4. Concluding remarks
4.2 Discussions
Despite a significant improvement in climate modeling, several regional SST biases from models spanning three CMIP phases found little general improvement. Precisely speaking, QBIAS is not necessarily the bias in surface heat flux itself but encompasses the effect of model errors, for example in convective parameterization, cloud schemes, boundary layer schemes, or vertical mixing scheme in an ocean model, on the SST. Although the origins of regional surface heat flux biases are beyond the scope of this study, several studies have noted long-standing regional SST biases are dependent on model resolution. For the tropically uniform cold bias, which is claimed to be the cause of equatorial dry bias, cloud biases in atmospheric models and 1-m near surface resolution in ocean model are suggested to be important for reducing cold biases (Li & Xie, 2012; Zhu et al., 2020). We show that the realistic simulations of SSTs in eastern boundary upwelling systems are critically important to improvement in regional precipitation bias in the tropics, and therefore, it is noteworthy that the biggest improvements in warm SST biases in coastal stratocumulus regions have been made with increased resolution in atmospheric model (Roberts et al., 2019). Meanwhile, pronounced cold model biases are persistent in the North Pacific and Atlantic, which is largely attributed to cloud albedo errors in atmospheric model (Burls et al., 2017) and can be reduced when the mixing strength is enhanced within ocean-only simulations (Zhu et al., 2020).
In efforts to find possible causes of the double-ITCZ, many previous studies found that the Southern Ocean cloud problem that leads to warm SST biases had little effect on shifting the ITCZ provided that the oceanic role in transporting heat northward is considered. Along this line of research, Hawcroft et al. (2017) further supports the idea that, with a focus on large-scale energetic controls, improvements in the representation of atmospheric processes should precede those in coupled models in efforts to correct the double-ITCZ problem. Though the results of this study only consider the effect of atmospheric teleconnection without a dynamical ocean, it is legitimate to assume that the double- ITCZ bias can be decomposed into roles of regional SST biases in atmospheric sense given that QBIAS
involves errors in the oceanic processes and regional slab ocean experiments confirm near-linearity.
It is yet to be seen how ocean dynamics may modulate the results reported here but our slab ocean experiments provide first-step guidance for where should be targeted to improve certain features of climate biases.
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