Large-Scale Patterns Linked To Low-Frequency Variability Of Daily Intensity Of Extreme Rainfall Over Argentina

ROBLEDO, FEDERICO ARIEL; OLGA C. PENALBA; VERA, CAROLINA

Foz do Iguazu

Conferencia; The Meeting of the Americas; 2010

American Geophysical Union

The main goal of this work is to analyzed planetary and regional features of the atmospheric circulation that impact in the extreme rainfall on Argentina. The analysis has been made on the basis of the leeding pattern of covariabilty between spring the daily intensity of extreme rainfall in Argentina and sea surface temperature (SST)for all the oceans from 17.5º N to 90º S. This analysis was performed using a Singular Values Decomposition (SVD) for springin the period 1962 to 2005. Two data sets were used to the SVD analysis: monthly SST from the Kaplan SST V2 from the NCEP/NCAR and high quality daily rainfall for 35 surface stations from the National Weather Service of Argentina distributed throughout the country.The monthly mean of daily intensity of extreme rainfall index (DIER) is the quotient between the monthly accumulated extreme rainfall (AE) and the number of days with extreme precipitation events per month (PE). We consider extreme daily precipitation when rainfall is greater than the mean 75th daily percentile for the period 1961 to 2000. Regression maps between SVD times series and relevant variables like meridional wind at 200 hPa and 850 hPa, and geopotentail height to 500 hPa were performed in order to describe leading patterns signal on the southern hemisphere circulation.The five leading SVD modes of the coupled SST and DIER variations account around 80 percent for spring of the total square covariance (TSC). The first mode of spring explains 45% of TSC. Spatial patterns of the leading mode are associated with SST anomalies resembling those typically linked to ENSO with 4-year temporal cycles and enhanced DIER in central and eastern Argentina. Regression maps of circulation anomalies depict Rossby-like wave trains emanating from central tropical Pacific and equatorial Indian oceans, in a similar way than those previously identified by other papers as promoted by ENSO events.The second SVD mode that explains 17% of TSC, presents an anomalous cooling in the tropical Atlantic Ocean as well as in the Indonesian Sea associated with low DIER over northeastern Argentina. This mode shows a significant decadal variability with two sub-periods: 8 years and 12 years. An upper-level cyclonic circulation anomaly is observed southward of the negative SST anomaly center located over the equatorial Atlantic. On the other hand, an anticyclonic circulation is identified over southeastern South America. Both features seem to be part of a Rossby-like wave train extending from western tropical Pacific Ocean arching toward South America. Evidences of an annular circulation structure are also observed over the Polar Regions in association with this particular SVD mode. The third SVD mode explains 10% of TSC and shows significant variability on periods around 14 years. A horseshoe-like shape characterizes the SST anomalies over the western Pacific related with this SVD mode while negative (positive) DIER extends over northern (central-east) Argentina. Circulation anomalies extend between New Zealand region and South America in relation to the third svd mode.