Seasonal mixed layer temperature and salt balances in the Banda Sea observed by an Argo float

Abstract

For the first time, the processes controlling the variations of mixed layer temperature (MLT) and salinity (MLS) in the Banda Sea are quantified using data from a single Argo float in combination with satellite and reanalysis outputs from August 2017 to August 2019. This augments previous studies that utilized ocean model data only. We document the presence of a barrier layer and quantify the roles of air-sea heat and mass exchanges, horizontal advection, and vertical entrainment in the seasonal variability of MLT and MLS. We find that heat gains and losses at the air-sea interface are the main contributor to the warming and cooling of the MLT. Seasonal changes in MLS are driven by advection of low salinity water rather than freshwater fluxes from precipitation and evaporation. This is particularly the case during the late northwest and monsoon transition period from February to April when low salinity is advected eastward from the Java Sea into the Banda Sea.

Observations of Barrier Layer Seasonal Variation in the Banda Sea

“…8b. The significant role of low salinity water horizontal advection to the MLD was also observed by Halkides et al (2011) and Ismail et al (2023). The resultant ILD thickening and MLD shoaling then favor a deep barrier layer in the Banda Sea forced by a shallow MLD that limits the momentum input to the near-surface mixed layer only.…”

“…A recently published study evaluating three global ocean reanalysis products demonstrates that BRAN2020 outperforms the Hybrid Coordinate Ocean Model and Mercator Ocean's Global Reanalysis in Southern Africa, especially for the MLD (Russo et al, 2022). Numerous researches in the Australasian region have been supported by BRAN2020 data, including analysis of the Fraser Gyre off southeast Queensland (Ismail et al, 2017), studies of intraseasonal variability of the ITF (Schiller et al, 2010), and mixed layer heat and mass budget in the Banda Sea (Ismail et al, 2023). Near-surface zonal and meridional wind data for the period January 1993 to December 2022 from the fifth generation (ERA5) of the global climate and weather outputs of the European Center for Medium-Range Weather Forecasts (Hersbach et al, 2023) were also used in this study.…”

“…The decaying phase of BLT in the Banda Sea during SON is in line with the changes in wind stress from southeasterly winds and an associated relaxation in the curl Ekman pumping, respectively. More saline waters appear at the surface, and also the surface ocean is affected by a net evaporation changing of salinity regime to salinization-dominated waters (Halkides et al, 2011;Ismail et al, 2023) and a remnant of positive relative vorticity in the same spot as in JJA.…”

“…At intraseasonal time scales, the Banda Sea sea surface temperature (SST) is modulated by the Madden-Julian Oscillation (MJO; Napitu et al, 2015;Pei et al, 2021). The upper ocean circulation in the Banda Sea is controlled by seasonally reversing monsoonal winds from the northwest during the austral summer and from the southeast during the austral winter (Gordon & Susanto, 2001;Ilahude & Gordon, 1996;Ismail et al, 2023;Kida et al, 2019;Sprintall & Liu, 2005;Zhu et al, 2019). The southeast monsoon drives basin-wide wind-induced mixing (Thomas et al, 2003), strong outflow through the Timor Passage and into the Indian Ocean (Kida et al, 2019), and quasi-stationary anticyclonic eddy in the southwest boundary of the Banda Sea (Liang et al, 2019;Zhu et al, 2019).…”

“…In contrast, the northwest monsoon current carries low salinity water from the Java Sea and Makassar Strait through rainfall and river discharge (Halkides et al, 2011;Ilahude & Gordon, 1996;Kida et al, 2019;Zhu et al, 2019). This freshwater influx, which has a pronounced low sea surface salinity signature (< 34), is overlying saline subsurface water and creates a strong salinity stratification in the Banda Sea that is forming a regional barrier layer (Ismail et al, 2023).…”