Publication Date

2018-08-15

Availability

Open access

Embargo Period

2018-08-15

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2018-06-12

First Committee Member

Mohamed Iskandarani

Second Committee Member

Brian Mapes

Third Committee Member

Chidong Zhang

Abstract

Troposphere-stratosphere dynamic coupling plays a crucial role in short and long term forecasts. In the tropics, the quasi-biennial oscillation (QBO) dominates stratospheric variability with zonal mean zonal wind alternating between westerly and easterly for an average period of 28 months. It has been shown in previous studies that the QBO can affect the tropical troposphere in different ways. Easterly phases of the QBO lead to stronger tropical deep convection in both observations and model simulations. Recent studies have indicated that activities of the Madden-Julian Oscillation (MJO) in boreal winter are also stronger in easterly phases of the QBO than its westerly phases. The MJO is one major component of the tropical intraseasonal variability and it is a key factor in bridging weather and climate. This QBO-MJO connection is of great importance for the following reasons: (1) Prediction of a single MJO event still suffers from many problems in most forecast models; (2) The QBO can be predicted more easily, thus there is large room for prediction improvement if the QBO-MJO connection is fully understood. In this study, a precipitation tracking method is used to identify individual MJO events, along with analysis of traditional Empirical Orthogonal Function (EOF) based global MJO indices that are independent of the tracking method. The results show that stronger MJO activities in QBO easterly phases are a consequence of more MJO days, not larger amplitudes of individual MJO events as previously thought. More MJO days come from more MJO events initiated over the Indian Ocean and their longer duration because of a weaker barrier effect of the Maritime Continent on MJO propagation. In addition, responses of general convective signals (e.g., total precipitation and cloud coverage) and tropical waves (Kelvin wave, Rossby wave and Mixed Rossby-Gravity wave) to QBO are also investigated in this study. The QBO-total precipitation connection shows both seasonal and spatial variations. Diversity among these responses reveals complexity in connections between the QBO and tropical variability of different scales. Physical processes behind theses connections are thus strongly demanded, and potential ones are discussed in this study.

Keywords

QBO; MJO; Seasonal cycle; air-sea interaction

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