Thongchai Thailand

AGW Drought Migrants

Posted on: February 17, 2020









  1. AGW climate change caused a contraction of the ITCZ that initiated a causation sequence as follows: ITCZ Contraction -> Drying of Neotropics -> Drying of Central America -> Social Unrest -> Mass Migration.
  2. Previous work based on limited data had suggested that global cooling during the Little Ice Age had caused a a southward migration of the ITCZ.
  3. However, the current research with climate models and limited observational data suggest the ITCZ does not migrate north or south in response to global warming and cooling but that it contracts and expands in response to warming and cooling.
  4. To resolve this contradiction, this research undertook a paleoclimate reconstruction study from the margin of the ITCZ and combined that with existing data from across the full annual north-south excursion of the ITCZ.
  5. The findings, reported in a paper published online [LINK] , are summarized in the abstract as follows: Large changes in hydroclimate in the Neotropics implied by proxy evidence, such as during the Little Ice Age, have been attributed to meridional shifts of the intertropical convergence zone (ITCZ), although alternative modes of ITCZ variability have also been suggested. Here, we use seasonally resolved stalagmite rainfall proxy data from the modern northern limit of the ITCZ in southern Belize, combined with records from across the Neotropics and subtropics, to fingerprint ITCZ variability during the Common Era. Our data are consistent with models that suggest ITCZ expansion and weakening during globally cold climate intervals and contraction and intensification during global warmth. As a result, regions currently in the margins of the ITCZ in both hemispheres are likely transitioning to more arid and highly variable conditions, aggravating current trends of increased social unrest and mass migration.
  6. The [FULL TEXT] of the paper is available online where the effects of ENSO on Central American climate is acknowledged as ” (ENSO) modulates Caribbean-sourced moisture through its influence on the Caribbean Low-Level Jet (CLLJ). In El Nino events, the CLLJ is stronger and in La Nina events it is weaker, although the resultant impact on precipitation can vary. The correlation between ENSO and precipitation in our study area is weak. ENSO influence on Central American precipitation is seen mostly in the Pacific Coast and not so inland.”
  7. SUMMARY: In essence, this paper provides empirical evidence in support of the widely held belief that the large number of Central American migrants who had entered the USA illegally in 2019 were climate change migrants because they were driven there by the climate impact of drought conditions brought about by a contraction of the ITCZ due to climate change. The empirical evidence consists of paleo data that show cooling trends cause the ITCZ to expand and that warming trends cause it to contract. Translated to the current AGW climate change, it implies that the warming trend had caused drought conditions in Central America by a contraction of the ITCZ and that therefore, the mass migration of Central Americans to the USA was a climate migration as claimed for example in this related post  [LINK] .



  1. The paleo data studied were interpreted only in terms of temperature such that the known confounding influence of other variables such as ENSO variability   was not considered. It is also noted that paleo data contain large uncertainties that may lend themselves to different interpretations [LINK] and that comparison of results with short term observational data may be helpful in verifying the conclusions.
  2. An additional consideration is that the paper limits the study of the Holocene’s six alternating warm and cool periods over about 10,000 years to the last 2000 years where only the Medieval Warm Period and the Current Warm Period remain with the Little Ice Age intervening between them. The strength of the reported findings are weakened by this limitation along with the limitation of possible causation variable to the ITCZ without consideration of SST and ENSO variability found in many of the papers in the bibliography below. These arbitrary limitations raises questions about the validity of the findings.
  3. Here we present a bibliography of papers on the subject of precipitation variability in Central America where a broad spectrum of variables and conclusions are found. Many of these methods and findings are inconsistent with the what has been presented above. For example, in the Aguilar(2005) paper is reported the data and findings of a climate change workshop held in Guatemala in 2004 to investigate climate change impacts and weather extremes in Central America. The full text of this paper is available for download online [LINK] . The paper finds that a relevant climate feature of Central America is that it is adjacent to both the Pacific and Atlantic Oceans and they find that precipitation trends there are therefore strongly positively correlated with trends in sea surface temperature (SST). This correlation implies that global warming should decrease and not increase the probability of the dry conditions that are thought to have driven the migration.
  4. An added consideration, seen in many papers {Chang(2015), Robertson(1998), Zhou(2001), Paegle(2002), and Gagnon(2002) are examples}, is the inclusion of ENSO variability as a factor in explaining changes in precipitation. There may be other variables in addition to SST and ENSO as for example local variables such as changes in population, agricultural practices, urbanization, and other factors. For example, development of hydro-power in Costa Rica is found to have caused significant environmental degradation that impacts the widespread availability of water {Anderson, Elizabeth etal “Quantifying the extent of river fragmentation by hydropower dams in Costa Rica.” Aquatic Conservation: Marine and Freshwater Ecosystems 18.4 (2008): 408-417.}.
  5. CONCLUSION: The weaknesses of the study listed above limits the interpretation of the findings and the validity of the claim that AGW climate change caused migration from Central America by imposing drought conditions there. The exclusive focus on the ITCZ variable and the limit of the paleo data to the last 2000 years invalidates the findings particularly so in light of the Aguilar(2005) paper listed in the bibliography below.  




