GEOACTA 01/2008;

ABSTRACT RESUMEN La variabilidad de la precipitación en distintas escalas sobre Argentina subtropical puede ser vinculada a las condiciones de la temperatura superficial del mar (TSM) en el Atlántico Sur (Venegas et al., 1998, Robertson y Mechoso 1998, Doyle y Barros 2002). Particularmente, la precipitación estival en el Centro-Oeste de Argentina (COA) presenta también vinculación con las TSMs en latitudes medias en el sector sudoeste del Atlántico Sur (Compagnucci y Agosta 2006). Las variaciones en la TSM sobre esta área oceánica está modulada por el flujo de masa de aire en superficie en torno a 40°S (Wainer y Venegas 2002). Por tanto el objetivo de este trabajo es estudiar los procesos en baja frecuencia del sistema atmósfera/océano sobre el sector sudoeste del Atlántico Sur que se asocian a la precipitación estival del COA. Los resultados muestran que la circulación de la atmósfera tiene un centro de acción sobre el sudoeste del Atlántico Sur, en torno a 45°S-60°O, durante gran parte del siglo XX, que modula las fluctuaciones interanuales a multidecádicas (cuasi-ciclo de 18 años) de la precipitación en el COA, estudiadas por Compagnucci et al. (2002). Este centro de acción de la atmósfera es el forzante común para la precipitación y la TSM sobre el sector sudoeste del Atlántico Sur. Palabra claves: Cambio climático-precipitación-transición 1976/77-teleconexión ABSTRACT Some authors find that the precipitation variability at diverse scales over subtropical Argentina can be linked to sea surface temperature (SST) conditions of the South Atlantic (Venegas et al., 1998, Robertson and Mechoso 1998, Doyle and Barros 2002). Particularly, summer precipitation in Central-West Argentina (CWA) shows relationship with the SSTs at mid-latitudes over southwestern South Atlantic (Compagnucci and Agosta 2006). SSTs variations over that oceanic area are modulated by surface air mass flow around 40ºS (Wainer and Venegas 2002). Therefore, the objective of this work is to study low-frequency processes in the atmosphere/ ocean system over southwestern South Atlantic associated with summer precipitation in CWA. Results show that the atmospheric circulation has a center of action over southwestern South Atlantic around 45ºS/60ºW, associated with precipitation during the whole 20 th century that modules the quasi-18-year oscillation of CWA precipitation (Compagnucci et al., 2002). This

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    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 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.
    Journal of Climate 12/2002; 15:3394-3410. · 4.90 Impact Factor
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    ABSTRACT:  Summer rainfall variability (October to March) shows inter-annual to multi-decadal fluctuations over a vast area of subtropical Argentina between 28°S–38°S and 65°W–70°W. Statistically significant oscillations of quasi-period in the bands of 18–21, 6, 4 and 2 years can be found throughout the region and intra-regionally, though the latter are variable. The lower frequency variation produces alternating episodes of above and below normal rainfall each lasting roughly 9 years. This quasi-fluctuation appears to be shared with the summer rainfall region of South Africa and were in-phase related one another until mid-1970s. The teleconnection between both subtropical regions could be generated by an atmospheric-oceanic bridge through the global sea surface temperatures (SSTs), particularly those of the equatorial-tropical South Atlantic. From mid-1970s, the alternating wet and dry pattern has been interrupted in the Argentine region producing the longest, as yet unfinished, wet spell of the century. Thus, a significant change of the long-term variation was observed around 1977 toward lower frequencies. Since then the statistical model that explains more than 89% of the variance of the series until 1977, diverges from the observed values in the 1980s and 1990s. In addition the Yamamoto statistical index, employed to detect a climatic jump, reaches its major value in 1973 at the beginning of the current long wet spell. Therefore the change could be located between 1973 and 1977. Application of the t-student's test gives significant differences of mean values for pre-1977 and post-1977 sub-samples from both individual time series and the regional index series. The spectral analysis also shows changes in energy bands in concordance with the features of the change that occurred from mid-1970s. The change gives rise to a significant increment of more than 20% in average of normal rainfall over the region. Conversely, a drought between mid-1980s and the 1990s has been observed in the South African counterpart with severe characteristics, thereby continuing the quasi-18-year oscillation. Consequently, the low-frequency coherent behaviour between both the Argentine and South African regions is lost from the mid-1970s. The analysis of association of wet/dry spells and warm/cold, El Niño/La Niña episodes appears to be not significant at scales of year-to-year variability although at decadal to multi-decadal scales the association could be relevant. More than one process of multi-decadal variability of global SSTs could influence the Argentine summer rainfall region and the former bi-decadal teleconnection. Finally, potential hypothetical factors of change are discussed, such as the strengthening of direct and indirect mechanisms of moisture flux transport associated with global warming, low-level atmospheric circulation changes and/or to SSTs mean condition long-term variations over tropical and subtropical South Atlantic and South Pacific oceans.
    Climate Dynamics 12/2001; 18(5):421-435. DOI:10.1007/s003820100183 · 4.62 Impact Factor
  • Cambios en el régimen interanual de la precipitación estival en la región Centro-Oeste Argentina. . 63-84.


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