Fifteen years of expendable bathythermograph (XBT) data from three lines that cross Australia's Northwest Shelf and the shallow part of the Arafura Sea, a region of broad continental shelf referred to below as the Shelf, have been used, with climatological salinity data, to estimate onshore geostrophic flow to the Shelf. A wind stress climatology is used for onshore Ekman transports. Onshore geostrophic flow in the top 200 m onto the whole Shelf has an annual mean of 3.1 Sv; it is weakest (0.8 Sv) in May-June and strongest (5.1 Sv) in January. The onshore Ekman transport has an annual mean of only -0.5 Sv, but its annual cycle almost completely cancels that of the geostrophic flow, so that the net onshore flow varies little through the year from its annual mean of 2.6 Sv. Most of this probably exits through the southern end of the shelf as the annual mean Leeuwin Current. The small annual cycle of the net onshore flow suggests that longshore flow at the northern end of the section has a similar seasonal cycle as flow at the southern end. We have not been able to test this idea or to obtain adequate mass budget closure owing to limitations and interannual variability in longshore current data. There is ~1 Sv of onshore geostrophic flow below 200 m in winter onto the northern Shelf. We have used climatological heat fluxes and temperatures on the Shelf to estimate mean seasonal net heat export from the Shelf; export is positive throughout the year on the northern Shelf and is negative for only 3 months or less on the southern Shelf. Despite the lack of mass closure we can infer (1) the Gentilli <1972> mechanism of alternating longshelf seasonal flow cannot explain all of the observations, (2) summer wind-driven upwelling can export substantial amounts of heat from both the northern and southern Shelf, and (3) the deep winter inflow to the northern Shelf probably upwells into the Arafura Sea, where nutrient-rich inflow has been observed <Rochford, 1966>. This flow has the right magnitude to supply westward Ekman transports in winter between Irian Jaya and Australia. We describe some striking observations of internal and bottom mixed layers; bottom mixed layer formation may permit the deep inflow to warm as much as 8 ¿C before it flows off the Shelf. It may also permit upwelling of the Leeuwin Undercurrent. ¿ 2000 American Geophysical Union