  1. Aguilar, Enric, et al. “Changes in precipitation and temperature extremes in Central America and northern South America, 1961–2003.” Journal of Geophysical Research: Atmospheres 110.D23 (2005)[FULL TEXT]    In November 2004, a regional climate change workshop was held in Guatemala with the goal of analyzing how climate extremes had changed in the region. Scientists from Central America and northern South America brought long‐term daily temperature and precipitation time series from meteorological stations in their countries to the workshop. After undergoing careful quality control procedures and a homogeneity assessment, the data were used to calculate a suite of climate change indices over the 1961–2003 period. Analysis of these indices reveals a general warming trend in the region. The occurrence of extreme warm maximum and minimum temperatures has increased while extremely cold temperature events have decreased. Precipitation indices, despite the large and expected spatial variability, indicate that although no significant increases in the total amount are found, rainfall events are intensifying and the contribution of wet and very wet days are enlarging. Temperature and precipitation indices were correlated with northern and equatorial Atlantic and Pacific Ocean sea surface temperatures. However, those indices having the largest significant trends (percentage of warm days, precipitation intensity, and contribution from very wet days) have low correlations to El Niño–Southern Oscillation. Additionally, precipitation indices show a higher correlation with tropical Atlantic sea surface temperatures.
  2. Dore, Mohammed HI. “Climate change and changes in global precipitation patterns: what do we know?.” Environment international 31.8 (2005): 1167-1181[FULL TEXT]  The objective of this paper is to synthesize the large literature recording changing patterns of precipitation in the observed data, thus indicating that climate change is already a reality. Such a synthesis is required not only for environmental researchers but also for policy makers. The key question is the broad picture at major regional and continental levels. Some interesting conclusions for this survey are emerging. For example, the review shows increased variance of precipitation everywhere. Consistent with this finding, we observe that wet areas become wetter, and dry and arid areas become more so. In addition, the following general changing pattern is emerging: (a) increased precipitation in high latitudes (Northern Hemisphere); (b) reductions in precipitation in China, Australia and the Small Island States in the Pacific; and (c) increased variance in equatorial regions. The changes in the major ocean currents also appear to be affecting precipitation patterns. For example, increased intensity and frequency of El Niño and ENSO seem associated with evidence of an observed “dipole” pattern affecting Africa and Asia, although this time series is too short so far. But the changing pattern calls for renewed efforts at adaptation to climate change, as the changing precipitation pattern will also affect the regional availability of food supply.
  3. Vera, Carolina, et al. “Climate change scenarios for seasonal precipitation in South America from IPCC‐AR4 models.” Geophysical research letters 33.13 (2006)[FULL TEXT]  A subset of climate simulations of the 20th century from the IPCC‐AR4 is analyzed to assess the ability of these models to reproduce the observed climatological seasonal precipitation in South America during the period 1970–1999. Changes of the model climatology in a climate change scenario (SRESA1b) for the period 2070–2099 are also discussed. Results show that models are able to reproduce the main features of the precipitation seasonal cycle over South America, although the precipitation in the SACZ region and the precipitation maximum over southeastern South America observed during the cold season are not well‐ represented. There is a general consensus among models that the precipitation changes projected are mainly: i) an increase of summer precipitation over southeastern subtropical South America; ii) a reduction of winter precipitation over most of the continent; and iii) reduction of precipitation in all seasons along the southern Andes.
  4. Maurer, Edwin P. “Climate model based consensus on the hydrologic impacts of climate change to the Rio Lempa basin of Central America.” (2009). [FULL TEXT]  Temperature and precipitation from 16 climate models each using two emissions scenarios (lower B1 and mid-high A2) were used to characterize the range of potential climate changes for the Rio Lempa basin of Central America during the middle (2040–2069) and end (2070–2099) of the 21st century. A land surface model was applied to investigate the hydrologic impacts of these changes, focusing on inflow to two major hydropower reservoirs. By 2070– 2099 the median warming relative to 1961–1990 was 1.9◦C and 3.4◦C under B1 and A2 emissions, respectively. For the same periods, the models project median precipitation decreases of 5.0% (B1) and 10.4% (A2). Median changes by 2070–2099 in reservoir inflow were 13% (B1) and 24% (A2), with largest flow reductions during the rising limb of the seasonal hydrograph, from June through September. Frequency of low flow years increases, implying decreases in firm hydropower capacity of 33% to 53% by 2070–2099.
  5. Campbell, Jayaka D., et al. “Future climate of the Caribbean from a regional climate model.” International Journal of Climatology 31.12 (2011)  [FULL TEXT]   Scenarios of rainfall and temperature changes for the period 2071–2100 under the A2 and B2 Special Report on Emissions scenarios are examined using the Hadley Centre Providing Regional Climates for Impacts Studies regional climate model. The model simulates ‘present‐day’ (1979–1990) rainfall and temperature climatologies reasonably well, capturing the characteristic bimodality of Caribbean rainfall and the boreal summer maximum and winter minimum temperatures. Seasonal spatial patterns are also reproduced, but rainfall amounts are underestimated over the northern Caribbean island masses, including Cuba, Jamaica, Hispaniola and Puerto Rico. Temperatures over the region are also overestimated by 1–3 °C. For the period 2071–2100, temperatures are projected to increase across the region by 1–4 °C for all months irrespective of the scenario. The rainfall response varies with season with one of the more robust changes being an intensification of a gradient pattern in November–January, in which the northern Caribbean (i.e. north of 22°N) gets wetter and the southern Caribbean gets drier. There is also a robust June–October drying signal. The results point to changes in the regional circulation patterns due to the human‐induced climate change and warrants further investigation. Copyright © 2010 Royal Meteorological Society
  6. Rajah, Kailash, et al. “Changes to the temporal distribution of daily precipitation.” Geophysical Research Letters 41.24 (2014): 8887-8894[FULL TEXT]  Changes to the temporal distribution of daily precipitation were investigated using a data set of 12,513 land‐based stations from the Global Historical Climatology Network. The distribution of precipitation was measured using the Gini index (which describes how uniformly precipitation is distributed throughout a year) and the annual number of wet days. The Mann‐Kendall test and a regression analysis were used to assess the direction and rate of change to both indices. Over the period of 1976–2000, East Asia, Central America, and Brazil exhibited a decrease in the number of both wet and light precipitation days, and eastern Europe exhibited a decrease in the number of both wet and moderate precipitation days. In contrast, the U.S., southern South America, western Europe, and Australia exhibited an increase in the number of both wet and light precipitation days. Trends in both directions were field significant at the global scale.
