Hydric extremes in Argentina: an updated review with special emphasis on the effects of El Niño and La Niña

Based on a widely used satellite precipitation product (TRMM Multi-satellite Precipitation Analysis 3B43), we analyzed the spatiotemporal variability of precipitation over the Pacific Ocean for 1998–2014 at seasonal and interannual timescales, separately, using the conventional empirical orthogonal function (EOF) and investigated the seasonal patterns associated with El Niño–Southern Oscillation (ENSO) cycles using season-reliant empirical orthogonal function (SEOF) analysis. Lagged correlation analysis was also applied to derive the lead/lag correlations of the first two SEOF modes for precipitation with Pacific Decadal Oscillation (PDO) and two types of El Niño, i.e., central Pacific (CP) El Niño and eastern Pacific (EP) El Niño. We found that: (1) The first two seasonal EOF modes for precipitation represent the annual cycle of precipitation variations for the Pacific Ocean and the first interannual EOF mode shows the spatiotemporal variability associated with ENSO; (2) The first SEOF mode for precipitation is simultaneously associated with the development of El Niño and most likely coincides with CP El Niño. The second SEOF mode lagged behind ENSO by one year and is associated with post-El Niño years. PDO modulates precipitation variability significantly only when ENSO occurs by strengthening and prolonging the impacts of ENSO; (3) Seasonally evolving patterns of the first two SEOF modes represent the consecutive precipitation patterns associated with the entire development of EP El Niño and the following recovery year. The most significant variation occurs over the tropical Pacific, especially in the Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ); (4) Dry conditions in the western basin of the warm pool and wet conditions along the ITCZ and SPCZ bands during the mature phase of El Niño are associated with warm sea surface temperatures in the central tropical Pacific, and a subtropical anticyclone dominating over the northwestern Pacific. These findings may be useful for prediction of seasonal precipitation anomalies over the Pacific Ocean during El Niño years and recovery years.

Abstract. The Community Climate Model (CCM3) from the National Center for Atmospheric Research (NCAR) is used to investigate the effect of the South Atlantic sea surface temperature (SST) anomalies on interannual to decadal variability of South American precipitation. Two ensembles composed of multidecadal simulations forced with monthly SST data from the Hadley Centre for the period 1949 to 2001 are analysed. A statistical treatment based on signal-to-noise ratio and Empirical Orthogonal Functions (EOF) is applied to the ensembles in order to reduce the internal variability among the integrations. The ensemble treatment shows a spatial and temporal dependence of reproducibility. High degree of reproducibility is found in the tropics while the extratropics is apparently less reproducible. Austral autumn (MAM) and spring (SON) precipitation appears to be more reproducible over the South America-South Atlantic region than the summer (DJF) and winter (JJA) rainfall. While the Inter-tropical Convergence Zone (ITCZ) region is dominated by external variance, the South Atlantic Convergence Zone (SACZ) over South America is predominantly determined by internal variance, which makes it a difficult phenomenon to predict. Alternatively, the SACZ over western South Atlantic appears to be more sensitive to the subtropical SST anomalies than over the continent. An attempt is made to separate the atmospheric response forced by the South Atlantic SST anomalies from that associated with the El Niño – Southern Oscillation (ENSO). Results show that both the South Atlantic and Pacific SSTs modulate the intensity and position of the SACZ during DJF. Particularly, the subtropical South Atlantic SSTs are more important than ENSO in determining the position of the SACZ over the southeast Brazilian coast during DJF. On the other hand, the ENSO signal seems to influence the intensity of the SACZ not only in DJF but especially its oceanic branch during MAM. Both local and remote influences, however, are confounded by the large internal variance in the region. During MAM and JJA, the South Atlantic SST anomalies affect the magnitude and the meridional displacement of the ITCZ. In JJA, the ENSO has relatively little influence on the interannual variability of the simulated rainfall. During SON, however, the ENSO seems to counteract the effect of the subtropical South Atlantic SST variations on convection over South America.

