Three Types of MJO Initiation Processes over the Western Equatorial Indian Ocean

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Three Types of MJO Initiation Processes over the Western Equatorial Indian Ocean

MEI Shuangli (1,2), Tim LI (3,4), and CHEN Wen (1)

(1) Center for Monsoon System Research,, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing,100029

(2) University of Chinese Academy of Sciences, Beijing 100049

(3) International Laboratory on Climate and Environment Change and Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing 210044

(4) International Pacific Research Center, Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

Abstract:

Thirty strong Madden–Julian Oscillation (MJO) events in boreal winter 1982–2001 are selected to investigate the triggering processes of MJO convection over the western equatorial Indian Ocean (IO). These MJO events are classified into three types, according to their dynamic and thermodynamic precursor signals in situ. In Type I, a remarkable increase in low-level moisture occurs, on average, 7 days prior to the convection initiation. This low-level moistening is mainly due to the advection of the background mean moisture by easterly wind anomalies over the equatorial IO. In Type II, lower-tropospheric ascending motion anomalies develop, on average, 4 days prior to the initiation. The cause of this ascending motion anomaly is attributed to the anomalous warm advection, set up by a suppressed MJO phase in the equatorial IO. In Type III, there are no clear dynamic and thermodynamic precursor signals in situ. The convection might be triggered by energy accumulation in the upper layer associated with Rossby wave activity fluxes originated from the midlatitudes.