Darren Engwirda: Freshwater Signals and Salinity Dynamics in Torres Strait
Torres Strait is a narrow, tidally-dominated waterbody located between Australia and Papua New Guinea and separating the Gulf of Carpentaria, Gulf of Papua, Northern Great Barrier Reef and Coral Sea. Punctuated by an array of shoals, islands and reefs, the Strait is a shallow shelf that gives rise to energetic mixing and circulation, with maximum current speeds exceeding 2m/s. Despite these locally-amplified dynamics, we find that Torres Strait is largely dominated by remote, seasonal effects, undergoing a ‘seasonal hysteresis’ in which flow through the Strait and surrounding Gulfs reverses course throughout the year under the influence of the Northwest Monsoon. In this work, high-resolution numerical modelling is used to understand the impact of this seasonal oscillation — examining the role of river flows, precipitation and saline water masses. A peak freshening is found to occur within the Strait in late summer due to the confluence of remote river flows propagating eastward along the West Papuan coastal shelf, southerly outflow from the Fly River as well as intense local precipitation. These dynamics reverse during winter, leading to a salinity maximum in Torres Strait in early spring associated with the north-westerly displacement of saline coastal waters originating on the Great Barrier Reef. These dynamics lead to the development and evolution of strong seasonal salinity fronts and freshwater plumes, resulting in a dynamically-rich circulation within the Strait that has implications for the local ecosystem and reef health.