This work performs a classification of sea level pressure fields for South America during summer and winter, in order to identify the main synoptic patterns associated with the Northwestern Argentinean Low (NAL) and the Chaco Low (CHL). To this purpose, the Principal Component Analysis technique in rotated mode was applied to the NCEP reanalysis for the period from 1985 to 2010. A second objective was to determine the main variability frequencies related to both systems, by employing Spectral Analysis. The results provide five circulation patterns, representative of about 80% of all cases. During summer, the CHL, located between northern Argentina, Paraguay, Bolivia and southern MS, is mainly associated with two synoptic patterns. The NAL is associated with a single synoptic mode, similar to the seasonal mean field. During winter, the CHL is not recognized, in agreement with its thermal nature related to latent heat released by tropical convection. The NAL is also associated with a unique synoptic mode during this season. This mode is similar to that obtained in the summer and is also related to synoptic scale disturbances but explaining a lesser percentage of variance. The Frequency Analysis show that the circulation mode associated to the CHL has an energy peak at around 17 days, while the NAL has an energy peak around 22 days in summer and a relative increase of synoptic variability in winter.
Synoptic classification; NA; CHL; Principal Components