  7. Chang, Ni‐Bin, et al. “Global nonlinear and nonstationary climate change effects on regional precipitation and forest phenology in Panama, Central America.” Hydrological processes 29.3 (2015): 339-355[FULL TEXT]  The inherent effects of global sea surface temperature (SST) anomalies on hydrological cycle and vegetation cover complicate the structure of tropical climate at the regional scale. Assessing hydrological processes related to climate forcing is important in Central America because it is surrounded by both the Pacific and Atlantic oceans and two continental landmasses. In this study, the use of high‐resolution remote sensing imagery in wavelet analysis helps identify nonstationary characteristics of hydrological and ecological responses. The wavelet‐based empirical orthogonal function (WEOF) further reflects the nonlinear relationship between the Atlantic and Pacific SST and the greenness of a pristine forested site in Panama, La Amistad International Park. Integrated WEOF and descriptive statistics for data analysis reveal a higher temporal variability in terrestrial precipitation relative to in situ land surface temperature and its probable effects on the presence of dry periods. Such teleconnection signals of SST were identified as a driving force of decline in tropical forest greenness during dry periods. The results of our remote sensing‐based wavelet analysis showed intra‐annual high‐frequency and biennial to triennial low‐frequency signals between enhanced vegetation index/precipitation datasets and SST indices in both Atlantic and Pacific oceans. A spatiotemporal priority search further confirmed the importance of the effects of the El Niño–Southern Oscillation (ENSO) over terrestrial responses in the selected study site. Coincidence of the effect of ENSO teleconnection patterns on precipitation and vegetation suggests possible impacts of El Niño‐associated droughts in Central America, accompanied by reduced rainfall, especially during the first months of rainy season (June, July, and August), and decline in vegetation cover during the dry season (March and April). Copyright © 2014 John Wiley & Sons, Ltd.
  1. Pisciottano, Gabriel, et al. “El niño-southern oscillation impact on rainfall in Uruguay.” Journal of Climate 7.8 (1994): 1286-1302[FULL TEXT]  The relationships between rainfall over Uruguay and the El Niño-Southern Oscillation phenomenon are investigated. Long time series of data from a dense network of rainfall stations are analyzed using an empirical method based on that proposed by Ropelewski and Halpert. The spatial patterns of the relationships and their temporal variability for the entire region and four subregions are studied in detail. It is found that years with El Niño events tend to have higher than average rainfall, especially from November to the next January. Further, years with high values of the Southern Oscillation index (501) tend to have lower than average rainfall, especially from October through December. These findings are in general agreement with previous studies. It is also found that the period from March through July tends to have higher than average rainfall after El Niño years and lower than average rainfall after high-SOI years. For the southern part of Uruguay, the wet anomalies during El Niño events are relatively weak, but the dry anomalies during high-SOI events are significant for the two periods identified. The dry anomalies disappear, and even revere, during January and February after high-SOI years. This feature does not have a symmetric counterpart during January and February after El Niño years. This study, therefore, provides both a verification and an extension of other studies that have emphasized southeastern South America but have used data from only a very few stations in the region.
  2. Robertson, Andrew W., and Carlos R. Mechoso. “Interannual and decadal cycles in river flows of southeastern South America.” Journal of Climate 11.10 (1998): 2570-2581.  [LINK]  The time series of annual streamflow of four rivers in southeastern and south-central South America (the Negro, Paraguay, Paraná, and Uruguay Rivers) for the period 1911–93 are analyzed. Application of the multitaper method shows that the following features are significant at the 95% level: 1) a nonlinear trend, 2) a near-decadal component, and 3) interannual peaks with ENSO timescales. The trend and near-decadal components are most marked in the two more central rivers, the Paraguay and Paraná, with ENSO timescale variability most pronounced in the Negro and Uruguay rivers in the southeast. Composites of SST are made for each of the statistically significant oscillatory components of river flow, by reconstructing each component using singular spectrum analysis. These composites confirm the influence of ENSO on the streamflow variability of the Negro and Uruguay Rivers, with El Niño associated with enhanced streamflow. On the decadal timescale, high river runoff is associated with anomalously cool SSTs over the tropical North Atlantic. A very similar near-decadal oscillation in SST over this region is identified separately from a rotated empirical orthogonal function analysis of gridded annual mean SSTs. The near-decadal component of the Paraguay and Paraná Rivers is strongest in the austral summer.
