Historic Meteorological Extremes as Indicators for Typical Scenarios of Holocene Climatic Periods in the Pampa Plain

IRIONDO M.,; KRÖLHING D.; BRUNETTO E.

Miramar - Mar Chiquita - Córdoba -Argentina

Congreso; Holocene environmental catastrophes in South America: from the lowlands to the Andes.Third Joint Meeting; 2005

ICSU Dark Nature - IGCP 490

Previous researchs (Iriondo, 1999) stated that the Holocene was marked by two different climatic periods in the Pampa, one (8.5-3.5 kyr B.P.) characterized by temperatures and precipitation higher than today and the other (3.5-1.4 kyr B.P.) semiarid, generating parabolic dunes. The humid period produced intense pedogenesis down to 40°S and a mobilization of iron oxides to 30°S, which means tropical climate with temperatures above 20°C and precipitation higher than 2,000 mm/yr. Sand deflation and development of parabolic dunes occur under climates with 300-400 mm/yr. This contribution is based on geological and physical proxies. Typical weather scenarios of those climates are sporadically reproduced today during extremely humid or dry periods. On that basis, years beyond the thresholds of 2,000 and 400 mm/yr were selected in order to know typical values of other meteorological parameters of the past climates. Extremely dry periods  were characterized by a large thermal amplitude, frosts and stronger winds, reproducing the continental anticyclonic circulation of the late Holocene proposed by Iriondo (1990). Humid extremes were warmer than normal (with lower thermal amplitude) and with higher relative air humidity; most rains occur by local convection processes. Typical scenarios with precipitation above 2,000 mm/yr, that is hypsithermal condition, are: 1,065 mm/yr potential evaporation; 21.1°C mean temperature; 25.9°C maximal monthly temperature; 16.6°C minimal monthly temperature; 38.7°C maximal absolute temperature; 3.6°C (no frost) minimal absolute temperature; 79.3% relative humidity; 6.3 km/h mean wind velocity. No year reached the deflation threshold in the last 90 years in the region. The nearest two periods (rains 100 mm above such condition) show the following scenario: 1,442 mm/yr potential evaporation; 15.4°C mean temperature; 23.9°C maximal monthly temperature; 5.8°C minimal monthly temperature; -8°C minimal absolute temperature (42 frosts/yr); 39°C maximal absolute temperature; 66.3% relative humidity; 10.4 km/h mean wind velocity and atmospheric pressure of 1005.4 hPa. References Iriondo, M. (1990). A Late Holocene dry period in the Argentine plain. Quaternary of South America and Antarctic Peninsula, 7:197-218. A.A. Balkema. Iriondo, M. (1999). Climatic changes in the South American plains: Records of a continent-scale oscillation. Quaternary International, 57/58: 93-112. Pergamon Press.