Seasonality of Intraseasonal Variability in Global Climate models

The tropical intraseasonal (30-90 days) oscillation (ISO) displays distinctive behaviors in boreal summer and winter. How well each mode is simulated in climate models has been investigated; however, very few studies have examined whether these modes are simulated in appropriate season. Here we developed diagnostics to assess this aspect and applied these diagnostics to numerous atmosphere-only and atmosphere-ocean-coupled models. We found out that all models share serious biases and that they sometimes incorrectly simulate the boreal summer ISO mode even in boreal winter and underestimate the appearance frequency of the boreal summer ISO in boreal summer. Nearly all atmosphere-ocean-coupled models show some improvements in the ISO seasonality representation compared to their atmosphere-only counterparts. It is suggested that good models for simulating the ISO seasonality have good life cycles for each ISO mode and that an accurate reproduction of the seasonal mean low-level zonal wind is crucial. Plain Language Summary The leading intraseasonal (30-90 days) oscillation (ISO) in the tropical atmosphere has different characteristics in the boreal summer and winter. Accordingly, it has different impacts on extreme weather such as heavy precipitation and tropical cyclone activity. Many studies have examined how well the boreal summer and winter modes are simulated in climate models; however, very few studies have examined whether climate models can simulate the boreal summer and winter ISOs appropriately in the boreal summer and winter, respectively. In this study, we developed a new method to examine this aspect and applied it to numerous present-day climate simulations using both atmosphere-only and atmosphere-ocean-coupled models. We found out that all models share serious biases and that they inadequately simulate the boreal summer mode even in boreal winter and underestimate the appearance frequency of the boreal summer mode in boreal summer. Nearly all atmosphere-ocean-coupled models show some improvements in the ISO seasonality representation compared to their atmosphere-only counterparts. It is suggested that good models for simulating the seasonality of the ISO show good life cycles for each ISO mode and a seasonal mean zonal wind in the lower troposphere. These results are helpful for further improving climate models.