This paper examines the oceanic mean states associated with the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) and their relations to the El Niño (EN). The mean ...states refer to the simultaneous occurrences of warm (W) and cold (C) phases of these oscillations: WAMO/CPDO, CAMO/WPDO and CAMO/CPDO. In general, the sea surface temperature (SST) anomaly patterns of the mean states show a combination of the PDO and AMO related anomaly patterns in the Pacific Ocean, and the strongest AMO‐related anomaly signals in the Atlantic Ocean with almost antisymmetric SST anomaly patterns in the northern and southern extratropical sectors of this Ocean. One of the most important results of this paper concerns the contrasting features between the CAMO/WPDO and the WAMO/CPDO mean states, which are noticeable for the SST, zonal circulation cell and precipitation during the austral autumn. Besides the opposite inter‐basin equatorial Atlantic/eastern equatorial Pacific cells, the CAMO/WPDO and WAMO/CPDO mean states also feature, respectively, a weakened and strengthened Walker cell in the tropical Pacific during the austral autumn. The results here indicate that the inter‐basin east–west cell between the equatorial Atlantic and eastern Pacific contributes to strengthen (weaken) the Walker cell during the austral autumn of the WAMO/CPDO (CAMO/WPDO) mean state. We also found that the mean states alter the EN features. The EN‐related largest positive SST anomalies occur in the central tropical Pacific during the WAMO/CPDO, and in the eastern tropical Pacific during the CAMO/WPDO. The Atlantic and Pacific oceanic mean states play an important role in modulating the EN features and their effects on the South American rainfall. The results presented in this paper might be relevant for climate monitoring and modelling studies.
The El Niño (EN) events and their effects in the South American rainfall vary depending on the oceanic mean states. The EN‐related anomalous dry‐wet dipole occurs between northern South America (SA) and southeastern SA (SESA) during spring to autumn of the CAMO/CPDO, between northern/northwestern SA and southeastern Brazil during summer and autumn of the CAMO/WPDO and between northern/northeastern SA and part of SESA during spring and summer of the WAMO/CPDO. The summer EN‐related precipitation anomaly patterns are illustrated below.
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.
The modulation of the Interdecadal Pacific Oscillation (IPO) over the teleconnections of the El Niño–Southern Oscillation (ENSO) with the Indian Ocean Basin‐Wide (IOBW) mode and precipitation over South America during the decaying phase of ENSO extremes was assessed for the 1901–2012 period. The relationship El Niño (La Niña) and positive (negative) IOBW during the positive (negative) IPO acts to prolong the scarcity of precipitation over tropical South America (subtropical South America). It carries important implications for the seasonal prediction of precipitation over SA.
The Colombian Biogeographic Choco (CBC) and the La Plata Basin (LPB) are regions with high biodiversity. However, these areas are characterized by scarce climatological information, complex ...orography, and rain-gauge network unevenly distributed. Interpolated data from the ground station might overcome these aspects. For this reason, is necessary to identify the best technique for the spatial interpolation of rainfall. Hence, the spatial interpolation techniques were applied to annual and seasonal rainfall in the CBC and LPB. Geostatistical results and deterministic approaches were compared by cross-validation. Cokriging with spherical (gaussian) model is the best interpolator in the CBC (LPB), as indicated by the lowest root mean square error (RMSE) and a standardized RMSE close to one. The CBC shows three rainfall cores: the northern, 9,000 mm/year; the central-southern, 10,000 mm/year; and the southern, 7,000 mm/year. The LPB shows a west-east rainfall gradient, with a minimum to the west (450 mm/year) and a maximum in the mid-west (2,000 mm/year). To the north of the LPB, rainfall reaches 1,500 mm/year, while in the south it reaches only 900 mm/year. The results in our study may be useful for scientists and decision-makers for use in environmental and hydrological models for the CBC and the LPB.
The effect of multiyear La Niña (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.
