Dynamical Constraint on Precipitation Biases Over the Indo-Pacific Region During Boreal Summer in AMIP6 Models

Climate models suffer from longstanding precipitation biases, much of which has been attributed to their atmospheric component owing to unrealistic parameterizations. Here we investigate precipitation biases in 37 Atmospheric Model Intercomparison Project Phase 6 (AMIP6) models, focusing on the Indo-Pacific region during boreal summer. These models remain plagued by considerable precipitation biases, especially over regions of strong precipitation. In particular, 22 models overestimate the Asian-Pacific monsoon precipitation, while 28 models underestimate the southern Indian Ocean Intertropical Convergence Zone precipitation. The inter-model spread in summer precipitation is decomposed into Empirical Orthogonal Functions (EOFs). The leading EOF mode features an anomalous anticyclone circulation spanning the Indo-northwest Pacific oceans, which we show is energized by barotropic conversion from the confluence of the background monsoonal westerlies and trade-wind easterlies. Our results suggest precipitation biases in atmospheric models, though caused by unrealistic parameterizations, are organized by dynamical feedbacks of the mean flow. The Indo-Pacific monsoonal rainfall provides life-supporting water to billions of people living in the region, and its associated latent heating drives changes in atmospheric circulation further affecting weather across the globe. The realistic simulation of rainfall over this important region remains challenging for generations of climate models. Many studies point to atmospheric models being the primary culprit, which motivates us to investigate the Indo-Pacific monsoonal rainfall simulated by the latest generation of atmospheric models in the sixth Atmospheric Model Intercomparison Project (AMIP6). We find that models examined here continue to struggle with, to various degree, simulating the observed distribution of Indo-Pacific monsoonal rainfall. We further find that variations in simulated rainfall among the AMIP6 models are not simply linked to their different model physics as previously thought, but are also strongly regulated by the atmospheric circulation over the Indo-northwest Pacific. AMIP6 models remain plagued by large precipitation biases in the tropical Indo-Pacific region during summer with large inter-model spread This inter-model spread is dominated by an anomalous anticyclone (AAC) circulation over the northwest Pacific This AAC arises via barotropic energy conversion from mean circulation and is energized by convective feedbacks