Abstract Author: Kyong-Hwan Seo, Wanqiu Wang, Jae-Kyung Schemm
Abstract Title: Tropical intraseasonal variability simulated
Abstract: This study investigates the capability of simulating the tropical intraseasonal variability, focusing on the MJO, in a series of atmosphere-ocean coupled and uncoupled simulations using NCEP operational general circulation models. The effect of air-sea coupling on the MJO is examined by comparing long-term simulations from the coupled Climate Forecast System model (CFS T62) and the atmospheric Global Forecast System model (GFS T62). Another coupled simulation with a higher horizontal resolution model (CFS T126) is performed to investigate the impact of model horizontal resolution. Furthermore, to examine the impact on a deep convection scheme, an additional coupled T126 run (CFS T126RAS) is conducted with the relaxed Arakawa-Schubert (RAS) scheme. From these runs, the most important factors for the proper simulation of the MJO are deduced. From empirical orthogonal function, lagged regression and spectral analyses, it is found that the interactive air-sea coupling much improves the coherence between convection, circulation and other surface fields on the intraseasonal time scale. A higher horizontal resolution run (CFS T126) does not show significant improvements in the intensity and structure. However, GFS T62, CFS T62, and CFS T126 all yield the 30-60 day variances statistically not distinguishable from a background red noise spectrum. Their eastward propagation is stalled at the “barrier” over the Maritime Continent and far western Pacific. In contrast to the model simulations using the simplified Arakawa-Schubert (SAS) cumulus scheme, CFS T126RAS produces statistically significant spectral peaks in the MJO frequency band, and the strength of MJO convection and circulation is much improved. Most of all, the MJO convection signal is shown to be able to penetrate into the Maritime Continent and western Pacific. In this simulation, surface moisture convergence is located persistently to the east of enhanced convection and gives rise to the frictional wave-CISK (conditional instability of the second kind) mechanism. In these NCEP operational models, the employed deep convection scheme and the ability of generation of proper convection-circulation interaction and so the frictional moisture convergence are revealed to be more dominant factors for the MJO simulation than basic-state sea surface temperature, vertical zonal wind shear and lower-level zonal winds. The improved simulation of the MJO does lead to the improved global circulation response to the tropical heating and extends the predictability over Asia and North America.
The spectral characteristics of model-simulated convectively coupled equatorial waves are also evaluated by a space-time spectral analysis.