Predictability and skill of the Southern Hemisphere summer circulation through the representation of the main modes of variability.

MARISOL OSMAN; CAROLINA VERA

Trieste

Workshop; Advanced school and workshop on subseasonal to seasonal prediction and application to drought prediction.; 2015

ICTP

The predictability of the summer tropospheric circulation in the Southern Hemisphere (SH) is assessed through the analysis of climate predictions available at the Climate Historical Forecast Project of the World Climate Research Program (WCRP/CHFP). Focus is made in the assessment of model skill in reproducing the main modes of variability of the geopotential-height anomalies at 200hPa. Forecasts valid at December-January-February (DJF) and made with initial conditions of November (Lead 1) over the period 1982-2006 are considered. A Multi-Model Ensemble (MME) using 11 models was constructed and its performance was evaluated using the NCEP/NCAR Reanalysis. It was found that the MME skill obtained computing the anomaly correlation coefficient, is higher at the tropics, reaching 0.9 and drops at midlatitudes to less than 0.3. The ability of the models in representing the leading patterns of circulation variability obtained from a rotated empirical orthogonal function (EOF) analysis was then assessed. ). The first REOF is related with the Southern Annular Mode (SAM), is well captured by the MME with a spatial correlation coefficient (SCC) up to 0.90 although is temporal evolution is not that well reproduced, a temporal correlation coefficient of 0.56. The fact that the SAM is mostly maintained by the atmospheric internal dynamics may explain the MME limitations to reproduce it. In the other hand, the spatial distribution of the second REOF is well reproduced by the MME spatially (SCC of 0.86) and temporally (TCC of 0.82). This mode is associated to teleconnections extended along the South Indian-Pacific Oceans and highly correlated with El Niño Southern Oscillation (ENSO). The third REOF, also related to tropical SST forcing from the Indian-Pacific Oceans, is well captured spatially (SCC of 0.69) and temporally (0.60), although simulated anomalies tend to be more zonally oriented than in the observations. Considering that the three leading REOF explain more than 60% of the variance in both models and reanalysis, it can be concluded that the current level of MME skill in representing the summer circulation in the SH mainly comes from the skill associated with these patterns.