• Late Holocene paleoclimate variability of coastal Karnataka. • Multiproxy analysis revealed four distinct climatic phases. • Strengthening of Indian Summer Monsoon during Late Holocene. • Increasing productivity, lake levels, and sedimentation rate. • Global teleconnections linked to TSI, ENSO, ITCZ, IOD, and regional SST variations. In recent decades, extreme weather events have become more frequent across the globe. It necessitates a deeper understanding of the underlying driving mechanisms. This study reconstructs the paleoclimatic variability of southern India, particularly the coastal Karnataka, over the past two millennia using a multiproxy approach (geochronology, environmental magnetism, sedimentology, inorganic geochemical analysis, Fourier Transform Infrared Spectroscopy, Diffuse Reflectance Spectroscopy, and loss-on-ignition). The study was conducted on a 1.54-m-long lacustrine sediment core covering the past 1566 years, from 2005 to 439 cal yr BP. Four distinct climatic phases were delineated, reflecting successive stages of the Indian Summer Monsoon (ISM) strengthening in the Late Holocene. Phase 1, which spans from 2000 to 1550 cal yr BP, experienced moderately low rainfall and weak pedogenesis. In Phase 2 (1550–1230 cal yr BP), the monsoon strengthened, leading to strong precipitation, intense weathering and pedogenesis, and high lake levels. Phases 3 (1230–570 cal yr BP) and 4 (570–439 cal yr BP) experienced stronger monsoons and a burst of rainfall that strengthened catchment streams, elevated lake levels, and increased productivity and sedimentation rate. A comparative study with regional records suggests a similar trend in broad climate variability, revealing a global teleconnection. The climatic evolution of coastal Karnataka aligns with the shifts in the Intertropical Convergence Zone (ITCZ), El Niño Southern Oscillation (ENSO), and Total Solar Irradiance (TSI). Additionally, the signatures of global factors like ITCZ, ENSO, and TSI have been overprinted by the signatures of regional factors such as sea surface temperature (SST), especially during periods of active Indian Ocean Dipole (IOD).

The South Atlantic Convergence Zone (SACZ)—a summertime band of convection extending from the Amazon southeastward into the southwest Atlantic Ocean—has generated considerable interest in the field of South American meteorology and climatology because of its substantial influence on summer rainfall in Southeast Brazil. It is responsible for the majority of rainfall in Brazil’s most agriculturally productive region, although an overly active SACZ can result in excessive precipitation with landslides in the mountainous regions and widespread flooding, causing major losses to society. The mean position of the SACZ seems to be controlled by the heat source of the Amazon basin, which induces a low-level trough that imports moisture from the Amazon, and, over the ocean, by convergence of moisture evaporated below the South Atlantic anticyclone into the SACZ, with additional influences from the local orography. It is, however, the less understood significant spatial and temporal variations that demand attention and are of potential concern. One important such fluctuation, known as the “South American Seesaw,” is a pronounced out-of-phase relationship between rainfall in the SACZ region and southern South America, which is most apparent on sub-seasonal timescales. Periods in which the SACZ is active tend to correspond to rainfall deficits in the south. The opposite phase, with a weakened SACZ but abundant precipitation over southern South America, is accompanied by a strong northerly low-level jet east of the Andes Cordillera that transports massive amounts of moisture from the Amazon basin southward. The mechanisms controlling the alternation and the activation of either phase are not fully understood but may be related to the phase of midlatitude synoptic wave trains as they impinge on South America. Some of these midlatitude waves that impact the SACZ undoubtedly form in the midlatitudes of the Southern Hemisphere, but others appear to emanate from the western Pacific tropics, arch southward along an approximate great circle route, and propagate into the vicinity of the SACZ. As such, they may be modulated by the Madden–Julian Oscillation (MJO), which shows some prediction skill and has been shown to be related to SACZ rainfall. The MJO influence on the SACZ may itself depend on the phase of El Niño/Southern Oscillation (ENSO) because a change in the longitude of maximum sea surface temperature (SST) alters both the background basic state and the location of maximum equatorial heating associated with an MJO event. In spite of the plausible ENSO modulation of the MJO impact on the SACZ, the direct influence of ENSO on the SACZ on interannual scales appears to be small. During an El Niño event, when large positive SST anomalies extend into the eastern Pacific and convective heating shifts eastward from its usual position in the western Pacific, there is a tendency for above-average precipitation in southern South America and a strong low-level jet. The concomitant weakening of the SACZ that occurs on short time scales, however, is not observed, resulting in a weak correlation between ENSO and the SACZ. One of the reasons for this lack of relationship may be that there is a tendency for the SACZ precipitation anomalies to change sign over the course of an El Niño summer. Although the representation of the South American Monsoon System in climate models is deficient in many regards, they are able to simulate a SACZ that extends south-eastward from the Amazon region, with a minimum just off shore. The Amazon maximum, however, is about 10º too far east in many models, so that the continental portion of the SACZ is significantly shorter than in nature. The misrepresentation of cloud physics contributes to this long-standing problem, although some improvement can be noted in the latest generation of CMIP models. The influence of Atlantic SSTs on the SACZ remains difficult to quantify. Most results suggest the ocean cools under an active SACZ because of reduced solar radiation, although vertical and horizontal advection in the ocean mixed layer may be of equal importance. The predictive skill of the SACZ in hindcast simulations is near zero on seasonal scales, which is thought to result partly from inaccurate representation of air–sea coupling processes. This notion is reinforced by a similar inability of AMIP models (atmospheric general circulation models forced with observed underlying SSTs) to reproduce precipitation variations in the SACZ region.

