Gabriel Silvestri

Differences in El Niño Response over the Southern Hemisphere

Southern Hemisphere (SH) circulation conditions during austral springs of ENSO events are examined. Based on previous knowledge that SST variations over the subtropical south-central Pacific (SSCP) region are linked to differences among El Nino (EN) events, a stratification of the springs associated with EN events was performed according to SST conditions over the El Nino-3.4 sector and over the SSCP region. The EN events associated with cold conditions in the SSCP (WC) exhibit enhanced convection not only in the intertropical convergence zone over the central equatorial Pacific but also in the South Pacific convergence zone (SPCZ) extended south- eastward into the subtropical regions. The resulting heating forcing intensifies a localized overturning cell, which is associated with an anomalous Rossby wave source in the central South Pacific extratropical region. Neither the Rossby wave source nor the associated wave pattern is evident during EN events associated with warm conditions in the SSCP and inactive SPCZ (WW). The basic features that characterize the differences in the EN response over the South Pacific can also be identified through the analysis of the SPCZ activity over the central South Pacific. The fact that variations in SPCZ activity lead SST anomaly changes in the SSCP would indicate that the differences in the EN response over the SH might be mainly driven by atmospheric changes, which induces extratropical SST anomalies. The differences in the circulation anomalies that characterize both types of EN response over the SH were further explored through the analysis of the activity of the three leading modes of circulation variability. The combined effect of the three leading patterns describes in some extent the intensification (weakening) of the cyclonic circulation anomaly observed over the southeastern Pacific in WC (WW), associated with an active (inactive) SPCZ. In particular, the interdecadal variability observed in the Pacific by many previous studies influences the circulation response to ENSO over the SH, mainly through changes in the activity of the SH annular mode.

Nonstationary Impacts of the Southern Annular Mode on Southern Hemisphere Climate

Abstract The temporal stability of the southern annular mode (SAM) impacts on Southern Hemisphere climate during austral spring is analyzed. Results show changes in the typical hemispheric circulation pattern associated with SAM, particularly over South America and Australia, between the 1960s–70s and 1980s–90s. In the first decades, the SAM positive phase is associated with an anomalous anticyclonic circulation developed in the southwestern subtropical Atlantic that enhances moisture advection and promotes precipitation increase over southeastern South America (SESA). On the other hand, during the last decades the anticyclonic anomaly induced by the SAM’s positive phase covers most of southern South America and the adjacent Atlantic, producing weakened moisture convergence and decreased precipitation over SESA as well as positive temperature anomaly advection over southern South America. Some stations in the Australia–New Zealand sector and Africa exhibit significant correlations between the SAM and prec...

Eastern Patagonia Seasonal Precipitation: Influence of Southern Hemisphere Circulation and Links with Subtropical South American Precipitation

AbstractSome aspects of the seasonal precipitation over eastern Patagonia, the southernmost area of South America east of the Andes Cordillera, are examined in this paper. Results indicate that the central-north areas, the southern continental region, and the southernmost islands are three independent regions of seasonal precipitation, and that each of them is associated with specific patterns of atmospheric circulation. Precipitation over the central-north region is significantly related to the precipitation over a wide area of southern South America east of the Andes during the four seasons. Enhanced (reduced) precipitation over this area is associated with weakened (intensified) westerly flow in the region. Precipitation over the southern continental area has a close connection with the dipolar pattern of precipitation over subtropical South America during spring, summer, and autumn. The anomalies of atmospheric circulation at low and upper levels associated with the subtropical dipole are also able to...

On the variability of seasonal temperature in southern South America

The aim of this paper is to investigate different aspects of the seasonal-to-interannual temperature variability in Eastern Patagonia, the southernmost area of South America, east of the Andes Cordillera. Homogenous regions of seasonal variability and the atmospheric circulation patterns associated with warm and cold conditions in each of them are described in this study. Relationships between temperature in Eastern Patagonia and that registered in other areas of southern South America are also addressed. Results show that the northern and southern areas of Eastern Patagonia have different temperature variability in summer and autumn whereas the temperature variability tends to be more homogeneous within the region during winter and spring. Warm (cold) conditions in the northern areas are associated with reinforced (weakened) westerlies in summer, winter and spring whereas northerly (southerly) advections of warm (cold) air toward the region produce such conditions in autumn. Temperature in the southern portion of Eastern Patagonia is affected by anticyclonic (cyclonic) anomalies that enhance (reduce) the incoming solar radiation and induce reinforced (weakened) westerlies promoting warm (cold) conditions in the region. Furthermore, cyclonic (anticyclonic) anomalies at subpolar latitudes hinder (favor) outbreaks of cold air increasing (decreasing) the temperature over areas of Eastern Patagonia. The circulation anomalies associated with warm (cold) conditions in Eastern Patagonia also promote cold (warm) conditions over areas of northern Argentina, Paraguay and southern Brazil. Consequently, a dipole of temperature is detected in southern South America with centers of opposite sign over these regions.

Accelerated greenhouse gases versus slow insolation forcing induced climate changes in southern South America since the Mid-Holocene

This paper is a pioneering analysis of past climates in southern South America combining multiproxy reconstructions and the state-of-the-art CMIP5/PMIP3 paleoclimatic models to investigate the time evolution of regional climatic conditions from the Mid-Holocene (MH) to the present. This analysis allows a comparison between the impact of the long term climate variations associated with insolation changes and the more recent effects of anthropogenic forcing on the region. The PMIP3 multimodel experiments suggest that changes in precipitation over almost all southern South America between MH and pre-industrial (PI) times due to insolation variations are significantly larger than those between PI and the present, which are due to changes in greenhouse gas concentrations. Anthropogenic forcing has been particularly intense over western Patagonia inducing reduction of precipitation in summer, autumn and winter as a consequence of progressively weaker westerly winds over the region, which have moved further poleward, between ca. 35–55°S and have become stronger south of about 50°S. Orbital variations between the MH to the PI period increased insolation over southern South America during summer and autumn inducing warmer conditions in the PI, accentuated by the effect of anthropogenic forcing during the last century. On the other hand, changes in orbital parameters from the MH to the PI period reduced insolation during winter and spring inducing colder conditions, which have been reversed by the anthropogenic forcing.