  3. Robertson, Andrew W., and Carlos R. Mechoso. “Interannual and interdecadal variability of the South Atlantic convergence zone.” Monthly weather review 128.8 (2000): 2947-2957[FULL TEXT]  Interannual variations of the summertime (January–March) atmospheric circulation over subtropical South America are examined during the period 1958–97 using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. It is found from an empirical orthogonal function analysis that an anomalous upper-tropospheric large-scale stationary eddy in the lee of the Andes tends to accompany a dipole in anomalous vertical motion. An anomalous cyclonic (anticyclonic) eddy accompanies an intensified (diffuse) South Atlantic convergence zone (SACZ), with anomalous descent (ascent) to the southwest. The cold-core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200-hPa vorticity balance are both characteristic of a stationary Rossby wave; the tendency for the eddy to be advected downstream by the mean westerlies is compensated by meridional advection of planetary vorticity and stretching associated with vertical motion. The anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the SACZ. A weak funneling of submonthly Rossby wave activity into this descent region is also identified. The interannual time series of the eddy is significantly correlated with north–south dipolar sea surface temperature (SST) anomalies over the southwest Atlantic; one standard deviation 200-hPa wind speed anomalies of up to 5 m s−1 are accompanied by SST anomalies of up to 0.3°C. A near-cyclic 15-yr component is identified, which the authors corroborate from independent analyses of southwest Atlantic SSTs and river flows; both are found to exhibit very similar oscillatory components. When the SACZ is intensified, the Paraná and Paraguay rivers in southern Brazil tend to swell, while the Uruguay and Negro rivers to the south tend to ebb; this north–south contrast in streamflow anomalies is most marked on the interdecadal timescale.
  4. Zhou, Jiayu, and K‐M. Lau. “Principal modes of interannual and decadal variability of summer rainfall over South America.” International Journal of Climatology: A Journal of the Royal Meteorological Society 21.13 (2001): 1623-1644[FULL TEXT]  Using the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) product together with the Goddard Earth Observing System (GEOS) reanalysis and the National Center for Environmental Prediction (NCEP) sea‐surface temperature (SST) data, we have conducted a diagnostic study of the interannual and decadal scale variability of principal modes of summer rainfall over South America for the period 1979–1995. By filtering the annual and short (<12 months) time‐scale variations, results of empirical orthogonal function analysis show three leading modes of rainfall variation identified with interannual, decadal and long‐term variability. Together, these modes explain more than half the total variance of the filtered data. The first mode is highly correlated with El Niño–Southern Oscillation (ENSO), showing a regional rainfall anomaly pattern largely consistent with previous results. This mode captures the summer season interannual variability, not only the Northeast Brazil drought but also its connection with excessive rainfall over Southern Brazil and the Ecuador coast in El Niño years. Another distinctive feature is the strengthening of the low‐level flow along the eastern foothills of the eastern Andes, signifying an enhancement of the South American summer monsoon in response to an El Niño anomaly.The decadal variation displays a meridional shift of the Inter‐Tropical Convergence Zone (ITCZ), which is tied to the anomalous cross‐equatorial SST gradient over the Atlantic and the eastern Pacific. Associated with this mode is a large‐scale mass swing between polar regions and the mid‐latitudes. Over the South Atlantic and the South Pacific, the anomalous subtropical high and the associated anomalous surface wind are dynamically consistent with the distribution of local SST anomalies, suggesting the importance of atmospheric forcing at the decadal time scale. The long‐term variation shows that since 1980 there has been a decrease of rainfall from the northwest coast to the southeast subtropical region and a southwards shift of the Atlantic ITCZ, leading to increased rainfall over northern and eastern Brazil. Possible links of this mode to the observed SST warming trend over the subtropical South Atlantic and to the interdecadal SST variation over the extratropical North Atlantic are discussed. Copyright © 2001 Royal Meteorological Society
  5. Haug, Gerald H., et al. “Southward migration of the intertropical convergence zone through the Holocene.” Science 293.5533 (2001): 1304-1308.  Titanium and iron concentration data from the anoxic Cariaco Basin, off the Venezuelan coast, can be used to infer variations in the hydrological cycle over northern South America during the past 14,000 years with subdecadal resolution. Following a dry Younger Dryas, a period of increased precipitation and riverine discharge occurred during the Holocene “thermal maximum.” Since ∼5400 years ago, a trend toward drier conditions is evident from the data, with high-amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the “Little Ice Age.” These regional changes in precipitation are best explained by shifts in the mean latitude of the Atlantic Intertropical Convergence Zone (ITCZ), potentially driven by Pacific-based climate variability. The Cariaco Basin record exhibits strong correlations with climate records from distant regions, including the high-latitude Northern Hemisphere, providing evidence for global teleconnections among regional climates.
  6. Nogués-Paegle, Julia, et al. “Progress in Pan American CLIVAR research: understanding the South American monsoon.” Meteorologica 27.12 (2002): 1-30[FULL TEXT PDF]  A review of recent findings on the South American Monsoon System (SAMS) is presented. SAMS develops over a large extension of land mass crossed by the equator with surface conditions that vary from the world’s largest tropical forest in Amazonia to a high desert in the Altiplano. The high Andes mountains to the west effectively block air exchanges with the Pacific Ocean, but plentiful moisture transport from the Atlantic maintains intense precipitation that is strongest over central Brazil. There is also abundant precipitation over the subtropical plains of South America in association with moisture transport from tropical latitudes. Furthermore, midlatitude systems are important modulators of the tropical precipitation. The combination of all these factors results in a unique seasonal evolution of convection and rainfall. The findings presented emphasize the system’s complexity, and highlight the importance of the South American continent as the core of atmospheric linkages with the adjacent oceans. A discussion on directions for research on SAMS is also presented. There are still outstanding questions on the relative roles played on the system evolution by the orography, local and remote heat sources, and sea surface temperature anomalies. Other remaining questions
    address the impact of Amazon-deforestation on water and energy cycles over the two largest river basins of South America.
