A Conceptual Model for the Indian Ocean Dipole-Mode
Fei-Fei Jin
Department of Meteorology School of Ocean and Earth Science and Technology University of Hawaii at Manoa, Honolulu, Hawaii
L.Yu
Department of Physical Oceanography Woods Hole Oceanographic Institute Woods Hole, MA02543
A conceptual model is constructed to illustrate the growth and oscillatory mechanisms of the recently discovered dipole mode in the tropical Indian SST anomalies. The SST dynamics combine thermodynamic feedbacks with emphasis on wind-evaporation feedback and ocean dynamic feedback. The later is facilitated by the annual coastal upwelling near Sumatra and annual Ekman upwelling in the west tropical Indian ocean. In these upwelling regions, the subsurface ocean temperature anomalies produced by wind stress forcing through ocean dynamic adjustment can be brought into the surface to alter SST anomalies. With the ocean dynamics playing an active role, the tropical Indian ocean-atmosphere system can support a damped oscillatory coupled mode which bears certain similarity to the ENSO mode in the Pacific. Over a wide range of model parameters, this mode has a period in the range of 2 to 3 years with a damping rate about a half year. This damped mode can be excited by stochastic forcing in general and forcing related to ENSO in particular. The basic characteristics of mode, including the relationship among the equatorial western Indian ocean thermocline depth and the eastern and western Indian SST anomalies, are in agreement with the observed dipole-mode variability.
