Predictability of the Southern Hemisphere Tropospheric Circulation in CHFP and SHFP models

MARISOL OSMAN; CAROLINA VERA

Reading

Workshop; 3rd SPARC / Dynamical Variability (DynVar) Workshop -1st SPARC / Stratospheric Network for the Assessment of Predictability (SNAP) Workshop; 2013

Stratospheric Process and their Role in Climate Project (SPARC)

The study documents the predictability of seasonal means of 500-hPa geopotential heights in the Southern Hemisphere during winter and summer. The study is based on ensembles of lead-1-month climate predictions outputs for DJF and JJA of 6 coupled models from both CHFP and SHFP, coordinated by WCRP. Total variance (V), signal variance (S2) and noise variance (N2) were computed for each model and for the multi-model ensemble. Predictability is defined as the ratio between the interannual deviation of the ensemble mean (S) and the ensemble spread (N). Predictability changes associated with ENSO events were also studied by computing the same indexes only for ENSO, El Niño and La Niña years.Two approaches were considered in multi-model ensemble treatment: for one side all members for all models were pooled together (standard ensemble mean). On the other side, all members were standardized with respect their own mean and standard deviation before combined together (unbiased ensemble mean). It is found that in DJF, both signal and noise variances are larger at high latitudes with maximum values over the Bellingshausen-Amundsen seas, and at around 45°S in the Indian Ocean and in New Zealand. Predictability is larger at tropical latitudes and drops abruptly poleward. Also, predictability resulted from unbiased ensemble is larger than that from the standard ensemble mean. In JJA, large signal values are found along the central South Pacific and poleward with a maximum west of the Antarctica Peninsula. Predictability is higher at the tropics and maximizes over West and Central tropical Pacific and the Maritime Continent. Predictability is also larger for the unbiased ensemble but smaller than that in DJF.Predictability increases during ENSO events, especially in the areas where the extratropical wave component of the ENSO response develops. This is mainly due to signal increase. A comparison of predictability of models with high- and low-vertical resolution was done. In DJF, CMAM high-resolution model presents increased (decreased) signal (noise) in the Amundsen-Bellinghausen sector and lightly increased predictability particularly off the coast of South America. ARPEGE high-resolution model exhibits increased predictability over the southern tip of South America and Antarctica Peninsula. In JJA, S2/V is higher in high-resolution models than in low-resolution models. In particular, CMAM exhibits higher predictability over extratropical South America and southeastern Pacific. On the other hand, ARPEGE high-resolution model shows lower predictability at the tropics and higher in the vicinity of the Antarctica Peninsula.