The Indonesia Seas is the only oceanic pathway that connects tropical ocean basins. It contributes significant inter-basin exchange of heat and freshwater through the well-known Indonesia Throughflow (ITF, Figure 1). The ITF is typically thought to be controlled by the pressure gradient between the tropical Pacific and Indian Ocean basins. However, the role of freshwater input and salinity variability is unclear yet even if there is strong mixing and significant freshwater input.

Figure 1. (a) Mean rainfall rate (color, unit in mm day^-1) from the Tropical Rainfall Measuring Mission (TRMM) data and averaged over January 1998 - December 2008, and (b) linear trend of Argo salinity (unit in 10^-1 PSU decade^-1) averaged over the upper 200-dbar during 2004-2014. The ITF pathways are indicated in black arrows according to Sprintall et al. 2015. The dashed black ellipse indicates where the ITF enters into the Indian Ocean (Hu and Sprintall, 2017b).

Scientists from the Institute of Oceanology at Chinese Academy of Sciences and Scripps Institution of Oceanography , Drs Shijan Hu and Janet Sprintall, recently worked on this essential issue to better understanding the variability of the ITF and hence the tropical Indo-Pacific Ocean environment. They successfully separate the salinity effect from the density and ITF transport calculations on the basis of the DBP-ITF theory proposed by Andersson and Stigebrandt (2005). They find that salinity effect contributes (36±7)% of the total interannual variability of ITF, which is mainly related to the El Niño-Southern Oscillation (ENSO) (Hu and Sprintall, 2016). Under the ENSO cycle, the Walker Circulation shifts longitudinally resulting in fluctuations in precipitation over the Maritime Continent that modulates oceanic salinity and subsequently modulates the ITF transport. Shijian and Janet further emphasized on the influence of rainfall enhancement over the Maritime Continent on the Indo-Pacific inter-basin exchanges. They revealed that the ITF water get freshening, the ITF heat and freshwater transports get increase, and consequently result in a significant warming and freshening trend in the eastern Indian Ocean (Hu and Sprintall, 2017a).

Figure 2. Schematic representation of the mechanism responsible for the enhancement of the ITF during the past decade. The linear trends of (a) precipitation in the tropical Indo-Pacific Ocean. Dashed black arrow-lines denote the tendency of ocean surface zonal wind stress over the three ocean basins. The linear trends in (b) temperature and (c) salinity in the western Pacific (along 4.5°N) and eastern Indian (along 12.5°S) Oceans. Contour lines show the mean (b) temperature and (c) salinity over 2004–2014 from RG Argo dataset. The direction of the ITF and the enhancement of the ITF is shown by the dashed blue arrow (Hu and Sprintall, 2017a).

The ITF community commented this study as an innovative progress in understanding the ITF dynamics and climate effect. The combined effect of the ITF transport of mass and freshwater along with tropical rainfall is found to plays a very important role in the tropical Indo-Pacific ocean environment and hence the marine ecosystem. Above results are published in recent papers by the Journal of Geophysical Research: Oceans, Geophysical Research Letters and CLIVAR Exchanges.

This work was supported by the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SYS023), the National Natural Science Foundation of China (grants 41406016 and 41330963), and the Physical Oceanography program of the National Aeronautics and Space Administration (NASA) under grant NNX13AO38G.

References:

Hu, S., and J. Sprintall (2017a), Observed Strengthening of Interbasin Exchange via the Indonesian Seas Due to Rainfall Intensification, Geophysical Research Letters, 44(3), 1448–1456, doi:10.1002/2016GL072494.

Hu, S., and J. Sprintall (2017b), A stronger Indonesian Throughflow Related to Enhanced Regional Rainfall, CLIVAR Exchanges, 71, 21-25.

Hu, S., and J. Sprintall (2016), Interannual Variability of the Indonesian Throughflow: the Salinity Effect, Journal of Geophysical Research: Oceans, 121, 2596-2615, doi:10.1002/2015JC011495.