Thermohaline Stratification of the Indonesian Seas-Model and Observations
Arnold L. Gordon
Lamont-Doherty Earth Observatory Palisades, N.Y. 10964, USA
Inter-ocean transport within the Indonesian Seas is the primary means of exporting excess freshwater from the North Pacific Ocean. The efficiency of this transfer dictates to a large measure the meridional overturning of the Pacific and Indian Oceans. The Indonesian throughflow is relevant to ENSO as it allows "seepage" of the western Pacific's warm pool water into the Indian Ocean, affecting the volume of the warm pool. For global models to faithfully reproduce the Earth's climate system, the inter-ocean fluxes of mass, heat and freshwater must be properly represented. The Indonesian Seas with it's complex topography falling near the division of the North Pacific and South Pacific water mass fields, represent a severe challenge to model validity.
Thermohaline observations within the Indonesian Seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) as part of the Arlindo program is compared to model results. The comparison model is the Los Alamos parallel ocean program 1/6 degree global model, forced by ECMWF winds. Two versions of the model are used: one is relaxed to Levitus monthly sea surface climatology in the upper 25m; and a surface thermal boundary formulation which allows for SST controlled 'corrective' feedback (Barnier conditions) replaced the Levitus boundary condition. An array of model "stations" are compared to the Arlindo CTD stations.
A general statement can be made; the model water masses pattern bears resemblance to the observed distribution, but the model water column is too salty, too much South Pacific thermocline water fills the Seas east of Sulawesi. West of Sulawesi, in the Makassar Strait the model results show a North Pacific water column, matching the observational stations, though with reduced core layer values. In the Flores Sea near the 12｡? isotherm, an infusion of salt from the Banda Seas mirrors a pattern revealed by observations. In the Maluku, Seram and Banda Seas the model stations are far too salty, though the Barnier conditions yield a more realistic stratification. In the Timor Sea, the model exports to the Indian Ocean a water column laced with too much South Pacific salt. In the Timor Sea below the Indonesian 1200m sill depth, the model lacks the observed intrusion of saline south Indian Ocean water.
Within the model, North Pacific thermocline water flowing through the Makassar Strait exits the Indonesian Seas in the Lombok Strait, whereas the observations show that the bulk turns eastward into the Flores and Banda Seas, before exiting the Indonesian Seas near Timor. The model South Pacific thermocline spreads uninhibited into the Banda and Timor Seas. It is suspected that the details of the geometry may be the cause of the differences, but larger scale aspects of the model may also be a contributing factor.
