Contrasting the Role of Regional and Remote Circulation in Driving Asian Monsoon Biases in MetUM GA7.1

Abstract

The Asian monsoon is a key component of the global atmospheric circulation. During the summer, the monsoon southerlies provide around two thirds of the annual precipitation to about half of the world's population, while during the winter northerly winds can lead to cold surges and severe weather (e.g., Li & Yang, 2010;B. Wang 2006). Given the complexity and spatial extent of the monsoon, its simulation has proven to be a challenging task for climate models, as well as a key testbed to evaluate their processes (Sperber et al., 2013).Numerous studies have documented the model monsoon biases, as well as their spatio-temporal evolution with the annual cycle (e.g., Kang & Shukla, 2006;T. Zhou et al., 2009). Many of these shortcomings have been persisting for decades (Ramesh & Goswami, 2014;Song & Zhou, 2014a). These include the excessive summer rainfall over the northwestern Pacific associated with an anomalously weak Western Pacific Subtropical High (WPSH), and a dipole precipitation anomaly over the Indian monsoon region with a rainfall deficit over the subcontinent and excess precipitation over the tropical Indian Ocean (e.g., Rodríguez et al., 2017;Sperber et al., 2013). The pervasiveness of these biases across both coupled and uncoupled models (M. Bollasina & Nigam, 2009;Song & Zhou, 2014b) suggests the underlying causes could be rooted in their atmospheric component. However, there is some evidence that coupled models can better reproduce monsoon precipitation than atmosphere-only models (Kumar et al., 2005;Zou, 2020) from two main reasons: the damped atmospheric interval variability due to negative feedback from the ocean (Z. Q. Zhou et al., 2018) and the compensating effects between atmospheric and sea surface temperature (SST) biases (Prodhomme et al., 2014;Yang et al., 2019).Model biases hinder reliable attribution of past monsoon variations to anthropogenic forcing (e.g., Wilcox et al., 2015), which in turn hampers our confidence in future projections. One of the limiting factors is the interaction between external forcing and internal variability, especially at interannual to decadal time scales (e.g., Deser et al. 2012). Large uncertainties stem from the atmospheric dynamical response Abstract Monsoon precipitation affects nearly half of the world's population, but monsoon biases are a long-standing problem in climate simulations. We apply dynamical nudging either globally or regionally to demonstrate the role of regional and remote circulation in generating Asian monsoon biases in an atmospheric general circulation model. Monsoon precipitation biases are substantially reduced in response to global nudging but may also be exacerbated over the warm oceanic equatorial areas because of unconstrained sub-grid convection. Regional nudging over Asia appears to be more efficient than nudging outside Asia in reducing seasonal precipitation biases over eastern China and India. This suggests a predominant role of local circulation anomalies in generating monsoon precipitation errors in these regions. An exception i...

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