  7. Paegle, Julia N., and Kingtse C. Mo. “Linkages between summer rainfall variability over South America and sea surface temperature anomalies.” Journal of Climate 15.12 (2002): 1389-1407[FULL TEXT]  A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined. The leading mode is related to El Niño–Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25° and 35°S. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50°W near the equator and negative loadings over Colombia and the subtropical plains. For December–January–February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displacements of the intertropical convergence zone. Northeast Brazil receives most rainfall in March–April–May (MAM) and it is modulated by both the Atlantic SSTAs and ENSO. In the interannual frequency band, the North Atlantic Oscillation (NAO) has very limited influence on rainfall. On the decadal timescales, the NAO leads REOF 2 by three years.Latitudinal variations of tropical convection are through the joint contribution of REOF 2 and REOF 4. REOF 4 is similar to REOF 2, but centers are displaced about 10° south. When these two EOFs are both positive, central South America is wet. The amplitudes of REOF 2 and REOF 4 are small during the mid-1950s to the mid-1960s and they are out of phase from 1968 to 1970, periods with persistent dry conditions over the upper La Plata River basin.
  8. Gagnon, Alexandre S., Karen E. Smoyer-Tomic, and Andrew B. Bush. “The El Nino southern oscillation and malaria epidemics in South America.” International Journal of Biometeorology 46.2 (2002): 81-89[FULL TEXT]  A better understanding of the relationship between the El Niño Southern Oscillation (ENSO), the climatic anomalies it engenders, and malaria epidemics could help mitigate the world-wide increase in incidence of this mosquito-transmitted disease. The purpose of this paper is to assess the possibility of using ENSO forecasts for improving malaria control. This paper analyses the relationship between ENSO events and malaria epidemics in a number of South American countries (Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname, and Venezuela). A statistically significant relationship was found between El Niño and malaria epidemics in Colombia, Guyana, Peru, and Venezuela. We demonstrate that flooding engenders malaria epidemics in the dry coastal region of northern Peru, while droughts favor the development of epidemics in Colombia and Guyana, and epidemics lag a drought by 1 year in Venezuela. In Brazil, French Guiana, and Ecuador, where we did not detect an ENSO/malaria signal, non-climatic factors such as insecticide sprayings, variation in availability of anti-malaria drugs, and population migration are likely to play a stronger role in malaria epidemics than ENSO-generated climatic anomalies. In some South American countries, El Niño forecasts show strong potential for informing public health efforts to control malaria.
  9. Doyle, Moira E., and Vicente R. Barros. “Midsummer low-level circulation and precipitation in subtropical South America and related sea surface temperature anomalies in the South Atlantic.” Journal of Climate 15.23 (2002): 3394-3410[LINK]  The midsummer interannual variability of the low-level tropospheric circulation and of the precipitation field in subtropical South America (SA) associated to the sea surface temperature (SST) anomalies in the western subtropical South Atlantic Ocean (WSSA) is investigated using reanalyses, regional precipitation datasets, and monthly SSTs. The region of the WSSA where SST has the strongest relation with precipitation in subtropical SA was identified using canonical correlation analysis. This region extends from 20° to 30°S and from 30° to 50°W. Composites corresponding to extreme SSTs in this area show two well-differentiated patterns in the low-level circulation and in the precipitation fields. In the composite, corresponding to the more positive SST anomalies in this area, the mainstream of the low-level flow and of the moisture transport from the continental low latitudes starts to follow a southeastward direction at 10°S, and converges with the west flow at 35°S over the Atlantic Ocean. On the other hand, in the composite corresponding to the more negative SST anomalies, the low-level flow and the moisture transport from the continental low latitudes turn eastward toward the South Atlantic convergence zone (SACZ) at about 20°S, converging with the flow from the north driven by the South Atlantic high. In this composite, there is an anticyclonic circulation with a westward flow between 25° and 35°S, which turns southward after reaching the proximity of the Andes Mountains.In the composite of the more positive anomalies, there are two regional maximums in the precipitation field. One maximum stretches along the continental extension of a southwardly displaced SACZ and another is centered at about 30°S and 55°W, in the path of the main stream of the low-level moisture transport. In the other composite, there is only one regional maximum in precipitation, which coincides with the continental extension of the SACZ shifted northward of its mean position, and with a relative minimum in northeastern Argentina and southern Brazil. In this composite, in western Argentina, there are positive anomalies in the precipitation field favored by the transport of moisture from the Atlantic Ocean. The low-level patterns of the monthly composites, corresponding to the more positive and more negative SST anomalies in the WSSA, are similar to the respective patterns of each of the phases of the active and inactive SACZ. This follows from the prevalence, according to the SST in the WSSA, of one or the other of the low-level patterns associated to the seesaw of the SACZ. A positive feedback between positive (negative) SST anomalies and weak (intense) SACZ activity might enhance the low-level circulation pattern associated with the SACZ seesaw. 