Aquarius observations feature a prominent zonal sea-surface salinity (SSS) front that extends across the tropical Pacific between 2–10°N. By linking to Argo subsurface salinity observations and satellite-derived surface forcing datasets, the study discovered that the SSS front is not a stand-alone feature; it is in fact the surface manifestation of a low-salinity convergence zone (LSCZ) located within 100 m of the upper ocean. The near-surface salinity budget analysis suggested that, although the LSCZ is sourced from the rainfall in the Inter-tropical convergence zone (ITCZ), its generation and maintenance are governed by the wind-driven Ekman dynamics, not the surface evaporation-minus-precipitation flux. Three distinct features highlight the relationship between the oceanic LSCZ and the atmospheric ITCZ. First, the seasonal movement of the LSCZ is characterized by a monotonic northward displacement starting from the near-equatorial latitudes in boreal spring, unlike the ITCZ that is known for its seasonal north-south displacement. Second, the lowest SSS waters in the LSCZ are locked to the northern edge of the Ekman salt convergence throughout the year, but have no fixed relationship with the ITCZ rain band. Collocation between the LSCZ and ITCZ occurs only during August-October, the time that the ITCZ rain band coincides with the Ekman convergence zone. Lastly, the SSS front couples with the Ekman convergence zone but not the ITCZ. The evidence reinforces the findings of the study that the Ekman processes are the leading mechanism of the oceanic LSCZ and the SSS front is the surface manifestation of the LSCZ.

