Natuna Off-Shelf Current (NOC) Vertical Variability and Its Relation to ENSO in the North Natuna Sea

Abstract

During the northwest monsoon (NWM), southerly flow off the Natuna Islands appeared as the extension of the turning Vietnam coastal jet, known as Natuna off-shelf current (NOC). NOC is generated by the interaction of wind stress and the North Natuna Sea’s bottom topography. The purposes of the present study is to investigate the vertical variability of NOC and its relation to El Niňo Southern Oscillation (ENSO) using Marine Copernicus reanalysis data. The vertical variability refers to the spatial distribution of NOC pattern at the surface layer, thermocline layer, and deep/bottom layer. in 2014 as representative of normal ENSO condition. To investigate the effect of ENSO, the spatial distribution of NOC in 2011 and 2016 were compared which represent the La Niňa and El Niňo conditions, respectively. The results show that NOC starts to generate at the southeast monsoon season to the transition I season and peaks in the northwest monsoon season. The occurrence of NOC is identified at all depth layers with the weakened NOC at the deep layer. Related to the ENSO effect, La Niňa tends to strengthen NOC while El Niňo tends to weaken NOC. These are releted with the strengthening and weakening northerly wind speed during La Niňa and El Niňo, Respectively. During La Niňa events, the NOC occurs more frequently than during El Niňo. Thus, beside affecting the magnitude of NOC, ENSO also influence the frequency occurrence of NOC.

Identification of eddy-induced chlorophyll in the Northeastern Natuna Sea

“…The La Niña, El Niño events, and the normal ENSO are in figures 4, 5, and 6. Figure 4 shows a similar result to previous studies [5], in which VCJ lies on the left (yellow box in figure 4a), eddy lies in the middle (green box in figure 4a), and NOC lies on the right (red box in figure 4a). VCJ was indicated by more than 0.6 m/s of current speed (brown color).…”

“…5). The absence of NOC in January 2010 supports [5] that stated El Niño event decreased NOC. Figure 6 shows that VCJ and Eddy still appear, although the ENSO index is in a standard category, with 1.67 m/s of maximum currents speed.…”

“…The process was detected in some areas, for instance, the South China Sea [6], [7], and the Southern Ocean near the Antarctic [8]. Furthermore, NOC strengthened during an extreme La Niña event [5]. Hence, this study used a data assimilation model to further see the role of mesoscale eddies in the North Natuna Sea this season, IOP Publishing doi:10.1088/1755-1315/1137/1/012008 2 especially in highly adverse ENSO conditions (La Niña events) in terms of an increase of chlorophyll-a concentration.…”

“…Meanwhile, around the area, there are three phenomena, namely Vietnam Coastal Jet (VCJ) [2], Natuna Island Eddy (NIE) [3], and Natuna Off-shelf current (NOC) [2], and [4], which is the NIE usually appears when VCJ and NOCs are strong. The NIE has a rotation with 0.6 m/s of maximum speed and an average of about 0.45-0.5 m/s in the core, cyclonic, and more stable in the Northwest Monsoon (NWS) [5] or Northeast Monsoon [6]. A cyclonic eddy is identified as a cold core that generates an upwelling process: a rise of colder water mass to the upper level and forming a lower height in the middle of the Eddy.…”

The Characteristics Of The Mixed Layer Depth During La Niña, El Niño, And Normal Years In The North Natuna Sea

“…The current plot results indicate that from March to May, the currents originate from the east and have small scales. According to Hariyadi (2021), during the first transitional season, weak ocean currents dominate around the South China Sea and the North Natuna Sea. By July-August, dominant currents shift from the west and tend to turn northward.…”