The Variability of Upper-Ocean Salinity in the Eastern Inflow Region of ITF

Abstract

The north and south Pacific waters influenced the water mass profile in the eastern inflow region of Indonesian throughflow (ITF), including Halmahera Sea, Maluku Sea, and Sangihe–Talaud Waters. Pacific waters are characterized by the strong salinity stratification in the upper-ocean (0–300m depth). Repeated annual oceanographic observations in the eastern inflow region of ITF found anomalous upper-ocean salinity profiles during the 2014–2017 periods. At the end of 2015 (El Niño condition), saltier salinity and nearly homogeneous salinity profile (>34.6 PSU) in the upper-ocean were found, and it is expected due to the decreasing precipitation rate. Conversely, freshening of upper-ocean salinity (33.5 – 34.6 PSU) was found during the weak La Niña condition in 2017. An increased precipitation rate might cause this condition for five months in a sequence (varies within +22% to 112% higher than its monthly mean). In between, the weak El Niño and weak La Niña condition in 2014 and 2016 are more likely to show a similar salinity profile to the strong El Niño 2015 and weak La Niña 2017, respectively. There is a slight reduction (increase) of precipitation rate during 2014 (2016), but it does not lead to a drastic increase (freshening) of salinity as indicated in 2015 and 2017. This study also suggests the less effect of MJO on the precipitation rate and salinity anomalies in the eastern inflow region of ITF, especially during its active phase in 2017.

Transformation of South Pacific Water Masses in the Halmahera Sea

“…A drastic decrease in salinity occurred in the South Halmahera Strait and Obi Strait, remaining the salinity of ~34.6 in the Maluku Sea (Stations 1). Recent studies also indicated that this water masses are the South Pacific Subtropical Water (SPSW) [17], [28], [27]. Even though the characteristic salinity range is not the same as in the area where it was formed, the maximum salinity characteristics are still visible in the Halmahera Sea [29].…”