Abstract
Abstract. Based on previous results obtained from observations and linear wave theory analysis, the hypothesis that large-scale patterns can generate extreme cold events in southeast South America through the propagation of remotely excited Rossby waves was already suggested. This work will confirm these findings and extend their analysis through a series of numerical experiments using a primitive equation model where waves are excited by a thermal forcing situated in positions chosen according to observed convection anomalies over the equatorial region. The basic state used for these experiments is a composite of austral winters with maximum and minimum frequency of occurrence of generalized frosts that can affect a large area known as the Wet Pampas located in the central and eastern part of Argentina. The results suggest that stationary Rossby waves may be one important mechanism linking anomalous tropical convection with the extreme cold events in the Wet Pampas. The combination of tropical convection and a specific basic state can generate the right environment to guide the Rossby waves trigged by the tropical forcing towards South America. Depending on the phase of the waves entering the South American continent, they can favour the advection of anomalous wind at low levels from the south carrying cold and dry air over the whole southern extreme of the continent, producing a generalized frost in the Wet Pampa region. On the other hand, when a basic state based on the composites of minimum frosts is used, an anomalous anticyclone over the southern part of the continent generates a circulation with a south-southeast wind which brings maritime air and therefore humidity over the Wet Pampas region, creating negative temperature anomalies only over the northeastern part of the region. Under these conditions even if frosts occur they would not be generalized, as observed for the other basic state with maximum frequency of occurrence of generalized frosts.
Highlights
During the past decades many observational and numerical works have shown how tropical thermal forcings can affect the atmospheric circulation
The relation that exists between the anomalous tropical heating and the circulation patterns in the extratropics explains a significant fraction of the flow variability in middle latitudes
Tropical convection regions are mainly confined to the Intertropical Convergence Zone (ITCZ), the equatorial region and southeastern Asia, due to the presence of the monsoon
Summary
During the past decades many observational and numerical works have shown how tropical thermal forcings can affect the atmospheric circulation. The interaction between tropical convection and global circulation can occur through the dispersion of Rossby waves which propagate to the extratropics. The propagation of these waves is essentially dependent on the structure on the basic state and the heat source (Ambrizzi et al, 1995, and references therein). This mechanism, known as the tropic-extratropic interaction, modifies the structure of the higher latitude mean flow modulating the characteristics of the synoptic systems. This region is of great importance for the Argentinean economy, since it is the main agriculture production region with an approximate area of 3/4 million km and that continues to expand
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