Investigations of Three Dominant Patterns of Large-Scale Circulation Variability in the Atmosphere

Abstract

This work covers the investigations of three dominant patterns of large-scale circulation variability in the atmosphere, namely the South Asian summer monsoon, the Madden-Julian Oscillation (MJO) and the annular mode.

The role of the orography on South Asian monsoon is comprehensively examined using numerical simulations. The results show that the Tibetan Plateau creates a strong monsoon mainly by insulating the thermal maximum south of the Himalayas from extratropical air, as the monsoon strength is not sensitive to the orography and surface heating perturbations north of the Himalayas. It is also found that free-tropospheric moisture anomaly may lead to deviations from the theory of strict convective quasi-equilibrium, implying complexities that need to be included in existing theories of monsoon strength.

The MJO is the dominant mode of intraseasonal variability in the tropics. A mechanism- denial study is conducted to examine the importance of different physical processes to the MJO using numerical modeling. The results show that the essential MJO dynamics are internal to the equatorial Indian and Pacific Oceans. The wind-evaporation feedback tend to slow down propagation of the MJO and the radiative-convective feedback is important to MJO amplitude. In an observational study, the vertical distributions of radiative heating anomalies associated with the MJO is constructed using radiative heating profiles from CloudSat. Bottom-heavy radiative heating is seen in actively convecting regions of the MJO, which is argued to strengthen the MJO.

The annular mode corresponds to the meridional shift of the eddy-driven jet. Using a linear response function (LRF), the present study confirms unequivocally a positive eddy feedback in the annular mode dynamics in an idealized GCM. The strength of the eddy-jet feedback is accurately quantified, and the result from the LRF is used to evaluate three statistical methods. Because of the quasi-oscillatory nature of eddies, the mean-state- independent eddy forcing reduces the accuracy of two statistical methods proposed by previous studies. The results from a new method proposed here converge to the value produced by the LRF as the mean-state-independent eddies are mostly filtered out. The statistical methods are then applied to the reanalysis data.