Climate models without an East Pacific Double ITCZ better simulate tropical Pacific climate variability and change

Climate-model mean-state biases such as the double-Intertropical Convergence Zone (ITCZ) bias and cold-tongue bias are long known, but a definitive solution to them has remained elusive due to the highly coupled nature of the climate system, and they remain a problem in the current generation of climate models (Tian & Dong 2020). Recently, as anthropogenically forced trends have more clearly emerged from the noise of internal variability, systematic biases in simulated trends compared to observations have also been demonstrated (e.g., Wills et al. 2022), notably in the tropical Pacific sea-surface temperature (SST) pattern and Walker circulation strength. This has raised interest in understanding the influence of mean-state biases on climate trends. Here we show that there are a handful of CMIP6 models with a much-reduced double-ITCZ bias in the East Pacific, a region that plays an important role in two-way teleconnections between the tropical Pacific and Southern Ocean (Dong et al. 2022). These low-ITCZ-bias models show tropical Pacific SST trend patterns that are more like observations, with bands of relatively little warming extending from the southeastern subtropical Pacific to the equator. Moreover, these models show much larger amplitude decadal SST variability throughout the Indo-Pacific, which is more in line with observations. We provide evidence that the more patterned forced SST trends and larger decadal SST variability both arise from a stronger propagation of southeast subtropical Pacific SST anomalies towards the equator when this propagation is not blocked by a double ITCZ. Our findings suggest that reducing the double-ITCZ bias in climate models has the potential to substantially improve climate projections.

Tian, B. & Dong, X. The double-ITCZ bias in CMIP3, CMIP5, and CMIP6 models based on annual mean precipitation. Geophysical Research Letters 47, e2020GL087232 (2020).

Wills, R. C., Dong, Y., Proistosecu, C., Armour, K. C. & Battisti, D. S. Systematic climate model biases in the large-scale patterns of recent sea-surface temperature and sea-level pressure change. Geophysical Research Letters 49, e2022GL100011 (2022).

Dong, Y., Armour, K. C., Battisti, D. S. & Blanchard-Wrigglesworth, E. Two-way teleconnections between the Southern Ocean and the tropical Pacific via a dynamic feedback. Journal of Climate 35, 6267–6282 (2022).