Subtropical Impact on the Tropical Double-ITCZ Bias in the GFDL CM2.1 Model

The double-intertropical convergence zone (ITCZ) bias has been an outstanding problem among climate models for two decades. However, it remains unclear how much of this tropical bias is attributed to the extratropics and tropics itself, respectively. Applying the regional data assimilation (RDA) method, we follow up a previous study with a more advanced model of GFDL CM2.1 to quantify the influence of extratropical atmosphere on the double-ITCZ bias. Our study reveals that this tropical bias is influenced to a large extent by the extratropics between 20 degrees and 30 degrees, with little impact from the extratropics poleward of 30 degrees. This vital role of subtropics in the double-ITCZ bias is likely determined by the meridional extent of Hadley circulation from zonal-mean perspective. Besides, the vital role of subtropics is also sup-ported by wind-evaporation-SST feedback in the subtropical southeastern Pacific from a regional perspective.