Impact of organized intraseasonal convective perturbations on the tropical circulation

Understanding processes responsible for the representation of intraseasonal convective perturbations in GCMs remains a challenge. Here, we study the role of large-scale organized convective perturbations in the triggering and the evolution of these perturbations. For this, we hypothesize that such large-scale convective perturbations can be triggered by forcing a GCM with observed intraseasonal SST anomalies. We thus compare two ensembles of simulations forced either with actual daily SST fields (high frequency, HF) or with low-pass filtered SST fields (T > 90 days; low frequency, LF). We obtain a more realistic representation of the intraseasonal variability in the HF simulations. Also, in HF simulations, there is a small increase of seasonal mean precipitation, but a strong and statistically significant increase of the seasonal mean low-level westerly flow. It is argued that it is mostly the ‘‘aggregation'' of the convection into a few large-scale organized intraseasonal events, and not the intensification of the seasonal mean precipitation, that acts to intensify the low-level westerly flow in the HF simulations. This non-linear behavior shows that a correct representation of intraseasonal variability in a GCM is required for a correct simulation of the average tropical circulation. This non-linear behavior is then further studied by inspecting the observed relation between the intraseasonal variability of the convection and the seasonal mean circulation.