Upwelling and Related Processes in the Banda and Northern Arafura Seas

Abstract

The Banda and Arafura Seas (BAS) are part of the Eastern Indonesian Seas (EIS), a tropical sea located between the Pacific and Indian Oceans. The seas are known to have abundant fishery resources and be rich in marine biodiversity. Due to their locations, the seas are subject to monsoonal winds and thus their resources are seasonally variable. Relatively higher productivity is observed during the southeast monsoon, when SST is lower and SSS is higher than during the northwest monsoon. Due to the complex topography of the EIS and the relatively large river discharges, tidal forcing and river runoff play important roles in the physical processes around the BAS.

In this study, a three-dimensional baroclinic nonlinear numerical model—HAMburg Shelf Ocean Model (HAMSOM)—is applied to investigate the effects of tidal forcing and river runoff on circulation during the upwelling period around the BAS by comparing different sensitivity runs from July 2004, i.e. “wind, river and tide” (WRT), “wind and tide” (WT), “wind and river” (WR), and “wind only” (WO) simulations. Furthermore, the role of adjacent oceans on upwelling intensity as associated with ENSO and IOD is investigated by running the WRT case over a 25-year period (1990–2014). The simulation results are validated with in-situ and altimeter data. Results show that seasonal variations in circulation, salinity, and temperature can be represented reasonably well by the model. Momentum analyses are further applied to explain the influence of tidal forcing and/or river runoff on wind-driven circulation during the upwelling period around the Northern Arafura Sea. Three vertical sections of this area (A, B and C) that represent pronounced upwelling signals are selected and investigated in detail.

The simulation results show that dry southeasterly winds (the southeast monsoon) generate upwelling around the BAS, and thus relatively low SST and high SSS are observed between May and October. Conversely, during the January–March period (i.e. the northwest monsoon), the wet northwesterly winds generate downwelling around the BAS. In addition, a subsurface salinity maximum of more than 34 within the thermocline layer is observed around the selected areas, indicating the influence of water masses from the Southern Pacific Ocean on the hydrodynamic conditions around the BAS area.

The important roles of tidal forcing and river runoff is indicated by the unrealistically high salinity maximum when both are excluded from the simulations. In contrast, the lower salinity maximum in the tide inclusion simulation indicates that tides enhance vertical mixing, mostly occuring around the Halmahera and Seram Seas. Here, relatively strong currents (known as the eastern Indonesian throughflow) flow within the thermocline layer, bringing with them Southern Pacific Ocean water masses that enter the Aru Basin (part of the Northern Arafura Sea). By comparing the WRT and WT cases, this study suggests that the vertical mixing intensity in these areas is relatively strong, and thus the mixing generated by tidal forcing and the rough topography reaches the surface, bringing relatively fresh water down to the subsurface, and then eroding the salinity maximum within the thermocline layer.

The simulation results show that Ekman surface currents are the main factors inducing upwelling over the slopes of all selected sections. The simulation results also show that tidal forcing generally leads to upwelling being enhanced near the Northern and Southern Aru Islands (section A and section B). In section A, enhanced upwelling in the tide inclusion simulation is related to the cell circulation formation and separation flow induced by nonlinear interaction between tidal flows and topography conditions. In section B, enhanced upwelling is mainly associated with the modified pressure gradient force that increases onshore subsurface flow. In contrast, tidal forcing weakens upwelling over section C in general; this is mainly caused by the relatively low residual surface currents toward off the Papua Coast, which are strongly related to the enhanced vertical viscosity coefficient induced by tidally enhanced vertical mixing. Furthermore, simulation results show that river runoff enhances cross-shelf circulation, as observed in all the sections. This is mainly related to enhanced stratification, that leads in weaker vertical eddy coefficients and interfacial stress. By considering the same wind forcing in the river runoff inclusion and exclusion simulation, the vertical viscosity force in the surface layer in the river runoff inclusion is subsequently enhanced.

Long-term simulations found that the El Nino/La Nina events contribute to the enhancement/weakening of upwelling intensities induced mainly by local wind forcing across the Arafura and Banda Seas. It is also observed that positive/negative IOD events lead to enhanced/weakened upwelling.