Tidal and Solar Radiation Impacts in the Southern Arafura Sea

Abstract:

In the shallow waters of the southern Arafura Sea near the Tiwi Islands, tides and the daily solar radiation cycle dominated the circulation in the form of tidal advection on the western side and internal tides on the eastern side as learned from observations in the region from the RV Investigator in November-December 2019. Slight changes in the topography resulted in the different dynamics between the eastern and western side of the Tiwi Islands, with one side baroclinic and the other barotropic. Extensive shallow water (< 30 m) regions exist on the western side; whereas, the eastern side has a lower percentage of shallow water. The daily solar radiation cycle on the western side increased the temperature, salinity, and density. The stratification as represented by the Brunt-Väisälä frequency increased due to the solar cycle and descended throughout the water column. Warm dense waters flowed out near the bottom. Cooler, fresher water occurred in deeper regions, particularly a trench. However, these waters were denser than the warmer saltier water and did not flow in deep on the western side. On the western side, tidal advection moved the entire water column together past the time series sites, so alternating warmer, saltier and cooler, fresher water passed the site. The result was a barotropic situation on the western side. The eastern side was baroclinic, with two clear layers separated at 20 m. The density of the lower layer was closer to that in the deeper water. Although there was a barotropic tide here too, it was weaker than on the eastern side and a baroclinic component was present with the two layers moving in opposite directions. Increases in the temperature due to surface radiation affected the entire water column, but upper 20 m were affected more and the increase in stratification was limited to the upper layer. The replenishment of the lower layer on the eastern side was reflected in an increase in fluorescence. Since fluorescence was limited to the lower water column on the eastern side but encompassed the entire water column on the western side, there are biological implications for these results.