Deep expression of the Indonesian Throughflow: Indonesian Intermediate Water in the South Equatorial Current

Abstract

The narrow westward flow of the South Equatorial Current (SEC), centered at 12°S and carrying freshened water from the Indonesian seas, is traced across the Indian Ocean using data from the World Ocean Circulation Experiment. The jet is remarkably zonal and quasi‐barotropic, following the potential vorticity contours characteristic of the tropics, separating higher‐oxygen and lower‐nutrient waters of the subtropics from the oxygen‐depleted waters of the tropics. The fresh surface waters are the usual Indonesian Throughflow Water reported previously. Less well studied is the intermediate‐depth SEC carrying fresher water from the Banda Sea and Pacific, known as Indonesian Intermediate Water (IIW) or Banda Sea Intermediate Water. The high‐silica signature of IIW is documented here, permitting us to (1) trace the spread of IIW from sill density at Leti Strait to higher density as it is diluted toward the west and (2) define an IIW core for transport estimates, of 3 to 7 Sv westward, using geostrophic and LADCP velocities. The high IIW silica is traced to the Banda Sea, arising from known diapycnal mixing of Pacific waters entering through Lifamatola Strait and local sources. New heat, freshwater, oxygen, and silica budgets within the Indonesian seas suggest at least 3 Sv of inflow through the relatively deep Lifamatola Strait, supplementing the observed 9 Sv through the shallower Makassar Strait. Both shallow and deep inflows and outflows, along with vigorous mixing and internal sources within the Indonesian seas, are required to capture the transformation of Pacific to Indonesian Throughflow waters.