Thermohaline Stratification of the Indonesian Seas: Model and Observations

Abstract

The Indonesian Throughflow, weaving through complex topography, drawing water from near the division of the North Pacific and South Pacific water mass fields, represents a severe challenge to modeling efforts. Thermohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Ocean Program (POP) 1/6 deg (on average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed profiles, such as between 200 and 1200 dbar where the model temperature is as much as 3°C warmer than the observed temperature. Within the 5°?28°C temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature profile, is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Makassar Strait, North Pacific waters are found down to about 250 dbar, in agreement with observations. The model sill depth in the Makassar Strait of 200 m, rather than the observed 550-m sill depth, shields the model Flores Sea from Makassar Strait lower thermocline water, causing the Flores lower thermocline to be dominated by salty water from the Banda Sea. In the Maluku, Seram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Pacific water. Observations show that the bulk of the Makassar throughflow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near Timor. In the model, South Pacific thermocline water spreads uninhibited into the Banda, Flores, and Timor Seas and ultimately into the Indian Ocean. The model throughflow transport is about 7.0 Sv (Sv ≡ 106 m3 s?1) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However, during the prolonged El Niño of the early 1990s the throughflow is suspected to be lower than average and, indeed, the model transports for the non?El Niño months of August 1988 and February 1989 are larger. It is likely that aspects of the model bathymetry, particularly that of the Torres Strait, which is too open to the South Pacific, and the Makassar Strait, which is too restrictive, may be the cause of the discrepancies between observations and model.