Seasonal Upwelling in the Northern Arafura Sea from Multi-datasets in 2017

Abstract

Seasonal upwelling phenomenon in the Arafura Sea plays an important role on supplying upwelled nutrient-rich water to sustain biogeochemistry processes and thus contributes to high marine primary productivity and fisheries resources in this region. The objective of this research was to investigate physical process and dynamics of upwelling by analyzing stratification of seawater properties, evolution of surface ocean-atmosphere parameters, and current structure and transport volume in the northern Arafura Sea. The multi-datasets in 2017 were used in this study, acquired from field CTD measurement, satellite-derived sea surface parameters, and the ocean general circulation model outputs, which were processed and analyzed using the available standard procedure. It was found that upwelling event was associated with a sharp subsurface thin layer that upsloping isotherms (23.5 - 25.5°C), isohalines (33.50 - 34.25 psu), and isopycnals (21.8 - 23.2 kg/m³) from the shelf-break region to the inner shelf region at a distance of approximately 167 km. This barrier layer separated the first surface mixed layer from the second mixed layer beneath the subsurface layer. The model suggests that the current in these two layers is in the opposite direction, to the west in the first layer as a response to the Ekman drift and to the east in the second layer as a current extension from deep Aru basin. Therefore, upwelling dynamics here is not only generated by the southeasterly monsoon winds from May (onset) to November (termination) that transport warm and fresh surface water away from the shelf, but also modulated by the presence of strong inflow currents beneath subsurface that supply colder saltier nutrient-rich water into the shelf. During the upwelling period, mean transport volume in the upper 25 m depth between Aru and Papua at 134.25°E was -0.28 (±0.34) Sv (westward), but the transport volume between 25m and 110m depth was +1.06 (±0.29) Sv (eastward), suggesting this inflow may regulate the upwelling and supply Arafura shelf water.

Deoxygenation of the eastern Indonesian waters and its variability

“…Seasonally, surface oxygen increased during the southeast (SE) monsoon (July-August-September) and decreased during the northwest (NW) monsoon (January-February-March). These patterns coincide with upwelling events of the eastern Indonesian water during the SE monsoon, which causes reduced heat at the sea surface and subsurface to reach minimum values [15,17]. There were indications of significant decreases in surface oxygen concentration in 1998, 2010, and 2016 (shown by gray shade in Figure 3).…”

“…This region is a complex marine system consisting of Molucca Sea, Halmahera Sea, and Ceram Sea in the northern area, Pacific Ocean and Arafura Sea in the eastern area, Timor Sea and Indian Ocean in the southern area, and Flores Sea and Savu Sea in the western area. The area crossed by A -A' vertical depth section, along the Flores -Banda -Arafura Seas, is strongly influenced by seasonal upwelling and plays a crucial role in supplying upwelled nutrient-rich water that contributes to high marine primary productivity [15]. The Arafura Sea in the eastern area is a relatively shallow water basin (< 200 m) affected by monsoonal winds and the Banda Sea circulation [16].…”

“…The upwelling event of the Indonesian seas occurred in the SE monsoon and characterized by the upward movement of deep waters shown in Figure 4, which replace nutrient-depleted surface waters. The vertical mixing from the deep sea to the continental shelf helps phytoplankton blooms and replenish the surface water nutrients that already high due to the massive river discharge [15,16].…”

The dynamic of sea level anomaly in the Papua coastal area and its associated response to the climate indices

“…This season promotes upwelling events which are identified by decreases in SST, shallowing of the thermocline, and decreases in sea level [38]. The Banda and Arafura seas are known for their extensive upwelling areas [39], [40], so the sea level decreases in this area are sharper compared to the northern waters of Papua.…”

“…Ekman upwelling along the southern waters of the Nusa Tenggara islands into the Banda and Arafura seas led to a reduced SST, high seawater density, and increased nutrient content that enhanced the productivity rates of the seawater [43], [44]. SST decreased to minimum values, especially in the Banda and Arafura seas in the southern waters of Papua Island (Figure 3), which are very intense upwelling areas [39], [40]. Meanwhile, the northern waters of Papua, which do not experience intense upwelling, did not experience a significant monsoonal forcing in SST in entire decades.…”