The Impact of Surface Flux– and Circulation-Driven Feedbacks on Simulated Madden–Julian Oscillations

Abstract

The intent of this study is to measure the total contribution of individual wind- and moist flux?induced feedback mechanisms on the generation and maintenance of simulated Madden?Julian oscillations (MJOs). This task is accomplished through the use of an idealized GCM that is also employed to examine the impact of sea surface temperatures on the robustness of the MJO signal. Among the mechanisms investigated are Conditional Instability of the Second Kind (CISK), Wind Induced Surface Heat Exchange (WISHE), and the feedback due to specific humidity differences between the surface and the boundary layer. A series of numerical experiments was conducted in which one or more of these feedbacks were suppressed. The resulting structure and periodicity of the simulated mode was analyzed using eigenvector and spectral analysis methods. The relative phases of surface wind, turbulent flux, and boundary layer moisture anomaly fields were then contrasted. It was found that the model produced a far stronger signal when the turbulent surface flux contributions were unsuppressed. However, the inclusion of turbulent surface fluxes yielded simulated modes possessing unrealistic frequencies of 16.79?28.83 cycles yr?1 compared with values in other studies ranging from 6.08 to 12.16 cycles yr?1 (30?60 days). Overall, the modes appeared to be most sensitive to the model sea surface temperatures and the availability of moisture modulated by wind-driven feedbacks.