While numerous detailed studies have been conducted of the annual cycle of convection over other regions (e.g., the Asian summer monsoon and the West African summer monsoon regions), the annual cycle and its modulation in the tropical South American region has received attention only relatively recently. Most of the annual total rainfall observed over tropical South America occurs during the austral summer and autumn months. The large-scale meteorological systems that modulate rainfall during these periods are linked to the strength and movement of large-scale climatological features—in particular, the Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone (SACZ). It is well known that the anomalous patterns related to the El Niño/Southern Oscillation (ENSO) influence the ITCZ and SACZ patterns, with strong interan-nual and seasonal variations over tropical and subtropical South America.The goal of this chapter is to analyze the influence of ENSO events on the regional Hadley and Walker cells and their respective impacts on South American seasonal rainfall. As is well documented, ENSO events influence regional precipitation patterns over South America, with the strongest influences in the Amazon/Northeast Brazil and southern South America.Basically, two separate responses can be composited for each phase of the ENSO cycle. El Niño (La Niña) Composite 1 is the canonical ENSO warm (cold) event with well-known impacts on large-scale atmospheric circulation and regional precipitation patterns overSouth America, indicating that the central-eastern Pacific sea surface temperature anomaly (SSTa) is the dominating feature in this case. On the other hand, the El Niño and La Niña Composite 2 analyses characterize the influence of the intertropical Atlantic SST gradients as being significant in modulating the influence of ENSO by intensifying the SACZ and ITCZ in some cases. For these latter composites, evidence of a completely reversed atmospheric circulation and regional precipitation patterns is found during the summer and autumn seasons. The analysis demonstrates that interaction of ENSO events with the South American monsoon produces changes in the time and space evolution of convection and circulation over northern South America, which can also be reinforced by the Atlantic. Thus, depending on conditions in the Atlantic, the South American rainy season may be strongly affected. These results suggest that some care always must be taken in producing precipitation (and impacts) forecasts based on ENSO indices and composites alone.KeywordsHadley CirculationSouth Atlantic Convergence ZoneSouth American MonsoonRegional Precipitation PatternThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The South American Summer Monsoon (SASM) system is a vital system that supports the existence of the unique biomes and ecology of tropical to subtropical South America, as well as the economies of the South American population. In order to improve our understanding of the South American climate system, this thesis uses a model-proxy data comparison approach to characterize and understand the mechanisms driving precipitation response during the mid-Holocene. First, a new speleothem oxygen isotopic record (δ18O) from Angelica Cave in central Brazil shows that precipitation under the South Atlantic Convergence Zone (SACZ) remained stable over the mid-to-late Holocene. The central Brazilian record shows that rainfall under the core SACZ is insensitive to changes in the monsoon strength within the core SASM region and that the latitudinal position of the convergence zone did not shift significantly over time. The steady precipitation trend is in contrast to hydroclimate datasets from the western Amazon and northeastern Brazil which respectively show a strengthening and weakening of rainfall over the same time interval in an east-west dipole pattern. Next, the spatial variation of hydroclimate records over the mid-to-late Holocene is investigated using an Atmospheric General Circulation Model (AGCM), the ECHAM4.6. Mid-Holocene and pre-industrial scenario experiments ran with the ECHAM4.6 coupled to a slab ocean model show that while reduced precipitation in the western Amazon and Andes is a result of a weakened SASM during the mid-Holocene, the wetter condition in northeastern Brazil is due to a weakened Nordeste Low, resulting in less atmospheric subsidence over the northeast, in addition to an intensified Intertropical Convergence Zone (ITCZ) towards the south. The model results also show an increase in precipitation rates along the northern region of the SACZ and a decrease along the southern regions, while precipitation along the core of the SACZ remained similar between the two scenarios with no latitudinal shift of the core axis. Therefore, the SACZ responds in a north–south dipole pattern to the increasing SASM strength over the mid-to-late Holocene, rather than a latitudinal shift, thus agreeing with the invariant speleothem δ18O trend from central Brazil. Further experiments with the ECHAM4.6 forced with prescribed sea surface temperatures (SST) are used to investigate the impact of the El Niño–Southern Oscillation (ENSO) on the climatological precipitation and precipitation δ18O over South America during the mid-Holocene. Results show that the removal of ENSO variability on an otherwise unchanged annual cycle in SST results in a relatively minor change in the climatological precipitation. This means that the reductions in ENSO variability (amplitude and frequency) during the mid-Holocene is likely a negligible factor in explaining mid-Holocene precipitation trends in tropical to subtropical South America. In contrast, mid-Holocene insolation and a ‘La Niña-like’ mean state change in SSTs result in a prominent east-west dipole pattern in precipitation response between western Amazon and the northeast of South America. However, only the insolation experiment manages to reproduce the dipole pattern in precipitation δ18O values. This is in part due to insolation changes having the greatest impact on western Amazon precipitation during the peak monsoon months, whereas, the change in the mean state SST has the greatest impact during the dry winter season, thus resulting in the varying δ18O responses. We further show that the location of anomalous subsidence in the insolation experiment is concentrated over the eastern tropical Atlantic (~30°E), compared to subsidence over the northeast of the continent (~60°E) in the mean state experiment, further resulting in different extents of drying in the region.

