Moira E. Doyle

Climate Variability in Southern South America Associated with El Nino and La Nina Events

A comprehensive view is given of the precipitation and circulation anomalies associated with the various stages of El Nino (EN) and La Nina (LN) events all over southern South America (SSA). This view comprises the delineation of coherent regions with respect to precipitation anomalies, the identification of the seasons of maximum anomalies, the indication of their magnitude, and the assessment of their consistency during those events. In addition, the spatial and temporal variability of these anomalies is detailed by calculating the expected precipitation percentiles and the consistency of wet and dry anomalies for each station and each three-month running season during EN and LN events. Composites of circulation anomalies and an assessment of their consistency are also presented and their connection with the precipitation anomalies is discussed. Southern Brazil presents the strongest average signal in EN events. The general behavior toward opposite signals in the precipitation and circulation anomalies over SSA during almost the same periods of the EN and LN events indicates a large degree of linearity in the response to these events. The timing of the anomalies changes throughout SSA, leading to the identification of eight different coherent regions in the EN case and six in the LN case. This regionalization is mostly caused by different processes leading to precipitation anomalies in SSA during those events. All these regions show a significant response in some part of each event. The magnitude and consistency of this response show a large spatial variability and some areas present very strong and consistent anomalies sometimes not disclosed when large coherent regions are analyzed. In spite of the differences in timing, some features of the precipitation anomalies are rather uniform throughout the region during EN and LN events. In EN episode, there is a tendency to lower than median precipitation in the year before the event, which continues until March of the year of the event. In a vast region, east of the Andes, the strongest positive precipitation anomalies occur in spring of this year, when the circulation anomalies concur to enhance rainfall over several regions. During the summer of the mature stage the positive precipitation anomalies almost disappear and then reappear in some regions in late summer-early autumn and in winter of the year following the starting year of the event. This description holds partially for the LN event, but with opposite signs, although there is a larger spatial variability in the LN-related anomalies in the following year and some shifts in timing. As for precipitation, the symmetry of the geopotential height anomaly fields with opposite signs between LN and EN cases is also remarkable, especially during the year (0).

Midsummer Low-Level Circulation and Precipitation in Subtropical South America and Related Sea Surface Temperature Anomalies in the South Atlantic

Abstract 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 t...

Recent Precipitation Trends in Southern South America East of the Andes: An Indication of Climatic Variability

Positive trends in precipitation were observed during 1916–1991, especially since the fifties, over most of the Argentine territory. The seasonal variation of the climatic parameters, including precipitation between 1956–1991, can be summarized as a displacement of the positive nucleus of precipitation to the northeast from summer to winter and a less systematic return to the southwest from winter to summer. Correlation studies between annual precipitation and hemispheric indices show that the correlation with the mean meridional gradient of temperature (MMTG) is of the same importance and in some areas greater than the correlation with the Southern Oscillation Index (SOI). Furthermore, the correlation field strongly suggests that the precipitation trends observed in the last 35 years are due to the decrease of the MMTG. In fact, the MMTG decreases around 1.5° C during that period. According to the theory of baroclinic instability this implies a displacement to higher latitudes of the planetary circulation systems. A displacement of 3° in latitude to the south has been reported by Gibson (1992) for the mean position of the maximum wind at 200 hPa in the Southern Hemisphere. Since 1976 a similar displacement of the Atlantic Subtropical High also can be inferred from data of the Atlantic coast. The shift to the south of the general circulation features produces trends in precipitation because of the close connection between precipitation and the latitude of the circulation systems. The data show relationship between the precipitation field and the latitude of the maximum wind speed at the altitude of 200 hPa. Consequently, the observed trends in the precipitation fields could be explained largely by a 5° displacement to the south of this latitude of maximum wind during the last 35 years. This means that in the study area an important component of the global circulation system changed its position in a statistically provable manner.

Geological evidence for a drought episode in the western Pampas (Argentina, South America) during the early–mid 20th century

Drought episodes during the early–mid 20th century were recognized and described in several places around the world, with extreme dry conditions and widespread landscape denudation, like during the famous ‘Dust Bowl’ in North America. However, there is scant documentation of droughts in southern South America, particularly from the Pampas, and none based on the geological record. In this article, we provide clear evidence of aeolian reactivation and sand deposition in some areas of La Pampa and San Luis provinces, western Pampas (Argentina), during early–mid 20th century in response to drier conditions, probably amplified, like historic droughts in North America, by anthropogenic factors (e.g. significant population increase and agriculture expansion into a fragile environment). Evidence includes widespread bare sand blowouts, extensive surfaces with active sand migration, steep dune lee slopes, and sharp crests covered by weak soil development (A/C profile), accompanied by historical documents. Optically stimulated luminescence (OSL) ages on aeolian beds confirm mobilization and sedimentation by wind processes c. 95–60 yr BP. Considering the dominant (over 70%) austral spring–summer precipitation, it is possible the rainfall deficit in western Pampas was linked to positive sea surface temperature (SST) anomalies in the western subtropical South Atlantic Ocean (20–30°S and 30–50° W), according to significant canonical correlation between the precipitation field in subtropical South America and the Atlantic Ocean SST anomalies.

