Observations and dynamics of sea-breezes in northern Australia

Observations of tropical sea-breezes in the Gulf of Carpentaria region of northern Australia are presented. Under suitable synoptic conditions these deep (1100 m) sea-breezes are shown to penetrate far inland (180 km by 2230 EST) and to possess a stronger circulation at night than during the day. The form of the 0600 EST wind and temperature profiles at Burketown, 25 km south of the Gulf, on the morning following a sea-breeze are typical of those observed prior to the arrival of an undular bore (known as the 'morning glory') which is generated on the Cape York Peninsula to the northeast. The two-dimensional form of the Colorado State University's mesoscale model is used to simulate the observations and reproduces well the stronger nocturnal circulation and deep inland penetration, with remnants of the sea-breeze, in a vastly modi-fied form, still evident 280 km inland at 0600 EST. The model wind and temperature profiles for 0600 EST in the coastal region show that the sea-breeze is responsible for the highly ageostrophic flow and deep stable layer observed at Burketown. An analysis of the dynamics involved in the nocturnal stage of the sea-breeze shows that horizontal advection dominates the processes involved in maintaining the frontal density gradient, but that the form of the sea-breeze changes quickly from a density current to a vortex once the advection of cooler sea air is cut off. The change in form occurs when air which crossed the coastline around sunset reaches the front. The vortex continues to move inland though steadily decelerating and decreasing in size, due mainly to subsidence warming in its descending arm. In assessing the importance of the sea-breeze in providing conditions for bore propagation in the region, linear internal wave theory is applied to observed early morning profiles with and without sea-breeze effects. It is found that both profiles will sup-port waves propagating at speeds within the observed range for morning glories, implying that sea-breezes in this region are not essential for the continued propagation of bores generated further to the east. However, there are indications that the typical pre-glory profile of a deep neutral layer overlying a low-level strongly stable layer — a situation necessary for the trapping of wave energy in the low levels and to which linear wave theory can be applied — does not often occur on non-sea-breeze occa-sions. The more common situation of marked stability aloft is probably associated with synoptic-scale subsidence.