John Luick

Circulation in the southern Great Barrier Reef studied through an integration of multiple remote sensing and in situ measurements

New mechanisms for stratification and upwelling in the southern Great Barrier Reef (GBR) are identified, and dynamic details of Capricorn Eddy, a transient feature located off the shelf at the southern extremity of the GBR, are revealed using the newly available surface current from High Frequency (HF) radar combined with other remote sensing and mooring data. The HF radar surface currents were used for tidal harmonic analysis and current-wind correlation analysis. These analyses, combined with Sea Surface Temperature (SST) data, mooring data, and altimetry-based geostrophic currents, enabled the effects of forcing from the large-scale oceanic currents (including the East Australian Current (EAC)), wind, and tides in a topographically complex flow regime to be separately identified. Within the indentation region where the width of the shelf abruptly narrows, current is strongly coupled with the EAC. Here strong residual flows, identified on current maps and SST images, fall into three patterns: southward flow, northwestward flow, and an eddy. Multiple data sets shed light on the prerequisite for the formation of the eddy, the reasons for its geometric variation, and its evolution with time. Intrusions of the eddy onto the shelf result in stratification characterized by a significant increase of surface temperature. Upwelling driven by wind or oceanic inflow is shown to cause stratification of previously well-mixed shelf water. The upwelling appears to be associated with equatorward-traveling coastal-trapped waves. The integrative method of analysis embodied here is applicable to other coastal regions with complex circulation.

Current measurements in the Halmahera Sea

A mooring with current meters at 400, 700, and 900 m at the 950-m isobath south of the 700-m-deep sill across the Halmahera Sea revealed many signals between June 1993 and July 1994. Strong tidal currents of 50 cm/s dragged the mooring down by as much as 80 m on occasions when the lunar perigees corresponded with new or full moons. At 400 m the nontidal currents were southward at up to 25 cm/s from October to April and northwestward at up to 20 cm/s at other times. At 700-m depth there was a near-continuous nontidal southward flow of 9 cm/s across the sill into the Halmahera Basin, which accords with findings by earlier Dutch and Indonesian workers. The current meter at 900-m depth showed the nontidal flow to be weak (∼1 cm/s) to the west. The southward transport between 350 and 700 m was inferred to reach a maximum of 5 Sv during the NW monsoon, with the annual mean being 1.5 Sv. There was a spring-neap effect on the nontidal currents at 400 m that was most pronounced in the last few months of the moorings life: these currents changed from ∼10 cm/s to the east during neap tides to ∼20 cm/s to the NNW during spring tides. Temperature and salinity profiles suggest that the waters of the Halmahera Sea are derived in part from the New Guinea Coastal Undercurrent.

Current measurements in the Maluku Sea

A current meter mooring was deployed in the eastern Indonesian archipelago, in the Maluku Sea, between the islands of Sulawesi and Halmahera, to contribute to estimates of the volume of the Indonesian Throughflow, for 13 months (June 1993-July 1994), with meters located at 740, 1250, 1750, and 2240 m below the surface. Hydrocasts for salinity and temperature were made at the time of deployment. The current meter and hydrographic data indicate a predominantly southward flow at the upper three meters throughout the year and a steady northward flow at the bottom meter. T-S and oxygen properties below 1800 m indicate a clockwise circulation in the Maluku Sea, with a vertical velocity of about 2 m/d. If the currents at the mooring, which is located on the western side of the Maluku Sea, are typical of the entire basin width, the average southward through flow between 740 m and 1500 m below the surface, 7 Sv (1 Sv = 106 m3/s ), is comparable in magnitude with estimates of upper layer transport and should be accounted for in estimates of the Indonesian Throughflow.

Closed Basin Modes of a Dual Basin Harbour

Closed basin seiches have been studied in Port Kembla, Australia, a compound harbour comprising two basins linked by a narrow channel. Despite the irregular geometry, there are well defined closed basin resonances which may be excited by long waves incident on the harbour entrance. The March 2011 Japanese tsunami excited open-basin modes, but did not significantly excite closed-basin modes. This is attributed to the very low incident wave energy at those frequencies. While the direct forcing of a closed basin mode has been extensively studied, the indirect forcing via an independent open-basin mode found here has not been extensively studied. It was found that single basin modes are more readily excited than the higher dual basin harmonics and the role of the irregular geometry in inhibiting some modes is discussed. The non-linear generation of a higher frequency mode unrelated to the forcing mode is demonstrated.