  10. Grimm, Alice M. “The El Niño impact on the summer monsoon in Brazil: regional processes versus remote influences.” Journal of Climate 16.2 (2003): 263-280[FULL TEXT]   The El Niño impact on Brazil’s summer monsoon has not been adequately assessed through seasonal analysis because it shows significant subseasonal variations. In this study, the El Niño influence on the summer monsoon circulation, rainfall, and temperature is analyzed with monthly resolution, using data from a dense network of stations. The expected precipitation percentiles during the monsoon season of El Niño (EN) events are calculated, as well as anomalies of surface temperature and thermodynamic parameters. This information is analyzed jointly with anomaly composites of several circulation parameters. The analysis shows that some precipitation and circulation anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. There are abrupt changes of anomalies within the summer monsoon season, suggesting the prevalence of regional processes over remote influences during part of the season. The probable role of remote influences and regional processes is assessed. The anomalous heat sources associated with El Niño perturb the Walker and Hadley circulations over South America and generate Rossby wave trains that produce important effects in the subtropics and extratropics. In the early summer monsoon season, remotely produced atmospheric perturbations prevail over Brazil. Anticyclonic low-level anomalies predominate over central-east Brazil, in the Tropics and subtropics, due to the subsidence over the Amazon and to Rossby waves in the subtropics. Easterly moisture inflow from the Atlantic is favored, but diverted toward northern South America (SA) and south Brazil. There are negative precipitation anomalies in north and central-east Brazil and positive ones in south Brazil. These precipitation anomalies are favored by the perturbation in the Walker and Hadley circulation over the east Pacific and South America, and by a Rossby wave train over southern SA that originates in the eastern Pacific. In January, with the enhancement of the continental subtropical heat low by anomalous surface heating during the spring, there is anomalous low-level convergence and cyclonic circulation over southeast Brazil, while at the upper levels anomalies of divergence and anticyclonic circulation prevail. This anomalous circulation directs moisture flux toward central-east Brazil, causing moisture convergence in this region. A favorable thermodynamic structure enhances precipitation over central-east Brazil, the dry anomalies in north Brazil are displaced northward, and the anomalies in south Brazil almost disappear. In February, after the above-normal precipitation of January, the surface temperature anomalies turn negative and the precipitation diminishes in central-east Brazil. There are negative rainfall anomalies in north Brazil and in the South Atlantic convergence zone (SACZ) and positive ones in south Brazil. Influence function analysis shows that while the anomalies of circulation over southeast Brazil in the spring of El Niño years are mostly due to remote influences from the tropical east Pacific, those in January are probably due to local influence. During this month the monsoonlike circulation is enhanced. Simultaneous and lagged correlation analysis of SST and rainfall in central-east Brazil shows that SST anomalies in the Atlantic Ocean off the southeastern coast of Brazil fluctuate on the same timescale as the circulation and precipitation anomalies.
  11. Grimm, Alice M. “How do La Niña events disturb the summer monsoon system in Brazil?.” Climate Dynamics 22.2-3 (2004): 123-138[FULL TEXT]  The rainy season in most of Brazil is associated with the summer monsoon regime in South America. The quality of this season is important because it rains little during the rest of the year over most of the country. In this study, the influence of La Niña events on the summer monsoon circulation, rainfall and temperature is analyzed with seasonal and monthly resolution, using data from a dense network of stations, giving a comprehensive view of the impact of these events. The expected precipitation percentiles during the monsoon season of La Niña events are calculated, as well as anomalies of surface temperature and thermodynamic parameters. This information is analyzed jointly with anomaly composites of several circulation parameters. The analysis shows that some anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. There are abrupt changes of anomalies within the summer monsoon season, suggesting the prevalence of regional processes over remote influences during part of the season. In spring there are positive precipitation anomalies in north and central-east Brazil and negative ones in south Brazil. These precipitation anomalies are favored by the perturbation in the Walker and Hadley circulation over the eastern Pacific and South America, and by perturbations in the rotational circulation over southern South America. Northerly moisture inflow from the Atlantic into northern South America is emphasized and diverted towards the mouth of the Amazon by the low-level cyclonic anomaly north of the equator. In December and January, probably triggered by anomalous surface cooling during the spring, there is an anomalous low-level divergence and an anticyclonic anomaly over southeast Brazil. This anomalous circulation directs moisture flux towards south Brazil, causing moisture convergence in part of this region and part of central-west Brazil. The thermodynamic structure in central-east Brazil does not favor precipitation over this region, and the wet anomalies in north Brazil are displaced northward. The dry anomalies in south Brazil almost disappear and even turn positive. In February, after the strongly below normal precipitation of January, the surface temperature anomalies turn positive over southeast Brazil. The low-level anticyclonic anomaly is much weaker than in January. There are positive rainfall anomalies in north Brazil and in the South Atlantic Convergence Zone, and negative ones return to south Brazil.
  12. Brown, Erik T., and Thomas C. Johnson. “Coherence between tropical East African and South American records of the little ice age.” Geochemistry, Geophysics, Geosystems 6.12 (2005)[FULL TEXT]  Comparison of high‐resolution paleoclimate records from Lake Malawi and the Cariaco Basin reveals a consistent tropical behavior during the Little Ice Age. These records, which are interpreted to represent southward excursions of intertropical convergence zone position over southern tropical East Africa and northeastern South America, are characterized by subcentennial variability superimposed on a ramp of increasing intensity from CE 1400 to 1800. This response appears similar to that of “cold” episodes during the Late Glacial. The synchrony of these records suggests a teleconnection in tropical climate that may be a manifestation of persistent ENSO‐like conditions.
