Seasonality of Intraseasonal Variability in Global Climate Models

Abstract

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.