Evolution of the Dipole Mode Events in an Ocean Model
P. N. Vinayachandran
Centre for Atmospheric and Oceanic Sciences Indian Institute of Sciences
Bangalore 560 012, India.
E-mail: vinay@caos.iisc..ernet.in
Satoshi Iizuka
National Research Institute for Earth Science and Disaster Prevention
Tsukuba, lbaraki, Japan
It has been discovered recently that strong east-west contrast in sea surface temperature develops in the Indian Ocean independent of El Nino. The positive dipole mode (DM) is characterized by warmer than normal sea surface temperature (SST) anomalies in the western Indian Ocean and cooler than normal SST anomalies in the eastern Indian Ocean. The DM events are accompanied by easterly wind anomalies along the equatorial Indian Ocean and upwelling favorable alongshore wind anomalies along the coast of Sumatra. The pattern reverses during negative DM events. The evolution of these events is examined in an ocean model that is driven by the NCEP fluxes during the period 1975 to 1998. The three significant positive DM events during 1997, 1994 and 1982 and negative DM events during 1984-85 and 1996 are captured by the model. The positive DM events are characterized weaker equatorial jets, shallower mixed layer and thermocline. Stronger equatorial jets and deeper thermocline and mixed layer characterize the negative DM events. Analysis of the heat balance in the model shows that, during the DM events, dominant terms in the temperature equation are zonal advection and upwelling. This is manifested in the westward propagating Rossby waves excited in the eastern and central equatorial Indian Ocean during the DM events.