  13. Vuille, Michael, and Martin Werner. “Stable isotopes in precipitation recording South American summer monsoon and ENSO variability: observations and model results.” Climate Dynamics 25.4 (2005): 401-413.  The South American Summer Monsoon (SASM) is a prominent feature of summertime climate over South America and has been identified in a number of paleoclimatic records from across the continent, including records based on stable isotopes. The relationship between the stable isotopic composition of precipitation and interannual variations in monsoon strength, however, has received little attention so far. Here we investigate how variations in the intensity of the SASM influence δ18O in precipitation based on both observational data and Atmospheric General Circulation Model (AGCM) simulations. An index of vertical wind shear over the SASM entrance (low level) and exit (upper level) region over the western equatorial Atlantic is used to define interannual variations in summer monsoon strength. This index is closely correlated with variations in deep convection over tropical and subtropical South America during the mature stage of the SASM. Observational data from the International Atomic Energy Agency-Global Network of Isotopes in Precipitation (IAEA-GNIP) and from tropical ice cores show a significant negative association between δ18O and SASM strength over the Amazon basin, SE South America and the central Andes. The more depleted stable isotopic values during intense monsoon seasons are consistent with the so-called ’‘amount effect‘’, often observed in tropical regions. In many locations, however, our results indicate that the moisture transport history and the degree of rainout upstream may be more important factors explaining interannual variations in δ18O. In many locations the stable isotopic composition is closely related to El Niño-Southern Oscillation (ENSO), even though the moisture source is located over the tropical Atlantic and precipitation is the result of the southward expansion and intensification of the SASM during austral summer. ENSO induces significant atmospheric circulation anomalies over tropical South America, which affect both SASM precipitation and δ18O variability. Therefore many regions show a weakened relationship between SASM and δ18O, once the SASM signal is decomposed into its ENSO-, and non-ENSO-related variance.
  14. Turney, Chris SM, and Douglas Hobbs. “ENSO influence on Holocene Aboriginal populations in Queensland, Australia.” Journal of Archaeological Science 33.12 (2006): 1744-1748[FULL TEXT]  In the Pacific region, the onset of modern El Niño/Southern Oscillation (ENSO) activity at approximately 5000 years ago may have played a significant role in the development of cultures in the Pacific basin. Within Australia, similar trends in population and resource use have been identified but largely ascribed to cultural changes. To test human responses to changing ENSO activity through the Holocene we analysed a comprehensive suite of 710 radiocarbon ages from archaeological sites in ENSO-sensitive Queensland. We observe a dramatic and sustained increase in landscape activity at inland sites from 4860 ± 15 years ago, statistically indistinguishable from the timing of the onset of modern ENSO activity. Subsequent changes in long-term activity directly impacted on human populations indicating that once established, ENSO maintained a continuous influence on disparate cultures throughout the Pacific basin.
  15. Grimm, Alice M., Jeremy S. Pal, and Filippo Giorgi. “Connection between spring conditions and peak summer monsoon rainfall in South America: Role of soil moisture, surface temperature, and topography in eastern Brazil.” Journal of Climate 20.24 (2007): 5929-5945[FULL TEXT]  A link between peak summer monsoon rainfall in central-east Brazil, composing part of the South American monsoon core region, and antecedent conditions in spring is disclosed. Rainfall in this region during part of spring holds a significant inverse correlation with rainfall in peak summer, especially during ENSO years. A surface–atmosphere feedback hypothesis is proposed to explain this relationship: low spring precipitation leads to low spring soil moisture and high late spring surface temperature; this induces a topographically enhanced low-level anomalous convergence and cyclonic circulation over southeast Brazil that enhances the moisture flux from northern and central South America into central-east Brazil, setting up favorable conditions for excess rainfall. Antecedent wet conditions in spring lead to opposite anomalies. The main links in this hypothesis are confirmed through correlation analysis of observed data: spring precipitation is negatively correlated to late spring surface temperature in central-east Brazil, and surface temperature in southeast Brazil is positively correlated with peak summer monsoon precipitation in central-east Brazil. The intermediary links of the surface–atmosphere feedback are tested in sensitivity experiments with the regional climate model version 3 (RegCM3). These experiments confirm that the proposed links are possible: the reduced soil moisture in central-east Brazil is shown to increase the surface temperature and produce a cyclonic anomaly over southeast Brazil, as well as increased precipitation in central-east Brazil. A crucial role of the mountains of southeast Brazil in anchoring the patterns of intraseasonal variability, and sustaining the “dipolelike” precipitation mode observed over South America, is suggested. The low predictability of monsoon rainfall anomalies in central-east Brazil during the austral summer might be partially ascribed to the fact that the models do not well reproduce the topographical features and the land–atmosphere interactions that are important for the variability in that region.
