Moisture-induced Diabatic heating enhances Baroclinic Annular Mode (BAM)

As the leading mode of variability of the extratropical eddy activity, Baroclinic Annular Mode (BAM) can modulate serial clustering of extreme weather events on intraseasonal time scale. In the future climate, BAM is projected to get enhanced, however, the physical mechanism driving such enhancement remains an open question. In this work, we will use the MERRA2 reanalysis product to quantify the role of moisture-induced diabatic heating and demonstrate evidence for the feedback between BAM and diabatic heating.

By adopting the finite-amplitude wave activity (FAWA) framework, we capture BAM events through diagnosing the variability of wave activity and single out the contribution from moisture-induced diabatic heating. We analyze the statistical relationship between moisture-induced FAWA tendency and BAM and find that moisture-induced diabatic heating exerts a positive contribution on enhancing the variance of BAM through the entire 20-30 day lifecycle. Since BAM can translate to regional scale, we further extend such BAM-diabatic heating relationship to regional scale as well by adopting the local wave activity (LWA) diagnostics. These results suggest that the enhanced BAM in future climate, as we previously reported, could be attributed to the enhanced moisture content and associated diabatic heating. Given the close connection between BAM and extreme weather events, this work suggests a more regular serial clustering of extreme events in a warmer and moister world.