Extratropical–tropical interaction model intercomparison project (ETIN-MIP): Protocol and initial results

In ETIN-MIP, nine climate modeling groups have performed common numerical experiments aimed at enhancing our understanding of the mechanisms for two-way extratropics–tropics interactions. The link between the extratropics and tropics is of high societal concern given its role in controlling regional patterns of climate change. Improving our understanding of the mechanisms that enable these connections would significantly enhance our ability to predict and prepare for future changes in regional hydrology. The spirit of ETIN-MIP is strongly in line with one of the four questions of the World Climate Research Programme’s Grand Challenge on Clouds, Circulation and Climate Sensitivity (Bony et al. 2015). In keeping with the original motivation for ETIN-MIP, namely, to provide guidance on identifying the origin of the double ITCZ bias, we have presented initial results focused on tropical precipitation and energetics. The results have practical implications for GCM development strategy and suggest that fixing tropical biases would be a more viable option for alleviating hemispherically antisymmetric components of tropical precipitation biases while fixing extratropical biases is more desirable for improving the hemispherically symmetric component of tropical precipitation biases. It also implies that the ability of extratropical biases to manifest the hemispherically antisymmetric component of the double ITCZ bias would depend on the strength of stratocumulus–SST feedback in the subtropics (Mechoso et al. 2016). For example, the effect of extratropical biases diminishes away from the source region, but the rate of damping would be weaker in models with a stronger coupling between the subtropical stratocumulus and SST that acts as a positive feedback; hence, extratropical biases are able to project onto the double ITCZ bias in some models. The limited ability of extratropical biases to meridionally displace the tropical precipitation compared to tropical biases is due to efficient heat uptake response by extratropical oceanic processes (Figs. 6 and 7). It suggests constraining tropical response to regional energy perturbations requires improved understanding of deep ocean circulation response. This project will enhance our understanding of the origin of longstanding double ITCZ bias, which is an essential first step in informing model developers. More generally, ETIN-MIP will advance our physical understanding of the atmospheric and oceanic circulation responses to regional energy perturbations in a fully coupled framework, and provides a resource for the climate dynamics community to understand the plausibility of different model responses to such regionally varying energy perturbations, including those expected from anthropogenic climate change.