We investigate global and regional changes in the intertropical convergence zone (ITCZ) position, width, and intensity during the last glacial maximum (LGM) relative to the preindustrial period using multiple simulations from Phases 3 and 4 of the Paleoclimate Modelling Intercomparison Project (PMIP3/4). On annual scale, most models show that LGM tropical precipitation decreases, and the deficit in the Northern Hemisphere is larger than that in the Southern Hemisphere, resulting in the southward shift, narrowing, and weakening of the ITCZ at the global scale. The arithmetic mean of 13 models shows that the global zonal mean ITCZ shifts southward by 0.85° (1σ = 0.86°), narrows by 1.05° (1σ = 1.33°), and weakens by 7% (1σ = 4%) during the LGM. Regionally, position and intensity changes are larger in the central and eastern Pacific, while width changes are most obvious in the Indian Ocean–western Pacific. Precipitation changes in the central and eastern Pacific and Atlantic oceans are dominated by the dynamic term. In the Indian Ocean–western Pacific, the thermodynamic term is the main cause for precipitation changes within 10°S–10°N, while the dynamic term plays a leading role at other tropical latitudes. Seasonally, the September–October–November and June–July–August mainly contribute to the annual ITCZ position, width, and intensity changes globally and in most regions. The convergence factor dominates both the dynamic and thermodynamic terms annually and seasonally. The model results are compatible with the existing site reconstructions on the southward shift of the LGM ITCZ.

This study uses multiple satellite data sets to evaluate seasonal simulations of the Weather Research and Forecasting (WRF) model over Central and Eastern Pacific. Experiments with five different convective parameterizations all show reasonably good performance for precipitation simulations. However, large discrepancies exist in the model-simulated ice clouds compared to CloudSat observations. Underestimations of ice clouds, mainly snow and graupel, are present in the Intertropical Convergence Zone (ITCZ) in all the experiments compared to CloudSat. In the ITCZ, all the experiments show a systematic overestimation of outgoing longwave radiation at the top of the atmosphere and downward shortwave radiation at the surface, along with biased cloud cooling in the middle and upper troposphere and biased cloud warming in the lower troposphere. Vertical motion is enhanced in the ITCZ compared to reanalysis. A weaker low-level circulation over the midlatitude oceans is evidenced in all simulations with an eastward overextension of the South Pacific Convergence Zone and overestimated moisture over the Southern Hemisphere oceans when compared to Special Sensor Microwave/Imager observations. Sensitivity experiment demonstrates that doubling the radiative effect of snow can reduce high biases in vertical motion within the ITCZ and improve the large-scale circulation and moisture over the midlatitude oceans.

In this study, precipitation in Tropical South America in the 1931–2016 period is investigated by means of Principal Component Analysis and composite analysis of circulation fields. The associated dynamics are analyzed using the 20th century ERA-20C reanalysis. It is found that the main climatic processes related to precipitation anomalies in Tropical South America are: (1) the intensity and position of the South Atlantic Convergence Zone (SACZ); (2) El Niño Southern Oscillation (ENSO); (3) the meridional position of the Intertropical Convergence Zone (ITCZ), which is found to be related to Atlantic Sea Surface Temperature (SST) anomalies; and (4) anomalies in the strength of the South American Monsoon System, especially the South American Low-Level Jet (SALLJ). Interestingly, all of the analyzed anomalies are related to processes that operate from the Atlantic Ocean, except for ENSO. Results from the present study are in agreement with the state of the art literature about precipitation anomalies in the region. However, the added strength of the longer dataset and the larger study area improves the knowledge and gives new insights into how climate variability and the resulting dynamics are related to precipitation in Tropical South America.

