Abstract

AGOSTA, Eduardo A  and  BARRUCAND, Mariana G. Mean winter conditions and quasi-stationary Rossby waves associated with the winter frequency of warm and cold nights in subtropical Argentina. Geoacta [online]. 2012, vol.37, n.2, pp.147-166. ISSN 1852-7744.

The study aims to examine the mean conditions of the tropospheric circulation and possible forcing associated with the occurrence frequency of cold nights (TN10) and warm nights (TN90) during winter (JJA) over subtropical Argentina, to the north of 40°S and surrounding areas (ASA) from meteorological stations and NCEP/DOE AMIP-II and ECMWF ERA-interim reanalysis data. It is found that the frequency of warm nights (Tmin over percentile 90, TN90) is modulated at interannual scales by quasi-stationary wave propagation induced by convection anomalies in the Indic and Pacific. The high frequency of warm nights is associated with anomalous warming over the central equatorial Pacific (positive phase of El Niño-Southern Oscillation, ENSO). The low frequency of warm nights is linked to anomalous convection over the monsoon Indian area and the western tropical southern Indic. Hence quasi-stationary Rossby wave activity propagation is favored over the Indic, Pacific and southern South America. Such a teleconnection favors in turn the high fequency of cold nights (Tmin below percentile 10, TN10). Instead, the low frequency of cold nights is linked to the low-frequency variability of the Southern Hemisphere high-latitude mode (SAM). It is found that winters with high (low) frequency of cold (warm) nights are characterized by a strengthening (weakening) of the subtropical jet over southern South America and adjacent areas. The current remote forcings are related with atmospheric/oceanic processes that are interconnected at seasonal-interseasonal scales, which could allow us to develop statistical-dynamical forecasts for the higher or lower occurrence of warm or cold nights in winter.

Keywords : Extremes; Warm nights; Cold nights; Quasi-stationary Rossby waves; Subtropical Argentina.

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