Indonesian Throughflow Volume Transport Research Articles

Roles of the Indo-Pacific subsurface Kelvin waves and volume transport in prolonging the triple-dip 2020–2023 La Niña

The rare triple-dip 2020–2023 La Niña event has resulted in a series of extreme climate events across the globe. Here, we reveal the role of tropical Indo-Pacific oceanic interactions in driving the first triple-dip La Niña of the twenty-first century. Specifically, we found that the eastern Indian Ocean subsurface warming anomalies were associated with the re-intensification of the subsequent La Niña event. The subsurface warming anomaly signals were propagated eastward by equatorial and coastal subsurface Kelvin waves from the eastern Indian Ocean to the western Pacific Ocean through the Indo-Pacific oceanic pathway, which contributes to the accumulation of heat content and deepens the thermocline in the western tropical Pacific. The westward Indonesian Throughflow (ITF) transported more heat during multi-year La Niña events from the western Pacific Ocean to the eastern Indian Ocean than during single-year events, resulting in the injection of more warm water into the eastern Indian Ocean. The combination of subsurface Kelvin wave propagation and increased ITF volume transport in the Indo-Pacific region acted to prolong the heat content in the western Pacific during the decay phase of La Niña, ultimately leading to the rare triple-dip 2020–2023 La Niña event.

Correlation analysis of the North Equatorial Current bifurcation and the Indonesian Throughflow

Based on monthly mean Simple Ocean Data Assimilation (SODA) products from 1958 to 2007, this study analyzes the seasonal and interannual variability of the North Equatorial Current (NEC) bifurcation latitude and the Indonesian Throughflow (ITF) volume transport. Further, Empirical Mode Decomposition (EMD) method and lag-correlation analysis are employed to reveal the relationships between the NEC bifurcation location, NEC and ITF volume transport and ENSO events. The analysis results of the seasonal variability show that the annual mean location of NEC bifurcation in upper layer occurs at 14.33°N and ITF volume transport has a maximum value in summer, a minimum value in winter and an annual mean transport of 7.75×106 m3/s. The interannual variability analysis indicates that the variability of NEC bifurcation location can be treated as a precursor of El Nino. The correlation coefficient between the two reaches the maximum of 0.53 with a time lag of 2 months. The ITF volume transport is positively related with El Nino events with a maximum coefficient of 0.60 by 3 months. The NEC bifurcation location is positively correlated with the ITF volume transport with a correlation coefficient of 0.43.

The Makassar Strait throughflow and its jet

A nested numerical model system has been set up to realistically simulate almost 40 years of the Indonesian throughflow (ITF). A global circulation model delivered the boundary values for sea surface height, temperature and salinity distribution to a fine resolution regional model of the ITF. The results of the regional model are in good agreement with measured data regarding velocity distribution, stratification as well as transported water masses, even though the division of the ITF volume transport into its western and eastern branches differs slightly from estimates from other model results or measurements. The results show a current system highly variable in space and time. Here, the analysis of model results focuses on the western branch of the ITF, the Makassar Strait throughflow, which is estimated to account for up to 50 to 80% of the entire ITF. The climatology and the mean vertical structure are presented. The model results show that the Makassar Strait throughflow occurs as a distinct current, which we termed Makassar Current. It is regionally developed as a subsurface jet, and it behaves like a western boundary current as it is attached to the western boundary (Sunda Shelf edge) along its entire path from the Sulawesi Sea through the Makassar Strait to its direct exit through the Lombok Strait. It appears from model experiments that the local wind stress counteracts the Makassar Strait throughflow.

Interannual to Interdecadal Variabilities of the Indonesian Throughflow Source Water Pathways in the Pacific Ocean

Abstract Some of the possible interannual to interdecadal variabilities of the Indonesian Throughflow (ITF) source water pathways in the Pacific Ocean are identified from an ocean reanalysis product provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) under the name Ocean Reanalysis, version S3 (ORA-S3). The data were used in an offline mode to evolve adjoint pathways of a passive tracer, which is injected from the key channels of the Indonesian straits where the ITF enters into the Indian Ocean. The adjoint pathways are simulated using interannually varying circulations for 41 yr starting from December 2000 to January 1960 with reversed currents and other physical parameters (control run). A climatological run for the 41 yr is produced with the reversed currents and other physical parameters as a monthly climatology. The adjoint pathway variability is found by subtracting the climatological run from the control run. The empirical orthogonal function (EOF) analysis carried out over the monthly differences between the tracer concentrations of the control run and the climatological run shows that the ITF is largely supplied from the northwestern tropical Pacific during a normal year, whereas the supply from the south equatorial Pacific is dominant during El Niño–Southern Oscillation (ENSO) years at a lag of 6 months. The interannual variability of the ITF source water pathways in the Pacific is largely determined by the ENSO variability and they are confined to the tropical Pacific, whereas the corresponding interdecadal variability is controlled by the meridional overturning circulations in the tropical and subtropical Pacific. The adjoint pathways hint that the ITF volume transport may have interdecadal variability; they are closely related to the variability of the subtropical cells (STCs) in the Pacific Ocean and can be quantified using the tropical convergence changes. The ITF is just an active member of the recharge–discharge of tropical warm waters at all time scales, and its role in the coupled climate variability of the Indo-Pacific needs to be assessed in that context.

Change in the Indonesian Throughflow with the climatic shift of 1976/77

A climate shift occurred in 1976 with Pacific equatorial temperatures experiencing a sharp rise and was first identified as a change in the background state of the El Niño Southern Oscillation. The associated weakening of easterly trade winds across the Pacific led to our hypothesis that the Indonesian Throughflow (ITF) had also weakened. The change in volume transport of ITF before and after December 1975 was estimated using all the available subsurface temperature data on the IX1 expendable bathythermograph (XBT) line between Australia and Indonesia. Sea surface temperature (SST) rose by 1–2°C, which could be due to increased air‐sea heat flux and/or a change in regional circulation. A subsurface cooling in the main thermocline was attributed to a weakening of the Pacific trade winds. The South Equatorial Current (SEC) diminished in size and weakened in strength. The net westward volume transport between Australia and Indonesia showed a decrease of ∼2.5 Sverdrups, or 23%.

Influence of Low-Frequency Indonesian Throughflow Transport on Temperatures in the Indian Ocean in a Coupled Model*

Abstract The relationship between 3- and 10-yr variability in Indian Ocean temperatures and Indonesian throughflow (ITF) volume transport is examined using results from a 300-yr integration of the coupled NCAR Parallel Climate Model (PCM). Correlation and regression analyses are used with physical reasoning to estimate the relative contributions of changes in ITF volume transport and Indian Ocean surface atmospheric forcing in determining low-frequency temperature variations in the Indian Ocean. In the PCM, low-frequency variations in ITF transport are small, 2 Sv (1 Sv ≡ 106 m3 s−1), and have a minimal impact on sea surface temperatures (SSTs). Most of the low-frequency variance in Indian Ocean temperature (rms > 0.5°C) occurs in the upper thermocline (75–100 m). These variations largely reflect concurrent atmospheric forcing; ITF-induced temperature variability at this depth is limited to the outflow region between Java and Australia extending westward along a band between 10° and 15°S.