Monsoon intra-seasonal variability in a high-resolution version of Met Office Global Coupled model

Yongjie Fang,Peili Wu,M.S Mizielinski,M.J Roberts,Bo Li,Xiaoge Xin,Xiangwen Liu

doi:10.1080/16000870.2017.1354661

Abstract

Intra-seasonal oscillation (ISO) is a key ingredient of the East Asia and western North Pacific (EAWNP) summer monsoon and particularly important for seasonal forecast. This paper evaluates the seasonal means and ISOs of the EAWNP summer monsoon simulated by the latest version of the Met Office Global Coupled Model (HadGEM3-GC2) with two different atmospheric model resolutions at ~130 and ~25 km coupled to a same 0.25° × 0.25° resolution ocean model. Results show that the mean states of sea surface temperature (SST), low-level specific humidity and the western Pacific subtropical high are all improved in HadGEM3-GC2 with higher atmosphere resolution. Moreover, although ISO variance is overestimated over the western North Pacific, the model has good fidelity in characterising ISO basic features over the EAWNP including the dominant EOF structure, northward propagation and cycle evolution, as well as the zonal displacement of western Pacific Subtropical High and South Asian High associated with the northward propagating ISOs. Increasing atmosphere model resolution yields improvements in most aspects of the Monsoon ISO over the EAWNP, especially for its northward propagation. Further analysis indicates that this improvement is mainly due to the better description of ISO-related air–sea interaction in higher resolution experiment, as evidenced by the enhanced intra-seasonal SST variance and more coherent northward propagation of rainfall, SST, and the associated surface dynamic and thermodynamic variables in the higher resolution model.

Highlights

The East Asian (EA) and the western North Pacific (WNP) summer monsoon is one of the most important land–atmosphere– ocean coupled climate system over the subtropics and tropics and exhibits substantial variability at seasonal and intra-seasonal time scales (Ding, 2007)
By using combinations of a high- and low-resolution atmosphere model coupled with the same high resolution ocean model, while keeping the basic model formulation, configuration, physics and dynamics settings constant, this study investigates the extent to which such models can reproduce aspects of the seasonal mean and Intra-seasonal oscillation (ISO) of the East Asia and western North Pacific (EAWNP) summer monsoon, with a focus on the impact of enhanced resolution
In view of such importance, we first investigate the simulation of the seasonal mean state of the EAWNP summer monsoon

Summary

Introduction

The East Asian (EA) and the western North Pacific (WNP) summer monsoon is one of the most important land–atmosphere– ocean coupled climate system over the subtropics and tropics and exhibits substantial variability at seasonal and intra-seasonal time scales (Ding, 2007). The monsoon ISO influences the timing of the onset and the withdrawal of monsoon and precipitation associated with the mei-yu in central China and the baiu in Japan (Hsu, 2005; Ding, 2007) It modulates sub-seasonal variability of the subtropical high and tropical cyclone genesis and tracks over the WNP region (Chen et al, 2009; Mao et al, 2010; Ren et al, 2013), the seasonal and the intra-seasonal variability of the EAWNP summer monsoon has significant influence on the socio-economic growth of East Asian countries.

HadGEM3-GC2

Data and methodology

Mean states

Monsoon intra-seasonal variability

Summary and discussion