Models of the Southeast Asian seas

The mean and seasonal variations in transport through and within the Southe ast Asian seas are investigated using a series of simple models. The result s are compared with results from a fine-resolution, 3D, numerical simulatio n of the global circulation from 1987 to 1995 [Parallel Ocean Climate Model (POCM)]. For the mean circulation, the models are based on Sverdrup dynamics with th e circulation around each island calculated according to the island rule wi th overlapping islands taken into account. Assuming all of the passages are wide and deep yields an archipelago circulation vastly at odds with observ ations. A large westward transport through Torres Strait provides the throu ghflow between the Pacific and Indian Oceans. a large westward transport th rough Luzon Strait passes southward through the South China Sea into the Su lu Sea and exits into the Pacific Ocean through the Celebes Sea. There is a northwestward transport through the remainder of the archipelago By succes sively blocking straits under the assumption that frictional effects are su fficient to arrest flow in the strait. an understanding is built up of why the mean circulation in the archipelago is as observed and as simulated in POCM. For example, blocking Torres Strait yields a more realistic circulati on with southward flow in the archipelago. Greater realism is achieved by b locking off the South China Sea, so making the dominant pathway for the thr oughflow from the Pacific westward through the Celebes Sea and southward th rough,Makassar Strait. The weak throughflow in POCM (7.5 x 10(6) m(3) s(-1)) is found due to the w ind stresses derived from the European Centre for Medium-Range Weather Fore casts 10-m twice-daily winds, which art much weaker than the Hellerman and Rosenstein climatology (HR) used in previous studies. Also, POCM's throughf low is wholly fed by the South Equatorial Current rather than predominantly by the Mindanao Current, as found in models forced by HR climatology. Anal ysis of the wind stress datasets and that of the Florida Start: University from 1961 to 1995 shows that the latitude of the zero-Sverdrup-transport st reamline near the Pacific entrance to the Celebes Sea has shifted poleward over the decades, so decreasing the absolute amount originating from the Mi ndanao Current. Regarding the seasonal cycle, there is negligible transport below 500 m at annual period within the archipelago in POCM. which suggests that the numer ous islands and sills within the archipelago enhance the adjustment to the applied wind stress locally. Assuming a local Sverdrup balance, island-rule -based models of the archipelago show that forcing by wind stresses over th e archipelago and Australia give reasonable agreement with POCM for the amp litude of the annual harmonic in depth-integrated transport. Better agreeme nt in phase within the straits and seas is obtained by recognizing that fri ctional effects within certain straits enabler the influence of wind stress variations to be felt in directions other than just to the west, as in the original island rule. It is further noted that the adjustment to semiannual period wind-stress fo rcing is incomplete within the seas: there is no local quasi-equilibrium re sponse. In POCM, the archipelago fills above 500 m in February-June and Sep tember-November, and drains in the remaining months. There is compensating Bow below. Also, seasonal variability of the currents in the west Pacific i s not sufficient to alter significantly the eyre closure in the west Pacifi c, and the depth-integrated throughflow is fed by the South Equatorial Curr ent throughout the year, either via a western boundary current or a broad z onal jet.