Generation and termination of Indian Ocean dipole events in 2003, 2006 and 2007 |
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| Authors: | Suryachandra A Rao Jing-Jia Luo Swadhin K Behera Toshio Yamagata |
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| Institution: | (1) Indian Institute of Tropical Meteorology, Pashan, Pune, 411008, India;(2) Frontier Research Center for Global Change, JAMSTEC, 3173-25 Showamachi, Kanazawa-ku, Yokohama 236-0027, Japan;(3) Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan |
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| Abstract: | Evolution of Indian Ocean Dipole (IOD) events in 2003, 2006 and 2007 is investigated using observational and re-analysis data
products. Efforts are made to understand various processes involved in three phases of IOD events; activation, maturation
and termination. Three different triggers are found to activate the IOD events. In preceding months leading to the IOD evolution,
the thermocline in southeastern Indian Ocean shoals by reflection of near equatorial upwelling Rossby waves at the East African
coast into anomalous upwelling equatorial Kelvin waves. Strengthening (weakening) of northern (southern) portion of ITCZ in
March/April and May/June of IOD years, leads to strengthening of alongshore winds along Sumatra/Java coasts. With the combined
shallow thermocline and increased latent heat flux due to enhanced wind speeds, the SST in the southeastern Indian Ocean cools
in following months. On intraseasonal time scales convection-suppressing phase of Madden-Julian oscillation (MJO) propagates
from west to east in May/June of IOD year, and easterlies associated with this phase of MJO causes further shoaling of thermocline
in southeastern Indian Ocean, through anomalous upwelling Kelvin wave. All these three mechanisms appear to be involved in
initiating IOD event in 2006. On the other hand, except the strengthening/weakening of ITCZ, all other mechanisms are involved
in activation of 2003 IOD event. Activation of 2007 IOD event was due to propagation of convection-suppressing MJO in May/June
and strengthening of mean winds along Sumatra/Java coast from March to June through changes in convection. The IOD events
matured into full-fledged events in the following months after activation, by surface heat fluxes, vertical and horizontal
advection of cool waters supported by local along-shore upwelling favorable winds and remote equatorial easterly wind anomalies
through excitation of upwelling Kelvin waves. Propagating MJO signals in the tropical Indian Ocean brings significant changes
in evolution of IOD events on MJO time scales. Termination of 2003 and 2007 IOD events is achieved by strong convection-enhancing
MJOs propagating from west to east in the tropical Indian Ocean which deepen the thermocline in the southeastern equatorial
Indian Ocean. IOD event in 2006 was terminated by seasonal reversal of monsoon winds along Sumatra/Java coasts which stops
the local coastal upwelling. |
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