Anomalies of the South American summer monsoon associated with the 1997–99 El Niño–southern oscillation

Lau, K.-M.; Zhou, Jiayu

doi:10.1002/joc.900

Cited by 30 publications

(24 citation statements)

References 32 publications

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“…Few studies using the PACS-SONET, reanalyses, and regional modeling have suggested enhanced SALLJ activity during 1998 as compared to 1999 (Douglas et al 1999;Misra et al 2000;Saulo et al 2000;Vernekar et al 2003;Marengo et al 2002;Lau and Zhou 2003) exhibits substantial interannual variability. Both reanalyses and observations suggest fewer episodes of SALLJs in 1999 as compared to 1998, but the lack of observations during other El Niño events does not allow for a general conclusion between the presence of El Niño and a more active SALLJ season.…”

Section: ) Annual and Diurnal Cyclementioning

confidence: 99%

Climatology of the Low-Level Jet East of the Andes as Derived from the NCEP–NCAR Reanalyses: Characteristics and Temporal Variability

Marengo¹,

Soares²,

Saulo³

et al. 2004

J. Climate

541

403

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A climatology of the South American low-level jet east of the Andes (SALLJ) is developed using the 1950-2000 circulation and moisture fields from the NCEP-NCAR reanalyses and available upper-air observations made in Bolivia and Paraguay since 1998. Upper-and low-level circulation fields were derived for seasonal means and SALLJ composites during the warm and cold seasons. The Bonner criterion 1 was applied for sites in central Bolivia and downstream near northern Paraguay, to determine the spatial and temporal characteristics of the SALLJ. On the circulation characteristics, SALLJ composites during the warm season show the enhanced low-level meridional moisture transport coming from equatorial South America as well as an upper-level wave train emanating from the west Pacific propagating toward South America. The intensification of the warm season SALLJ follows the establishment of an upper-level ridge over southern Brazil and a trough over most of Argentina. The circulation anomalies at upper and lower levels suggest that the intensification of the SALLJ would lead to an intensification of the South Atlantic convergence zone (SACZ) later on and to the penetration of cold fronts with an area of enhanced convection ahead at the exit region of the SALLJ.Regarding the time variability, the SALLJ seems to occur all year long, bringing tropical moist air masses from the Amazon into southern Brazil-northern Argentina more frequently in the warm season, and bringing tropical maritime air, which is less humid than the tropical air masses coming from the subtropical Atlantic high, more frequently during the cold season. SALLJs are detected mostly during the warm season to the north of ϳ20ЊS, while to the south the SALLJs seem to occur all year long. The diurnal cycle shows that SALLJs are more frequent and intense between 0600 and 1200 UTC for the warm season north of 20ЊS, while at the region downstream the maximum is detected between 0000 and 0600 UTC during the cold season. At interannual time scales, even though there is a weak tendency for stronger and more frequent warm season SALLJ episodes in years with anomalously warm surface waters in the tropical Pacific, it cannot be affirmed with a large degree of certainty that there is a strong relationship between the occurrence of El Niño events and the number and/ or intensity of SALLJ episodes. However, the 1998 El Niño featured more frequent and intense warm season jet episodes than occurred during the 1999 La Niña, and this has been demonstrated by the reanalyses, the available Pan American Climate Studies-Sound Network (PACS-SONET) upper-air observations, and by other studies using independent datasets and regional modeling.

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Section: ) Annual and Diurnal Cyclementioning

confidence: 99%

Climatology of the Low-Level Jet East of the Andes as Derived from the NCEP–NCAR Reanalyses: Characteristics and Temporal Variability

Marengo¹,

Soares²,

Saulo³

et al. 2004

J. Climate

541

403

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“…For instance, Berbery and Barros (2002) suggested that during the austral summer of the three more relevant El Niños (EN) that occurred in the 1980-2000 period, the large-scale patterns showed increased southward component of moisture flux over La Plata Basin. Lau and Zhou (2003) observed a reduction of rainfall over the Amazon associated with a poleward shift of the South American monsoon circulation and rainfall system during the 1997/98 El Niño. They indicated that the Bolivian high was hydrostatically enhanced by the anomalous tropospheric warm ridge extending from the Niño-3 region to the Altiplano Plateau.…”

Section: Introductionmentioning

confidence: 96%

“…They found a much stronger SALLJ penetrated deeply into the extratropics along the eastern side of the subtropical Andes, coinciding with the reinforcement of the upper tropospheric subtropical westerly and the local meridional overturning. Marengo et al (2004), Lau andZhou (2003) andNieto Ferreira et al (2003) have also indicated that the 1997/98 EN had more frequent and strong SALLJ episodes when compared to the 1999/2000 La Niña (LN) event. Silva and Ambrizzi (2006) verified that during the event of 1997/98 a low-level anomalous anticyclonic circulation over the central part of Brazil enhanced the wind in the core of the SALLJ and displaced the jet to the Northern Argentina and South of Brazil.…”

Section: Introductionmentioning

confidence: 99%

Observational evidences on the modulation of the South American Low Level Jet east of the Andes according the ENSO variability

Silva¹,

Ambrizzi²,

Marengo

2009

Ann. Geophys.