To study the seeding mechanism of ionospheric irregularity occurrences, a correlation study has been carried out between the global monthly/latitudinal (m/l) distributions of irregularity occurrences and the deep atmospheric convective clouds in the intertropical convergence zone (ITCZ) indicated by the outgoing longwave radiation (OLR) measurements. Seven longitude sectors - the African, Indian, West Pacific, Central Pacific, East Pacific, South American, and Atlantic sectors - are selected to study the correlations between the two distributions. The results indicate that good correlations exist only in the South American sector and to some extent in the African sector. For the other five sectors, no correlations are found in the m/l distributions between the irregularities and OLRs. This implies that the gravity wave induced in the ITCZ cannot be the sole seeding agent for the Rayleigh-Taylor (RT) instability in the global irregularity occurrences every season. We suspect that the post-sunset ionospheric electrodynamic perturbations could be the prevailing seeds for the RT instability globally year long. Together with the favorable post-sunset ionospheric condition, the global m/l distributions of irregularity occurrences could be adequately explained.

A La Niña took place in the equatorial Pacific in 1999, and strong tropical instability waves (TIWs) developed, causing large meanders of a sea surface temperature (SST) front between the equator and 3°N. High‐resolution satellite measurements are used to describe the variability of SST, surface wind velocity, column‐integrated water vapor, cloud liquid water, and precipitation associated with these strong TIWs in 1999. Coherent ocean‐atmosphere patterns emerge in both the Pacific and Atlantic, revealing rich regional characteristics. In the far eastern Pacific, southeasterly trades strengthen (weaken) over positive (negative) SST anomalies, apparently due to enhanced (reduced) mixing with high‐speed winds aloft. In the central Pacific we find evidence that SST‐induced sea level pressure changes also contribute substantially to wind fluctuations. Similar SST‐wind covariability also exists in the Southern Hemisphere along 2°S, but the wind variability induced by unit SST anomaly is much larger than that north of the equator. In the central Pacific where the equatorial front is broad, the northern SST pattern has a large meridional scale and reaches as far north as 6°N. Further to the north in the Intertropical Convergence Zone (ITCZ) where local SST anomalies are small, significant variability is found in clouds and precipitation, which is further correlated with surface wind convergence. In the Atlantic, TIW signals in SST are strongly trapped near the equator, but they induce significant remote response in the ITCZ, which takes a more southern position than its Pacific counterpart and thus more susceptible to TIW influence.

Several numerical experiments have been conducted using the NCAR Community Atmosphere Model, version 3 (CAM3) to examine the impact of the time step on rainfall in the intertropical convergence zone (ITCZ) in an aquaplanet. When the model time step was increased from 5 to 60 min the rainfall in the ITCZ decreased substantially. The impact of the time step on the ITCZ rainfall was assessed for a fixed spatial resolution (T63 with L26) for the semi-Lagrangian dynamical core (SLD). The increase in ITCZ rainfall at higher temporal resolution was primarily a result of the increase in large-scale precipitation. This increase in rainfall was caused by the positive feedback between surface evaporation, latent heating, and surface wind speed. Similar results were obtained when the semi-Lagrangian dynamical core was replaced by the Eulerian dynamical core. When the surface evaporation was specified, changes in rainfall were largely insensitive to temporal resolution. The impact of temporal resolution on rainfall was more sensitive to the latitudinal gradient of SST than to the magnitude of SST.

