The influence of different El Niño types on the South American rainfall

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.

Section: Discussionmentioning

confidence: 99%

Pacific and Atlantic multidecadal variability relations to the El Niño events and their effects on the South American rainfall

Kayano

Andreoli

Souza

2019

“…The weakening of the convective activity and the decrease of the water vapour content over northern South America, which is connected to the development of above normal dry and hot conditions at the surface, is the result of exceptional intense northward ITCZ displacements (Tedeschi et al, ; Tedeschi and Collins, ). Northward ITCZ displacements are related to SST anomalies over the North Tropical Atlantic Ocean (Zeng et al, ) and to ENSO, that tend to influence the normal wind pattern, the levels of temperature and atmospheric convective activity throughout this region (Grimm, ; Andreoli et al, ).…”

Section: Discussionmentioning

confidence: 99%

Climatic and synoptic characterization of heat waves in Brazil

Geirinhas

Trigo

Libonati

et al. 2017

According to the latest global and regional circulation models, the probability of occurrence of large heat waves (HWs), such as the 2003 European or the 2010 Russian events will increase significantly in the following decades under most climate scenarios. Currently, there are numerous studies for the Northern Hemisphere characterizing HWs and evaluating their impacts in several areas such as public health, economy, and agriculture. However, over South America, and in particular for Brazil, similar analysis is lacking despite its large geographical extension and numerous population potentially affected. Here we perform an assessment of HW events and characteristics recorded in six large Brazilian cities during the last five decades. The performed analysis reveals the existence of positive and significant trends in HW frequency since the 1980s, particularly for the cities of São Paulo, Manaus, and Recife. Over the last decades, Brasília was the city that recorded the highest number of days per year under a HW regime, contrasting with Rio de Janeiro that recorded the lowest value. The assessment of the large‐scale atmospheric circulation patterns associated with summer HWs, indicated for Porto Alegre, São Paulo, Rio de Janeiro, and Brasília the presence of well‐marked concentric and positive 500 hPa geopotential height anomalies followed by positive 850 hPa temperature anomalies. These anomalies are likely associated with quasi‐stationary anticyclonic systems promoted by anomalous westward displacements of the South Atlantic Subtropical High System which are related to a weakening of other transients (and non‐transient) systems such the Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone. For Manaus, the identified anomalies are linked to a northward displacement of the ITCZ. This configuration is compatible with an increase in solar radiative pattern and decreased soil moisture, enhancing surface temperature values, possibly associated with positive feedback mechanisms between soil and the atmosphere.

“…The upward motions around 80 W indicative of a double east-west cell, as previously described (Ashok et al, 2007), makes the downward motions to be positioned further east over equatorial South America for this EN type than for EPEN. Indications of a double cell during CPEN events were also observed by Andreoli et al (2017). In MAM(+1), there is a weakening of this cell, however the characteristics of a double cell remain.…”

Section: Vertical Longitude Cross Section Of the Vertical Velocity mentioning

confidence: 69%

The role of the Atlantic Multidecadal Oscillation precondition in the teleconnection of different El Niño‐Southern Oscillation types and impacts on the 15°N–15°S South American sector precipitation

Figliuolo

Andreoli

Kayano

et al. 2019

The Eastern Pacific (EP) and Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) types and their impacts on the tropical North Atlantic (TNA) SST variability and 15°N–15°S South American precipitation during the warm and cold phases of the Atlantic Multidecadal Oscillation (WAMO and CAMO) were evaluated during the 1901–2012 period. The results show more frequent ENSO events during the CAMO. The El Niño (EN) (La Niña [LN]) events, regardless of type (EP or CP), during the WAMO (CAMO) were accompanied by a warming (cooling) in the TNA after its mature phases. In these cases, extratropical teleconnection patterns are established through variations in the Pacific/North America (PNA) teleconnection pattern and are accompanied by variations in the Walker circulation. For the EN (LN) in the CAMO (WAMO), the tropical teleconnections occur predominant, through the Walker cell and the zonal inter‐basin gradient, which is intensified due to the SST gradient between the eastern equatorial Pacific (non‐neutral anomalies) and the equatorial Atlantic (neutral anomalies). These circulation pattern changes affect the precipitation patterns in the 15°N–15°S South American sector during December–January–February (D(0)JF(+1)) and March–April–May (MAM(+1)). The EP EN (EPEN) events are associated with the intensification of the negative precipitation anomalies in northeastern Brazil (NEB) during the WAMO and in the central part of the Amazon during the CAMO. In the case of CP EN (CPEN) events, the greatest differences between the AMO phases occur during MAM(+1), with reverse sign anomalies over northwestern South America. In the case of LN events, the largest differences occur in NEB, with reduced rainfall in the WAMO, regardless of type EP or CP. The results presented here highlight the role of low frequency oscillations in defining the teleconnection patterns between tropical Pacific and Atlantic Oceans, not discussed previously.