Unidad de Ciencias de la Atmósfera – Facultad de Ciencias - Universidad de la República - Uruguay

1. Processes that enhance predictability of surface temperature in southeastern South America during May-July Marcelo Barreiro barreiro@fisica.edu.uy Unidad de Ciencias de la Atmósfera – Facultad de Ciencias - Universidad de la República - Uruguay Abstract: The main source of potential predictability of surface temperature in SESA [65°W-47°W,19°S-37°S] during May-July is related to El Niño-Southern Oscillation (ENSO), which increases the northerly flow bringing warm air to the region. The air-sea thermodynamic coupling off-SESA plays a role, increasing the persistence of the atmospheric anomalies. Potential Predictability We run experiments using an AGCM (Speedy, T31L8) forced with prescribed sea surface temperatures in different basins. A 10-member ensemble was constructed for each basin: GOGA - Global Ocean-Global Atmosphere: historical SST everywhere POGA - historical SST between 50S-30N in Pacific Ocean, climatology elsewhere. AOGA - historical SST between 50S-30N in Atlantic Ocean, climatology elsewhere. Model results are compared with the NCEP-NCAR reanalysis. Analysis of Variance (ANOVA) of surface temperature for MJJ The maps below show the percentage of variance due to SST-influence. Even in GOGA the forced variance over SESA is relatively small, and the results for POGA confirm the importance of ENSO forcing during this season as a source of potential predictability for southeastern South America, in costrast to SST anomalies in the Atlantic Ocean. Correlation of observed surface temperature with the ensemble mean shows a maximum in May-June-July for GOGA and POGA, the same season when Niño34 has large correlation. GOGA POGA AOGA Circulation anomalies associated with variability of surface temperature over SESA during MJJ In observations warm MJJ seasons are associated with a surface anticyclonic circulation located off SESA that induces northerly flow. In the upper atmosphere there is also a well defined anticyclonic circulation. Tropical Pacific SST anomalies and a warming off south Brazil accompany these circulation features. Model simulations show that the observed anomalies can be decomposed into a component due to ENSO captured in the regresions for GOGA, and a component due to internal atmospheric variability shown in the regresion for an experiment where the AGCM is coupled to a slab ocean (where ENSO is absent by construction). GOGA Both ENSO and internal variability induce northerly surface flow, but the anticyclone in upper levels is only associated with internal variability. AGCM+SLAB OCEAN OBSERVATIONS U850 U200 U850 U850 U200 U200 + = SST SST SST Role of thermodynamic coupling Correlation of simulated with observed temperature Persistence of temperature anomalies over SESA We perfomed two additional experiments: GOGA-SACL – as GOGA except in S. Atlantic where climatological SST is imposed. GOGA-SAML- as GOGA except in S. Atlantic where the AGCM is coupled to a slab ocean. OBSERVATIONS GOGA-SACL GOGA-SAML The thermodynamic air-sea coupling in the South Atlantic increases the potential predictability of the surface temperature over SESA for all seasons (above left). This is related to an increase in the persistence of the surface temperature anomalies (above right).