Identification of the principal patterns of summer moisture transport in South America and their representation by WCRP/CMIP3 global climate models

The goal of this study is to assess the ability of a set of global climate models (GCMs) to represent the main regional spatial patterns of austral summer moisture transport in South America in order to evaluate if one of the possible causes of GCMs misestimating summer precipitation in this region could be associated with an erroneous representation of these patterns. For this purpose, NCEP/NCAR reanalysis and 20 GCMs from the WCRP/CMIP3 multi-model dataset for the period 1960–1999 were considered. Extreme cases of moisture transport patterns were selected to assess their association with rainfall anomalies. Results obtained indicate that only some aspects of water vapor transport and convergence in South America as well as the associated precipitation anomalies can be reproduced adequately by GCMs. Finally, a case study is presented showing that one of the moisture transport patterns identified was observed during December 2008–February 2009.

Experiments using new initial soil moisture conditions and soil map in the Eta model over La Plata Basin

An effort towards a more accurate representation of soil moisture and its impact on the modeling of weather systems is presented. Sensitivity tests of precipitation to soil type and soil moisture changes are carried out using the atmospheric Eta model for the numerical simulation of the development of a mesoscale convective system over northern Argentina. Modified initial soil moisture conditions were obtained from a hydrological balance model developed and running operationally at INPE. A new soil map was elaborated using the available soil profile information from Brazil, Paraguay, Uruguay, and Argentina and depicts 18 different soil types. Results indicate that more accurate initial soil moisture conditions and incorporating a new soil map with hydraulic parameters, more representative of South American soils, improve daily total precipitation forecasts both in quantitative and spatial representations.

Twenty-first century projections of extreme precipitations in the Plata Basin

Distributions of monthly rainfall averaged spatially over three regions of the La Plata Basin (LPB) were projected for 2011–2040 and 2071–2100 using outputs of four regional climate models (RCMs) nested in three different general circulation models, run with the SRES A1B emission scenario. Tuning of simulations with observations was done at the control period 1981–1990. During the past 50 years, in part of LPB, there was a positive trend in annual precipitation. Two of the models indicate the maintenance of this trend over the northeast of Argentina and south of Brazil, while over the southernmost region of LPB, all models show increasing precipitation throughout the twenty-first century. Trends are less ambiguous for extreme precipitation, especially in the southernmost part of LPB, where huge and long-lasting floods take place over plains with small drainage. Months with extreme precipitation in LPB present a pronounced annual cycle with higher frequency from October to April. According to the RCM projections, this pattern would persist during the twenty-first century. Although, other factors cannot be discarded, the projected trends towards higher extreme monthly precipitation seem to be caused by an increase in the moisture convergence in the lower atmosphere over the east of LPB.

On how Cattle Egret (Bubulcus ibis) spread to the Americas: meteorological tools to assess probable colonization trajectories

The Cattle Egret (Bubulcus ibis) is native to the old world. Before 1877 no Cattle Egrets had been sighted in the Americas. There are no written records of this species being transported to or escaping from captivity in South America and there is enough evidence to suggest that individuals are capable of making the crossing from Africa to the Americas unaided. Since long-distance movements of species are partly dependent on meteorological events we analyze the possibility of B. ibis crossing the Atlantic Ocean aided only by wind conditions evaluating existing theories and shedding light on their feasibility through the analysis of weather patterns and atmospheric circulation. Zonal and meridional wind components taken from the Twentieth Century Reanalysis Project were used to calculate trajectories between different points along the West African coast and South America between 1871 and 1920 in two seasons (March–April and September–October). From a total of 192,864 trajectories analyzed, 1,695 with origin in the west coast of Africa reached the NE coast of South America or the Caribbean islands in less than a week (successful trajectories). The probability of these successful trajectories originating in Central Africa was above 0.65 for the majority of the destinations analyzed. Particularly, in Guyana and Suriname where B. ibis was first sighted, the probability of the origin being Central Africa was 0.84, most of them occurring during March. Several weather events favored not only the colonization of the Cattle Egret but also its establishment and spread all throughout the Americas.