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Glacial geologic studies in the Southern Hemisphere (SH) mid-latitudes (40-54°S) indicate renewed glacial activity in southern South America (Patagonia) and New Zealand's (NZ) South Island starting at ∼7 kyr, the so-called neoglaciation. Available data indicate that neoglacial advances in these regions occurred during a rising trend in atmospheric CO and CH concentrations, lower-than-present but increasing summer insolation and seasonality contrasts. In this paper we examine the climatological context in which neoglaciations occurred through analysis of the complete Paleoclimate Modelling Inter-comparison Project (PMIP2) database of simulations at 6 kyr for the SH. We observe that the amplitude of the annual insolation cycle in the SH did not change significantly at 6 kyr compared to the pre-industrial values, the largest difference occurring in autumn (MAM, negative anomalies) and spring (SON, positive anomalies). The simulated changes in temperatures over the SH respond to the insolation changes, with a 1-2 month delay over the oceans. This results in a reduced amplitude of the annual cycle of temperature and precipitation over most continental regions, except over Patagonia and NZ, that show a slight increase. In contrast, large-scale circulation features, such as the low and upper level winds and the subtropical anticyclones show an amplified annual cycle, as a direct response to the increased/decreased insolation during the transitional seasons SON/MAM. In the annual mean, there is a small but consistent equatorward shift of the latitude of maximum wind speed of 1-3° over the entire SH, which results in a small increase of wind speed over the South Pacific and Atlantic Oceans north of ∼50°S and a widespread decline south of 50°S. PMIP2 simulations for 6 kyr, indicate that in the annual mean, the SH mid-latitudes were colder, wetter and with stronger winds north of about 50°S. These conditions are consistent with the observed neoglacial advances in the region, as well as with terrestrial paleoclimate records from Patagonia that indicate cooling and a multi-millennial rising trend in Southern Westerly Wind intensity starting at ∼7.8 kyr. [ABSTRACT FROM AUTHOR]

In the second fortnight of December 2013, the longest heat wave registered until then occurred in Buenos Aires city and over a large region of Argentina, with large socio-economic impacts. Excess heat indexes were used to characterize this heat wave, which occurred within the warm season with largest number of heat wave days. This extreme event resulted in the longest consecutive period of heat wave conditions between 1979 and 2014. This event was the result of the combined activity of short intraseasonal (10–30 days), long intraseasonal (30–90 days) and larger (more than 90 days) time scales. Accounting for the lower frequency, dry soil moisture anomalies were observed in Argentina during autumn, winter and spring of 2013 probably favoring more extreme values in the temperature anomalies. Weekly geopotential heights anomalies computed during the event showed that the combination of the negative phase of the Southern Annular Mode and a wave-4 pattern at midlatitudes favored the development of positive geopotential height anomalies over southern South America which promoted subsidence motions there. Intraseasonal variability played a key role in the persistence of the heat wave. The development of an MJO event over the Indian Ocean (RMM phase 3) 12 days prior to the beginning of the heat wave may have contributed to organize the extratropical wave train which in turn favored the anticyclonic upper-level anomaly development over southern South America. Furthermore, an intense SACZ event during the first 12 days of the heat wave favored clear-sky conditions and diabatic heating as well as subsidence, while during the last days it was the northerly advection of warmer air in the 10–30-day time scale that maintained the intensity of the temperature anomalies. [ABSTRACT FROM AUTHOR]

A new climatology of South Atlantic cyclones is produced to provide new insights into the conditions leading to genesis in different regions of the domain. Cyclones are identified and tracked based on the relative vorticity at 850 hPa computed from the NCEP-CFSR winds. The characteristics of the cyclones are obtained by diagnostic variables sampled within a radial distance from the cyclone centers to produce the spatial distribution of cyclone properties at the time of genesis. Also, cyclone centered composites are used to analyze the cyclone structure and evolution during their genesis. There are four main cyclogenesis regions in the South Atlantic Ocean: the Southern Brazilian coast (SE-BR, 30 ∘ S ), over the continent near the La Plata river discharge region (LA PLATA, 35 ∘ S ), the southeastern coast of Argentina (ARG, 40 ∘ S – 55 ∘ S ) and the Southeastern Atlantic (SE-SAO, centered at 45 ∘ S and 10 ∘ W ). We found that cyclogenesis northward of 35 ∘ S occurs mainly due to low-level forcing associated with moisture transport in the summer, and is associated with upper-level forcing in the winter due to a strong baroclinic environment. Southward of 35 ∘ S , cyclones develop in a high baroclinic environment throughout the year with only a small influence from moist processes. The cyclone composites reveal that SE-BR and SE-SAO cyclones are associated with secondary development, the LA PLATA cyclones development is influenced by an orographic low in their early stages, and ARG cyclones are influenced by thermal advection as an essential mechanism in the reduction of static stability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]