The Yanai Wave in the Asian Monsoon: Results From an Observational and Modeling Study

Climate simulations using a multi-scale modeling framework (MMF), or “super-parameterization” may offer new insights into the role of convection, air-sea coupling, and equatorial mixed Rossby-gravity waves (e.g. Yanai waves) on the simulation of the Asian monsoon system.

In this study, we compare the monsoon simulation in the Community Climate System Model (CCSM, v3.0), which uses a traditional cumulus parameterization, to super-parameterized coupled and uncoupled versions of CCSM, in which the traditionally parameterized effects of cumulus convection are explicitly simulated with multiple realizations of a cloud resolving model. The atmosphere-only super-parameterized model is referred to as SP-CAM, and the fully coupled super-parameterized model is SP-CCSM. CCSM simulates a very weak monsoon. The monsoon in SP-CAM is more robust, with improved simulation of the eastward- and westward-propagating components, but the variability is too high. SP-CCSM offers the best simulation of the monsoon, including the signature NW-SE tilted rainband structure that arises from the combined eastward- and northward-propagating components of the monsoon.

We focus on the role of the n=1 equatorial Rossby (ER) and Yanai waves in the northward-propagating component of monsoonal precipitation. Yanai wave activity is highly correlated with northward-propagating monsoon precipitation in observations and SP-CCSM but not in SP-CAM and CCSM, since neither of those models produce a significant peak in the Yanai wave-space region. Furthermore, ER wave activity precedes Yanai wave activity by ½ to ¼ cycle in observations and SP-CCSM, suggesting that ER waves precondition the atmosphere for Yanai wave activity.

Comparison of each model's basic state climate and high-frequency wave activity lead us to conclude that successful simulation of the Asian monsoon depends on a model's ability to simulate a realistic basic state climate, as well as a variety of wave types, especially ER and Yanai waves. Explicitly simulating convection is important for increasing variability, but convection only organizes into modes similar to those seen in nature when the resulting waves propagate through a realistic basic state atmosphere. Another conclusion drawn from this work is the benefit of examining low-frequency (intraseasonal) variability in terms of the interactions of higher-frequency waves.