Modulation of North Atlantic Tropical Cyclones by the Madden-Julian Oscillation in the HighResMIP AGCMs

The last decade has witnessed notable advancements in operational forecast systems to predict the Madden-Julian Oscillation (MJO) and its impact on tropical cyclones (TCs). However, limited studies have examined the TC-MJO relationship in long-term climate simulations, particularly over the North Atlantic. Gaining a deeper understanding of model ability in replicating the observed MJO-TC relationship could offer insights to further improve their overall performance in simulating and predicting seasonal-to-sub-seasonal variability of TCs. This study evaluates how well the TC-MJO relationship is captured in 14 realizations from 8 models that participated in HighResMIP (CNRM-CM6-1-HR, EC-Earth3P-HR, MPI-ESM1-2-XR, HadGEM3-GC31-HM, GFDL-CM4C192, MRI-AGCM3-2-S, ECMWF-IFS-HR, and IPSL-CM6A-ATM-ICO-VHR). The experiments were performed with prescribed sea surface temperatures with horizontal resolutions ranging from 20 to 50 km. Notably, only one model—MRI-AGCM3-2-S—reasonably reproduced the observed intra-seasonal TC variability. Specifically, it showed significant contrast in basin-wide TC activity between MJO phases 2-3 and phases 6-7. The primary obstacle preventing most HighResMIP models from accurately reproducing the observed impacts of the MJO on Atlantic TCs was the mean state bias in TC climatology, especially over the Atlantic main development region (MDR). All models exhibited negative biases in TC genesis over the tropical North Atlantic. Furthermore, the underestimated MDR TC activity can be attributed, in part, to the weaker-than-observed African easterly wave activity around 10 N, which is hypothesized to result from the weak bias in the regional Hadley circulation. In addition to biases in TC climatology, most HighResMIP AGCMs face challenges in realistically representing the intra-seasonal variability linked to the MJO and consistently exhibit an underestimation of the MJO eastward propagation power. In models characterized by a higher ratio of eastward-to-westward-propagating variance, the simulated MJO demonstrates a stronger capacity to modulate sub-seasonal TC activity. Overall, our recent work suggests the urgent need for continuous efforts aimed at improving model performance to achieve a more realistic simulation of current climatological TC activity over the tropical North Atlantic. Furthermore, there is room to improve the eastward propagation skill of the MJO in HighResMIP models. These improvements are essential for the models to satisfactorily capture the observed modulations of Atlantic TCs by the MJO.