The effect of multiyear La Niña (LN) events on precipitation in South America (SA) was assessed using observational data of the 1901–2012 period. The drier (wetter) conditions during austral spring and summer (winter and spring), particularly over southern and southeastern Brazil (Colombia) in the second year of LN, might have more severe effects on the regional hydrological cycle than those in the first year of LN. The results shown here indicate that accurate prediction of LN duration is crucial in a climate‐monitoring context.
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 Ward hierarchical clustering method is used to select representative areas of the South American monsoon system (SAMS) and of the northern tropical America regions. Using these areas, regionalized antisymmetric outgoing long‐wave radiation (AOLR) indices are constructed to determine the onset and demise dates of the rainy season in the SAMS. The AOLR indices keep the climatological features of the convection annual cycle. Therefore, these indices are useful for the SAMS rainy season monitoring purposes.
Abstract
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.
This study analyzes the variability of the Choco jet (CJ) and Caribbean low-level jet (CLLJ) with consideration of the simultaneous Pacific interdecadal oscillation (PDO) and Atlantic multidecadal ...oscillation (AMO) low-frequency mean states and their effects on the atmospheric circulation and rainfall in northwestern South America and Central America for the 1900–2015 period, during the seasons with the highest intensities of the CJ (September–November (SON)) and the CLLJ (June–August). Variations in the sea surface temperature (SST) anomaly positioning in the eastern Pacific, tropical North Atlantic (TNA)/Caribbean Sea during different mean states restrict the anomalous circulation, and, consequently, the intensity of the CJ and CLLJ. During the warm AMO (WAMO)/cold PDO (CPDO), the SST gradient from the tropical Pacific into the TNA, accompanied by a cyclonic circulation near the east coast of the Americas, intensifies the west–east circulation in the region, strengthening the CJ and weakening the CLLJ during SON such that rainfall increases over Colombia, Central America and in adjacent oceans. During the cold AMO (CAMO)/warm PDO (WPDO) phase, a relative east/west SST gradient occurs in TNA, consistent with a cyclonic circulation in western TNA, establishing an anomalous southwest–northwestward circulation from the eastern Pacific into the Caribbean basin, forming a well-configured CJ, increasing precipitation over Central America and its adjacent oceans. For the CLLJ, during CAMO phases, the anticyclonic circulations extended over most of the TNA favor its intensification from 30° W to the Caribbean Sea. In contrast, during WAMO, the cyclonic circulation near the east coast of the United States restricts its intensification to the Caribbean Sea region. To the best of our knowledge, the results presented here are new and might be useful in atmospheric modeling and extreme event studies.
The seasonal precipitation contrasts in South America (SA) associated with two types of multiyear El Niño–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 Niño (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 Niña (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.
From April 2014 to January 2015, ozone (O3) dynamics were investigated as part of GoAmazon 2014/5 project in the central Amazon rainforest of Brazil. Just above the forest canopy, maximum hourly O3 ...mixing ratios averaged 20 ppbv (parts per billion on a volume basis) during the June–September dry months and 15 ppbv during the wet months. Ozone levels occasionally exceeded 75 ppbv in response to influences from biomass burning and regional air pollution. Individual convective storms transported O3-rich air parcels from the mid-troposphere to the surface and abruptly enhanced the regional atmospheric boundary layer by as much as 25 ppbv. In contrast to the individual storms, days with multiple convective systems produced successive, cumulative ground-level O3 increases. The magnitude of O3 enhancements depended on the vertical distribution of O3 within storm downdrafts and origin of downdrafts in the troposphere. Ozone mixing ratios remained enhanced for > 2 h following the passage of storms, which enhanced chemical processing of rainforest-emitted isoprene and monoterpenes. Reactions of isoprene and monoterpenes with O3 are modeled to generate maximum hydroxyl radical formation rates of 6×106 radicals cm−3s−1. Therefore, one key conclusion of the present study is that downdrafts of convective storms are estimated to transport enough O3 to the surface to initiate a series of reactions that reduce the lifetimes of rainforest-emitted hydrocarbons.
•In the rainforest, convective storms transport ozone-rich air to the surface.•Ozone levels remain enhanced for more than 2 h after the passage of storms.•Enhanced ozone drives oxidation of rainforest-emitted hydrocarbons.
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