An investigation of the Coral Sea with an ocean general circulation model

Field: Earth and Planetary Sciences (Physical Sciences)

An ocean general circulation model covering the global domain is used to determine the flow regimes of the southwest Pacific, and the relationship between the Indonesian throughflow (IT), the East Australian Current (EAC) and the Coral Sea dynamics. A number of physical oceanographic surveys in the Coral Sea, using newly acquired acoustic Doppler current profiling instrumentation, show this area to be rather more complex than was at first thought. Earlier investigations of the Coral Sea led to the conclusion of a closed cyclonic gyre as the dominant pattern. The model shows a subsurface western boundary current, the New Guinea Coastal Undercurrent (NGCU), which begins on the Australian east coast near 16◦S and circulates along the coast of Papua New Guinea. The field surveys confirm the existence of this current and show the average volume to be about 25 Sv, being in fair agreement with the model. The observed flow turns northwards at the eastern tip of Papua New Guinea and is probably the source of the Vityaz Strait throughflow. An additional strong current is predicted to lie north of New Ireland during winter, and strong westwards flows through St. George's Channel have already been observed. The bifurcation of the South Equatorial Current (SEC) occurs between 14◦S and 23◦S, at varying depths along the Australian coast. The southern branch forms the EAC and the northwards flow forms the NGCU. The bifurcation has a notable baroclinic component with more of the deep water moving northwards. The Great Barrier Reef Undercurrent, flowing northwards in the Queensland Trough, is a deeper component of the SEC that bifurcates around the Queensland Plateau, while the shallower southwards flowing EAC originates from the surface bifurcation near 15◦S. The model NGCU is found to retroflect eastwards at 140◦E during summer, whence it forms the equatorial undercurrent, in good agreement with historical surveys. However, during winter, the model NGCU flows further to the west where it encounters Halmahera Island, which deflects the current northwards to near 3◦N, and it then finally retroflects again to form the Equatorial Undercurrent. If this island is moved southwards in the model, or removed, the NGCU supplies more than half the IT during winter, which is in direct contradiction to all observations. It is asserted that it is Halmahera Island which is the sole cause of the NGCU's non–participation in the IT, and not the broader scale dynamics. The model IT is found to be sensitive to modifications of the Antarctic Circumpolar Current (ACC), while the South Pacific wind stress perhaps does no more than seasonally modulate the throughflow. If the world model has its southern wall at 64◦S the IT is about 15 Sv during spin–up, but falls to 10 Sv after relaxation is switched off. With the wall at 75◦S the IT remains fairly steady during all phases on integration. Thus, it is assumed that the ACC has a primary role in driving the IT. Sverdrup models predict that the IT is driven by the South Pacific wind stress since there is a 16 Sv northwards residual in the curl of the wind stress field. It is postulated that the basic assumptions of Sverdrup models are violated to some extent by real and modelled ACC. The ACC effectively sets the depth–averaged pressures at the southern tips of both Africa and South America, so the single reference pressure set in Sverdrup models in not a valid assumption. A survey in September–91 by the R.V. Franklin (Fr0791) found intense offshore flows originating from the EAC at about 24◦S, moving to more than 500 km off the coast. This is well north of the EAC's normal departure latitude (33◦S), and no such findings appear to have been reported before this survey. Using Smagorinsky viscosity in the model, intense gyres that are elongated against the western boundary are found to develop, but only if the boundary itself is meridional. Haney & Wright (1975) reported this behaviour in a barotropic quasigeostrophic model with a rectangular domain, but they concluded that it constituted an unrealistic numerical mode. However, intense recirculations of the EAC are apparent in the Fr0791 data and in the data of the March–60 cruise of the H.M.A.S. Gascoyne (Ga0260). The model gyres are very similar to the observations, and it is concluded that they have at least some degree of realism.