  16. Evangelista, H., et al. “Evidences linking ENSO and coral growth in the Southwestern-South Atlantic.” Climate Dynamics 29.7-8 (2007): 869-880.  Physical and biological changes in the marine environment, induced by oceanic-atmospheric processes, can be imprinted in massive coral skeletons. Herein, we present an evidence of potential El Niño impacts at the Southwestern South Atlantic Ocean (SWSA) inferred from the sclerochronology of the reef coral Favia leptophylla. The application of spectral analysis (wavelet decomposition and the iterative regression) to coral growth length and to meteorological-oceanographic parameters (air temperature, sea surface temperature and precipitation) as well as to Southern Oscillation Index (SOI) and solar irradiation indicated a major significant inverse relationship between SOI and coral growth length at the 4–8 years frequency band. We propose here that coral growth length from the SWSA could be affected by El Niño Southern Oscillation (ENSO) events through an “atmospheric bridge”, in contrast to its direct effect at the Pacific Ocean, related to the increase in sea surface temperature.
  17. Yancheva, Gergana, et al. “Influence of the intertropical convergence zone on the East Asian monsoon.” Nature 445.7123 (2007): 74-77The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits1,2,3, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bølling–Allerød warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons1,2,3. We conclude that this anticorrelation is best explained by migrations of the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period AD 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.
  18. Donders, Timme H., Friederike Wagner-Cremer, and Henk Visscher. “Integration of proxy data and model scenarios for the mid-Holocene onset of modern ENSO variability.” Quaternary Science Reviews 27.5-6 (2008): 571-579.  Climate model studies have shown a gradual insolation-forced intensification of the El Niño–Southern Oscillation (ENSO) during the Holocene. Proxy records of past climate variability provide important test cases for such model simulations, and are needed to determine the exact mechanisms and dynamics of the ENSO system. We provide an integrated overview of marine and terrestrial paleoclimatic proxy data relevant for detecting ENSO variability. We reconstruct spatial climate patterns during two time-slices, 6–5 and 4.5–3.5 ka cal BP, to examine the mid-Holocene intensification of ENSO. The proxy data consistently indicate that a state change occurred at ∼5 ka cal BP towards active ENSO cyclicity in the equatorial Pacific. Furthermore, from around 3 ka cal BP the ENSO-teleconnected regions are characterized by an increased impact of ENSO, comparable to the present-day high-amplitude fluctuations of ENSO. Model studies have thus far explained the late-Holocene intensification of ENSO by insolation-forced Pacific trade wind reduction during summer. Our review shows that this single mechanism cannot completely explain the observed Holocene changes. An additional mechanism is proposed, involving increased Indo-Pacific Warm Pool (IPWP) heat charging, which is a possible explanation for the late-Holocene increase in ENSO amplitude.
  19. Garreaud, René D., et al. “Present-day south american climate.” Palaeogeography, Palaeoclimatology, Palaeoecology 281.3-4 (2009): 180-195[LINK]  This paper documents the main features of the climate and climate variability over South America, on the basis of instrumental observations gathered during the 20th Century. It should provide a modern reference framework for paleoclimate research in South America, targeting high-resolution proxies over the past few centuries. Several datasets suitable for present-day climate research are first described, highlighting their advantages as well as their limitations. We then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans. In particular, the diversity of precipitation, temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large-scale circulation caused by the Andes cordillera, the contrasting oceanic boundary conditions and the landmass distribution. Similarly, the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence, as some regions are more influenced by large-scale phenomena rooted in the tropical oceans while others are more influenced by high-latitude phenomena. The impact of these large-scale phenomena over South America is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields. We have included a discussion on the seasonality and long-term stability of such impacts, and complemented our general description by an updated review of the literature on climate variability over specific regions.
  20. Arriaza, Bernardo T., et al. “Possible influence of the ENSO phenomenon on the pathoecology of diphyllobothriasis and anisakiasis in ancient Chinchorro populations.” Memórias do Instituto Oswaldo Cruz 105.1 (2010): 66-72[FULL TEXT]   Current clinical data show a clear relationship between the zoonosis rates of Diphyllobothrium pacificum and Anisakis caused by the El Niño Southern Oscillations (ENSO) phenomenon along the Chilean coast. These parasites are endemic to the region and have a specific habitat distribution. D. pacificum prefers the warmer waters in the northern coast, while Anisakis prefers the colder waters of Southern Chile. The ENSO phenomenon causes a drastic inversion in the seawater temperatures in this region, modifying both the cool nutrient-rich seawater and the local ecology. This causes a latitudinal shift in marine parasite distribution and prevalence, as well as drastic environmental changes. The abundance of human mummies and archaeological coastal sites in the Atacama Desert provides an excellent model to test the ENSO impact on antiquity. We review the clinical and archaeological literature debating to what extent these parasites affected the health of the Chinchorros, the earliest settlers of this region. We hypothesise the Chinchorro and their descendants were affected by this natural and cyclical ENSO phenomenon and should therefore present fluctuating rates of D. pacificum and Anisakis infestations.
  21. Schneider, Tapio, Tobias Bischoff, and Gerald H. Haug. “Migrations and dynamics of the intertropical convergence zone.” Nature 513.7516 (2014): 45-53Rainfall on Earth is most intense in the intertropical convergence zone (ITCZ), a narrow belt of clouds centred on average around six degrees north of the Equator. The mean position of the ITCZ north of the Equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere. On seasonal and longer timescales, the ITCZ migrates, typically towards a warming hemisphere but with exceptions, such as during El Niño events. An emerging framework links the ITCZ to the atmospheric energy balance and may account for ITCZ variations on timescales from years to geological epochs.  

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