<p>Rapid climatic reorganizations during the last Termination (i.e. Heinrich Stadials 0-1) had major impacts on the Atlantic Meridional Overturning Circulation (AMOC) strength and on global atmospheric circulation patterns. However, if and how this high-latitude forcing affected low-latitude climate variability is still poorly constrained. Here we present a high-resolution multi-proxy record from marine sediment core M125-3-35 recovered in the western tropical South Atlantic combining foraminiferal Mg/Ca, Ba/Ca ratios, stable oxygen isotope measurements and organic biomarker-based sea surface temperature (SST) proxies (TEX86 and UK’37). The near-shore core position of M125-3-35 off the Paraíba do Sul river mouth in southeastern Brazil and the means of foraminiferal Ba/Ca ratios, which depends on the quantity of continental freshwater input, enables us to investigate direct coupling of continental hydroclimate and oceanographic changes.</p><p>The data show a complex interplay of oceanic and atmospheric forcing dominating the tropical South American climate, which is mainly controlled by the strength and position of the Intertropical Convergence Zone (ITCZ) and South Atlantic Convergence Zone (SACZ). During times of weakest AMOC in Heinrich Stadial 1 (HS1) , a distinct SST peak in the tropical South Atlantic points to an enhanced Brazil Current and strong recirculation of heat within the southern hemisphere. Further, wet conditions prevailed during this time in tropical South America caused by a maximum southward shift of the ITCZ. This happened in coincidence with a temperature drop and weakening of the North Brazil Current (NBC) in the tropical North Atlantic (Bahr et al., 2018) as result of maximum AMOC slowdown. Therefore, for the first time, we reveal a clear seesaw-like pattern of the NBC and BC during times of abrupt AMOC variability.</p><p>While HS1 is generally characterized by a warm and wet anomaly in our record, Ba/Ca ratios and SST show a distinct centennial-scale alternation between warmer (colder) and wetter (drier) phases indicating a distinct climate instability during this climatic phase. A distinct offset exists between SST reconstructed using Mg/Ca, TEX86, and UK’37 which points to strong seasonal differences in the oceanographic settings and/or changes in the terrestrial input from the south American continent. These findings illustrate the strong sensitivity of hydroclimate variability in tropical South America to oceanic forcing as expected also during future climate change, in line with recent studies that showed a severe impact on modern South American climate by changes in (tropical) South Atlantic SSTs (Rodrigues et al., 2019, Utida et al., 2018).</p><p> </p><p>Bahr, A., Hoffmann, J., Schönfeld, J., Schmidt, M. W., Nürnberg, D., Batenburg, S. J., & Voigt, S. (2018). Low-latitude expressions of high-latitude forcing during Heinrich Stadial 1 and the Younger Dryas in northern South America. <em>Global and Planetary Change, 160</em>, 1-9.</p><p>Rodrigues, R. R., Taschetto, A. S., Gupta, A. S., & Foltz, G. R. (2019). Common cause for severe droughts in South America and marine heatwaves in the South Atlantic. <em>Nature Geoscience, 12</em>(8), 620-626.</p><p>UTIDA, Giselle, et al. Tropical South Atlantic influence on Northeastern Brazil precipitation and ITCZ displacement during the past 2300 years. <em>Scientific reports</em>, 2019, 9. Jg., Nr. 1, S. 1698.</p>

The Northern Northeast Brazil (NNB) has two rainy periods, namely Pre-Wet Season (PWS) and Wet Season (WES), which are usually treated as one system. The precipitation pattern on NNB is influenced by sea surface temperature (SST) anomalies in the Atlantic and Pacific Ocean on interannual timescales particularly by the Interhemispheric Gradient of SST anomalies (IGS) and El Niño Southern Oscillation (ENSO). On intraseasonal time scales, the MJO is especially important. This study investigates the variability of the PWS/WES. The PWS is largely associated with the development of the South America Monsoon System and South Atlantic Convergence Zone (SAMS/SACZ); the onset is depicted by incursion of the SAMS/SACZ northward. Anomalous atmospheric cyclonic circulation over the southeastern Brazil along with easterlies over the northern Tropical Atlantic marks the early onset of the PWS, while easterlies over the southern Tropical Atlantic are related to late onset episodes. The demise of the PWS is significantly associated with propagation of the MJO, specifically during phases 4-5 of the MJO lifecycle. A Rossby wave train in 200-hPa geopotential height with positive anomalies over central-southern Brazil is depicted during transition between PWS and WES.