NEWS LETTER    第12号 (2006年 春)


    竹中 宏平    たけなか こうへい




 Pathways and effects of the ITF in the Indian Ocean
     Vinu K. V.     ビニュー

  Indonesian Throughflow (ITF) is a system of currents flowing from the Pacific to the Indian Ocean via Indonesian Straits. The ITF is the upper (warm) branch of the Global Conveyor Belt, while much less is known on its pathways within the Indian Ocean, whose understanding is necessary, to find the impact of the ITF in the Indian Ocean climate as well as the response of the conveyor belt in long term climate change. This research is focused on the detailed three dimensional trajectories of the ITF in the Indian Ocean and to categorize its impact on the Indian Ocean climatology.
The 3-Dimensional pathways of the ITF in the Indian Ocean are identified using an OGCM, with a combined set of tools (1) Lagrangian particle trajectories, (2) passive tracers and (3) active tracers (temperature and salinity). The Lagrangian particles suggest that at the entrance region the surface ITF (< 60m) subducts off-northwestern coast of Australia (releasing ~40 Wm-2), and then travels across the Indian Ocean along the thermocline depths. The subsurface (> 60m) ITF more directly departs westward and crosses the Indian Ocean. Using the passive tracers, which are mixed vertically under convection as well as horizontally due to diffusion, the ITF is shown to undergo vigorous mixing as soon as it enters the Indian Ocean and modifies its upper T-S characteristics.
Upon reaching the western boundary, more than 70% of the ITF turn northward and reroutes into three distinct depth ranges, owing to the seasonal reversal of the Somali region, as Route-1 (26%): across the Indian Ocean just to the south of the equator (200m-300m), Route-2 (24%): across the Indian Ocean to the north of the equator (100m-200m) and Route-3 (20%): upwells in the Somali region and spreads all over the surface of the northern Indian Ocean. Route-3 dominates during summer monsoon (Apr-Oct) while Route-2 dominates during winter monsoon (Nov-Mar).
The effects of the ITF on the temperature and salinity are mainly accompanied with the major pathways shown by the Lagrangian particles and the passive tracers. The direct effect of the ITF is responsible for a warming (~30C) and freshening (~0.2 psu) of the thermocline depths (100 m) along the main stream (100S to 200S).

Figure: Schematic view of the ITF spreading pathways in the Indian Ocean