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Abstract. The differences on the phase and wavelength of the quasi-stationary waves over the South America generated by El Niño (EN) and La Niña (LN) events seem to affect the daily evolution of the South American Low Level Jet east of the Andes (SALLJ). For the austral summer period of 1977-2004 the SALLJ episodes detected according to Bonner criterion 1 show normal to above-normal frequency in EN years, and in LN years the episodes show normal to below-normal frequency.During EN and LN years the SALLJ episodes were associated with positive rainfall anomalies over the La Plata Basin, but more intense during LN years. During EN years the increase in the SALLJ cases were associated to intensification of the Subtropical Jet (SJ) around 30 • S and positive Sea Level Pressure (SLP) anomalies over the western equatorial Atlantic and tropical South America, particularly over central Brazil. This favored the intensification of the northeasterly trade winds over the northern continent and it channeled by the Andes mountain to the La Plata Basin region where negative SLP are found. The SALLJ cases identified during the LN events were weaker and less frequent when compared to those for EN years. In this case the SJ was weaker than in EN years and the negative SLP anomalies over the tropical continent contributed to the inversion of the northeasterly trade winds. Also a southerly flow anomaly was generated by the geostrophic balance due to the anomalous blocking over southeast Pacific and the intense cyclonic transient over the southern tip of South America. As result the warm tropical air brought by the SALLJ encounters the cold extratropical air from the southerly winds over the La Correspondence to: G. A. M. Silva (gyrlene@model.iag.usp.br) Plata basin. This configuration can increase the conditional instability over the La Plata basin and may explain the more intense positive rainfall anomalies in SALLJ cases during LN years than in EN years.

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“…In ENSO warm phases (El Nino), it is moved eastwards as far as to NEB, resulting in a descending dry air mass. At the same time, the subtropical anticyclones on both sides of the ITCZ are strengthened [26].…”

Section: Study Area: Northeastern Brazil (Neb)mentioning

confidence: 99%

“…Extreme climate conditions, such as prolonged droughts due to climatic anomalies, are associated with an eastward shift of the Walker circulation and, at the same time, with a warming of the middle and upper troposphere, due to increasing SST in the Eastern Pacific [26]. Both processes lead to a temperature increase in the troposphere east of the Andes.…”

Section: Study Area: Northeastern Brazil (Neb)mentioning

confidence: 99%

Vegetation Greenness in Northeastern Brazil and Its Relation to ENSO Warm Events

et al. 2014

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Abstract:The spatio-temporal variability of trends in vegetation greenness in dryland areas is a well-documented phenomenon in remote sensing studies at global to regional scales. The underlying causes differ, however, and are often not well understood. Here, we analyzed the trends in vegetation greenness for a semi-arid area in northeastern Brazil (NEB) and examined the relationships between those dynamics and climate anomalies, namely the El Nino Southern Oscillation (ENSO) for the period 1982 to 2010, based on annual Normalized Difference Vegetation Index (NDVI) values from the latest version of the Global Inventory Modeling and Mapping Studies (GIMMS) NDVI dataset (NDVI3g) dataset. Against the ample assumption of ecological and socio-economic research, the results of our inter-annual trend analysis of NDVI and precipitation indicate large areas of significant greening in the observation period. The spatial extent and strength of greening is a function of the prevalent land-cover type or biome in the study area. The regression analysis of ENSO indicators and NDVI anomalies reveals a close relation of ENSO warm events and periods of reduced vegetation greenness, with a temporal lag of 12 months. The spatial patterns of this relation vary in space and time. Thus, not every ENSO warm event is reflected in negative NDVI anomalies. Xeric shrublands (Caatinga) are more sensitive to ENSO teleconnections than other biomes in the study area.

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Anomalies of the South American summer monsoon associated with the 1997–99 El Niño–southern oscillation

Cited by 30 publications

References 32 publications

Climatology of the Low-Level Jet East of the Andes as Derived from the NCEP–NCAR Reanalyses: Characteristics and Temporal Variability

Climatology of the Low-Level Jet East of the Andes as Derived from the NCEP–NCAR Reanalyses: Characteristics and Temporal Variability

Observational evidences on the modulation of the South American Low Level Jet east of the Andes according the ENSO variability

Vegetation Greenness in Northeastern Brazil and Its Relation to ENSO Warm Events

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