Observed and simulated summer rainfall variability in southeastern South America

DÍAZ, L.; VERA, C.; SAURRAL, R.

Qingdao

Congreso; CLIVAR Open Science Conference; 2016

The changes in regional climate observed in the last decades have raised concern among policy and decision makers about the importance of the knowledge about how climate would evolve in the next decades. In particular southeastern South America (SESA) has experimented large positive austral summer precipitation trends during the 20th century that impacted many socio-economic sectors. However, regional studies show that natural decadal variability has a large influence in SESA precipitation. As a consequence, a deeper knowledge of the combined influence of both decadal climate variability and global warming is needed in order to project near-term future climate changes with a lower degree of uncertainty. The availability of the different simulations included in the Fifth phase of the World Climate Research Program-Coupled Model Intercomparison Project (WCRP/CMIP5) Experiment provides an excellent opportunity to investigate physical processes involved in SESA precipitation changes and their predictability.The evolution of precipitation during the last 150 years exhibits in SESA considerable multi-decadal variations that have been identified in previous works as forced by the tropical ocean variability. Therefore, in order to better understand the influence of the observed large-scale interannual variability of the sea surface temperatures (SST) on austral summer rainfall in SESA in a global warming context, a singular value decomposition analysis was performed over the 1902-2010 period. The leading mode (SVD1) shows a clear global warming signal, mainly related to warming in the Pacific and Indian Oceans, in association with a rainfall increase in SESA. The temporal series of the mode exhibits significant variability ranging from the interannual scale to long-term trends. In particular, the decadal variability signal is remarkable, with a particular phase shift at around the middle 1970s. After detrending the series, the spatial distribution of both SST anomalies and precipitation anomalies in SESA associated with the first mode resembles that typically related with El Niño-Southern Oscillation. Moreover, the mode temporal evolution has a remarkable variability on decadal scales, which shows that the relationship between SST anomalies, especially in the Tropical Pacific Ocean, and SESA precipitation is non stationary. It is speculated that the latter is related with decadal variations of the Southern Hemisphere circulation conditions that constructively or destructively influence the teleconnection development between the tropical Pacific-Indian oceans and SESA. Historical and Decadal WCRP/CMIP5 simulations are considered in order to make a preliminary evaluation of the representation of austral summer rainfall variability and trends in SESA and its connection with ocean variability. Models are able to represent a significant positive trend over SESA in agreement with the observations, although weaker than observed. The model representation of the SVD1 was also assessed. Preliminary results using short-term climate predictions show that some models are able to reproduce the main spatial features associated with the mode, on both SST anomalies and precipitation anomalies in SESA. In particular, these models are able to reproduce the teleconnections linking the tropical Pacific-Indian Ocean sector with SESA. An assessment of the temporal evolution of this mode provided by those simulations will be also presented in the Conference.