Walker circulations also show differences between the AMO phases, which justify the observed precipitation anomalies over tropical South America for the analysed cases. The strong anomalous Atlantic Hadley cell determines the dipolar structure of the precipitation anomalies between NEB and northern South America for the DRY-WAMO and WET-WAMO cases. The strong anomalous Atlantic Walker cell defines the dryness over NEB for the DRY-CAMO, and both the strong anomalous Atlantic Hadley and Walker cells act together to yield an anomalous dry condition over NEB and wet condition over northern South America. Therefore, the results here provided clear indications that for climate monitoring and forecasting tasks, the AMO phases should be considered. These are new aspects of the tropical Atlantic variability that might be useful for climate monitoring purposes. [ABSTRACT FROM AUTHOR]

Some teleconnection patterns characterized through various hydro‐climate indices (HCIs) have been shown to influence regional rainfall regimes over a range of time scales in Brazil. However, our current knowledge of how various HCI–rainfall relationships vary across regions in Brazil is still incomplete. In this study, simple correlation analysis is performed using sliding window sizes and lag times on the order of months to years to substantially reduce the effects of high‐frequency variability inherent in HCI and precipitation time series and reveal any lower‐frequency relationships that may exist. This analysis is applied to monthly rainfall and HCI data during the time‐span of 1961–2015. The strength and significance of each correlation were tested. HCIs considered in this analysis include those characterizing the El Niño–Southern Oscillation (ENSO), those characterizing the Madden–Julian Oscillation (MJO), as well as the Tropical Southern Atlantic index (TSA), the Caribbean index (CAR), and several others. A cluster analysis based on the k‐medoids algorithm was also applied to explore the relationship between local factors and the distribution of HCIs exhibiting maximum correlation at each site. A 60‐month sliding window and a 12‐month lag time were found to optimize the results of the correlation. The MJO was found to have a strong and significant link to annual rainfall throughout much of eastern Brazil, while other HCIs showing strong influence in other regions include ENSO, the CAR, and the TSA. The cluster analysis revealed a more spatially homogeneous response over the Amazon River basin and the southern regions than other regions of Brazil with complex topography, where the orographic effect attenuates the influence of the main atmospheric mechanisms at a local scale. In general, the rainfall–HCI relationship could be exploited for some operational applications (e.g., a regional drought early warning system), where the strongest correlations were observed. [ABSTRACT FROM AUTHOR]

This study provides an updated dynamical analysis of the observed climate conditions during the longest, most severe and continuous drought (2012–2016) ever recorded in Northeast Brazil (NEB). This investigation was carried out based on temperature, precipitation, sea surface temperature, outgoing long-wave radiation, and ERA-Interim reanalysis data from 1981 to 2016. This was done by first looking for the area of the NEB where the drought was most severe, and then calculating the main rainy season anomalies and atmospheric parameters from 2012 to 2016 in relation to the climatological conditions (1981–2010). Results show this drought event was influenced mainly by two distinct oceanic conditions that induced anomalous atmospheric circulation patterns. The observed negative precipitation anomalies were firstly related to the northward displacement of the Intertropical Convergence Zone (ITCZ), followed by an anomalous upward motion over the Western Amazon, as well as the anomalous precipitation deficit along the South Atlantic Convergence Zone, that was slightly displaced southward of the climatological position. From 2015 to 2016 the subsidence over NEB Brazil was linked to the descending branch of the Walker circulation, as well as to the strong anomalous flow induced by convection in the ITCZ over Africa and Gulf of Mexico. [ABSTRACT FROM AUTHOR]

Copyright of Revista Brasileira de Engenharia Agricola e Ambiental - Agriambi is the property of Revista Brasileira de Engenharia Agricola e Ambiental and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

ABSTRACT Severe droughts have caused serious impact on water supply and agriculture in São Francisco River Basin ( SFRB), Brazil, especially during the rainy season. Observational evidence suggests that droughts in this region could be driven by some large-scale ocean-atmospheric patterns. This study provides a general description of linkage between the ocean-atmospheric circulation patterns and the droughts during the rainy season (November-December), in the SFRB's upper sub-basins. The NCEP reanalysis I and monthly gridded precipitation dataset of the Global Precipitation Climatology Centre from 1948 to 2010 were used. SPI was used as drought index. All analyses were performed in a domain that covered South America and southern-tropical portions of the Atlantic and Pacific Oceans. Both the wind speed and vertical velocity fields at 200 hPa and specific humidity at 700 hPa were analysed. Results indicate that more extensive short-duration droughts are linked to: (1) El Niño conditions highly concentrated inside the Niño 3.4 region; (2) ITCZ with a northernmost-than-normal position; (3) weaker-than-normal convective activity over the SFRB coupled to a weaker-than-normal upper-level westerly jet stream. For decadal scale, the atmospheric anomalies patterns are best correlated to long-duration extensive droughts than the oceanic anomalies patterns ( r = −0.84 and −0.14, respectively). This suggests that droughts in SFRB are strongly dependent on large-scale moisture transport and upper-level atmospheric circulation. [ABSTRACT FROM AUTHOR]